SonexBuilders.net

Thanks Mark for that - super helpful!

GordonTurner wrote:For compRison, do you know how the numbers look with some typical form of exhaust heat based system?

Gordon

I don't. We could calculate it easily if we knew the CFM and temperature rise of the air.
The exhaust-based systems have a big advantage in that the temp difference between the heated surface (exhaust pipes) and the working fluid (heated air) is very high ( ? 800 deg F?). In a typical radiator core, the temperature difference will be much less, so proportionally more area is required to exchange the same amount of heat (given similar CFM of the air).

...a couple of basic HVAC equations to estimate what you need...

heating application so sensible heat only...

Start on the water side...

the formula you need on the water side is
Q (btuh) = GPM * 500 * (T1-T2)

T1 - T2 is the water temp drop across the rad coil.

Similarly, having calculated Q...for the air side...

Q (btuh) = 1.08 * CFM * (T1-T2)

T1 - T2 is the air temp drop across the rad coil.

Assume an efficiency loss... meaning the Q actually transferred to air will be something less that the heat in the water. Guessing 70 - 80%. Coils used in HVAC apps have tables with this data...you're just looking to get close.

You can make it more complicated by including actual density of air and water but for simple space heating its just not necessary. The formulas apply for the heat rejection coil as well.

peter
Doylestown, Pa

...T in deg F

It finally helped me with heating problems. Nice suggestion given in this thread.