Our house came with a pretty standard, roughly 20 year old gas furnace in the basement, maybe 75-80% efficient, which delivered heat through ductwork to a register in each room. We had no air conditioning; hardly anyone in San Francisco does, because traditionally it’s never gotten hot enough to need it. Yay Pacific Ocean.
Then climate change kicked in, and now most summers (still not every year, thankfully) we get at least one day when A/C is really useful. Compounding the problem, the house has an attic crawlspace which soaks up heat on hot days and radiates it back out at night, countering most of the effect of the cool California summer nights. The Labor Day heatwave of 2017, when it got over 100 degrees even in our neighborhood on the west side and the house stayed way too hot to sleep all night, made me think more seriously about a cooling solution.
Regular fans didn’t do much; they just can’t move enough air fast enough in the right direction to overcome the building’s thermal lag. A portable A/C unit worked well for the room we put it in, but was extremely loud and inefficient. Installing central air would have been expensive, complicated, and still useless the vast majority of the year. A whole house fan might have helped a lot if it were workable— my research says you should probably get one if your house can accommodate it— but ours couldn’t; the crawlspace wasn’t tall enough.
So in 2019 when our furnace started making weird fan noises, I was raring to go and get it replaced with a heat pump. Quiet, super-efficient, fossil-fuel-free heat for most of the year, plus air conditioning for the few days we’d need it: what’s not to like? A few annoying things, as it turned out, but overall the heat pump has been great and I strongly recommend getting one if and when your furnace, and/or your existing A/C setup, gets toward replacement age. It’s the HVAC of the future, and we need that future to get more evenly distributed soon.
Ducted or ductless?
Heat pumps transfer heat between the inside and outside of your house, but they don’t transfer air: that’s part of what makes them so efficient. The conduit for the heat is a refrigerant line that runs from the compressor outside the house to one or more air handlers inside. In a “ductless mini split” system, the air handlers are wall-mounted units, typically one to a room or couple of rooms, a few feet long and fairly slender. You might have seen one of these if you go to Europe regularly— they’re much more common over there. Ductless systems, because they deliver the heat exchange directly to where you need it, are typically the most efficient and effective choice for new construction, and for retrofitting older homes that have no existing ductwork, e.g. those heated by boiler/radiator systems or baseboard heaters.
But for homes with forced-air HVAC that have ducts to every room already, it’s usually easier and cheaper, and good enough, to retrofit the heat pump onto the ductwork by putting an air handler where the furnace used to go. The air handler is typically smaller than the furnace and its fan is much quieter. This is the approach we went with for our retrofit, and it made for a nice drop-in replacement.
The installation experience
We got our heat pump installed by Air Flow Pros who I can highly recommend: good explainers, straight shooters, well-executed quality work. They initially visited us in January 2020, got us an estimate right away, and ended up installing in June— an unusually long delay that was not their fault, due first to tremendous pre-Covid demand and then to the spring 2020 Covid lockdown. They typically install heat pumps made by either Mitsubishi or Fujitsu, the two big players in the current market. We got a Mitsubishi which has been trouble-free so far.
The work took a full week, during which time we had no central heat in the house. This is apparently typical, so installing in the warmer months is advisable. They had to demolish and remove the existing furnace; remove and replace some of the older parts of the existing ductwork; put the new air handler in place of the furnace; pour a small concrete pad about one foot by four in the backyard and mount the compressor on top of it, braced to the outside wall of the house; and run the refrigerant conduit from compressor to handler, drilling a small hole through the wall of the house.
There was also a fair bit of electrical work involved to run the necessary 240V circuit out to the compressor and to wire up the air handler. We were lucky that we had juuuust enough spare capacity in our breaker panel to make it work, despite already having a hot tub, an EV charger, and solar panels; the next time we do a major electrification upgrade like this we’ll likely need a new panel. San Francisco being San Francisco, this all required a post-install city inspection too. We had to contract separately with the electrician, and the guy Air Flow Pros referred us to (Val Dimitrov of Value Electrical) was great.
Costs and options
The install cost us $23K in all: $18K for the basic heat pump functionality, plus $3K for optional add-ons described below, plus another $2K for the electrical work. That’s a lot, and it’s why Saul Griffith says in Electrify that we need more credit programs to help people spread out the cost of upgrades like this. In a city with cheaper labor than San Francisco (i.e. almost any other city) it’d probably be a few thousand less. By comparison, we got quotes for a replacement gas furnace and conventional central air system at about $4K and $8K respectively in 2017, though they would probably cost significantly more now.
About $1500 of the cost went to giving us dual-zone control, where the bedrooms use a separate air handler from the rest of the house so that in theory they can be heated and cooled separately. My thought was that it would be more efficient on a hot night to only cool the bedrooms, for example. In practice we’ve never used this and it’s annoying to have two thermostats to reset whenever you need to change the heating/cooling schedule. If I had to do it over again I’d have gone with a single zone, though I think a second zone could be much more useful for a larger and/or multistory house.
I am much happier with the whole house air filter, another $1500 add-on. This basically puts a HEPA filter on the air handler so that whenever you run the fan to heat or cool the house, it cleans the air as it goes, just as a freestanding air filter would. It’s good for peace of mind in wildfire season, probably helps at least modestly with Covid transmission risk too, and in general I have become something of an air quality fanatic since reading the research on the bad health effects of particulates, so I want every kind of household air cleaning I can reasonably get.
The additional option I wish we had gotten is a Nest thermostat connection. The heat pump isn’t Nest compatible out of the box and wouldn’t work at all with the first generation Nest that came with the house. So we just have a standard Mitsubishi wireless thermostat for each zone, which does the job well enough but is really clunky and hard to use compared to the Nest, and can’t be monitored or adjusted remotely when we’re away from home. I may yet inquire with Air Flow Pros about a retrofit.
So how well does it work?
For heating in the easy environment of San Francisco, it’s terrific, much nicer than the old furnace. The warm air comes almost silently out of the vents, and you can’t hear the compressor blowing cool air outward at all, unless you go in the backyard. It has no trouble keeping us toasty on the coldest winter nights, which are maybe 35-40 degrees Fahrenheit. Heat pumps used to be much less effective in subfreezing winter temperatures, but they’ve apparently gotten better recently. We have several friends in Minnesota who have installed them alongside their old steam boilers, expecting to still need the boilers through the winter, and they find that the heat pump is enough except on the very coldest midwinter days.
The A/C function is nice to have and definitely takes the edge off the hottest days, but it’s not nearly as effective overall as the heating function. This is probably an idiosyncrasy of our house’s thermal lag, though apparently it is also common for forced-air ductwork retrofits, because cool air doesn’t naturally rise like hot air so you get less cooling power than heating power from the air handler in the basement. If I were in a climate with hotter summers I’d probably pay the extra for ductless head units, at least in the bedrooms.
The heat pump uses about 3 KW when it runs, and as best I can tell from our highly confounded before-and-after electricity use totals, probably something in the range of 3-5 MWh per year. That puts the annual electricity cost in the mid-hundreds of dollars a year. On the other hand, it’s cut our total household gas usage by more than half: from 469 therms in the 12-month period ending June 2019 to 213 therms in the 12-month period ending June 2022. That’s probably an underestimate since we were home much more in the latter period, thus using more midday heat as well as more hot water and cooking gas, and it represents several hundred dollars of annual gas bill savings at current PG&E rates. And of course people in areas with higher heating needs will see proportionately larger savings.
It may not be a net operating cost win for us, and certainly is not enough of a win to pay back the additional installation cost, at current electricity and gas prices. But in the future we should hope and work for, clean electricity will get cheaper and fossil gas more expensive. And a better-sealed and insulated house should need less power to run that heat pump: one more thing for me to keep working on!