Comparing cost of oil and electricity for heat

FlyingScot · 12-28-2021, 04:53 PM

Quote:

Originally Posted by randalnh

I installed a Goodman split system 9 years ago with a 16 SEER heat pump. The heat pump is super efficient, but only good down to around 32 degrees F. In the fall and spring months, it heats the house at a cost of around $25-30 a month.

The propane side of the heating equation is not so good, although the furnace is rated at 95% AFUE (Annual Fuel Utilization Efficiency) and it is maintained annually.

BH, curious if the geothermal system that cost $150 a month to run includes the proportional installation cost of the GTH system.

Yes, this kid of hybrid system is a good improvement over propane-only, but it is not really in the same league as Mitsubishi mini splits, which are effective even at 5 or 10 below zero.

I appreciate these posts--I have mini splits in Mass, but only traditional in NH. I'm going to check out both the hybrid furnace and the condensing water heater as Summer projects.

Woody38 · 12-28-2021, 05:13 PM

When I was working one of my offices was in Sandwich, MA. Not a huge office heated by hot air electric with a heat pump. The monthly bill was around $50 year around. I had always heard that heat pumps were not good in cold weather and I went over one day despite the snowy, icy roads. The temperature was -10. We set the thermostat at 50 when we were not there.
This day I slowly turned up the thermostat so as to not turn on the furnace.
I set it at 70 and it took about 10 minutes to reach 70 using the heat pump.
Surely today that monthly bill would be higher.

_____________________________________

I am a retired workaholic and continuing aquaholic

randalnh · 12-28-2021, 06:07 PM

Looking around the IoT, I see a lot of variation as to how low a Heat pump can generate heat. I was told that my heat pump was good down to around 32 degrees F, but I am now not sure - I guess some more research is required.

DickR · 12-28-2021, 07:38 PM

Regarding the minimum operating temperature of an air-source heat pump (ASHP), it's a matter of how it was designed. All heat pumps, including ones used for heating or cooling a house, providing air conditioning for your car, or a refrigerator, use a refrigerant fluid chosen for its thermodynamic properties. Two heat exchanger surfaces are involved. In heating mode, the refrigerant must evaporate by absorbing heat at some low pressure (but above atmospheric) and corresponding temperature somewhat below that of the outside air (or ground or well water in the case of geothermal). The refrigerant vapor is compressed to a pressure at which it condenses by losing heat to the inside air (or water if that is the purpose of the heat pump); the condensation temperature must be somewhat above that of the air or water absorbing the heat. The high temperature/high pressure liquified refrigerant then passes through a throttling valve to the low side (evaporator) coil to complete the cycle. The heat rejected at the high side is the total of heat absorbed at the low side plus energy input by the compressor. The Coefficient of Performance (COP) is the ratio of heat energy delivered divided by the energy input to the compressor.

An ASHP designed for operation in a milder climate than ours might well be limited to outside air temperatures above freezing. For New England, a cold-climate heat pump can indeed operate well to -13 F or even lower. As has been mentioned, the COP drops as outside air temperature is colder. It's like pumping water up hill; it takes more pump energy to move a given amount of water higher up the hill. So with an ASHP, the greater the "lift" in temperature, the more compressor power needed to move a given amount of heat absorbed to the destination temperature. Or, given the maximum compressor input power, less heat can be absorbed and delivered over a greater temperature difference. Good ASHPs can achieve a seasonal average COP of around 3, although at very low outside temperatures this drops to 2 or lower.

By choosing the right refrigeration equipment and refrigerants, it's quite possible to absorb heat at cryogenic temperatures (think air liquifaction) and reject it to the "low" side of another complete refrigeration system, which then can reject the heat at more moderate temperatures. It's all a matter of operating conditions and refrigerant selection.

More than you wanted to know, huh?

tummyman · 12-28-2021, 07:50 PM

Quick...look out the window and see if you can see my head spinning from the cold air breeze! Ha...great info, but yes, more than I think I can absorb.

Thread Tools
Show Printable Version Email this Page
Display Modes
Switch to Linear Mode Hybrid Mode Switch to Threaded Mode

12-28-2021, 05:13 PM	#2
Woody38 Senior Member Join Date: May 2016 Posts: 584 Thanks: 45 Thanked 110 Times in 81 Posts	When I was working one of my offices was in Sandwich, MA. Not a huge office heated by hot air electric with a heat pump. The monthly bill was around $50 year around. I had always heard that heat pumps were not good in cold weather and I went over one day despite the snowy, icy roads. The temperature was -10. We set the thermostat at 50 when we were not there. This day I slowly turned up the thermostat so as to not turn on the furnace. I set it at 70 and it took about 10 minutes to reach 70 using the heat pump. Surely today that monthly bill would be higher. _____________________________________ I am a retired workaholic and continuing aquaholic

12-28-2021, 06:07 PM	#3
randalnh Senior Member Join Date: May 2007 Posts: 50 Thanks: 1 Thanked 19 Times in 8 Posts	Looking around the IoT, I see a lot of variation as to how low a Heat pump can generate heat. I was told that my heat pump was good down to around 32 degrees F, but I am now not sure - I guess some more research is required.

12-28-2021, 07:38 PM	#4
DickR Senior Member Join Date: Apr 2008 Location: Moultonborough Posts: 765 Thanks: 4 Thanked 260 Times in 172 Posts	Regarding the minimum operating temperature of an air-source heat pump (ASHP), it's a matter of how it was designed. All heat pumps, including ones used for heating or cooling a house, providing air conditioning for your car, or a refrigerator, use a refrigerant fluid chosen for its thermodynamic properties. Two heat exchanger surfaces are involved. In heating mode, the refrigerant must evaporate by absorbing heat at some low pressure (but above atmospheric) and corresponding temperature somewhat below that of the outside air (or ground or well water in the case of geothermal). The refrigerant vapor is compressed to a pressure at which it condenses by losing heat to the inside air (or water if that is the purpose of the heat pump); the condensation temperature must be somewhat above that of the air or water absorbing the heat. The high temperature/high pressure liquified refrigerant then passes through a throttling valve to the low side (evaporator) coil to complete the cycle. The heat rejected at the high side is the total of heat absorbed at the low side plus energy input by the compressor. The Coefficient of Performance (COP) is the ratio of heat energy delivered divided by the energy input to the compressor. An ASHP designed for operation in a milder climate than ours might well be limited to outside air temperatures above freezing. For New England, a cold-climate heat pump can indeed operate well to -13 F or even lower. As has been mentioned, the COP drops as outside air temperature is colder. It's like pumping water up hill; it takes more pump energy to move a given amount of water higher up the hill. So with an ASHP, the greater the "lift" in temperature, the more compressor power needed to move a given amount of heat absorbed to the destination temperature. Or, given the maximum compressor input power, less heat can be absorbed and delivered over a greater temperature difference. Good ASHPs can achieve a seasonal average COP of around 3, although at very low outside temperatures this drops to 2 or lower. By choosing the right refrigeration equipment and refrigerants, it's quite possible to absorb heat at cryogenic temperatures (think air liquifaction) and reject it to the "low" side of another complete refrigeration system, which then can reject the heat at more moderate temperatures. It's all a matter of operating conditions and refrigerant selection. More than you wanted to know, huh?

The Following User Says Thank You to DickR For This Useful Post:
FlyingScot (12-29-2021)

12-28-2021, 07:50 PM	#5
tummyman Senior Member Join Date: Aug 2009 Location: Moultonborough Posts: 889 Thanks: 278 Thanked 764 Times in 271 Posts	Quick...look out the window and see if you can see my head spinning from the cold air breeze! Ha...great info, but yes, more than I think I can absorb.