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tuulikki · 10-02-12, 09:12 AM

---ETA: found a mistake. Left this post uncorrected, but read following posts below--

I've been doing a lot of math, and it looks like this whole idea may work! Many thanks to Xringer for his usage data and the detailed thread about heat pumps.

Comparing his house to our house:

For last year, we used 926 therms of natural gas. This breaks down into ~20 therms per month for hot water and the rest for heat (so ~690 therms for heat).

We also used 4280 kWh of electricity. Some of that was powering window A/C units.

Xringer used a total of 8195 kWh of electricity. He estimates his house baseline at his June usage (458 kWh for 30 days). Scaled up for a year, this gives 5572 kWh.

His heat & A/C via heat pump is therefore 8195 - 5572 = 2623 kWh.

We are located very close to each other, and both homes experienced the same mild winter last year, so heat usage is based on the same outside temperature data.

Now to check how heat production compares:

1 therm produces 10e5 BTU.
690 therm produces 6.90e7 BTU.

1kWh produces 3414 BTU.
2623 kWh produces 8.95e6 BTU.
but it's via a heat pump, so multiply by the HSPF factor ~9
gives 8.08e7 BTU
so we're in the same ballpark!

Cost savings for gas: 690 therm x $0.80 /therm = $552 / year.
Cost of heat pump system like Xringers: $1600 to $2000 plus installation.
Still working on cost of solar system.

If we can produce enough electricity to power the heat pump at this level, we would save (690 therms)(13.5 lb CO2/therm) = 9315 lb CO2. More CO2 than we use with our current driving.

We would have steam heat powered by natural gas as our back-up system and as our hot water system (they're tied together, same boiler heats water which is stored in a SuperStor).

Heat pump would be installed as to be approximating a second heat/cool zone, probably indoor unit would be on the second floor.

Not taken into account:
--boiler efficiency (high 80's or low 90's, can't remember --it's whatever the best available is for steam)
--some of our electricity usage is for A/C
--difference in house size
--difference in insulation

Conclusion: I'm going to talk to the solar contractor about what our maximum production capacity would be given roof size and shade factors.

10-02-12, 09:12 AM	#5
tuulikki Lurking Renovator Join Date: Sep 2012 Location: Boston Area, MA Posts: 6 Thanks: 2 Thanked 1 Time in 1 Post	---ETA: found a mistake. Left this post uncorrected, but read following posts below-- I've been doing a lot of math, and it looks like this whole idea may work! Many thanks to Xringer for his usage data and the detailed thread about heat pumps. Comparing his house to our house: For last year, we used 926 therms of natural gas. This breaks down into ~20 therms per month for hot water and the rest for heat (so ~690 therms for heat). We also used 4280 kWh of electricity. Some of that was powering window A/C units. Xringer used a total of 8195 kWh of electricity. He estimates his house baseline at his June usage (458 kWh for 30 days). Scaled up for a year, this gives 5572 kWh. His heat & A/C via heat pump is therefore 8195 - 5572 = 2623 kWh. We are located very close to each other, and both homes experienced the same mild winter last year, so heat usage is based on the same outside temperature data. Now to check how heat production compares: 1 therm produces 10e5 BTU. 690 therm produces 6.90e7 BTU. 1kWh produces 3414 BTU. 2623 kWh produces 8.95e6 BTU. but it's via a heat pump, so multiply by the HSPF factor ~9 gives 8.08e7 BTU so we're in the same ballpark! Cost savings for gas: 690 therm x $0.80 /therm = $552 / year. Cost of heat pump system like Xringers: $1600 to $2000 plus installation. Still working on cost of solar system. If we can produce enough electricity to power the heat pump at this level, we would save (690 therms)(13.5 lb CO2/therm) = 9315 lb CO2. More CO2 than we use with our current driving. We would have steam heat powered by natural gas as our back-up system and as our hot water system (they're tied together, same boiler heats water which is stored in a SuperStor). Heat pump would be installed as to be approximating a second heat/cool zone, probably indoor unit would be on the second floor. Not taken into account: --boiler efficiency (high 80's or low 90's, can't remember --it's whatever the best available is for steam) --some of our electricity usage is for A/C --difference in house size --difference in insulation Conclusion: I'm going to talk to the solar contractor about what our maximum production capacity would be given roof size and shade factors. Last edited by tuulikki; 10-04-12 at 12:32 PM.. Reason: found a mistake in my math