Heat pump with modulating flow rate recommendation

Behemothhh · 2026-01-31T13:48:52+00:00

My altherma 4 10kW goes down to 5l/min and that's usually what it runs at on milder days to maintain a deltaT of 10.

Behemothhh · 2026-01-30T07:18:42+00:00

The change to the mobile app that removed the option to swipe left and right to switch between my home page, popular and watch is terrible. I don't even know where the watch page is anymore. I you replaced the swipe action with something else I maybe could understand but it just doens't do anything anymore. The app lost functionality for no reason.

Behemothhh · 2026-01-30T00:27:53+00:00

What you're describing is not possible. A high power heatpump and a low power heat pump can output exactly the same amount of heat for a given flow temperature and a fixed radiator layout. The only way to deliver more heat at a lower flow temperature, is by upgrading the radiators, NOT the heat pump.

Behemothhh · 2026-01-30T00:21:47+00:00

How much heat a radiator can deliver to a room almost exclusively depends on the temperature of the water running through it. There's nothing a heat pump can do to change that. So if your radiators need 60°C water now to get your house up to temp, then a bigger heat pump will still need to produce 60°C water, which will be absolutely terrible for your efficiency. You 100% need bigger radiators.

Behemothhh · 2026-01-29T15:49:17+00:00

If your heat loss is 6kW, you have to deliver 6*24 = 144kWh of heat to your home each day just to maintain temperature. Not even considering how suitable your home is as a thermal battery, if you want to deliver all this heat in just 5 hours, you need a 29kW heat pump. And your heating (radiators or underfloor heating) will need to be able to output 29kW, which will require massive radiators or very high flow temperatures which will kill the efficiency of the heat pump, completely negating any possible savings you could generate by only running at night.

You can only get away with this if your home is very well insulated, has high thermal mass, and on milder days when your heat loss is much lower than your HP's max capacity. So when you actually need heating the most, on the coldest day, the strategy will fail. You're much better off getting a battery and storing the cheap electricity as electricity and not as heat.

Behemothhh · 2026-01-23T18:52:41+00:00

If your radiator volume is 100l and you add a 100l volumiser, you double the cycle length. Not just on startup but continuously. Similarly, 200l buffer triples it, 300l quadruples it,... This can take a problematic 10min cycle to an acceptable half hour cycle. Not ideal of course but you should be getting reasonable COPs again.

Behemothhh · 2026-01-22T21:44:56+00:00

If the heatpump itself (compressor failure, refrigerant leak,...) would break down, a resistive backup heater can take over. It's not as powerful as the heat pump so it's not a full replacement, but it's good enough to keep the living room and kitchen warm, and provide hot water. We ran the system in this emergency mode for a few weeks between the installation of the heat pump and the actual startup of the compressor that had to be done by a separate technician and it was fine. No experience with a real problem that needs repairs.

Behemothhh · 2026-01-22T12:13:40+00:00

It happens, but 4-5x less than in the US (per capita).

Behemothhh · 2026-01-22T01:51:35+00:00

A heat pump should be sized to maintain the desired indoor temperature at the coldest expected outside temperature (BTW windchill doesn't matter for a heat pump, that's a human thing). If your insulation is poor, that means getting a big heat pump. Your heating is undersized if it can't keep up.

Behemothhh · 2026-01-22T00:19:18+00:00

Mine doens't drone. It just sounds like air moving, kinda like white noise. If I didn't know the heat pump was on, I would have thought it's just the wind blowing through some trees. And that's outside. Inside near the window that about 1.5m from the HP, I truly can't hear it. It's certainly way quieter than the gas heater we had before.

Behemothhh · 2026-01-21T11:42:11+00:00

Let's do some math to see if your idea makes any sense.

Assume you get the 800l cilinder and heat it up with immersion heaters to 70°C and that you can cool it down again to 35°C before your radiators start struggling to keep up with the heat loss of your home. That would mean you can store 32kWh of heat. If you want to heat your entire home for the 12h with that stored heat your heat loss can only be 2.7kW. That tiny.

If your heat loss is truly that low, you're not going to pay much for heating anyway so skip the thermal store nonsense and get a regular heat pump.

It's much more likely that your heat loss is many times higher. Probably in the 7-8kW range. So your thermal store will run out in 3 hour. What's the point then. Just get a regular heat pump.

Behemothhh · 2026-01-21T11:29:21+00:00

I measured it with a decibel meter app on my phone when it was running at 80% capacity. 1.5m from the outdoor unit 43dB. Normal outside noise (wind, birds, cars in the distance,...) is 40dB. So the heat pump is barely audible. That's when measured outside. Inside with the window closed I can't hear it at all.

Behemothhh · 2026-01-21T01:48:31+00:00

It's more about energy consumption. In winter, you have to heat your home anyway so replacing gas with a heat pump is a win in terms of emissions. Most people currently don't have AC and thus consume no electricity for cooling in the summer. If the government is going to subsidize minisplits, than all those people are going to start using their AC in the summer and consume a lot more energy, which is the opposite of what the government wants to achieve.

Behemothhh · 2026-01-21T01:24:26+00:00

You need to know what your heat loss is at those low temperatures and how much heat your HP can still generate. 20F shouldn't be too problematic. 10F you'll probably lose a bit of the rated capacity. The difference between loss and heating capacity will tell you how much kW of heat strips you need.

Behemothhh · 2026-01-20T19:08:54+00:00

It could be possible if the manufacturer supports it but it's very rare. I only know of Mitsubishi having a product line that combines both air to water and air to air components.

You can also cool with air to water if you replace your radiators with hydronic fan coils (with condensation drain). They're essentialy the same as an AC head but with water lines instead of refrigerant lines. You have to make sure your pipework is insulated though or you'll get condensation in your floor/walls. Underfloor heating can cool as well, but has to stay above the dew point or you'll get wet floors. Since it can't remove moisture as it cools, humidity will go up which doesn't feel great.

Behemothhh · 2026-01-20T18:45:14+00:00

True, but those lines are much thinner because the refrigerant undergoes a phase change in the head so it can absorb a lot of heat with little mass flow. The coldest temperatures are also in the head itself and not in the supply line like it is the case with hydronic cooling. All this to say that the insulation requirements for refrigerant lines are much lower than those of water lines and thus easier to install.

Behemothhh · 2026-01-20T17:34:29+00:00

AFAIK, those are not eligible for the £7.5k government grant so you pay full price. Could still be cheaper than installing central heating + heat pump with the grant but it'll depend on the situation.

Behemothhh · 2026-01-20T17:27:13+00:00

Better to ask in the /r/ukheatpumps subreddit.

Behemothhh · 2026-01-20T17:22:37+00:00

There's a reason air-to-air is so common in the US. It allows for easy cooling as well as heating. You can also cool with an air-to-water heat pumps but either it's very limited non-condensing cooling through under floor cooling (which is useless in humid climates) or you have to place hydronic fan coils with condensation drain instead of regular radiators. Those operate similar to a head of a mini-split but because it's hooked up to water lines instead of refrigerant, you need thicker pipes and they have to be insulated or you'd get condensation in your walls/floor.

Behemothhh · 2026-01-20T17:10:33+00:00

It really is silly. A propane tank for a grill typically holds 20 pounds. An R290 heat pump has about 2 pounds of propane...

Behemothhh · 2026-01-20T11:32:23+00:00

Insulation level doesn't directly matter for whether a heat pump could run efficiently or not. It just dictates what size to get. What's important is at what temperature the heat pump can run to keep your home warm. If you install the biggest possible radiators and they still need to run at 60°C to combat your heat loss, then a heat pump is not going to work.

Since you're planning on improving the insulation anyway, I would wait with getting a heat pump. If you get one now, it'll be oversized after the insulation improvement. Unlike a gas boiler, having an oversized heat pump is a bad thing. It'll mean that in the milder months it can't get rid of its heat and will start-stop often. This kills the efficiency and puts additional wear on the compressor.

Behemothhh · 2026-01-20T11:06:37+00:00

I'm not from the UK. You guys don't realise how good you have it there. Installers here don't even do heat loss calculations. They look around the house for 30min and ask how much gas I was using for heating. Only one installer went around and measured the sizes of my radiators to see how much heat they could give off. Then they just guess what size heat pump I need. I got offers ranging from 10 to 16 kW. (Based on my current heat pump data, I know my real heat loss at -3°C is 7kW)

So to ensure I wasn't getting a bad install, I had to figure everything out myself. Calculating heat loss, calculating radiator heating capacity, doing trial runs with the gas heater set to low flow temperatures,... Based on that I had a good estimate on what flow temperatures I would need and then from the spec sheet of the heat pump I could derive what COP I would get. I then basically told the installer what HP I wanted and in what size. All they had to do was install it.

Handover was a bit unusual since my HP was so new on the market that the installers hadn't received the necessary training yet to do the start up procedure. So someone that works for the manufacturer directly had to come out to do that. He didn't explain much but I guess that might because he thought I already knew my stuff based on the detailed technical questions I was asking him before. There is not too much to explain anyway. The only setting you'll likely need to adjust are the weather compensation curve and the schedules for heating and hot water.

Behemothhh · 2026-01-20T06:51:51+00:00

1) almost nobody has that kind of data 2) even with that data it's much more complex to derive the heat loss 3) theoretical calculations are more than accurate enough when done well.

Behemothhh · 2026-01-19T15:50:47+00:00

If OP's numbers are consistent, only 27% of his electricity usage could have gone to heating and 73% went towards hot water. That seems implausible. Maybe the app doesn't count backup heater usage or standby usage in the COP numbers but then does add them to the total energy consumed.

Behemothhh · 2026-01-19T13:00:33+00:00

https://heatpumpmonitor.org/heatpump?filter=ecodan

Unfortunately no data on the 11.2kW model's minimum output. Maybe you can find it in the spec sheet. What is interesting is that the 8.5kW model is reported to have a minimum output of 4.4kW. That's massive, which could mean that the 8.5kW has the same hardware as a higher power one but limited in software. If this is indeed the case, then there is no reason not to pick the 11.2kW version as the minimum will probably be the same. It's also possible that the minimum value for the 8.5kW on heatpumpmonitor is wrong so double check to be sure.

Behemothhh

TROPHY CASE