Advice with new furnace purchase - 1 or 2 units?


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Old 08-28-08, 10:51 AM
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Advice with new furnace purchase - 1 or 2 units?

We live in a home with 2 old furnaces (1 for up, 1 for downstairs). House is about 2500-3000 sq ft. House has poor insulation, but temperature drops to 35-40 f at low in winter.

We've been advised to replace both with 2 two-speed Carrier 92 efficiency 60 or 80 btu units rather than a single furnace 120 BTU unit with a zone system - advantages being easier to control temperature from one floor to the next, fewer technical issues, avoid oversized furnace overheating from only heating one floor. However I know no one who has two furnaces.

My question is what would be the ideal configuration in your opinion. I'd like to be able to control two zones - upstairs and downstairs - separately, and would like the most efficient, quiet solution. More quality focused - dont want to skimp. Want a system that will be problem free, as much as possible, as efficient. DONT want to under-buy - and DONT need to heat the entire house when the heat is on. Thnx
 
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Old 08-28-08, 12:48 PM
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I'd like to be able to control two zones - upstairs and downstairs - separately, and would like the most efficient, quiet solution. More quality focused - dont want to skimp. Want a system that will be problem free, as much as possible, as efficient. DONT want to under-buy - and DONT need to heat the entire house when the heat is on. Thnx
Then you want to have two furnaces. Zoning of residential forced air systems is usually a mistake, often a very BIG mistake. Zoned systems are not simple nor are they trouble free. They usually are not even close to being as efficient as two separate furnaces.

In my opinion it would be a HUGE mistake to try to retrofit a two-zone system into your existing home. I strongly recommend that you invest in two-stage furnaces IF you can get them in the proper sizes. The problem here is that you probably need really small furnaces, especially if you upgrade the insulation in your home and properly seal it against air infiltration which should be done before the new furnaces. The 90+% AFUE furnace may also be difficult to find in small sizes also and with your mild winter temperatures is likely not a good investment, certainly not as good as the insulation/infiltration upgrade. Be sure that your selected heating contractor does a complete heat loss calculation and does not size the new furnaces from the existing units. More likely as not your existing furnaces are oversized.
 
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Old 08-28-08, 05:46 PM
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One or Two Furnaces

I completely agree with Furd. Another big advantage to 2 separate systems is if one goes down, you still have some heat from the remaing furnace. You may not be as comfortable as you would like but some heat beats no heat every time.
Zoning with dampers is a terrible way to try to heat a home.
 
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Old 08-28-08, 06:34 PM
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I could not have said it better!!
 
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Old 08-28-08, 10:53 PM
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You know, when you first hear about zoned systems, in theory, you'd think what a great idea. And frankly when I first came here and read about them I thought wow I'd love that, until I read what is now common knowledge - that these zoned systems are inherently troublesome.

I also agree with keeping the two furnace system, and choosing a nicely efficient couple of machines. You should not have to worry about oversized units - you simply must have Manual J heat loss/load calculation performed as mentioned. Using that, you will have two great perfectly sized units, saving the most energy, and bringing you the most comfort. Plus you have the redundancy and LACK of headache sans the dampers and associated issues of a zoned system.

P.S. The heat loss/load calculation mentioned does include measuring windows, doors, rooms, checking insulation, etc. It is not a walk and look rule of thumb type thing, and it is absolutely necessary. Plus as also mentioned, your existing units are most likely oversized as are most furnaces. Get 3 bids for the job too, minimum.

Good luck!
 
 

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