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Old 04-17-2010, 12:31 PM   #1
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Replacing one furnace with two new ones


We need to replace one of our two forty year old original furnaces in our southern Alberta two floor home. The house is 2400 square feet and both thermostats are set to the same program. We are gradually upgrading the basement and attic insulation. We would ideally like to replace both furnaces with one new high efficiency furnace. We have had one installer advice for it and one advise against it! Any un biased opinions would be welcome!

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Old 04-17-2010, 01:16 PM   #2
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Replacing one furnace with two new ones


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We have had one installer advice for it and one advise against it! Any un biased opinions would be welcome!
What were their reasons?
I have heard many people noticing no difference in their utility bills going from low to high efficiency. Ideally I guess you'd be billed for 100(83/95) = 87% of the amount you used to pay.

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Old 04-17-2010, 04:01 PM   #3
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Replacing one furnace with two new ones


Thanks for your response. We would of course welcome smaller gas bills but would like lower initial cost of only putting in one new furnace rather than two. One installer felt that the lack of flexibility in the two zones in the house would eventually frustrate us. The one who was happy to change the two for one felt that mechanically adjusting the dampers to the individual levels would work just fine.
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Old 04-17-2010, 05:28 PM   #4
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Replacing one furnace with two new ones


Two would be better as that is what the house ductwork is designed for. I professionally balance ductwork and doubt if your situation will work well. The ductwork for the whole house would need to be designed for one furnace. The largest high efficiency furnace is 130,000 BTU which may be too small for your house along with the physical fan size. If you want to spend some $$ and get a professional engineer to assess your ductwork size and do a heat loss calculation that is the only way to know for sure. 2 furnaces are better than one as when 1 breaks down you have another one which will at least keep the pipes from freezing.
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Last edited by yuri; 04-17-2010 at 05:31 PM.
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Old 04-17-2010, 06:43 PM   #5
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Replacing one furnace with two new ones


If one furnace is 10% likely to break down when you need it, there is only a 1% chance that both will break down when you need a furnace.
If two small ones costs less than or equal to one big one and you are concerned about reliability this choice is easy.
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Old 04-17-2010, 08:37 PM   #6
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Replacing one furnace with two new ones


Even if you have both thermostats running the same program. Your first floor unit is doing most of the heating.

Heat rises.

So you will find trying to manually balance the duct system a bit harder then you think. To almost impossible.

Have a load calc done. And see what size "Furnaces" you really need.

You should still be using heat. So see how often your first floor unit runs compared to your second floor unit. That will give you an idea of how hard it would be to balance 1 furnace, since your duct is set up for 2 units.

I have heard many people complain about temp problems because they went from 2 furnaces to 1. Never heard anyone complain because they went from 1 furnace to 2.
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Old 04-18-2010, 06:41 PM   #7
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Replacing one furnace with two new ones


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Two would be better as that is what the house ductwork is designed for. The ductwork for the whole house would need to be designed for one furnace.
agreed
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Old 04-19-2010, 01:22 PM   #8
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Replacing one furnace with two new ones


As a previous poster said, hot air rises, cool air falls. This means that with a single system, you will be adjusting your dampers in the summer to blow more cool air upstairs and adjusting them again in the winter to blow more warm air downstairs. Go with the two systems your house and ductwork were designed for.

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