Water to Air heat exchanger sizing

How do I size the heat exchanger? The plenum coming off the air furnace is 18x20 and goes up to a 26x26 Not sure on what to look for in heat exchangers. My gas heat is 70,000 btu ouput if that helps (not enough improperly sized)

For an air system, btuh=1.08 (delta T)(CFM)
I would guess 50 degree heating delta (120-70)
so the CFM = 1300 (about), but it could be higher.

get the largest one to fit your opening as you have to make sure you close off any air flow except thru the ex. You want the air flow to not be restricted and not bypass at all. a 18x20 will do it.
leaddog

I agree with leaddog, don"t go small, I have a 75,000 btu furnace with a 20 x20 exchanger, which I think it is capable of 100,000 btu"s if I"m not mistaken. Go bigger not smaller . Have no problems with a/c due to air flow , and very nice heat.
Keith

Answered perfectly Wasn't sure about restriction.. My furnace is 88000 btu in and 70,000 out. according to the label.