Two furnace house, one abandoned, duct work cobbled together

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Two furnace house, one abandoned, duct work cobbled together

Postby melvin2345 » Wed Nov 20, 2013 11:03 am

The house my wife and I recently bought has two furnaces, both natural gas forced air. The 'front furnace' supplied the original house, the 'back furnace' supplies the addition and second floor. It appears as though somewhere along the way, the front one died, and instead of fixing or replacing it, the previous owners decide to cap off the supply and return connections to the 'front furnace' and installed 32' of insulated 8" flex duct between the main supply ducts.

The vents being fed from the 'back furnace' system are fine, and those rooms are heated well. The rooms that were supplied by the 'front furnace' have little to no air flow, and are between 5-12 degrees cooler than the rest of the house. The 'front' duct work is quite warm to the touch, so it seems like hot air is getting into the main 'front' supply duct, but it's not moving into the rooms. We closed all the 'back' vents in the house as a test, and it marginally improved the 'front' vent air flow.

The 'back' furnace is 100,000 BTU, and the house is 4100 sq ft.

I built a quick vent booster out of some old cpu fans and some scrap lumber to test with. I put it in one of the 'front' rooms and after 2 cycles of the 'back' furnace, that room was 3-4 degrees warmer.

Rather than make a vent booster for each room, I was wondering about the effectiveness of putting a duct booster fan at the end of the flex duct as it feeds into the main 'front' supply duct. We live in Michigan, so it's going to get colder before it gets warmer and something needs to be done soon. If I can get away with a quick fix for this winter and have it fixed proper in the spring, that would be ideal as money is tight having just closed and moved in.

Any suggestions, comments or opinions would be greatly appreciated.
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- Two furnace house, one abandoned, duct work cobbled toge

Postby Freon » Wed Nov 20, 2013 3:06 pm

There is only one answer that will make your house comfortable... replace the front furnace.

Here are the -
1. 100,000 BTU is way to little for your house.
2. You need air to flow INTO returns in order for air to flow OUT the supplies. Buy capping the front returns, you have no additional air to flow to the front supplies.
3. 32' feet of 8" flex has a huge coefficient of friction, even if it is all a straight (no bends) run. All the duct air flow help can not change that fact.

I don't know where you live but there are some very reasonably priced 85% efficient gas furnaces. Since all the ducting is still in place the installation should be easy. If money is an issue you can return the front ducting back to its original state and have an HVAC company install a furnace. If you're handy and have some help (strong back), you can install the furnace yourself then have an HVAC company do a annual service to make sure it's all working as it should.
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