When “beneficial electrification” advocates began suggesting that cold-climate heat-pumps reduce carbon emissions when compared to high-efficiency natural-gas furnaces, we were skeptical at first. After all, with more than a decade working in the residential and commercial energy-retrofit industry, heating with electricity was a worse-case scenario; the emissions were about four times higher, and the monthly utility costs were dramatically higher than heating with natural-gas.
Fast forward to 2022, cold-climate heat pump performance, and efficiency has improved, while the electric-grid is significantly greener than just a decade ago. Even so, as consistent sceptics of the next “Big Idea” (anyone remember when we were all going to be driving cars with compressed natural-gas?), we stubbornly decided to grab a calculator, and do that math ourselves. Here’s what we figured out:
For starters, we have to know the efficiency of the two different pieces of heating equipment, and the associated emissions of each fuel source of energy. Let’s start with the ol’ trusty natural-gas furnace…
Modern, high-efficiency natural-gas furnaces are between 95% and 97% efficient. These furnaces are so efficient that exhaust gas temperatures1 are barely warmer than the home itself, say 75°. Compared to the old 80% furnaces, with exhaust-gas temperatures in the 250° to 300° range (with all that wasted energy going up the chimney), there’s practically no wasted heat going up the flue.
We calculated the emissions of a 60,000 BTU (per hour) natural-gas furnace, knowing that a therm of natural gas (100,000 BTU’s) = 11.7 pounds of CO2, to find the carbon emissions from one hour of use:
60K BTU (Input) Furnace / 100,000 BTUs per therm x 11.7 lbs = 7 lbs CO2 / .96 AFUE = 7.3 lbs CO2
We then calculated the emissions of a high-performance cold-climate air-source heat-pump (CC-ASHP), with the knowledge that heat-pumps lose some heating capacity as outside temperatures decrease. Since the average overnight low in Denver in the coldest months of winter is around 25°, we went with the corresponding performance data at that temperature, and also a desired indoor temperature of 70°.
We also know that in Colorado, according to Xcel Energy’s recent 2021 Annual Sustainability Report2, one KWh (a kilowatt hour is 1,000 watts for one hour) of electricity produces about one pound of CO2.
Our test-case 60k BTU CC-ASHP makes only 41k3 BTUs per hour at 25°, while using 3,600 watts of electricity, so it will have to run for 30% longer to make the same 60k BTUs of heat as the gas furnace:
60K BTU CC-ASHP = 3,600 watts (3.6 KWh) = 3.6 lbs CO2 / (41k BTUs/60k BTUs) = 5.27 lbs CO2
To our surprise, the cold-climate heat pump not only has 28% lower emissions, but at today’s prices for energy (natural gas vs. electricity), heat pumps produce as much warmth as gas furnaces for about the same monthly utility costs. Consumers and contractors would be wise to warm up to heat pumps, and take advantage of numerous incentives to adopt this technology while reducing their carbon footprint.
1 Exhaust gas temps are so low, we now use PVC venting instead of metal flues, because one by-product of burning natural-gas is water vapor, and at these low temps, acidic water condenses, and runs back down the flue, hence the condensate drain on your high-efficiency furnace trickling water when your furnace is running.
3 Carrier 25HPB6 Performance Series Heat Pump: max COP 4, operates down to -10°, produces 44K BTUs at 25°, and 41K BTUs usable heat after “defrost cycles” are calculated. Scalable, mid-grade price range and performance.