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Two furnaces? One upstairs?

Posted by livingreen2013 (My Page) on
Fri, Aug 31, 12 at 22:05

We were just told by our contractor that it looks like we may need to put a second furnace upstairs. Have any of you had experience with this? Our plan is a little over 4,000 sf. and two-level. The question is, our options would be to put it in a seperate closet that would need to find its way into our loft/tv space, or in one of the kids' bedroom closets (the furnace/closet space needing to be 3ft. x 3 ft. We're curious about the noise factor and safety in particular. Any recommendations? Thanks in advance!


Follow-Up Postings:

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RE: Two furnaces? One upstairs?

It depends on the house, heat load, geo location.

Big waste of money and maintenance. I would request hvac calcs and see what they really say. Maybe upgrading envelop, attic insulation will be cheaper.

On the other side two furnaces may be able to keep house more comfortable, w/o expense of zoning.


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RE: Two furnaces? One upstairs?

Code in NC requires it. So I've never had a house with one furnace. Do you have a basement? You can put the furnace there and run ducts to the upper floor. Where is the duct work for the upper level?


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RE: Two furnaces? One upstairs?

Code in NC requires two furnaces? NC homes are covered by the IRC, so I'd be very sceptical of any such requirement.

I build in a much colder climate, which also gets mid C.30s humid and sticky in the summer and have never installed more than one furnace for up to 4,400 sf. for two floors.

The OP should hire an independent HVAC designer. And in no circumstance install the furnace or ducts in an unconditioned (vented) attic or crawlspace.


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RE: Two furnaces? One upstairs?

NC code does not require two furnaces. We only have one.


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RE: Two furnaces? One upstairs?

2010 rule requires less than 4 degree differential between floors which is nearly impossible to maintain in both a heating and cooling environment without zoning or 2 systems. Since there is a also a Man J sizing requirement, it is difficult to zone to get this. And since zoning would often be more expensive than 2 basic systems, 2 systems is the norm. Zoning 2 floors from 1 furnace with this requirement would of course involve staged systems to meet the man J requirement (with sizing variance).

So true - 2 furnaces are not required. I misspoke. It is just that it is a de facto requirement because it is the cheapest way to meet the requirement. And when you build better, it is rare that zoning represents the easiest way to go beyond that. You would have to trust a HVAC person a lot to have a 6 ton a/c in the basement that can just be cooling the 2nd floor without noisy registers. It would almost have to be a Carrier Infinity system and you would still need pretty large registers.

Of course I have lived in older houses than 2010 but they have all had 2 furnaces as that is the normal around here even before it was a requirement. There are plenty of houses that were built with 1 system but anything more than basic spec have 2 systems. I had a 1999 house tract house with linoleum kitchen floor and carpet everywhere else that had 2 systems.

2012 requirements are sounding a little like Title 24 in CA with 75% high efficiency lighting requirements, blower door testing, etc.

Ans as far as ducts not unconditioned spaces - while it isn't ideal, there are far worse sins. As far as I can tell, 2012 code still allows it.


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RE: Two furnaces? One upstairs?

Why can't he do mannJ with manual dampers and set correct flow?


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RE: Two furnaces? One upstairs?

"manual dampers and set correct flow?"

Not a popular solution since they almost always have to be adjusted between heating and cooling modes.

Of course you could go with power dampers, and then deal with their reliability (or lack of...).


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RE: Two furnaces? One upstairs?

I did not build our house, but we have a furnace on the second floor. It's housed in its own closet within the closet of our loft.

I am unaware of any safety issues as it meets/met code at the time it was placed (2002) and our home inspector didn't have a problem with it (2005), but that's all I am going on there.

As far as noise, it does make noise - because it's a furnace :-) We used the loft area for a nursery twice, so two different babies with bionic hearing were OK with the noise level for 18 months apiece. Also, because it was the nursery, I spent many, many nights on the couch in the room myself and it didn't bother me either. Ours has a very predictable noise pattern - a click or two followed by a whoosh when you turn it on. The best way I can describe it is: if the furnace were in the closet of a hotel room you were staying in for a day or two, it would likely irritate you, but for the long haul, it's completely fine. Almost a white noise.

This furnace is 10 years old as well, they may be quieter now. One thing I love is that because it's only heating/cooling the second floor (Master Bedroom/bath and loft only -maybe 700 sq. ft tops) it cools down/heats up in literally 5 minutes. I often turn it off completely in the morning and turn it on just before bed in hopes of saving energy.


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