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[Rudy151]

I need some help sizing my water to air Heat exchanger for my furnace heat duct work. First I have a new H2 Hardy Stove it is about 40 ft from the house. The house is Log 30X40 with full unfinished basement, half loft with cathedral ceilings and open floor plan built in 1996. It has a 3 ton heat pump and the duck work coming out of the top of the inside unit where the Heat exchanger goes is 11 1/4 X 17 3/4. If you need more info just let me know. Any help would be appreciated

[bajonesy77]

 :post:Your home sounds very similar to mine. I'm using a 18x20 that came with my used stove and it fit perfectly into my air handler just above the return as my duct work is small like yours.

[Scott7m]

If you are installing it into duct work, metal or duct.  I would use a 12x18.  Don't think for one second a 12x18 won't do the job, I have 12x18's on houses over 4000 sq ft.

If your putting that into your duct work, it is the closest fit you will find.  Cut it out, slide it in and tape around it with furnace tape. 

If you need help let me know

[Rudy151]

Scott7m
Is that 12 X 18 the finned part or whole thing over all?

[Scott7m]

12x18 is the part that goes into the duct.  The pipe portion sticks outside the duct, and the plate fastens up to the duct

Pm sent

[yoderheating]

A 12x18 coil is rated at 80,000 btu.  On a 3 ton unit I normally don't go below a 16x18. Under sizing the coil will choke down the whole system. I may go overboard on coil size but I want people to brag on the extraordinary difference between the heat pump and the outdoor furnace. Besides, if the customer is going to drop 8 grand on a system it better knock their socks off. 

[Scott7m]

You think the fan running an extra 20 seconds or something is going to cause the electric bill to be a noticeable differnence? 

When a fan kicks on it draws far more juice than after it's rolling.  I think folks often do oversize coils a lot, I installed a couple 12x18's because it's all that would fit into 2 large homes.  One was a log home over 4800 sq ft, customer loves it and says his home has never been so comfy. 

What I learned, it's not always best to blast a home with a huge coil, the fan wholes it's running is circulating air through the entire home, eliminating hot and cool spots,, and keeping the upstairs from being dramatically warmer than the bottom floor. 

Then, in further trainings I learned that a lot of top hvac guys have fans that never shut off, ever.  They operate at a really slow speed, and have a pump that kicks on with the thermostat to put heat into the coil when need be, u can still do this one one loop and use a 20 plate as well if it's plumbed properly.

The other day I was called out to do an estimate and found a retired hvac business owner.  He showed his unit, it was built in the early 70's, obviously top of thr line, he said his fan hadn't shut off in 40 years except when the electric was out.  It was an oil furnace with ac added on.  He said he would never live in a home that didn't have a low speed fan operatiion that ran 24/7. 

I am going to build a house in the spring and plan to use this technique, I am also encouraging my customers that "can" do this to do so, I try to sell comfort as much as I do "cheap heat"

[yoderheating]

One of the other things to consider is the restrictions in the duct. If the coil isn't large enough you could actually be reducing the efficiency of the existing unit.

[Rudy151]

A 12x18 coil is rated at 80,000 btu.  On a 3 ton unit I normally don't go below a 16x18. Under sizing the coil will choke down the whole system. I may go overboard on coil size but I want people to brag on the extraordinary difference between the heat pump and the outdoor furnace. Besides, if the customer is going to drop 8 grand on a system it better knock their socks off.

How much is a 16 X 18 coil run ?

[Scott7m]

193 to your door on 16x18 is what I get...