Two Furnaces one duct system

14 years ago

I have a 850 sq. ft. cabin that presently has a goodman heat pump system.

The airhandler and ductwork are under the cabin. It works very well except in the winter when the temp. gets below 30 degrees. It does have the heat strips that come on, but it sures eats the power when the strips are on. I recently bought a new 95% 40,000 BTU Coleman gas furnace. I have converted it to propane.

I want to put the new furnace on the same cold air return and ductwork that the heatpump is on. I will have two thermostats. One will control the heatpump the other the gas furnace. My question is when I use the propane furnace wont it blow thru the heatpump air handler and loop back through the cold air return duct? Is there some kind of automatic damper that will close off the furnace not being used so it wont loop around? Thanks for your help!

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Comments (7)

brickeyee
14 years ago
Just put the heat pump coil on top of the gas furnace and disconnect the electric heat elements.

You are making this way more complicated than it needs to be.
twinjett
Original Author
14 years ago
Thanks for your reply. Yes, I thought about doing that but it would not be easy. I would have to have a box custom built to contain the A-coil on top of the new coleman furnace. Also the refrig. lines would have to be changed to a different height and lastly the control board for the heat pump is built into the present air handler.As it is now I just have to hook up the return air duct and the supply airduct to the new furnace. Would the blower fans in each furnace be enough to keep the supply air from looping back through the unit not being used. Thanks
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I would use mastic and a chip brush to apply. I would seal all of the returns and supply vents. The time you waste will pay you back in $$$$ and comfort. The more you seal other things like pipe runs and where wires come up into the house and into the attic area the less air you will not waste time heating and cooling. I would have to say for the average builder if you spent an extra $1,000.00 in sealants and time (yours or the builder's) it would probably pay itself back in less than 5yrs. This is talking going beyond what quite a few other builders do. I have a quote in front of me for sealing things that I am doing myself after the 'builder thought it was a waste' home was built. If these things were done at the time of building it might have cost an extra weeks labor and less than $1,000.00 materials. Seal returns in walls and floor joists $400.00 seal around baseboard trim (remove and reinstall and paint) on outside walls $3,000.00 wrap and seal supply ducts $1,840.00 Seal recessed lights $75ea x14 add one 6x30 return in hall wall $800.00 Doing this after the home is completed is a total pain in the rear compared to taking a little time and doing it at the time of the build. It might seem to be a waste of his time or money but he isn't the one that will be paying the utilities. I would say from the sounds of this guy that you might be best to do some research on the net and ask us and others what to do and how to do it cause I doubt he cares to do it right. Best of luck,
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The infinity system is an automatic zoned system. There is so much more to it that you describe. But from the user standpoint, I'll give some examples. Each zone has a stat. The master stat can change temps for the different zones. Each zone can turn on the system. A basic manual damper system might only direct air when the system is on but couldn't turn the zone on from anywhere. The best example is with basements in shoulder seasons. We don't run the heat here in late March but the basement is cold still. In a simple system, the basement dampers might be all open but it can't call for heat so it stays cold. Now in January, it can direct all the heat it wants (to a limit) down to the basement so it is fine. The Infinity system measures the pressures in the air handler and cuts down the motor if they get too high. So if a small zone is calling for heat, it runs the motor at a lower speed. This eliminates the need for dump or bypass zones. Inifinity stats are small computers that are shockingly more sophisticated than the stats you see in 95% of homes. Not simply programmable but a lot more. They have LCD screens and menus. They tell you when the filters need to be changed. They have digital humidity readouts. They can set heat pump/furnace cutoffs. etc etc The zone dampers on an infinity system relay back their position to the controller. So it isn't a simple relay and a command to open a damper. It is a nearly foolproof system. No one can give you an Inifinity system for $1200 unless they don't know what they are doing and lose a chunk of money. I paid about $3000 per floor.
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PRO
CJ Mechanical of North jersey llc.
14 years ago
fabricating a plenum to handle the evaporator coil is normal prodeure in the hvac industry. And to suggest haveing 2 pieces of equipment on the same single duct system is not gonna fly. Do as brickeyee suggested
twinjett
Original Author
14 years ago
Thanks guys for the replys. I am going to do it the way you suggested. I called a sheet metal fab shop today. I will be taking all the measurments to him. I will mount the control board for the heat pump close to the control board for the furnace. The only thing they will have in common will be they use the same blower fan. Thanks
ryanhughes
14 years ago
Not sure why you think you need to control them off two different controls. A heat pump is commonly used with a furnace in what is known as a dual fuel setup. The heat pump provides most of the heat, and the furnace is used when the heat pump can no longer keep up. You can use a dual fuel kit to control this setup, or a dual fuel thermostat. I would suggest having a professional do the work unless you're in the field.
countryboymo
14 years ago
The money spent having a professional set this up will be regained in lower bills later I am sure. I have no doubt you can set it up and make it work, but you want the end result to leave as much money in your wallet as the months go by.
Lee Kirkland
2 years ago
last modified: 2 years ago
Of course you can have 2 units tied into 1 duct system. As to your actual question, i'll answer it. You will need to either manually or mechanically block off the unit not being used...Fof instance, install the gas furnace facing the existing heat pump etc, then slide a rigid baffle in front of the heat pump, blocking any pass through air flow when using the gas furnace...lol, simple as moving one baffle incorporated into the supply plenum. 2 separate T stats, separate power supplies, same existing duct system, supply and return. I never did like the way 98% of people ask you more questions than you ask them.....Why, Why, Why.....dunno "Why" they need to know Why, just answer the question OP posted, lol.

Two Furnaces one duct system

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brickeyee

twinjett
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Two Furnaces one duct system

Comments (7)

brickeyee

twinjettOriginal Author

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