What's the most efficient way to use a heat pump?

I would have thought heat pumps use buried cooling pipes, not the ambient air.

I knew someone who had a cottage up north where the temperature can occasionally hit -40°F (whatever that is in Celcius). They said the simplest thing was to take the big coil of piping and throw it in the lake. Your external temperature for the pump never goes below about 4°C.

Both buried pipes and air are used for heat pumps. The air ones are cheaper because there’s no need to dig a hole or trench for the pipe. However, if you’re in a really cold climate, you’re going to need the buried pipe type for winter.

Ah yes. The lake.
I’m sure I had one lying about here somewhere.

You do need a reasonable sized body of water to make this work. It isn’t true that you can’t get below 4ºC. Not enough water and you freeze the entire mass of water around your pipework. I have seen 2 million litres suggested as a good body of water for a single household to use as a thermal soak. So say one Olympic swimming pool. That is a pretty small lake, but a seriously big pool. Works just as well for cooling in summer if you need it.

Mine goes down to 5F, I could have gotten the unit that goes down to -20F, but I am also happy with the 5F as I burn a minimal amount of fuel now and the calculations showed the price difference would never be paid back with fuel savings, though increased efficiency of the higher SEER unit at lower temps was not taken into account.

Defrost gets triggered when the output indoor temperature starts dropping. I’m not sure that’s the trigger itself, but I have a temperature monitor directly on the output air stream as I was curious how it works with it’s multi stages.

Many defrost systems are completely dumb, running just on a timer with a set interval and maybe one or two simple checks/cutoffs. That’s why you sometimes see a unit go into defrost seconds after it turns on, or when there’s no frost buildup, or only after it’s been frosted up for seemingly too long. They may at least have termination switches that can detect when the refrigerant has warmed enough to melt all the frost, assuming that’s even enabled, and that the defrost timer isn’t set for too short a time. There’s certainly ways to be smarter about it, and some manufacturers do it (for more $ of course). A heat pump can defrost in above-freezing weather just by keeping the outdoor fan unit running after the system has shut down (air defrost), but in practice that level of “sophistication” is rare for the commodity junk installed in most of the US.

Demand charge is quite different from time-of-use billing. They are 2 separate charges on a bill. Time of use will be billed in KWH. Demand charges are based on peak KW use. But with these new electronic meters it may be possible record both on the same meter.

Our rate plan has both.

We live on Puget Sound. The temperature of the water is 53 degrees right now. Could I use this body of water somehow to design a better heat pump system? The air temp right now is in the 40s. So maybe it’s not worth it. But some days it does go below freezing.

As I said, a heat pump pumps heat uphill, takes from the cold and gives to the hot. So the least work is to use the warmest cold source. I guess the question is - how often does the air temperature dip below the sea temperature?

There’s also an efficiency question which occurs to me but I can’t answer which is pretty relevant, is how good a heat exchange is air vs water? If your heat pump piping is immersed in air vs water, I imagine water more effectively warms the coolant to current temperature. Is air a problem in this regard, does it not as easily warm the coolant to ambient outside temps?

quite frequently from November through February. But what do I have to do, run a pipe into the sound?

That might not be feasible. We have dramatic tides of up to 14 feet vertical (which can be over 100 feet horizontal.)

Water is a much better conductor of heat than air, so a few loops of pipe can achieve a lot of heat transfer, so long as you don’t freeze the water. It also has a much more consistent temperature than air, which helps the efficiency as well. 53 degree water is great for both heating and for cooling. The problem is that you may not be legally allowed to just chuck a closed-loop pipe into some random body of water that’s not your own private pond. Even more onerous would be sucking water into the system and then discharging it back into the lake/river/ground (open-loop). That gets the regulators involved (EPA, DNR, etc.).

I’m sure that’s true here, from what I know about shoreline regulations. I’m mostly interested in the theoretical. And I am finding this all very interesting.

The heat pump will use less electricity, and will have a lower operating cost, if you reduce the temperature setpoint at night as opposed to leaving it at a constant higher temperature.

This is only partly because it will run more efficiently (higher COP) with a smaller temperature lift. It’s also simply because you are using less heat.

I believe the main reason for the advice to leave a heat pump running on a constant setpoint is that it typically takes much longer to heat a home using a heat pump than a conventional heating system, because of the low flow temperature. The heat pump therefore works most effectively (not “efficiently”) if you just leave it alone to keep the house warm and cosy rather than turning it on and off like a conventional system.

If the project is a big, public-sector one, though, the calculus changes a bit. The city of Toronto cools many downtown buildings with a glycol/water mixture cooled to 4°C by the deep waters of Lake Ontario:

It seems to me that, in theory, there’d be a benefit to allow not just direct cooling in summer but also heating in winter. This would require a heat exchanger between the heat pump and the glycol/water mixture. I suspect that Ontario is cold enough that (a) they have very few installed heat pumps and (b) the increased efficiency of adding heat pumps is small enough that it’s not worth the cost.

Regardless, Ontario’s system is nifty.

The Toronto system looks like “free cooling” in that no mechanical refrigeration cycle is used. It’s just like tapping into geothermal (i.e. volcanic) heat sources, except they’re tapping into cold rather than hot. All they’re doing is circulating water around. That’s fantastic if you can do it, but for proper cooling you need your refrigerant/glycol/whatever to be in that 40ºF range (4ºC), and for heating it needs to be over 100ºF (38ºC). Most available heat sinks, whether air, water, or ground, are between those temperatures, so a refrigeration system is necessary to boost it one way or the other. The Toronto system would freeze up the bottom of the lake if they tried to extract the amount of heat needed in winter, and they’d need a massive refrigeration plant to do it, instead of just a pumping station. Temperate climates have the benefit of roughly equal heating and cooling seasons, so heat extracted in winter gets put back in the summer. In more extreme climates you risk saturating your heat sink if it’s not relatively mobile like a river or the air. Also, the bigger the project the more potential environmental impacts come with it.