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Old 08-25-2008, 09:53 AM   #1
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Optimal Install of A-Coil


Hello all - first post. I pulled a 3 yr/old nat gas forced air furnace from a 1200 sq/ft house that got torn down. Now need to replace my furnace with this one and add central air. Purchased new evaporator coil from AlpineAir and need advice to instal it in box (plenum?) above furnace.

Looking for info regarding optimal location of A-Coil (orientation and distance from furnace). I don't want to burn out compressor or increase utility costs unneccesairily. I have a couple of challenges:

1 - Plenum is 20 1/4 (front to back) x 19 (side to side) 22 in hight. Furnace is 18 (front to back) x 20 (side to side).

2 - I have only 2 small returns 3x8 eventually conncted to 12 round duct(both to 1st floor).

Right now need to decide how far above furnace A-Coil I am going to fabricate shelf. Directions say 2 inches - could do 6 if that is better.

Also need to understand if airflow through coil should be restricted and if so is some type of winter/summer damper a good idea ?


House is 2 story 1200 sq ft near Chicago.

I have not been afraid to cut and bend sheet metal - it probably won't look pretty but it will be stron and functional.

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Old 08-25-2008, 01:54 PM   #2
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Optimal Install of A-Coil


Sperry, If you can bend up what ever it takes to enclose the A coil go for it! But your return air has me worried. You should have 200 sq/in of free return air. A 12" is only 100 sq/in. Hoy many supply runs and there size do you have?

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Old 08-25-2008, 08:31 PM   #3
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Optimal Install of A-Coil


I only have two returns. One runs up to the top (7 feet) of the 1st flr kitchen though the studs - the other runs in to the play room (8" round) which is lower than the first floor.

Heat works OK - in fact I had to restrict the run to one bed room when I gutted kitchen. Was straight shot up throught studs (3x 15 run ?) and got hot in that room. Changed to 3" round - and that was about right (should have gone with a 4" round).

All in all, for heating (it is a small house - 2 br/1 bath) I think natural convection seems to make up for the lack of returns.

Now that with the NEW central air - I am a little worried. AC by definition means conditioning air that I now think I need to "pull" from 2nd floor to condition it (no returns on 2nd floor). I am considering grabing the run to the 2nd floor bathroom to use as a retrun in the summer and heat run in the winter.

Any ideas on a summer/winter damjper switch ? The only downside I see to this approach is that I need to be alive to make the switch twice a year.
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Old 08-25-2008, 10:35 PM   #4
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Optimal Install of A-Coil


First, no, you don't want to put a 'summer/winter damper' that will reduce to airflow across the A-coil. In summer you want all the air to go across the coil to cool the house. In winter you want all the air to go across the heat exchanger (which will then go through the A-coil...no harm) to heat the house. If you dampen the air flow...it won't cool well in summer, and not only not heat well in winter, but you can overheat the heat exchanger, as it relies on the air passing over it to keep it within operating tempature.

If the instructions state a minimum of 2 inches above the heat exchanger, then definately don't go lower than that. If you can get 4-6, that should be fine, but I don't think you'd have to go higher than that. Make sure your 'shelf' that the coil will sit on is sealed as best as possible, but a couple small air leaks shouldn't hurt.
When you get to the point of test-firing the furnace, I'd use a manometer to set the output pressure of the gas valve, and also do a tempature rise. (and do a temp drop after you get the AC running).

As for the air return....does sound like you need to get more...and pulling from the 2nd flr is a good idea.

2 basics things to remember:

You can't give a furnace too much air...but you sure can starve it

You can't get air into a room unless you first get air out (unless you compress it...see how long your walls will withstand that...lol)
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Old 08-25-2008, 10:49 PM   #5
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Optimal Install of A-Coil


SD - Thanks for the input. No damper I get it. My shelf ended up about 4 inches from top of furnace. Good input on sealing the shelf - good to know small leaks are not too big of a deal.

As far as return - if you can't give a furnace too much air - is pulling from the basement OK ? That of course is the easiest way to add air flow - consideing I am way too short on return air - I wonder if adding some from the basement (say a 5 in round) is better than not doing anything. I realize that when cooloing this is not optimal but when heating - it might make things more efficient ?
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Old 08-25-2008, 11:15 PM   #6
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Optimal Install of A-Coil


Pulling air (for return) from the basement in general isn't the way to go. Basically, you want to pull the air from 'the conditioned rooms'...the room that are heated and/or cooled. If you have a finished room in the basement that you are htg/clng, then you can pull from there and add that into you calculations. You don't want to 'just pull air from the basement'...you're not doing the rooms upstairs any good. Plus, you sure don't want to pull any fumes and whatnot from the laundry, wookshop, etc.

Re-reading your prior post, you can use the cavity in a wall as a return, but don't put the grill in a bathroom (or a kitchen...you really don't want to circulate those smells). Use another rooms. And if you're going to pump heat up to the 2nd, it'll have to be in ductwork...can't use an open cavity for the heat run.

I know...I know...I can see the $ signs in your eyes...lol

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