COMMUNITY FORUM

michaelserra

09:04AM | 02/16/05
Member Since: 02/15/05
6 lifetime posts
Bvhvac
I was recently asked to look into a failing furnace. It is one of five furnaces in a large multi-zone home. Upon looking I found that the failing furnace is sharing its central return with another furnace adjacent to it.

Rather than being mounted in the furnaces, the furnace filters are stacked, two in-a-row in the shared return duct before the tee to the two furnace inputs.

This results in zero air flow resistance between the two blower inlets and 2 filter resistances between either fan and the common return.

The result is that when one blower is on, the blower that is off is spun in reverse by the suction from the on blower. When the off blower then attempts to turn on it gets stuck in reverse, even when the other is turned off.

This may all sound confusing...and believe me, it is!

My question is: Is there any practical explanation for why someone would have installed a shared return duct between two separate furnaces?

- Baffled

michaelserra

11:50AM | 02/16/05
Member Since: 02/15/05
6 lifetime posts
Just what I was thinking!

MBTUHS

04:55PM | 02/23/05
Member Since: 12/15/04
16 lifetime posts
I believe you are looking at a poor representation of a "Twinning" setup.

michaelserra

05:34PM | 02/27/05
Member Since: 02/15/05
6 lifetime posts
Okay, so there is some method to the madness. I will find out more about the reasoning behind twinning setups.

Thanks for the tip.

MBTUHS

02:47PM | 03/03/05
Member Since: 12/15/04
16 lifetime posts
If they are not twinned properly with a factory twinning kit, they will do exactly as you have described. They are not to common but they are out there. The twinning kit includes a board that turns both blowers on at once so you are 'pulling - pushing'. That is a total waste.

MBTUHS

02:50PM | 03/03/05
Member Since: 12/15/04
16 lifetime posts
I need to correct myself, if a twinning kit is not installed you will push and pull air through a unit that is not pushing air. You do not want that, a twinning kit is engineered specifically for the purpose of two furnaces.

michaelserra

05:34PM | 03/03/05
Member Since: 02/15/05
6 lifetime posts
Then that is definitely what is missing from this installation. There is absolutely no mechanism installed to prevent one blower from pushing/pulling the other system. I'll try to track down the original installer. Else I'll find a twinning kit or just separate the returns.

Thanks a million. This has been the most help so far.

- Mike

michaelserra

06:22PM | 03/03/05
Member Since: 02/15/05
6 lifetime posts
I have just learned that twinning is not only sharing a return, but also a supply trunk. The system in question has only common return, but separate supplies as they go to completely separate areas of the house. So, there is a common return and no twinning kit to simultaneously control the fans. I guess you guys were all correct. Whoever installed this was out to lunch!

Thanks guys.
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