Heat on a blizzard day

With a blizzard upon us my thoughts turn to heat. Basically, the most economical/sustainable form of heat is a geothermal heat pump powered by solar photovoltaics. These extract heat from a field or well about four feet below the top soil (in New England) or deep enough to be below the frost line. Two feet in many locations with a big enough field is fine. Anything around 55 degrees is warm enough to extract heat. Earlier systems relied on deep wells with a coil sunk into the "well." Newer systems are efficient enough to work with a horizontal field of pipes (PEX typically) running back and forth in a shallower pit. Heat and AC is distributed through vents and unlike old hot air furnaces these systems run continuously eliminating the hot/cold cycles forced hot air systems create.

My own house has mini split heat pumps. These are air to air heat pumps that extract heat from the outside air. Though it sounds counterintuitive, they can actually extract heat from even freezing cold air. They are extremely efficient and the lowest cost way of creating electrical heat. As the outside temperatures drop they become less efficient and they begin to struggle at temperatures below about 20 degrees F. We've had several below zero days here in Massachusetts this year and the heat pumps still work. They're fully powered by solar photovoltaic and essentially provide free heat. (Geothermal systems have the advantage that the ground temperatures below two feet vary very little hovering constantly around 55 degrees in the north, while outside air temperatures can vary by 100 degrees over the course of seasons and by 40 degrees or more in a given 24 hour period.)

There is no doubt that on cloudy days like today when the air temperature is in the teens and the wind chill below zero, our mini splits struggle to produce enough heat. Ours are set to 68 degrees but a thermometer shows room temperatures are actually 65 to 66 degrees most of the time. We could turn them up and probably produce enough heat to get the inside spaces up to 70 but they would be working extra hard to do so.

I am particularly sensitive to cold and so supplement this system with small space heaters (which are energy hungry). We have enough solar capacity that on average over the year we're still Net Zero. But I decided, as an experiment, (a very expensive experiment) to install radiant floor heat on the first floor of our house through a "staple up" sub floor system of PEX pipes in aluminum transfer plates placed in the exposed joist wells between joists in our basement. Working with PEX in a cold basement is difficult. It doesn't want to bend cleanly and sometime crimps. Those crimps have to be cut out and spliced but every splice creates an opportunity for a future leak. Radiant floor heat works best when there is a significant thermal mass like a concrete slab, but our system runs under the subfloor and finished hardwood floors on our first floor. One problem is that it tends to open up small gaps between the hardwood floorboards due to expansion/contraction. These gaps aren’t big enough to be problematic but they are visible. Hardwood floors are definitely not ideal for radiant floor heat.

The much bigger problem is the cost of heating the hot water (actually water and glycol antifreeze) that runs through the radiant tubing. In a concrete slab system, you only need to heat water to 110 to 120 degrees and often a lot less. You can do this easily with a simple on demand modulated domestic hot water heater but in a subfloor system sending heat up through subflooring and ¾ to 1-½ inch hardwood floor, you need at least 130 degree F water. That means installing an actual boiler capable of 180 degree water. Electric boilers to heat water are very energy intensive and running our system adds about $600/month to our electric bill. If we only run it December through February that’s nearly a $2,000 addition to our electrical demand and enough to take us out of Net Zero territory. The heat itself is wonderful. It’s even, comfortable, and has no trouble maintaining 70 degree inside temperatures. But as a sustainability enthusiast I haven’t run the system for the last two years. It’s just too “hungry.” I do recommend, however, that every house with mini splits has some form of backup heat in the event of a heat pump failure. In the sustainable rehab I recently completed I recommended a pellet stove for occasional backup heat but the new owner has not installed one.

Aluminum under floor heat transfer plate in my house with PEX tubing in red.

Another solution would be to install a natural gas on demand condensing boiler which would be a lot more energy efficient. However that means more combustion, carbon and pollution. And on our street there is a moratorium on new natural gas hookups. We would have to install a propane system which has its own costs and is more expensive than natural gas. Future propane availability is questionable and many community regulations are leading people away from combustion and its greenhouse gases. Given that it would only be used for a coupe of months a year we may consider that.

Last, we may look at a pellet stove. They are “set it and forget it” other than filling with pellets. However the pellets are made from compressed sawdust and “resin” and are less environmentally friendly. Although they burn cleanly, (much cleaner than wood), they are still a source of carbon and greenhouse gases.

In summary, geothermal heat pumps powered by solar panels are best. Heat pump mini splits are second, though a backup source of heat is recommended. Third, if you have sufficient electrical generation, subfloor or in-floor radiant heat is absolutely the most even and comfortable but requires a lot of energy. In a slab, rather than subfloor it can be driven by a hot water heater instead of a boiler for greater efficiency.