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[lasor1]

Hello I hve a 2 story house with full basement I am wanting to heat just main floor level it is close to 1200 sqft the existing duct size off of air handler is 17 x 20. Would a heat exchanger around that size work for the sq ft I have or do I need to make a transition to be able to fit a bigger heat exchanger I will also hook up the 2nd level later on down the road it is roughly 820 sq ft. I a had a heat pump at 1 time up there but was taken out before we took over this house all duct work is there I should be able to to make an air handler for it and what size do you recommend for the upstairs. Aslo the house has plenty of windows and descent insulation it is a 25yr old house. Any recommendation would be apprecaited thanks!

[RSI]

It is going to be hard to find a heat exchanger that is the 17" dimension. You will probably need to either use a 16x20" or if possible it I would try stretching the duct to 18" so you can use a 18x20. To get the most possible surface area you need access to both sides of the duct. Pretty much any heat exchanger you will find will have a 1" lip at each end where the pipes loop around. The heat exchangers are sold by finned area and do not include these 2" in the dimension.
If you leave 1" of sheet metal on the duct you can bend it out 9 degrees and screw to the heat exchanger flange. Since cutting both sides out, just slicing the corners a bit farther should easily get you an extra 1" of width.
Otherwise, if you only have access to one side of the duct you will probably have to use a 16" x 18" heat exchanger. It should put out enough heat for what you need but will cause some restriction. Probably not too big of a deal unless you have A/C running through it.

[lasor1]

Thanks for the tip and info ihave access all the way around duct and it does have a coil in bottom of air handler yeah I might try to stretch it out or have a transtion piece made to fit maybe a 20 x 20 in it would that be ok to do if not I will do the 18 x 20 like you was talking about doing thanks for your help.

[RSI]

I don't think there would be any need to use a 20x20.
Most people would probably just throw a 16x18 in there but I personally would try to get the 18x20 in and if that won't work out then I would use a 16x20.
The size probably varies somewhat between brands but here are some dimensions that might help.
Finned Area 16"x18"    Outside dimensions    16 1/4"x20"x3 1/2"
Finned Area 16"x20"  Outside dimensions   16 1/4"x22"x3 1/2"
Finned Area 18"x20"    Outside dimensions    18 1/4"x22"x3 1/2"
The outside dimensions are only for the tin and do not include the copper manifolds

[lasor1]

Thanks rsi!

[Treewinder]

An 18 x 20 will give you around 140K BTU, more then enough for your SF.

[willieG]

i am no heat ing guru and i know there are some on here and they can straighten me out if i get too far off line here but it is my belief that most gas or oil furnaces  have burners set to shut off if the plenum gets too hot (and i think i was told about 170) and if you have yoru OWB set to that high or higher and your old furnace was able to heat your home then if you fill the plenum with rad so no air can slip by withour being heated then you should have no problem heating your home as you should be constantly supplying 160 or better water temps (if you OWB is sized properly)and that can heat the air going by as fast as your old furnace could? the only time you could'nt heat your home would be if the heat loss of your home was greater than your supply and if you never had a problem with your old furnace and ductwork than you should not have one with your OWB thats my take on it anyway ,,right or wrong