We’re at 4000 ft in northern AZ, so we can get hot (105-107F) in the summer and cold (some 2 ft snowfalls, lows around 20 F) in the winter. Our Carrier Infinity heat pump (2013 vintage) works well year-round, with only occasional use of the 10 kW electric add-on heater in the air handler during the colder weather and programmed-thermostat transitions, as well as the inevitable heat pump “thaw” cycles in winter (which are not too often, since we usually have dry air). We have not experienced any refrigerant leaks in the 9 years we’ve had it, although the previous owners went through 5 outdoor units in 25 years because they never changed out the leaking heat exchanger in the air handler. We did.

There is a claim that a modification to create a higher-efficiency heat pump has recently been invented, which could be incorporated into heat pump designs pretty easily if it is confirmed to be significant and cost-effective. It would be easy for manufacturers to use the same heat pump models as today, but offer this as an upgrade feature (read: higher cost and higher profit) by just adding it into their existing line(s) of products.

Like you, we’ve also “upgraded” our Andersen windows with tightly-friction-fit 1/4″ “optically clear” acrylic interior panels, which I custom ordered to fit where the casement screens used to go from an on-line supplier. I also ordered automotive trim with a hollow foam bubble that can be compressed, and a 1/4″ push-on fitting that goes on the acrylic, in black, so it largely disappears and provides the seals for the panels. These have improved the windows’ resistance to outside noise as well as our energy efficiency. So far, so good.

We have a condensing gas boiler that also makes domestic hot water, so we burn about 670 ccf of gas/ year (40 kBtu/ SF-yr). We do have a mini-split heat pump with one head on each floor, that we are running right now for heat in mild months. An article I read recently said that the HSPF rating for heat pumps is not a good predictor of energy use for variable speed compressors (it underestimates actual use). I have made an estimate based on the HSPF with a reduction for that fact (using HSPF 9.5 instead of 10.3 it is rated for) and I get a result that we would need 6600 kWh more each year (over the 3500 kWh now) to replace the 62.2 million Btu’s provided by burning 670 ccf of natural gas, and that would cost $465 a year more than the gas. I expect a big increase in gas cost this year so maybe that difference will drop a bit to $330 or so, but it will definitely be more expensive to run the heat pump all winter, and I am not so sure the heat distribution will be acceptable (oil and especially propane users would see savings, though not a great payback). Several single zone heat pumps might do better, because I have seen HSPFs as high as 13 kWh/kBtu, and that would save about $110/ yr. IF you don’t have access to solar and do have the option for a condensing boiler, I don’t think an air source heat pump can compete on operating cost or, if it’s very efficient will have a VERY long payback. I don’t know how we will achieve mass electrification of existing homes with the current air source heat pumps that have little or no operating cost savings and a rather high installation cost to provide good distribution in 3 and 4 BR homes. Only higher gas costs and lower electric costs or more efficient heat pumps can solve that puzzle.
IMHO, what we need are high temperature hydronic heat pumps that can use the existing baseboard heat terminals in older north east homes. They would have to make 170 degrees F. hot water and probably use CO2 as a refrigerant, but they would keep the conversion cost down and have a cheap resistance backup in the indoor buffer tank. Relying on leak prone heat pumps for winter heat in a cold climate with no backup seems like a bad idea. Imagine a 4-hour refrigerant-leak repair in 18″ of snow; not good! A hydronic model could be quickly removed and repaired in a shop. Also we have to stop using R-410A refrigerant that has 2,085 times the GWP of CO2. Two leaks in my 3 year old mini-split had the global warming impact of two years of burning natural gas. And R-32 with a GWP of “only” 700 isn’t much better. The one-piece construction of hydronic heat pumps make them less leak prone too; no field joints are needed. We have low-temp hydronic heat pumps available now, but I suspect the Dutch will beat us to the high temp model.

I seem to have gotten a bit off the track of home electricity monitoring, didn’t I ? But with my new Emporia Vue, I can track my heat pump power use and maybe I’ll do a little experiment in November and December to heat the house. I’ll still be getting gas bills for domestic hot water (and the $18 customer charge), so will be able to track gas prices too. Rising gas prices have driven electric prices up 40% here since January and for the first time in 20 years or so, no vendor is offering 100% renewable power in CT. My previous vendor pulled out of the state in August. So right now I can only buy 33% renewable power.

When we bought the house, we converted all of the lights to CFLs or LEDs, and as the CFLs have gotten older, and some have burned out, I’ve used LEDs for all replacements. We had a 4 ft 4-light fluorescent fixture on the ceiling in the kitchen when we bought, and immediately replaced that ugly old piece of junk with a nice thin edge-lit LED 2 ft by 4 ft panel that puts out 7000+ lumens, and recently added six 4″ LED recessed lights for another 3500+ lumens around the perimeter (the panel is over a central island). These use so little energy (the panel is only 70W) that the Sense has not (yet?) recognized them in the low-level noise. It is picking up the start-up spikes of the small wine fridges, but their compressors are so small and efficient that their running power is just a little bit higher than the background from the computers, wireless home phone, cable modem/wifi, clocks, and other very low level users (including the few watts for the Sense module). As I write this, the lights are all off, and Sense is reporting that our “always on” load is 275W, 89W of “other” (i.e., unidentified), and 112W for a fridge, which I suspect is our 4 cu ft wet bar fridge, but our computers are still buried in the “always on” category; our solar output is at 4 kW (it is morning here; we’re currently peaking at about 5 kW in mid-day). The lowest-energy devices that I see in its artificial-intelligence-identified list are the garage door, a “motor,” two “lights” (I don’t know if that is correct or not), a “vacuum” (we have a central vac), and my laser printer, which is small and only used on random occasions. So, I think, depending on the energy load and “waveform” associated with any given light it may or may not be able to identify it. Based on my interpretation of the description of the tech in the instructions, a reproducible and differentiable waveform pattern in the energy use is a key component for their algorithm. In our low-energy-use-by-intention house it does not seem to find most of our lights. For that matter, it has not (yet) identified our heat pump water heater, which must have a built-in soft-starter since I don’t see any start-up spikes that I’d associate with it (but I have not done the intentional experiments to see it, either).

Dave, we had a Sense system installed with our new solar panels last summer. It actually works very well, and closely matches the data from the utility and the solar power Enphase Enlighten system that is also regulatory-grade monitoring. While it has not (correctly) identified all of our energy-use sources by their power “signatures,” it has gotten most of them. I can also look at the past, recent, and current real-time data from the Sense anytime that I want, and display the power-consumption (or solar-power-generation) details down to the sub-minute level (the smallest-timeframe display is 15 minutes on my desktop screen). It seems to be fast enough to get almost all of the heat pump surge current (and other surge currents) in the usual on-off cycles, and has shown me the power consumption of the 12.5 kW tankless unit that I had installed for the master bath (which is on the opposite end of the house from the water heater). Hair dryer, hair curler, clothes iron, computers, induction cooktop, microwave, Ninja Foodie mini-oven, big wall ovens, and motor-running devices like our heat pump water heater, refrigerators, a freezer, and mini wine fridges…it sees all of them on the demand side. On the solar supply side, I can see the effect of variable-density clouds passing overhead (the panels keep working even when unevenly shaded), or the shading effect from nearby trees, or the daily blue-sky differences in production as the sun’s course varies in the sky. They also have a feature which detects faults in the incoming power line or within the home (in the “Sense Lab”), which have fortunately been not activated in our case, and the display can be set up with your personal electric power rate structure (including “time of day” and “day of week” and “time of year” rate adjustments), as well as your particular billing cycle, to estimate the electricity-use portion of your bill. A comparison of your use with “similar home” and “within your state” users is also available, as well as a real-time graphical breakdown in a type of bubble plot that shows your current use and how it is attributed to certain devices/uses or to “other.” I used its daily and monthly summaries of information to compare with the Enlighten and utility data. Lots of information that you can use, if you want to.

Marty: Thanks for letting me know about that system. I like the idea of not having a separate button or occupancy sensor to start the recirculation. I’ll have to check it out.

A new system that I’ve seen and plan to install uses a flow sensor to start up the demand pump. You turn on the hot water for 1 sec, then off. The pump is activated for 30 sec, or until hot water use is stopped. An “advanced bridge valve” can allow up to about 5 gal/min flow, and closes automatically in the 100 +/- 5 F temperature range. With such a system, hot water is not continuously fed into the lines, only to cool off. As such, it should be far more efficient than a recirculating timer or the system that you described (maintaining hot water in the lines all the time), and much more convenient than on-demand systems operated by push buttons or motion sensors. Just open for a second, wait a few more seconds for the hot water to arrive, and go!

The Watts and similar bridge valves are low-volume-rate plastic devices that seem to limit the crossover flow to relatively low rates, based on the one I bought several years ago to use with our recirculation system (with under-slab uninsulated hot water lines). A newly patented (in 2021) all-metal temperature-controlled device that probably uses a bimetal closure to function like a thermostat, has a higher flow rate for on-demand system use (they call it an “advanced bridge valve”). A company called AquaMotion has developed this, with the high-rate one (apparently good for up to 5 gal/min) being for on-demand systems and a lower-rate version also available to function more like the older plastic ones. The nominal closing temperature is in the 95-105 F range, so that fully-hot water does not flow into the cold water line, but is still warm enough to be used by itself if you are washing hands or getting ready to shower. Combined with new on-demand system controls that detect the use of hot water (turn on the valve part way for 1 second, then off), they start the demand pump on a 30-second minimum timer (it stays on if hot water is indeed needed), and shut it down when the hot water use has stopped, The company says that this kind of need-detection on-demand system can be used with multiple such bridge valves, if desired, since each will automatically close once hot water arrives. This avoids the hassle of push-button starting or the annoyance of motion-sensor starting… it only starts up when hot water is actually being wanted and used, all by itself. As if this is the way it is supposed to be, not kluged!