How to size heat exchanger.. I think this will work

[Bricks]

Never could find anything to help me figure out heat exchanger size for my garage

Would this formula work, I believe it would get me close.

I have an old LP furnace that puts out 100,000 BTUs Figure it is only % 50 efficient. I take 50,000 devided by 60 minutes which gives me 844 BTUs a minute. Pick a day that is close to 0 F outside and time my furnace how long it runs for during an hour. Say it runs 8 minutes. 8 Minutes X 833 BTUs = 6,664 BTUs per hour required to keep at set temp. Add 20 % percent 1333 BTUs + 6,664 BTUs = 7997 BTUs My water to air heat exchanger would need to be in this range....Correct ? Comments....

 

[Medman]

I would suggest using a true heat loss calculator like slantfin - its free at www.slantfin.com. Your reasoning sounds correct but in reality you have to size the heat exchanger for the actual heat load, not the current heat input to the load.

In my case, I was running an electric forced air furnace at 68,000 Btu/h and it was not able to keep up to the load at all, even though my calculated load worst case was 46,000 Btu/h. I installed a new air handler with a 120,000 Btu/h heat exchanger sized to fit in the plenum. My boiler output is only 85,000 Btu/h max, in reality probably about 70,000. I achieve a 15*F drop across the exchanger with an air flow of 800 CFM, 20*F drop at 1200 CFM. This is able to heat my house with capacity to spare.

Once you do a heat loss calculation on your house, you can size the heat exchanger to your needs. I would suggest going at least as big as your plenum dimensions to mazimize air flow in the system.

Also, remember to have the water flow through the exchanger running counterflow, to maximize heat transfer.

There has been much discussion here regarding size and construction of water to air exchangers - do a search. Buy the best one you can afford - you don't want to be messing with it later.

 

[Bricks]

The problem I am having is I can`t find a program that will allow heat loss calcs for 2 10 X 16 insulated commercial over head garage doors. At least I have not had any luck.

For the garage I think I just need to at least get kinda close for HX size.

 

[Medman]

Hydronic garage space heaters, as stand-alone units, are usually in the 30 - 50,000 Btu/h range. I am considering a 30,000 Btu unit for my car bay which is 24x24, just to take the edge off when I need to do work in the winter. Insulation helps, but is not enough on the 20 below days.