I need to replace my gas furnace
Hello to everybody at GBA.
My gas furnace and AC system is starting to get old and I’d love to replace it with a heat pump. As I already have ducts, it seems to me that a ducted system would be best, but I can’t figure out if that would really work.
I live just north of Boston in a 1,475 sq. ft. 1970s ranch. I did put more insulation in the attic, but this is no high performance house, I have 2×4 walls and inexpensive windows with storms. Working backwards from my gas bill (using Dana Dorsett’s excellent formula – Thank you Dana!) and an outside design temperature of 8 deg. I came up with an hourly heat load of just over 22,000 btu/hr. The current furnace is rated 90,000(!) and has a 92% AFUE, the AC is a 3 ton, 10 SEER.
The ducts are in the basement and as far as I can tell, it’s a good system. All sheet metal with straight runs. The main duct is 8×17″ with 6″ round duct branches. They’re neither sealed nor insulated but the “waste” heat heats the studio and workshop so I’m reluctant to change that.
Can I just switch to a heat pump and air handler with this set up? Are there other things I should be thinking of? Would this system be more expensive to run? If the basement could be a little warmer, that would be great.
Thanks in advance for any advice you can give.
David McCoy
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
David,
If you have access to natural gas, it's hard to believe that you would save money with a heat pump. However, a heat pump will be able to provide cooling during the summer.
Any competent HVAC contractor should be able to provide you an estimate for installing a heat pump. Make sure that you ask whether the heat pump operates at cold outdoor temperatures -- and if so, how cold.
If you're on the gas main, it'll be cheaper to run condensing gas than a typical ducted heat pump in an eastern MA climate. But if you can spring for a modulating heat pump like the Carrier Greenspeed, it would deliver mini-split type efficiency.
Play around with their online capacity/sizing tools(click the "Heating Capacities" tab) with different compressor & air handler options:
http://www.tools.carrier.com/greenspeed/
If you plug in +8F for a design temp and 20,000 BTU/hr for a load (both heating & cooling) you'll find several combinations that have you covered- even with the 2 ton compressor for the Greenspeed would have you covered in heat-pump only mode for at least 97% of all heating hours, and the 3-ton compressor would cover you down into negative-digits. Since it's a modulating unit with a pretty-good turn down ratio, the 3 tonner would probably give you slightly higher seasonal efficiency. The HSPF of some of these combinations is right up there with mini-split.
Even though it's all inside of conditioned space, it's worth sealing the ducts, since duct leakage induces pressure imbalances between rooms, and drives outdoor air infiltration, increasing load. If you want to tap in some registers to heat your basement studio, do it in a balanced way, with both supply AND return duct openings. Counting on dumb luck duct leakage to provide pressure-balanced heating with hot air systems is a bit like "faith based HVAC design". With perfectly balanced ducts and no air-handler driven infiltration losses your heat load would probably measure closer to 20K, than the measured 22K. With a modulating air handler the air-handler driven infiltration losses in the imperfect system would be less on average, but still the same at high-speed. Seal the duct seams & joints with duct-mastic whereever you have access to them. You may be surprised at how much quieter it is, since the leaks have a lot of associated whistle & hiss. Tape the seams of the air handler with FSK tape.
If you're on one of the municipal electric utilities you may have been spared some of the brunt of this year's winter price hikes. NStar & National Grid residential retail is running about 25 cents/kwh right now (grid delivery charges included). But if you have the solar access you can get that down to 15 cents or so even under a $0 down leased solar option, cheaper still if you opt to own rather than least. Since MA has a very solar-friendly net-metering policy support, going with rooftop PV (whether leased or owned) makes a lot of sense when considering heating with heat pumps. At 15 cents a GreenSpeed would beat condensing gas on operating cost.
Before replacing any equipment, a more serious air sealing & weatherizing approach may be warranted. If you have no foundation insulation, bringing that up to IRC 2012 code min (R15 continuous insulation, for MA) would cut a good chunk off that heat load number, and should bring it under 20K, at which point the 2-ton GreenSpeed makes more sense.than the 3-tonner. Insulating the foundation would also increase the basement temperature in winter. My basement in Worcester doesn't drop below the mid-60sF in winter since adding 3" of reclaimed polyiso to the interior of the foundation walls & band joist, sealing it all up. It's at it's coolest when I'm heating primarily with the wood stove, with less distibution loss heat from the hydronic heating system or the hydronic air-handler ducts, but it's never really "cold". In MA there are multiple vendors of reclaimed foam board, which makes an all foam solution cheaper than a virgin stock R5 foam + R13 studwall solution to hit that performance point.