Sizing heat exchanger

[KRB64]

Finally bought an OWB (Taylor). I'm tying it in to my existing forced air propane system. I've done some searches but can't really find what I need to know, so:

What size heat exchanger to buy? Do I go by BTUs based on the existing propane unit (100K) or just fill up the existing duct (20"X24") or do I need to be factoring in anything else? I'm assuming my propane unit gives me the data I need to know as far as BTUs or am I missing something here? Its been working for 6 years now.

Looking for words of wisdom.

Will also be looking for aquastat/thermostat advice but I've found enough old threads on that to keep me busy for awhile!

 

[Butch(OH)]

While I am sure there some differences in makes and models I just filled the duct with mine and its about the same size as your duct. Not at home so I cant measure but mine is close to your measurements and believe me there is no lack of BTUs to heat the average house. Our furnace is a natural gas Amana hi ef and the fan runs less often and for shorter periods with the OWB and heat exchanger than it did when burning gas. The duct temps are also hotter. Again dont know the furnace BTUs but can get them if need be. I am sure there is science that can be applied but I just filled my duct since it would be major work to use one that is larger or much smaller than the duct. I also made mine on a slide so it can be slid out of the duct in the summer. I didnt want the additional restriction to my A/C. Some say its not necessary but it added little work or expense. Sorry my camera is down so I cant provide a pic of the slide.

 

[Madspeed]

My heat exchanger was installed just like Butch's. Fit the largest exchanger in that would fit. Here's alink to the same one on ebay... click here. These guys have been very helpful to me and I plan on purchasing the rest of my goodies from them once my burner arrives. They are more than willing to answer your question. I have no stake in them, just recommend them due to my experiences.

 

[Madspeed]

They have a 20x20 listed at $228 with free shipping that is quoted to be 185,000 btus.

 

[projectsho89]

Do not under-size (physically) the HX. Make certain that all the duct air flows through it. If necessary, buy a larger HX and either let it hang out of the plenum or enclose the "extra".

If you're getting too much heat output, you can always restrict or reduce the flow of hot water.

Steve

 

[mistergreentree]

I used a 20 x 25 in mine. My house is 3300 sq. ft. including the basement and it provides plenty of heat. I have a Shaver 165 furnace hooked to it. I am wondering about restricting the water flow due to the heat that I am getting at the registers. I want to get an infrared thermo to test the temp and see what I am getting out of the registers.

I basically got that size because that's what fit.

 

[KRB64]

OK thanks. So it sounds like I will just fill my duct up then. The phlenum (SP?) necks down from 20x24 to 20x12 after the furnace so I can fit practically any size HX but I'll go for the biggest based on this feedback. Thanks for the link too by the way.