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I live in Upstate NY and in the winter it gets below 0° F.

I currently have baseboard water heating throughout my home. Prior to this I believe there was a gravity furnace which supplied the house with heat.

I am hoping to switch to a regular furnace for two main reasons:

  1. I would really like to get central A/C in the future
  2. I strongly dislike the idea of water lines everywhere in my house; copper does fail eventually

Right now we have 2 zones, upstairs and downstairs, which would be nice to maintain.

My 1940 home has a full basement and a full attic in which I can stand upright.

I would like to have a basement furnace which heats the downstairs. It will be trivial to run the supply lines and cold returns in the walls above the baseboard trim. This would allow me to remove the downstairs zone from my boiler.

I would also like to have an attic furnace to heat the upstairs. I feel that this would be easier than trying to run HVAC upstairs from the basement and it would make dual-zones easier to achieve. I think the supply lines would be do-able but properly doing the cold air returns would be a headache.

My understanding is that a high efficiency furnace has a condensate reservoir which gets pumped out through a 1/4 tube whenever it fills up; much like a dehumidifier with a built-in pump. Would this be a freezing risk? My attic does get below freezing.

Assuming I have insulated supply runs, would my attic get heated by the furnace and cause ice dams on my roof?

I do have some space on the second floor which can fit a furnace, would this be a better choice?

  • I'm not a qualified HVAC guy, but I have a furnace in my attic (central Illinois). The only issues I've had are the condensate line freezing up, which I solved with some heat tape. I should add that putting two furnaces in a restored home is common. – Duston Jan 6 at 15:16
  • Would there be enough room in your attic to build a small "room" around the furnace and insulate it? All it would need to be is some framing at 16" or 24" on center and then some paper-faced insulation installed with a continuous vapor barrier. That way, the furnace would be in a heated space so to speak. – PhilippNagel Jan 6 at 15:16
  • @PhilippNagel Yes. The peak of my attic is about 7 feet tall. From eave to eave is about 30 feet and the depth is about 30 feet. – MonkeyZeus Jan 6 at 15:20
  • @PhilippNagel This image represents it quite well thecordovatimes.com/wp-content/uploads/2018/08/… and the floor joists are all covered with 1/2 inch plywood. We do not currently use it for storage because I'm trying to solve a mouse problem so everything is wide open. – MonkeyZeus Jan 6 at 15:23
  • @Duston Would you be willing to share a picture of your attic furnace? I would love to get an idea of how the install should look. – MonkeyZeus Jan 6 at 15:25
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Since you have some space in your attic, and you seem set on installing the furnace up there, I think it would be easy enough to make a small room just for the furnace. I did that in our new addition, as the dedicated furnace for the addition was an afterthought after running ductwork over from the original furnace location turned out to be impractical. We put it in the attic next to the second level, but if the attic is above, it should work just the same.

We simply made a room large enough for the furnace and to have some work space around it. Coordinate with the furnace installer in how they're going to place it, and keep in mind which direction the filter needs to come out to be changed. Then simply build some 2x4 stud walls around the furnace at either 16" or 24" spacing. This allows you to use paper faced batt insulation so you can leave the one side if the wall just open. The other side should then have a continuous vapor barrier installed and probably sheetrock for fire protection. If you're going to install a high efficiency furnace, it will have its own combustion air supply, so you don't really need to worry about letting air in too much, otherwise, obviously, you will need to make sure there is an air supply there. If you're taking the room all the way up to your roof, make sure to leave space for venting in between your insulation and the underside of your roof.

Depending on how you will be situating things, think about the easiest method to seal around the ductwork. It may be best to install the ductwork first, then build walls, then install the furnace, but based on your situation, a different order might be better.

With this setup, your furnace will be at least in an insulated space, and it will keep itself warm in there to some extent. It might be worthwhile to remove the floor insulation underneath your room to get more heat in there as well. Keep the condensate drain plumbing within the insulated areas to keep it from freezing. You may end up running ductwork through non-insulated areas, in that case, consider getting the ductwork spray-foamed after installation.

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  • With it being inside a room code may require permanent access. Not hard to do, the thing some don’t realize is with permanent access this affects the electrical code (protection of the wires) but I think I read the attic is decked so it won’t be hard in any case. – Ed Beal Jan 6 at 17:36
  • Yes, OP posted a picture in a comment of an attic that looks like it has a decked floor and presumable access, since it is used for storage. Of course, the furnace room should have a separate, preferable insulated door as well. – PhilippNagel Jan 6 at 17:38
  • @EdBeal Yes. We have a pull-down stair case which provides attic access. I believe mine is as old as the house so I get a little concerned when I use them due to my heftier size. I will be replacing them with aluminum as my next project. – MonkeyZeus Jan 6 at 18:21
  • I am really not sure why I did not think of this but I am really happy that you did. All of the other answers are excellent considerations but I think your answer solves the bulk of my issues and hesitation. I sincerely thank you for presenting this solution :-) – MonkeyZeus Jan 6 at 19:46
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Yes you can put a high efficiency furnace in an attic hose if allowed by your local code. I use self regulating heat tape if there is any chance of the condensate line freezing.

Have you considered a mini split system? I have installed high efficiency furnaces in attics in the past but you would still need a separate compressor unit outside for AC.

The newer mini splits are extremely efficient and zone control can’t be beat as each unit or zone can be independently controlled.

With larger homes I recommend multiple outside units each outside unit can serve from 1 up to 8 inside units depending on the size and brand. Mini split systems don’t require duct work and this is where installing a new forced air system gets expensive, in your case with multiple heat sources and ac you should at least consider a split as it won’t take up space in the attic or in the 2nd floor if you choose that location , no duct work is a big plus when changing heating styles like you are planning.

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  • I've considered those but I'd rather not have those units on my walls and the pipes on the outside of my home. I'm a big fan of do it once and do it right even if it costs a bit more. I am okay with installing an outdoor compressor for A/C. As for efficiency I am pretty sure that electric is still the most inefficient heating choice. – MonkeyZeus Jan 6 at 16:02
  • Electric is actually the most efficient typically, but it tends to be the most expensive because electricity costs more per kw/h than gas in most areas. With any type of gas heating, you will have some loss out the exhaust, while electric is essentially 100% efficient (no loss to the exhaust because there is none). – PhilippNagel Jan 6 at 17:05
  • Heat pumps are actually more efficient than straight electric and you get cooling, I understand not wanting the units on the walls. I was just suggesting to look into them if I had gas in would come down to the install cost but can understand preferences my wife has some areas she doesn’t care how much it costs she wants it her way, happy wife happy life. – Ed Beal Jan 6 at 17:27
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Another consideration is installing the unit on vibration isolators. When the fan turns on, it will cause the unit to move (slightly). Likewise, when it’s running, the fan will cause some vibration in the unit. Make sure the unit is isolated from the framing.

I prefer hanging the unit from the roof rafters rather than sitting it on the ceiling framing. They make vibration isolators for both, but the hanging isolators are better than the floor mount isolators. However, you may need to increase some roof joists in order to support the unit...you get some large amounts of snow you’ll need to accommodate too.

Also, install isolators between the ducts (supply and return) and the unit. Otherwise you’ll hear vibration, especially when the unit kicks on.

Electric Heat pumps are efficient and inexpensive to operate...down to a certain temperature. Heat pumps work on the principle of taking heat out of the air (or cool air during the air conditioning cycle). This works down to about 38-40 degrees. Then resistance heat turns on and that is expensive.

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  • Actually many modern heat pumps are rated for single digit temps some in the - range. Hanging a high efficiency unit may void the warranty as they specify the mounting with all the ones I have installed. – Ed Beal Jan 6 at 18:23
  • Wow, good call on the isolators. I will definitely look into those. – MonkeyZeus Jan 6 at 18:24
  • @MonkeyZeus I would also suggest installing an adjustable volume damper on your outside air intake. I fiddled with mine for a couple of years before I felt comfortable. I’m an empty nester, so it’s quite around here now. However, when the kids were home there were lots of doors opening and closing and I needed less outside air. – Lee Sam Jan 6 at 18:48
  • @LeeSam Thanks, I will keep it in mind! – MonkeyZeus Jan 6 at 19:32
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The catch about these new units is that they are sometimes condensing units. I had two installed. One has a direct feed to the drain, while the other has a pump that pipes it to the same place. We had a super-hard freeze (uncommon where I live) and the pump line froze up. That, in turn, tripped the failsafe switch, which cut power to the unit (required by code). I awoke to half the house being cold as a result (I figured it out when I heard the condensate pump running non-stop). Had to defrost the line with a hair dryer and that fixed it. The HVAC guy then insulated the plastic tube and it has not frozen since.

If you deal with freezes on a regular basis like that, just be sure to use an appropriate heating device on the condensate line.

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  • Thanks, do I have to worry about the condensate reservoir freezing as well or just the drain line? Err, is there even a reservoir at all? – MonkeyZeus Jan 6 at 19:09
  • The reservoir has the benefit of somewhat heated liquid entering it. The plastic tube (which is always filled with water) did not. My unit that has no pump did not freeze. – Machavity Jan 6 at 19:10

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