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

[Just Dallas]

Removed

[JohnEd]

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.

Thanks again,

John

[Busted Knuckle]

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!

I know it's true because they say so in their commercials!
 BK  

[Len Silva]

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.

[bryanhes]

Back from the X-Mas party  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!!  You sure miss allot here in a day 

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

[buswarrior]

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

[kyle4501]

"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 . . . 

[NewbeeMC9]

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? buy more cars??  work twice as much???  maybe you should just get hamsters

[JohnEd]

Kyle,

If that is your worst performance you walk on water.

Thanks,

John

[kyle4501]

It ain't walking . . .

More along the lines of $___ floats


   

[PCC]

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.

[kyle4501]

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 . . .

[PCC]

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)

[JohnEd]

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 

[kyle4501]

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 . . .)