By Fred Porter
What: The heat pump is a heat multiplier
Cold climate heat pumps are probably the most significant method to greatly reduce greenhouse gas (GHG) emissions from heating our buildings.
For a home with ducted forced air, a whole house heat pump will look like an air conditioner and function as air conditioner. Then, in the winter, it uses one unit of electricity to move more heat from outside to inside. It can work alone or with a gas furnace or electric strip heat.
In the past, these “air source” heat pumps quit working when the outside air was below 30 F or so. Beginning 15 years ago, “cold climate” heat pumps became available. These deliver heat at much lower temperatures, some even down to -20 F. According to the assessor’s listings, 60% of the residential floor area in Carbondale, including most modular homes and “trailers,” is heated with forced air, and about 65% of this already has central AC.
There are other alternatives for homes with baseboard electric or hot water, which can be quite simple or involved.
How: Multiplying heat
Inside a heat pump, an electric motor drives a compressor in a piping loop filled with refrigerant. This loop is controlled to create different pressures along it. The refrigerant boils and absorbs heat in one “coil” at low pressure and condenses and releases heat in the other at high pressure. For air conditioning, heat is absorbed inside, in the duct, and ejected outside. In heating mode, a valve reverses the action, and heat is absorbed outside and released in the duct.
Why: Emissions and cost
When heated with gas, a typical 2,000 square-foot residence in Carbondale emits 10,000 lbs of CO2 each year from the flue and another 5,000 lbs of indirect greenhouse gas equivalents from upstream processing and leaking. This is about the same as driving a 25 mpg vehicle for 17,000 miles.
Electricity in Colorado is sourced 40% from wind and solar. That fraction is higher for Xcel and Holy Cross Energy; typically higher in fall, winter, and spring; and is increasing. This means that the same house using a heat multiplier will decrease heating GHG emissions by around two-thirds. As far as the cost to heat, this past year, this kind of heat pump would save a few hundred dollars per year. Gas costs went up much more than electricity recently.
When: Before your furnace or AC breaks
When a furnace or AC breaks, a one-one replacement for each is usually immediately available for several thousand dollars. A cold-climate heat pump, or even a more efficient furnace, may require electrical or HVAC upgrades, and may not be possible to schedule when a broken unit needs to be replaced quickly. To get a heat pump by next winter, this is a good time to have an HVAC contractor determine the work needed and schedule deliveries and crews.
A cold climate heat pump will cost several thousand more than simple replacements but is eligible for tax credits and utility rebates, almost offsetting the additional cost.
So, it pays to plan ahead. Assistance is available from CLEER and CORE. And if your home has not been fully “weatherized,” that’s a basic first step which saves money and emissions.
In Maine, heat pump retrofit programs are starting to target mobile homes, and a pilot in Eagle County fully electrified five mobile homes in the Dotsero trailer park in 2021.
In New England, a few gas distribution utilities are diversifying into providing and installing ground loop heat pump components.
Why not just distribute and burn clean hydrogen or “RNG” (renewable natural gas or biomethane)?
There are claims the gas system can distribute low-emissions heating gas made from biomass and hydrogen. But supplies of the former will always be limited and the latter is expensive. The opportunity for heat multiplication does not exist when burning fuel.
Hydrogen fuel can be made from water using wind or solar electricity. With significant effort, the gas grid and furnaces could be modified to burn this. However, the conversion and distribution and combustion steps lose half the energy, and the heat multiplication process is unavailable. So instead of one wind turbine producing enough heat for 2,000 heat pump homes, it would only heat 500 homes.