Uncovering some decarbonization complexities

September 27, 2022

AirtightnessDecarbonizationEco DesignGreen ArchitecturePassive House Architecture

Recently a friend asked me over to see if switching from his gas furnace to heat pumps would make sense—he’d gotten advice from one heat pump installer that his house wasn’t a good candidate, and also got a separate quote of over $60k for an elaborate heat pump system. Why can’t you just switch from a gas furnace to a heat pump?

Thermal Envelope

The thermal envelope, it turns out, really matters. The house in question is a poorly insulated, leaky house—in other words, sorry to say, a typical older house. Houses like that will have rapid changes in temperature inside with rapid changes and heavy winds outside, and require a lot of energy to heat and cool. Most gas furnaces are made to blow a lot of air and heat, so they can respond to sudden demands; but heat pumps are designed to hum along at a fairly even temperature, and don’t have the capacity to ramp up quickly. In other words, residential heat pumps do really well, and are most efficient, in buildings with decent-or-better thermal envelopes.

Courtesy buildingscience.com

Heating Capacity

The other simple problem is capacity: this house had an existing 8-ton heating system (about 100k BTUh) connected to big ducts. There is no 8-ton heat pump you could just plug in to switch from gas to efficient electricity—the biggest I know of is about 4 tons. So you would need to redesign the ducting system to break up the house into two smaller zones, and as you can imagine, complexity, expense, and disruption of the house would increase. The $60k+ quote he got was for—get this—3 outdoor heat pumps with EIGHT indoor units—linesets everywhere, lots of equipment to maintain, and, yeah, expensive. All to heat a poorly insulated house.


What to do– choose a bigger hose, or a bucket that doesn’t leak? This is a good case for insulating first: with insulation and air sealing, the heating load can be reduced drastically, often by 50% or more. This will require some expense and disruption on the house, but once you do it, you need less expensive (smaller) HVAC equipment, and you’ll spend less on heating and cooling forever– you will have locked in that efficiency for the life of the building.

The Expense Equation

Let’s not kid ourselves—the insulation job won’t be cheap either. To do it right means dealing with the roof—probably by spray foaming the underside of the roof deck to reduce stack effect and heat flow, get a few inches of the correct type of insulation on the existing foundation walls, and in this case, with a brick structure, consider putting exterior insulation over the brick with new siding over it. All of that will run in the neighborhood of $60k I should think, and then the HVAC system might now be down around 30k. Good news is there are many incentives out there to offset those costs—with new on top of existing federal incentives for efficiency upgrades, and a $10k retrofit incentive through our local village, the bill may come to more like $50k: less than the wasteful options, with way lower energy bills and a more comfortable house. Maybe I was guessing too low and the total may come out to around 70k after incentives—even so, there would be a tradeoff of lower energy bills for increased loan cost, maybe close to a wash.

Future Energy Costs?

None of the above even takes into account where energy prices are going. Almost as certain as death and taxes are rising energy costs. Between extreme weather, supply chain, and war in Ukraine, it’s been volatile lately, and my hunch is it’s not going to just quietly settle down quickly. To reduce energy use at home, especially with renewables and backup batteries, brings some peace of mind as well as lower bills. I would say it’s worth it.

Courtesy EIA.gov
Courtesy EIA.gov

Watch this video below of the Green Built Home Tour to learn more about this project.