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Maybe more fluid flow thru the 60,000 btu model? Tom Y
Sometimes you can get really good info from the manufacturer's tech support people. There could be different tube designs that effect the flow of the fluid, different fin designs effecting the air flow, and even different alloys. Of course, there just could be different rating parameters that lead to inflated claims like one is rated assuming 70 degree air and the other is rated using 40 degree air. In summation, I don't know.
Hi Dallas,
BTU ratings have many factors. One being, surface area, second being, CFM accross the surface, third being,
thickness of the aluminum fins that transfer heat, and fourth being, volume/rate of liquid traveling through
the coils..
So, my question is, what are you trying to heat?
Nick-
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My expectation is that any heat exchanger or core that has a BTU rating, must have some underlying assumptions that should be stated along with the claim. They probably figure that the liquid being pumped through it is a vehicle coolant (with 50/50 antifreeze) at a temperature around 200 degrees F. Next, they have to pick an ambient temperature for the BTU test. They might use 50 degrees F, 32 deg F, or something else. Who knows? Then there is the question of the flow rate through the core. Is there a standard GPM for a heater or water pump? I dunno. Dallas raises some valid questions and an interesting topic for discussion. ....I'm ready to learn.......
oh, where have the engineers gone?
Used to be some smart guys who knew and worked with this stuff hang out on the boards.
happy coaching!
buswarrior
The BTU rating of a radiator (heater core) is the same as a CFM rating of a fan. Without knowing other parameters, they mean absolutely nothing.
With fans (I'm talking about real fans, not the cheap ones), you get a fan curve that shows the fan's performance by relating the CFM flow to the pressure to the power required.
With heat exchangers, you need to know the temp of the water in, temp out & the flow rate to determine the BTU's given off. Increasing air flow will increase the heat transfer. Of course there is the point of diminishing returns . . . so there is a practical limit.
You are limited on the inlet temp due to the boiling point. The exit temp is limited by the environment.
It has been a while since I've had to do the calculations, but it is fairly easy to calculate the installed capacity of a heat exchanger if you know the flow rate & temp change of the water.
If the Webasto is rated at 40kBTU, that is probably how much fuel it can burn at max setting. Some of that heat is wasted out the exhaust, some is lost due to radiant & conductive losses in the piping & Webasto housing. So, you won't be able to get any more than that out of the heat exchangers. The reason the first one got so hot was because it had hot water going in & as the air was heated, the water temp was lowered. The next heater had a lower water inlet temp than the first, therefore, it couldn't heat the air as much as the first one could. If they were plumbed in parallel with the same water flow to each one, you'd see the same air temp on each. That is why some house systems use thermo valves before each radiator - to better control the heat output.
Hope this makes sense - been a long day at work with idiot vendors making messes & showing utter contempt for my customers needs. You'd think they'd be more concerned in this economy . . . :(
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Hi Dallas,
I know you didn't want too much tech but, there is also BTU Input and BTU Output..
Here is Wiki BTU.
The British thermal unit (BTU or Btu) is a traditional unit of energy equal to about 1.06 kilojoules. It is approximately the amount of energy needed to heat one pound of water one degree Fahrenheit. It is used in the power, steam generation, heating and air conditioning industries. In scientific contexts the BTU has largely been replaced by the SI unit of energy, the joule (J), though it may be used as a measure of agricultural energy production (BTU/kg). It is still used unofficially in metric English-speaking countries (such as Canada and the United Kingdom), and remains the standard unit of classification for air conditioning units manufactured and sold in many non-English-speaking metric countries.
In North America, the term "BTU" is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills, and air conditioners. When used as a unit of power, BTU 'per hour' (BTU/h) is understood, though this is often abbreviated to just "BTU".
The unit MBTU was defined as one thousand BTU presumably from the Roman numeral system where "M" stands for one thousand (1,000). This is easily confused with the SI mega (M) prefix, which multiplies by a factor of one million (1,000,000). To avoid confusion many companies and engineers use MMBTU to represent one million BTU. Alternatively a therm is used representing 100,000 or 105 BTU, and a quad as 1015 BTU.
U.S. Department of Energy - Energy Efficiency and Renewable Energy
Energy Savers
Sizing Heating and Cooling Systems
Older space conditioning systems (more than 10 years old) are often unreliable and much less efficient than a modern system. When it's time for a new replacement, choosing one of the correct size (heating and/or cooling output) is critical to getting the best efficiency, comfort, and lowest maintenance and operating costs over the life of the new system. Some national surveys have determined that well over half of all HVAC contractors do not size heating and cooling systems correctly.
The most common sizing mistake is in oversizing. This not only makes the new system more expensive to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. Oversized heating equipment also often creates uncomfortable and large temperature swings in the house. Oversized air conditioners (and heat pumps) do not run long enough to dehumidify the air, which results in the "clammy" feeling and unhealthy mold growth in many air-conditioned houses (see dehumidifying heat pipes as one solution to this problem).
Incorrect Sizing Methods
It is the installer/contractor's job to perform the correct sizing calculation for the building. However, many installers only check the "nameplate" (the label on the unit that has the Btu per hour output among other things) of the existing system and sell you one just like it, or even worse, one that's larger. This is a not a correct sizing method and not in your best interests! Other methods include simple "rules of thumb" based on the size of your home or using a chart that accounts for a variety of factors. While these methods might provide a first estimate, they should not be used to size your system.
Why Most Older Systems are Oversized
Before the era of tightly constructed homes, it was not uncommon to install furnaces and air conditioners that had two to four times the necessary capacity. Since many people have added new windows, caulking, weather-stripping, and insulation to their homes, going by the nameplate is likely to result in an oversized system. Making improvements such as these to reduce heat loss in the winter and heat gain in the summer should allow you to install a smaller systems while still being comfortable, as well as saving large amounts of energy.
Manual J and Manual D: The Correct Way to Size a System
Correct system sizing requires considering many factors other than simply reading the nameplate of the existing unit. Key factors for correctly sizing a heating and cooling system include the following:
The local climate
Size, shape, and orientation of the house
Insulation levels
Window area, location, and type
Air infiltration rates
The number and ages of occupants
Occupant comfort preferences
The types and efficiencies of lights and major home appliances (which give off heat).
Homeowners should insist that contractors use a correct sizing calculation before signing a contract. This service is often offered at little or no cost to homeowners by gas and electric utilities, major heating equipment manufacturers, and conscientious heating and air conditioning contractors. Manual J, "Residential Load Calculation," published by the Air Conditioning Contractors of America (ACCA), is the recommended method for use in the United States. There are also many user-friendly computer software packages or worksheets that can simplify the calculation procedure. You should make sure that the procedure used by the contractor follows Manual J.
If ducts are part of the installation, they should be sized using the ACCA's Manual D, "Residential Duct Design." The ACCA also offers a comprehensive guide for choosing home heating and cooling systems, called Manual S, "Residential Equipment Selection."
A Special Case: Sizing Steam Heating Systems
One exception to the above is in steam heating systems. For these systems, the boiler should be sized to match the radiators. However, there is still room for energy savings. First of all, the original boiler may be oversized for the radiators, so the contractor shouldn't just order the same capacity boiler, but instead should match the boiler to the radiators. Second, if you've increased the energy efficiency of your home, it may have more radiators than it needs.
It may be possible to remove radiators in the core of the house and shift the others around, replacing larger radiators with smaller ones. Since radiators are modular, it is theoretically possible to downsize a radiator by removing sections; in practice, this is usually difficult to do without damaging them. In many parts of the country, used radiators are available cheaply, so you can potentially buy small radiators to replace large radiators; if you do so, be prepared to replace the shutoff valves as well, since they often won't match. Newly manufactured steam radiators are available as well.
In any case, you should work with a heating and cooling professional when downsizing your system. Your house's heating needs should be calculated using Manual J, and your radiators should be downsized appropriately. Match the new boiler to the remaining radiators. Note that balancing steam heating systems is more an art than a science; ideally, you will find a heating professional with experience in steam heating systems.
Steps a Good Contractor Should Take to Size Your System
Many factors affect a home's heating or cooling requirement, or "load." A good estimator will measure walls, ceilings, floor space, and windows to determine the room volumes, and will assess the R-value of the home's insulation, windows, and building materials. A close estimate of the building's air leakage is also necessary. A blower door test is the best way to measure air leakage.
A good estimate will also include an inspection of the size, condition of seals on joints and insulation, and location of the distribution ducts in forced air systems. The placement of supply and return registers should be appropriate for the system type and size.
The orientation of the house also affects heat gain and heat loss through windows. Overhangs can reduce solar gain through windows. Make sure the contractor uses the correct design for the outdoor temperature and humidity in your area. Using a higher summer design temperature results in oversizing air conditioners.
When the contractors are finished, get a copy of their calculations, assumptions, and the computer printout or finished worksheet. This is your only proof that they did the job right. To summarize, when designing your new heating and air conditioning system, the contractor you choose should do the following:
Use a computer program or written calculation procedure to size the system
Provide a written contract listing the main points of your installation and includes the results of the heating and cooling load calculation
Give you a written warranty on equipment and workmanship
Allow you to hold the final payment until you are satisfied with the new system.
Learn More
Codes & Standards
Manual J Residential Design Center
Air Conditioning Contractors of America
Now your really lost! Lol
Nick-
Nick, whats with the einstein stuff? I thought You were the air conditioning Man? LOL, the next time I have to call the heat pump guru I am going to ask Him about the term Joule. Joule is a common term in electronics and welding especially in the electron beam welding world. Anyway had to harass you, sorry the debil made Me do it. Regards John
Quote from: Now Just Dallas on December 11, 2009, 05:37:41 PM
Nick,
This is more of a learning process for me and maybe others.
What I'm trying to learn personally is how they come up with the figures for Btu on each of different cores, but both made of copper and brass.
Here's another for instance: A friend just installed a 40KBtu Webasto/Espar/Proheat.
He had it set up with 3 forced air cores each capable of 30KBtu. (we'll call them in series).
The first core would get hot enough to burn your hand, but the second and third would only get lukewarm. Coolant temp at the return was in the 115° range. Why?
Well, each unit fan coil should be piped "home run" or piped equally from a manifold.
Your first unit fan coil that is run in "series" is disapating heat into the room. Thus a temp drop is occuring and leaving the
next unit fan coil left with a lower temp to disapate into the room. And so on and so fourth..... Running each unit fan coil
"home run" from a manifold will deliver equal temps to each unit fan coil giving you even temps at all locations. One factor
to concider is resistance! Your coolant/water will also flow to the route of least resistance. Meaning, when you have many
elbows, or tee's or even a smaller size pipe in a particular zone, you are creating resistance and restricting flow.
Nick-
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Too many variables to make it simple.
Some of those variables are too difficult to determine the true value for.
Bottom line - most can cool their coaches with 2 roof airs, so 60kbtu cooling is minimum starting point.
I'd start with 40kbtu when searching for a heater.
If you want to perform an experiment to determine your actual heating needs, see how many 1500 watt electric heaters are required to keep the bus at a comfy temp when it's really cold outside - then it is a simple conversion from watts to btus . . .
Quote from: Now Just Dallas on December 11, 2009, 08:40:09 PM
That's great Nick, but.....
How do I know if I'm overdriving or underdriving a given heat exchanger in a given application without having a formula that tells me that if I have a heat source with X Btu output connected to a heat exchanger with Y Btu transfer with N cfm of air moving across it, R gpm coolant moving through it and A surface area, I can expect Z results?
My comment to use small words was a bit tongue in cheek, but I think many of us would like to know that a particular system will or will not work. It shouldn't be rocket surgery to find a generic, workable formula that will give a general idea of usability.
Dallas,
When I measure a bus conversion in the real world, I take into concideration Heat Loss more then anything esle because our busses
are nothing like residential homes where we can calculate with the Manual J method. Sooo, Try this. Bring your bus to 70 degrees
inside with outside ambiant temp of 30 to 35 degrees. Then turn the heat off and calculate how long it takes to drop 10 degrees inside.
If the 10 degree drop is within 15 mins, you now have to look outside your bus from inside from your drivers window area. If you see
snowflake shaped crystels on the windshield, you can concider yourself one cold SOB!! and need a bigger heater!
LOl sorry, I couldn't help myself..... I know, I'm mean! So, where did you think this was going anyway!....... ;D
Me!
Nick,
Smart@&% :D :D ;D. And here we thought you were getting ready to teach us something, LOL! Was funny though ;D
I have been reading through this post as I will be trying to figure out how many heat exchangers I would need to run as well in mine. It does make sense though to have the source such as a manifold that feeds all exchangers but what would you do about restricting return
flow?
I just played a little and put this together based on how I interpreted the posts (nothing fancy, its to late for that ;D). Is this what you were meaning? I would also think with it set up like this it would have to flow back to the supply with little restriction?
Bryan
Hi Bryan,
Your engine should also be a "Zone" or at least valve it so it can be taken out of the loop for stationary use.
Your returns should be piped to a manifold for equal flow back to the webasto.
Nick-
Dallas, the formula for a HX rate is Qrate= mrate * Cpof liquid * (Tin - Tout)
Qrate will be in BTU/hr
Mrate is how much mix you have going through the HX it is usually in gpm and you need to convert to pounds/hr you can measure this by taking the return line loose at the return header and time the flow of it going into a bucket. or if you can find a flowmeter even better.
you can weigh a gallon and assume 1 lb = 1lbm for ease of calc and close enough for bus
For Cp(Specific heat capacity) of antifreese, units BTU/lbm-F degrees
http://www.engineeringtoolbox.com/ethylene-glycol-d_146.html (http://www.engineeringtoolbox.com/ethylene-glycol-d_146.html) page down to table on Specific heat capacity
You can use you handy HF IR thermometer to check the Temperature in and Temperture out.
This will get you a close estimate to what your getting out in your setup. :)
Good luck and let us know what you come up with. :)
kyle4501, you meant 30K BTU of cooling minimum, I think...
You doubled your double.
Tim Horton's?
happy coaching!
buswarrior
Nick,
Is this what you are talking about? I changed the diagram to include an additional manifold and two shut off valves.
Bryan
I like that.
How about some isolation valves for each circuit?
If some thing leaks or needs to be replaced...
happy coaching!
buswarrior
BW,
I will put that in my next revision ;D Heading to OKC for X-mas party.
Bryan
I can recommend some books to read if you want to learn a bit more about this:
- Handbook of Air Conditioning and Refridgeration (2nd ed.) (http://www.amazon.com/Handbook-Conditioning-Refrigeration-S-K-Wang/dp/0071189815/ref=sr_1_1?ie=UTF8&s=books&qid=1260644536&sr=1-1)
- Modern Hydronic Heating (2nd Ed.) (http://www.amazon.com/Hydronic-Heating-Residential-Commercial-Buildings/dp/0766816370/ref=sr_1_1?ie=UTF8&s=books&qid=1260644706&sr=1-1)
- Testing and Balancing HVAC Air and Water Systems (http://www.amazon.com/Testing-Balancing-Water-Systems-Fourth/dp/0849393590/ref=sr_1_1?ie=UTF8&s=books&qid=1260644309&sr=8-1)
-T
Quote from: bryanhes on December 12, 2009, 09:15:09 AM
Nick,
Is this what you are talking about? I changed the diagram to include an additional manifold and two shut off valves.
Bryan
Very good bryan!
Now, try and add either a tank with a fill to add coolant to your system or just a fill port that will hold presure in the return
line before your webasto. This fill or tank will need to be above the heat exchangers to eliminate air in the system. Another
member here on the BBS made a real neat fill port that he constructed behind his kitchen sink. A panel lifted up and there
it was.... Real clever!
Nick-
"Most can cool with two roof airs." True. However only 27K or 30K BTU's. Not 60K.
Roof air is 13.5K or 15K BTU's.
Webasto spec is heat output. Not based on fuel input. And also new output is 45K BTU's. Fuel pressure was increased about three or four years ago.
Side note: Webasto doesn't spray the fuel. It produces an extremely fine fog.
For heating systems in your coach Sure Marine assumes 175F fluid and a flow rate and 12+VDC to fan motors to get the rated output.
I have noticed that the heaters take about 5-6 mins to really get to full output. This correlates nicely with the Webasto coming up to operating temperature.
Nick, excellent explanation on the sizing of a house heating system. We went through the exercise about 6 years ago to change out a 1967 heating boiler. Considerable downsizing of the boiler. Running a slightly smaller nozzle than the old one had a end of life. And we're using a lot less fuel oil.
If I run a cube heater rated at 1,500watts in my coach it will raise the inside temp 30 degrees above the outside. I assumed that it would do that no matter how hot or cold it was outside. It seems to work out as a theory and I am comfy down to (74-30) 44 degrees, The watts translates to BTU and I assume that I could accurately determine my furnace capacity requirements. My propane furnace is rated at 48KBTU and it will keep the interior sauna warm down to the -8 degrees I have been in.
Dallas,
If your second heat exchanger has 90% of the fin area of the one that has a BTU label then the unknown most probably will have a 90% of the BTU rating. Logical? Find a yardstick!
John
Bryan your latest revision is more or less how our system is plumbed. The problem with a parallel system is that you need to maintain a sufficient pressure differential between the two manifolds to ensure equal flow through all the heat exchangers. Otherwise the flow will be preferentially to the lowest resistance circuit. In our case the ProHeat pump simply isn't adequate to maintain that pressure differential. I have added a slave pump in the highest resistance loop which is a work around that does pretty well but the right solution would be to have enough excess pump capacity to maintain the pressure drop between the two manifolds. Some engineer could calculate how much that pressure drop needs to be to sustain maximum flow through all your heat exchangers. You could probably accomplish the same goal with throttling valves in the low resistance circuits (which would also restrict your heat output) and in theory you could also do it with flow dividers but I don't know whether they exist for this application.
This is kinda related and just FYI. When we installed our ProHeat, we wired all the heat exchanged 12 volt circulating fans to an aquastat that is installed in the hot water line from the ProHeat. The fans do not come on until the water temperature is 120 degrees. When the ProHeat shuts down the fans continue running until the water temperature is 120. This prevents that initial blast of cold air when the ProHeat first comes on. Jack
JohnEd, you got that 30 degree rise with your cube heater under the outside temp and wind conditions at that time. If it is colder outside, it will take more heat/more cube heaters to do the same job.
Some law of physics, rats!
Wind is a huge player in air infiltration. Exchanging air via the cracks is the biggest devil for HVAC.
The interesting part of this discussion, it might determine that many of us have a lot more heating capacity than we need, depending on how and where we use the coach.
FWIW, 40K BTU is roughly equal to 8 of those 1500 watt cube heaters.
Now, before someone goes and fine tunes themselves a little too close, remember, same as the engines in our vehicles, excess capacity is what gives us better acceleration...
So, lots of furnace capacity will warm the coach up from cold faster, the same way lots of AC will cool down a hot coach faster.
But, excess power typically uses more energy to cruise, once the chosen "speed" has been reached, than if a power plant rated closer to the energy needs of cruising is chosen.
Another vote for multiple small devices instead of a single large one?
Same as the cylinder shut off in all the big V8's now for efficiency, and the greatly sought after redundancy if one of them breaks.
Great fun!
happy coaching!
buswarrior
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BW,
Great post and a really useful clarification/expansion of my overly brief and simplistic example. All of your points are valid and the info you provided brought it all together for me. And as Dallas said yours is the Spirit...
My thirty degree rise was seen in a deep and steep little valley in Pennsylvania. No wind ever. Kansas would be different, I guess. :P ;D ;D ;D
Thanks again,
John
All great info!
But when I need answers to high tech stuff like this, I jest drop by Home Depot or Lowes! They only hire highly trained experts that have all the answers! ;D
I know it's true because they say so in their commercials! ;D
;D BK ;D
Quote from: buswarrior on December 13, 2009, 06:26:54 AM
JohnEd, you got that 30 degree rise with your cube heater under the outside temp and wind conditions at that time. If it is colder outside, it will take more heat/more cube heaters to do the same job.
Some law of physics, rats!
Wind is a huge player in air infiltration. Exchanging air via the cracks is the biggest devil for HVAC.
The interesting part of this discussion, it might determine that many of us have a lot more heating capacity than we need, depending on how and where we use the coach.
FWIW, 40K BTU is roughly equal to 8 of those 1500 watt cube heaters.
Now, before someone goes and fine tunes themselves a little too close, remember, same as the engines in our vehicles, excess capacity is what gives us better acceleration...
So, lots of furnace capacity will warm the coach up from cold faster, the same way lots of AC will cool down a hot coach faster.
But, excess power typically uses more energy to cruise, once the chosen "speed" has been reached, than if a power plant rated closer to the energy needs of cruising is chosen.
Another vote for multiple small devices instead of a single large one?
Same as the cylinder shut off in all the big V8's now for efficiency, and the greatly sought after redundancy if one of them breaks.
Great fun!
happy coaching!
buswarrior
Maybe we should return to the days of the early Scenicruisers. Two small engines with today's sophisticated controls might give us both economy and redundancy.
Back from the X-Mas party ;D won about $68,000.00 in chips. Unfortunately I could only exchange them for raffle tickets and did not win the new flat screen for the bus :'( the wife had one to many glasses of wine and in the dead silence of about 40 people from work when the winning raffle number was called she cries out Noooo! It was funny!! :D ;D You sure miss allot here in a day :o
So if I tie the system into the engine loop and then cut the engine out of the loop when not needing to preheat should I have a secondary reservoir with a fill to keep from over pressurizing? And maybe a second pump after the return manifold?
Does this revision look as though it would solve over pressurizing and flow?
Bryan
You may retain the engine's overflow tank for expansion by only closing one valve to the engine.
Saves duplicating the function of a pretty much failure proof part of the system.
For normal running, you don't have to seal the engine off, you just need to block flow.
However, install both valves in order to isolate for repairs or failure.
happy coaching!
buswarrior
Quote from: Bill B /bus on December 12, 2009, 08:20:07 PM
"Most can cool with two roof airs." True. However only 27K or 30K BTU's. Not 60K.
Roof air is 13.5K or 15K BTU's.
Yep, as was already pointed out, I doubled after I had doubled. The hazards of posting at the end of a long day . . . ;)
Quote from: kyle4501 on December 13, 2009, 08:37:20 PM
Yep, as was already pointed out, I doubled after I had doubled. .....
Seems you've done that with buses too, :-\ Recognizing a pattern ??? are you about to have more kids with your other wife? ??? :D buy more cars?? work twice as much??? maybe you should just get hamsters :D
Kyle,
If that is your worst performance you walk on water. ;D
Thanks,
John
It ain't walking . . .
More along the lines of $___ floats
;D ;D ;D
Quote from: Now Just Dallas on December 13, 2009, 06:56:49 AM
This has been a great thread so far. The reason I posted it in the first place was to get all those excellent minds we have available working on a subject that really impacts all of us in not only our hobby but in our day to day lives.
This thread has really shown what this board is all about~ learning, teaching, garnering information and helping.
I would like to thank everyone who has posted or just read, we have a great bunch of people here.
And this is what I have learned to appreciate about this site. Thank you all for helping me.
I do have one question about BTUs - Does it take the same number of BTUs to heat as it does to cool? Is the BTU a measure of whatever is needed to change the temperature up or down?
I am not asking to consider efficiency or the method of getting the heat/cool into the coach, just want to understand the concept of measuring BTUs and how that measurement fits into the equations.
A BTU is simply a basic measure of thermal (heat) energy. (One BTU is the amount of energy needed to raise or lower one pound of water one degree Fahrenheit - assuming no phase change FROM solid to liquid or liquid to vapor).
Yes, it works in reverse & is the same thing for cooling as heating. Some of the confusion comes from the (usually) different methods used for heating vs cooling.
Burning something for heat is usually more efficient than burning something for refrigeration.
Heat pumps use the same method for heating or cooling - BUT - the efficiency depends on the outside temp & the inside temp . . .
So if I need 150,000 BTU to cool this coach (12.5 ton), do I need to estimate the same, or a little less because of solar influences and the greenhouse effect, but that needs to be my "magic number" to heat the coach when it is -20F outside ???
Open the floor and let the heat just rise in?? LOL (I am mid-engined)
PCC,
I don't have the authoritative, cut and dried, answer for you. Some things I have considered to be part of the factors in determining the answers are. For AC/cooling: Solar heat loading thru windows...Ambient (ambient 120) heat conduction thru the floor, walls, and roof with ambient 120 degrees,,,the number of people is important as each produces BTU...Air permeation through coach when stopped....air permeation when moving(could be an enormous number)....appliances heat generation incandescent lights/cooking/entertainment. For heating: BTU thru the walls, ceiling and floors at -20F, ambient air permeation when stopped and ambient air permeation when moving. They split hairs for cooling but took a broader view for heat and the temp they wanted to hold was apparently 74* F. I think a critical factor is "how fast the temp can be changed" from 140 degrees or -20 in heat soak or cold soak conditions. I would think 30 min would be satisfactory to recover.
For my home estimate they had zones that applied to your location in the US, the square footage, dual pane windows and whether my insulation was "poor or good or excellent". With that abbreviated data set the number of tons of BTU capacity for my heat pump was determined.
John
Where did the 12.5 tons of cooling come from?
If that is the size of the stock AC, you shouldn't need that much as a RV.
The stock system was over sized to accommodate a full load of passengers producing heat, plus the huge amount of fresh air (to allow said passengers to breathe). Not to mention cooling the bus quickly for the passengers comfort. . . . .
Just some random thoughts . . . .
For what it's worth, the Scenicruiser came with a 7.5 ton AC.
(lots of glass - 4 sky lights, rear glass & a second windshield . . .)
Here's something to think about...
My home needs 100,000 BTU's of heat capacity to heat it and 48,000 BTU's [4 tons] of A/C to cool it!
Nick-
I don't think we've mentioned straight out that a big difference between between heating and cooling has to do with the temperature differential.
Cooling the coach to 70 degrees while sitting in 100 degree weather, a 30 degree spread, is a whole different animal than heating the coach to 70 degrees while sitting in 0 degree weather, a 70 degree spread.
That's why the furnace is so often sized so much larger than the AC, the further from the equator you are.
And why campgrounds are more likely to charge for electricity if heating season is involved?
happy coaching!
buswarrior
I said much the same thing but my post got stuck in Gmail.
It went:
If you are cooling then the most you might need to change the temp is, 120 degrees minus 74 is a delta of 46 degrees. For heating you might get a low of -20 and you will need to raise that to 74 and that is a 94 degree delta. Given that that is even remotely true, it isn't hard to follow that your heating capacity should be approx double the BTU power. But of course that is dramatically affected, the logic, depending on where you live....what zone.
Still, what Nick says is certainly good logic if we want max flexibility to go anywhere in any season. You simply cannot live in a bus that is either too hot or too cold.
John's 2 cents( and that may be over valued)
Close, huh?
Quote from: Nick Badame Refrig. Co. on December 14, 2009, 05:53:21 PM
Here's something to think about...
My home needs 100,000 BTU's of heat capacity to heat it and 48,000 BTU's [4 tons] of A/C to cool it!
Nick-
Is that partly because the Mrs. keeps turning the thermostat up? ;)
Bryan
In your last diagram you had two pumps -- one supplying the heater and one supplying the heat exchangers. In home boiler systems the normal setup is to install the circulation pump pushing the coolant into the heater and allowing the the pressure to return the coolant.
With the two pumps one could act as a restriction for the second even if both pumps are running and are the same make and size.
The last system I installed the diagram called for the pump to be placed pushing the coolant into the distribution manifold. I called the company and asked for clarification because I was not used to seeing it set up that way. They wanted the expansion tank between the heater and the pump and manifold. I was told that it would balance the pressure better and give a more even heating when all the zones were open.
I am not sure it works any better that way but just something more to throw into the mix here. (By the way I did it just like they wanted)
Melbo
Hmmm. The boilers in buildings that I work on are normally using the circulator on the return, pushing water into the boiler. Also, the zone valves are installed on the return.
The reason for this is to keep all of these parts as cool as possible by having them at the end of the loop. This makes for a longer life for these items. Honeywell Has used a plastic gear in the zone valve motors that gives up when it gets too hot for too long.
I haven't studied any of the bus heating systems enough to find out how they are plumbed.
Tom Caffrey
Yes Tom
The zone valves are on the return as is typically the pump
That was my observation was the repositioning of the pump
The diagram has it just before the distribution manifold
Melbo
Quote from: pvcces on December 16, 2009, 07:37:22 PM
Hmmm. The boilers in buildings that I work on are normally using the circulator on the return, pushing water into the boiler. Also, the zone valves are installed on the return.
The reason for this is to keep all of these parts as cool as possible by having them at the end of the loop. This makes for a longer life for these items. Honeywell Has used a plastic gear in the zone valve motors that gives up when it gets too hot for too long.
I haven't studied any of the bus heating systems enough to find out how they are plumbed.
Tom Caffrey
I use a outside wood boiler ( OWB ) to heat my shop and have done quite a bit of experimentation on the posistion of the circulation pump.
My final layout uses a primary / secondary piping system.
I also have a oil burner boiler also in the loop.
The circulators are both on the hot side by my choice, mainly to ensure they are freeze protected.
The system typically runs at 180 degree water more than enough of the cooling effect.
I have also duplicated the shop system in the bus. I took a circulation pump that GM used ( in the hot line) to send the water to the front of the bus 40 ft away. Now typically the running temp of the bus water is 180* to 200*
I removed the webasto circulator and installed a circulator salvaged from and old GM bus and it pumps hot water to the webasto then
thru my water heater side heat exchanger then back to the engine.
Bottom line is the placement of the circulator is usually left up to the installer.
Do you recgonize this heat exchanger in the shop :)
Quote from: Nick Badame Refrig. Co. on December 14, 2009, 05:53:21 PM
Here's something to think about...
My home needs 100,000 BTU's of heat capacity to heat it and 48,000 BTU's [4 tons] of A/C to cool it!
Buses, however, and RV's in general, have a different set of issues. With a well-sealed and well-insulated coach, solar heat gain through the glass and even the metal, combined with the various heat loads inside the coach from occupants and appliances, make cooling the coach more of a challenge than heating it.
We have real-world data: Our coach has 45,000 BTU/hr of hydronic heating, and ~55,000 BTU/hr of air conditioning. We've been more than comfortable in temperatures well below freezing for days on end, and even then, the furnace does not run full-time but, rather, cycles on and off. By contrast, on a 115°+ day, the A/C's struggle to keep up, running full-tilt. Admittedly, we have a lot of glass, but we do have dual-cell thermal shades along with silvered mylar foamed window inserts, 20' awnings on both sides, and a polar white roof.
We have found, in fact, that in temperatures down even into the 40's, we can often avoid using the heat simply by parking with one side facing south, and opening the window coverings on that side on a sunny day. Of course, we also have five mammals aboard, and our inverter is in the living space.
Since I have not yet weighed in on the earlier question about hydronic runs, let me add that I strongly recommend separating engine and household systems. You will have the added expense of a heat exchanger and another small pump (for engine preheating), but there are many advantages:
- Engine coolant changes, which are required more frequently than hydronic changes, will involve far less coolant.
- The hydronic system can use inexpensive antifreeze, as opposed to the more expensive SCA-laden antifreeze required for diesel engines.
- A leak that develops in the household system will not threaten the more critical engine cooling.
- During normal parked heating operations, heat will not be wasted on the engine block, which otherwise accounts for a large loss (the exchanger itself represents a much, much smaller loss).
- The main engine water pump is not being asked to circulate through the much larger hydronic system, possibly overtaxing the pump.
- Likewise, the electric hyrdonic pump is not being asked to circulate through the main engine loop all the time; the smaller pump used for engine preheat will only be run for short periods on days where preheat is called for.
I will be covering hydronic systems during the BYOB workshop in Arcadia in January, following Bussin' 2010.
-Sean
http://OurOdyssey.BlogSpot.com (http://ourodyssey.blogspot.com)
Dallas,
This doesn't answer your question as to the formula, but it answers it in a way in what works for me. I knew nothing about how to heat my coach when I converted so I deferred to Dick Wright from Wrico . He has done hundreds, so he knew what works. He sold me the 40k Webasto with 6 7k fan forced 12v heaters. I don't know how they rated them at 7k, but that is what the stock number was.
I also bought the 45k heat exchanger to hook to the engine coolant to scavenge heat from the engine and preheat the engine.
All my heaters are in series, 3 in the lounge/galley, 1 in bathroom, 1 in bedroom, 1 in mechanical bay. The front of the bus gets the water first then head back toward lounge, then back to galley, then back to bathroom, then back to the bedroom, then to the mechanical bay, then back to furnace. So, as you can see the water will be cooler in the bedroom than the lounge. That's ok because, as Sean said, lots of windows up front means more cold air thus needing more heat.
Now, after all this was installed and we took some trips to the cold country, we found we were still cold in the lounge and plenty warm in the bed and bath. Hmmm, what to do??? I began searching for more heat exchangers. I didn't want any more 12v fans to draw battery juice, so I opted for finned radiators. I bought a 1000btu tube for behind the couch, a 500btu tube for the wall adjacent the lounge chairs, and a 500 btu tube under the dinette against the wall. This greatly improved the comfort of the lounge/galley.
Now, there is one problem with my setup, though I've overcome that to my satisfaction. Even when the front zone t-stat reaches its temperature and the back zone t-stat is calling for heat in the bed/bath, there is still heat coming off those 3 finned radiators. That is okay because its not enough to make the front uncomfortable and it is rare, because the bed/bath needs so little heat for comfort because of so few windows.
Now, when I am underway and heat is not needed I still need to heat my hot water supply, so I have a 3 way valve easily accessable in the bay I turn to bypass all heat exchangers and only make the loop through the water heater, thus no heat in the coach living area.
I guess to sum up my system--I have a few more heat exchangers (btus needed) than a furnace (btus made) can put out. If I run everything and the t-stat settings are never reached then obviously the webasto will never turn off. I've been in some minus 0 places where that has happened, but we were comfortable in the coach.
In your friend's application where it was burning hot at the first heater and cooler down the line, I would just damper the air output to send more btus down the line. I think this is easier than designing a multiple parallel system.
David
One thing I forgot to mention. My driver's bus heat and defroster are still in the coach and separate from the Webasto. When driving in freezing cold and snow at 60mph it takes the driver's heat, defroster and the whole house system to stay warm in the coach. It gets cold up front near those windows.
David