Heat pumps, marvels of physics that defy understanding, advised against by HVAC companies who prefer to stick with the old gas-fired boilers and furnaces or electric heat. Air-to-air will get you a lecture. But nothing will get them fired up to dissuade you than asking about air-to-water heat pumps.
Number one reason to get a heat pump: costs less to operate, more comfortable heating.
Air-to-air is taking heat out of outside air and blowing heated indoor air through your ductwork to warm up your place.
Air-to-water is taking heat out of outdoor air, heating up a tank of water, and circulating that water through your radiators — or directly heating up the water in your rads — to warm up your place.
HVAC experts will inform you that your gas-fired boiler heats up the water to 60 or 70C, and no heat pump can do that. You’ll freeze!
Huh? I scratched my head over that — my circulating pump’s dial said the water temperature in my rads was 40. Yes, four-zero. Well, they intoned. Your pump was off; those dials can get stuck. Yeah, sure. If my dial was stuck, and the water temp was really 60 or 70, I’d never be able to touch those rads. I’d hear singeing if I so much as reached a finger towards them!
Besides which, I had a new pump installed. Did the HVAC company install one whose dial was already stuck at 40C? I don’t think so!
If I hadn’t been babying my furnace through a couple of winters, knowing it was long past its death date, and if I hadn’t already once lived through a dead furnace mid-winter, the experts may’ve persuaded me to stick to my gas-fired one. But if I was going to spend money I didn’t have on an expensive appliance that would last decades, I was going to spend it on technology that wouldn’t contribute to climate change. I may be one person, but I can have an impact in my own way. I can be a force for good, not lazy thinking afraid of change.
Plus grants and loans would give me the money. I wouldn’t have to spend my own like if I replaced my furnace with another gas-fired one.
Heat Pump Physics
The other huge objection to heat pumps is that they don’t work in Canadian winters, even Toronto’s milder ones. I mean, the air is frozen, right? Well, actually…The only time air becomes frozen — when the molecules come to a complete standstill — is at absolute zero. That’s zero Kelvin or -273C. No Canadian winter has achieved that cold hell! Until that point, air molecules keep on moving. Even on a completely still day, molecules dance and jitter, creating heat. Think of how when you move, even when you stroll or sway, you generate heat. The slower you move, the less heat. But even stamping your feet on a cold day is creates friction from movement; the friction generates heat to warm up your feet. Same with air molecules.
Decades ago, some genius inventor figured out how to get that heat from molecules stamping their metaphorical feet, into our homes. That’s basically what a heat pump does.
Since heat from outdoor air is free, the only energy a heat pump requires is hydro to run the compressor. That makes it more than 100% efficient, way more.
Cool Heat Pump Feature
Every now and then, the heat pump will go into defrost mode. That’s like when your fridge defrosts the freezer to prevent ice buildup. The heat pump’s fan stops; the pump defrosts itself; steam wisps out; the fan begins spinning again; steam billows out, wreathing the outdoor unit of the heat pump. Pretty cool to see.
This video shows how the heat pump defrosts itself. The sound of the compressor about 2 minutes in may be unpleasant, but the guy describes what’s happening well. One difference between air-to-water and air-to-air is that when the heat pump defrosts and stops heating the water in the buffer tank, the circulating pump keeps circulating the heated water into the rads and back into the buffer tank.
Air-to-Water
The only difference between air-to-air and air-to-water heat pumps is the medium that carries the heat indoors. The physics and the method is basically the same from outside to the inside. It’s what happens to the heat once it’s brought indoors that differs. Air-to-water seems to still be in its infancy here. But in the UK, they have whizbang heat pumps that come with remote controls and controller units that homeowners can customize to their heart’s content. Unlike furnaces, heat pumps are individualized to the building. Insulation, geographical orientation, local weather, floor size, etc. can determine how your heat pump is set up. I’ve learnt that weather plays a big role in setting buffer tank maximum and minimum temperatures. It’d be a heck of a lot easier if Canadian installers would computerize this weather compensation function like UK ones do.
I know, you’re confused. Since when does your heat source need to know the outside temperature? Let me back up.
Cold climate heat pumps are installed in Canada so that the pump can extract heat from air down to -20 and now -30. It’ll probably be a matter of time before they get to -50. People always assume that current is future; people forget that just as current tech is improved on past tech, so will future be better than now.
In air-to-water heat pump, the pump heats up a buffer tank. Basically, a hot water tank with many holes in it for the piping into and out of it. You need a pipe to go into it from the heat pump; a pipe to come out of it to circulate the heated water into the rads; a pipe to circulate the water from the rads back into the buffer tank; a pipe for the expansion tank; and a pipe to the heat exchanger. Yes, it looks rather bristling. It also doesn’t need to be as big as a standard hot water tank. About half or less the size.
The installer sets up the algorithm of the maximum water temperature and the minimum. The maximum is based on the heat pump’s capacity. Mine is 50C. The maximum temperature is what the tank temperature is heated to. The minimum temperature triggers the heat pump to come back on to heat the buffer tank water back up to the maximum temperature.
Now, we get to the part where the HVAC experts show their ignorance when they claim that heat pumps can’t work as well as gas-fired boilers because they can’t raise the temperature to 60 or 70C.
The lower the water temperature, the more efficient and more comfortable the heating.
Yup, you got that right.
The maximum water temperature depends on the outside weather temperature.
No, not the way you’re thinking.
The colder it is outside, the higher you set the maximum temperature.
But you want to set it as low as you can. So instead of 50 when it’s -15 outside, you’ll set it to 45C.
“It is very important to understand that the hotter the water produced by a heat pump, the poorer the energy-efficiency. So, running at a lower temperature can save a lot of energy. The following ratings, for a typical ground source heat pump system, illustrate this point:(note – the larger the COP, the better)
Water heated to 55°C (95°F), COP = 2.4 ( e.g. a 6kW heat for 2.5kW input)
Water heated to 45°C (113°F), COP = 3.2 ( e.g. a 6kW heat for 1.9kW input)
Water heated to 35°C (131°F), COP = 4 ( e.g. a 6kW heat for 1.5kW input)
(COP is the energy efficiency ratio. See Glossary for better description).
By keeping the heat-output temperature low, the energy-efficiency will be high. This is generally achieved by having large radiators or good underfloor heating.”John Cantor Heat Pumps, UK.
Here’s a handy chart to show optimum maximum temps correlated to outdoor temps.
See the infogram in action: https://infogram.com/1pp03jrkypexe3ir6w5qej2z1ysw7mkgmm
Heat Pumps Heat Differently Than Furnaces
A gas-fired boiler blasts flames and heats up cold water really fast. But it’s not instant heat. Once the water gets hot, it flows through the rads. When the thermostat tells the boiler, hey, man, it’s my set temperature now, the gas shuts off. The circulating pump stops. That takes about 20 to 30 minutes. The rads get cold; room temperature drops below the thermostat’s set point; and the thermostat triggers the boiler to ignite the gas again. On really cold days, the gas goes on and off about every 20 minutes or so, and the rads stay hot.
In contrast, a heat pump heats the water independent of the thermostat. When the thermostat calls for heat, the circulating pump turns on and instantly hot water circulates into the radiators. But the room temperature water flows back into the buffer tank as the pump circulates the water from the buffer tank into the rads and back into the tank. As you can imagine, the buffer tank’s water temperature drops when you have room temperature water at, let’s say 20C, mixing with the 45C water. When the tank temperature drops down to the minimum temperature your installer set, the heat pump turns on. Let’s say the minimum temperature is 37C. It seems the standard range between max and min is 8 degrees.
But the water temperature in the buffer tank will continue to drop as the circulating pump continues to circulate colder rad water into the tank. It may drop down to 30, or as happened to me a couple times, 27C. The heat pump heats the water up slowly against this continual influx of colder water.
When the circulating pump isn’t on, the heat pump will warm that tank back up in 30 minutes or less, depending on how low the tank temperature had dropped to. But when the circulating pump is pumping water from the colder rads into the tank, it takes hours for the heat pump to heat the tank temperature back up to the maximum set point of, let’s say, 45C.
Even so, 26 degrees is still hotter than 20. You’ll feel your rads warm. It’s just that they’re not at 45C immediately. Instead, you get a slow ramp up. As the heat pump keeps sending heat into the water, the water in the rads slowly increases.
That slow ramp up that keeps the rads warm for hours is why heat pump warmth is a lot more comfortable than the on-off swings of gas-fired heat. Note: when the heat pump is first turned on and on frigid days, the rads reach 45 and stay there or close to it for a long time. This blog post describes how weather compensation — adjusting the tank temp in sync with outside temps — leads to better comfort: https://www.linkedin.com/pulse/why-your-heat-pump-weather-comp-set-37-degrees-lizzie-graham-hendra
Slow Steady Feels Warmer Than On-Off
I noticed a few differences.
1. Somehow, without having adjusted the radiators’ valves, the heat pump created a better balance of heat between floors.
2. With the rads warm continually, the air feels warmer, and I’m more comfortable, even when the rads are barely warm.
3. Rads won’t stay warm continually if the maximum tank temperature is set too high for the temperature outside. The warmer it is outside, the lower the maximum tank temp should be to achieve warm rads for hours, otherwise you get gas-fired-like on/off cycling.
4. Computers do a better job of compensating for weather fluctuations than a human who’s never had a heat pump and is wary of changing maximum tank temperatures for fear of freezing.
5. Heat pumps don’t need gas-fired backups. For those days that drop below -20 or -30 (depending on which cold climate heat pump you have), a backup electric heater in the buffer tank can be automatically triggered on, which your installer sets.
6. My indoor air is noticeably cleaner and my energy levels better with no more natural gas appliances (water heater, furnace, stove and oven).
7. Heat pumps are so silent that you don’t know when the heat is on. You have to learn to go near the closest window when the place is dead quiet or check your smart thermostat settings.
And that comes to the last big lie/myth of heat pumps.
Smart Thermostats Work BETTER
I was told infinite times to set my thermostat to one temperature and leave it at that. Heat pumps don’t like changing thermostat settings. You’ll decrease its lifespan if you do and spend more on hydro. Seriously? What kind of primitive tech is that? Isn’t this supposed to be better than traditional? And who expects me to be able to sleep at 22C?! Or even 20?! Open a window was the huffy reply. What?! I’ve spent all this loan and grant money on something energy efficient, just to blow it out a night window? That’s nuts. Besides which, only a man would tell a woman to have a window open at night. (If you don’t understand how bad that advice is, sit for a moment and think or read a newspaper.)
So I did an experiment. I’ve had a basic Honeywell thermostat since forever. It worked fine with my boiler furnace. But with the heat pump, hoo boy, it kept my place HOT. It didn’t matter that I set it to 20C — or 18 in desperation. Every time, it read that my hallway temperature had dropped to 21, it called for heat. Rads got warm, the thermostat showed the temperature rising to 23…24…even 25 one time. Although with the heat pump, room differentials are about 1 to 2 degrees (lower than with the boiler), that meant the warmest rooms were climbing to 25 or 26. The only thing that dropped it was opening doors wide!
After some assertiveness with the installer, I had an ecobee smart thermostat installed. After a few weeks, I bought room sensors. After a few more weeks, I figured out where to place them to create the best balance of heat between rooms and floors to keep me comfortable wherever I am. That lead to my lowest energy consumption.
A smart thermostat saves energy and me money.
“Minimize Temperature Setbacks. Heat pumps have a slower response than furnace systems, so they have more difficultly responding to deep temperature setbacks. Moderated setbacks of not more than 2°C should be employed or a “smart” thermostat that switches the system on early, in anticipation of a recovery from setback, should be used. Again, consult your installation contractor on the optimal setback temperature for your system.” Natural Resources Canada
An engineer friend told me, although a 2-degree setback at night is fine, yes, varying the room temperatures will decrease heat pump lifespan. BUT. Taking care of my heat pump through regular maintenance will increase its lifespan far more than a smart thermostat varying temperatures will decrease it — just like my boiler lasted years and years longer than its average lifespan because I had it checked and cleaned annually.
More on smart thermostats and tracking energy consumption in a later post.
So the next time some HVAC expert lectures you for a solid non-stop 15 minutes about heat pumps being bad, send them this post or tell them UK manages it perfectly well and so have hundreds of Torontonians who are warmer and more comfortable with their non-hybrid air-to-water and air-to-air heat pumps than they were with their gas-fired furnaces.
Comments