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.
我可以在设置的情况下切换到热泵和空气处理程序吗？我还应该考虑其他事情吗？该系统运行会更昂贵吗？如果地下室有点温暖，那就太好了。
Thanks in advance for any advice you can give.
David McCoy
GBA Detail Library
由气候和房屋部分组织的一千个建筑细节的集合
答复
大卫，
如果您可以使用天然气，很难相信您会用热泵省钱。但是，夏季，热泵将能够提供冷却。
任何有能力的HVAC承包商都应该能够为您提供安装热泵的估算值。确保您询问热泵是否在寒冷的室外温度下运行 - 如果是这样，则有多冷。
如果您在气体主机上，与东部MA气候中的典型管道热泵相比，运行冷凝气的运行要便宜。但是，如果您可以像载体Greenspeed这样的调制热泵弹簧，它将提供迷你切片类型的效率。
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.
即使这一切都在条件空间内，值得密封管道，因为导管泄漏会导致房间之间的压力失衡，并驱动室外空气浸润，从而增加负载。如果您想点击某些寄存器以加热地下室工作室，请以平衡的方式进行供应和返回管道开口。依靠愚蠢的运气管道泄漏来为热空气系统提供压力平衡的加热，有点像“基于信仰的HVAC设计”。由于管道完美平衡，没有空气处理器驱动的渗透损失，您的热负荷可能比测量22K更接近20k。通过调制空气处理程序，不完美系统中的空中处理器驱动渗透损失的平均水平将较小，但在高速下仍然相同。无论您可以访问它们，都可以用管道湿润密封管道接缝和关节。您可能会对它的安静感到惊讶，因为泄漏有很多相关的哨声和嘶嘶声。用FSK胶带将空气处理程序的接缝胶带胶带。
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.