BTu Question for the Science Guy Types - Please Use Small words

[Just Dallas]

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[Tom Y]

Maybe more fluid flow thru the 60,000 btu model?   Tom Y

[Lin]

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.

[Nick Badame Refrig/ACC]

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-

[Just Dallas]

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[WEC4104]

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.......

[buswarrior]

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

[kyle4501]

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 . . . 

[Just Dallas]

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[Nick Badame Refrig/ACC]

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-

[junkman42]

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

[Nick Badame Refrig/ACC]

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-

[Just Dallas]

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[Just Dallas]

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[kyle4501]

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 . . .