Heat Pump - Is it working or is it just rubbish?

FreeBear

matt_drummer said: We are used to gas and oil boilers that have much bigger heating outputs that are capable of raising indoor temperatures very quickly.

I have a heat loss of about 4kW at -3c but had a 28kW gas boiler.

Obviously, the house would warm up more quickly with 28kW of heat compared to only 4kW from the heat pump.

If your radiators were bigger than they are now then your gas boiler could put in even more heat, it is just restricted by the radiator outputs.

Yup. Many people are used to having roasting hot radiators and assume that luke warm means the system isn't working.

With around 11kW@∆T50°C, the highest output I've seen from the boiler is 16.7kW during the last service. Flow temperature was up to around 90°C Much of the time, I'm running at 45-55°C and use 4-6kW of heat. Boiler is currently range limited to 7.5kW as I don't really need any more.

Really should do a full heat loss survey if only to see if theory matches data I've been logging from the heat meter. I suspect I'm not far off your 4kW@-3°C.

michaels

matt_drummer said:

FreeBear said:

Bad_Pump said:

So I had a new heat pump installed in November. I have two issues:
1. I appreciate they have to work harder in winter but my house always feels cold. Occasionally it's 'not cold' but it's never warm. My radiators never feel hot like they did with my old gas boiler. They just feel warm at best. Is that normal? Is the idea constant low level heating or is the HP just not doing it's job?

I've been running my gas powered central heating with low flow temperatures this winter - At one point, as low as 30-45°C just to see if I could. Took a long time for the house to heat up after cooling down overnight, but it never felt cold. The radiators felt as if they were barely warm, yet room temperatures steadily increased - Have temperature sensors throughout the house and log the data..

Low temperature heating systems are slow & steady (think tortoise & hare), taking time to heat a place up. But to do it effectively requires a system that has been properly specified and configured. One would assume that your installation was specified by an MCS accredited installer, so radiators should be large enough. Quite possible that the system hasn't been tuned properly to maximise efficiency & temperatures. In which case, get them back to adjust the settings.

It is not actually the low flow temperature that makes the home take longer to warm up with a heat pump compared to the gas/oil boilers they often replace.

The emitters (radiators/ufh) if big enough put out plenty of heat even at very low flow temperatures. It's actually the dT between flow and return temperatures and the flow rate that determine the heat output, the actual flow temperature is irrelevant to the heat output as long as the emitters can cope.

The main reason it takes longer to heat up with a heat pump is that their maximum output is usually pretty close to the heat loss of the home at the coldest temperatures.

That means not much extra heat is available when it is coldest to raise the indoor temperature.

We are used to gas and oil boilers that have much bigger heating outputs that are capable of raising indoor temperatures very quickly.

I have a heat loss of about 4kW at -3c but had a 28kW gas boiler.

Obviously, the house would warm up more quickly with 28kW of heat compared to only 4kW from the heat pump.


If your radiators were bigger than they are now then your gas boiler could put in even more heat, it is just restricted by the radiator outputs.

Once you replace your gas boiler with a heat pump your limiting factor will be the maximum heat output of your chosen heat pump. When it is close to design temperatures it will need to run continuously to keep the home at the designed indoor temperature, if you don't, and with no spare heating, you'll never raise the indoor temperature with the heat pump alone.

Am not disagreeing but from your description it is not clear that you can get a bigger gap between flow and return temps with a higher flow temp because with a bigger difference between the air temp and the rad temp the same rad releases more heat energy.  So yes with the same flow rate and flow-return temp difference the rad output is the same, regardless of flow temp but with a higher temp the same rads will generate a bigger flow-return delta and thus emit more energy.

Mr.Generous

I read an independant energy report on a landlord forus that said something like an air source heat pump costs about £800 more per year than gas heating for a typical 3 bed semi, while a groundsource will save approx £10 per year. When you consider the instalation costs ...

matt_drummer

michaels said:

matt_drummer said:

FreeBear said:

Bad_Pump said:

So I had a new heat pump installed in November. I have two issues:
1. I appreciate they have to work harder in winter but my house always feels cold. Occasionally it's 'not cold' but it's never warm. My radiators never feel hot like they did with my old gas boiler. They just feel warm at best. Is that normal? Is the idea constant low level heating or is the HP just not doing it's job?

I've been running my gas powered central heating with low flow temperatures this winter - At one point, as low as 30-45°C just to see if I could. Took a long time for the house to heat up after cooling down overnight, but it never felt cold. The radiators felt as if they were barely warm, yet room temperatures steadily increased - Have temperature sensors throughout the house and log the data..

Low temperature heating systems are slow & steady (think tortoise & hare), taking time to heat a place up. But to do it effectively requires a system that has been properly specified and configured. One would assume that your installation was specified by an MCS accredited installer, so radiators should be large enough. Quite possible that the system hasn't been tuned properly to maximise efficiency & temperatures. In which case, get them back to adjust the settings.

It is not actually the low flow temperature that makes the home take longer to warm up with a heat pump compared to the gas/oil boilers they often replace.

The emitters (radiators/ufh) if big enough put out plenty of heat even at very low flow temperatures. It's actually the dT between flow and return temperatures and the flow rate that determine the heat output, the actual flow temperature is irrelevant to the heat output as long as the emitters can cope.

The main reason it takes longer to heat up with a heat pump is that their maximum output is usually pretty close to the heat loss of the home at the coldest temperatures.

That means not much extra heat is available when it is coldest to raise the indoor temperature.

We are used to gas and oil boilers that have much bigger heating outputs that are capable of raising indoor temperatures very quickly.

I have a heat loss of about 4kW at -3c but had a 28kW gas boiler.

Obviously, the house would warm up more quickly with 28kW of heat compared to only 4kW from the heat pump.


If your radiators were bigger than they are now then your gas boiler could put in even more heat, it is just restricted by the radiator outputs.

Once you replace your gas boiler with a heat pump your limiting factor will be the maximum heat output of your chosen heat pump. When it is close to design temperatures it will need to run continuously to keep the home at the designed indoor temperature, if you don't, and with no spare heating, you'll never raise the indoor temperature with the heat pump alone.

Am not disagreeing but from your description it is not clear that you can get a bigger gap between flow and return temps with a higher flow temp because with a bigger difference between the air temp and the rad temp the same rad releases more heat energy.  So yes with the same flow rate and flow-return temp difference the rad output is the same, regardless of flow temp but with a higher temp the same rads will generate a bigger flow-return delta and thus emit more energy.

They certainly will, up to the power limit of the heat source.

The point was, as I know you understand, with big enough radiators, they barely need to be warm to put out as much heat as much smaller radiators at a higher flow temperature.

The problem with heat pumps is that unless they are massively oversized which causes other issues, they do not generate enough heat at their minimum design temperatures to heat up a house quickly.

It doesn't matter what the flow temperature is, if your heat loss is 7kW and you have an 8kW heat pump it won't be capable of heating a house up quickly. They need to run continuously.. It is not the flow temperature that is the issue in how long a house takes to warm up with a heat pump, it is the limited power output that makes it slow.

And an 8kW heat pump can put out 8kW with a 30c flow temperature as long as the radiators are big enough.

My 8kW heat pump could never run with my radiators actually hot, it just isn't powerful enough.

Reed_Richards

Can you tell us more about the AI setting on the thermostat, please?  My heat pump, an LG Therma V, has an AI setting on the manufacturer's controller/room thermostat (it's the LG terminology for Weather Compensation).  But that only works properly if the underlying parameters are set correctly.

My other question is whether you got the £7,500 grant towards your heat pump installation.  If so it should have been done to MCS standards and the radiators should have been upgraded as necessary.  If not it may be a cowboy installation and your old radiators could be too small to keep your house warm enough.   

QrizB

Mr.Generous said:

I read an independant energy report on a landlord forus that said something like an air source heat pump costs about £800 more per year than gas heating for a typical 3 bed semi, while a groundsource will save approx £10 per year. When you consider the instalation costs ...

I'm not sure anyone here would agree with that, which shows what you get when you don't quote your sources!

The "average" dual fuel property - a group which I suspect includes a fair few 3-bed semis - uses 11500kWh of gas a year. If we assume that gas is all burned for heat in a 90% efficient boiler (none used for cooking) that's 10350kWh of heat.

A moderately well designed and operated ASHP will deliver 3kWh of heat for each kWh of electricity fed into it. Some (like @matt_dummer 's) will do much better. But if we assume 3, those 10350kWh of heat will need 3450kWh of electricity.

At the current Ofgem capped tariff of 6.24p/kWh, 11500kWh of gas will cost £717.60. On the same capped tariff, 3450kWh of electricity at 24.5p/kWh will cost £845.25.

That's an increase of £127.65, which is pretty close to the current annual standing charge for a gas supply of £115.56 - a cost you'll avoid once your gas supply has been removed.

So it's nearly cost-neutral.

matt_drummer

Bang on @QrizB

energynerd99

One thing that is being ignored is that a house heated to 20 degC 24/7 will lose a lot more heat than one heated for 40% or less of the time so the actual electricity used will be considerably higher.

matt_drummer

energynerd99 said:

One thing that is being ignored is that a house heated to 20 degC 24/7 will lose a lot more heat than one heated for 40% or less of the time so the actual electricity used will be considerably higher.

That is true in part.

But heat pumps do not work like other forms of heating so the same logic doesn't apply.

Yes, heating a house to 20c all day and night needs more heat than only heating to 20c for part of the day.

I know somebody with the same heat pump as me and his house is very similar to mine. We both had Octopus heat pump installations and we both had 9kW Daikin heat pumps replaced with 8kW units about a year ago. We both have Open Energy Monitor MID certified metering fitted.

My heating is on continuously and his has been on since 06.30

My house is 22c/23c or so constantly, his is heated to 21c.

So far today I have needed 32kWh of heat at an average 4.3c outside to keep my house at 22c/23c

The other house has needed 24kWh of heat to get back to 21c at an average outside temperature of 4.6c. His house was at 20c at midnight and dropped to a low of 18.5c

So, with the same house heat loss, the same outside temperature and the same heat pump who do you think used more electricity?

Me with it on for 11 hours or him with his same heat pump on for only 4.5 hours?

Well, it is me of course!

But my house is a little warmer so that might explain it, it's actually much closer than you might think.

He has used 5.7kWh of electricity and I have used 6.2kWh, only 0.5kWh more and that is largely due to my house being nearly 23c rather than 21c.


So the electricity use is not considerably higher, it's about the same.


The same house with the same heat pump, it's more efficient to just leave it running, it consumes more heat but no more electricity.

My COP is 5.2 and his is only 4.2, the heat pump has to work harder to recover the house temperature (it took 4 hours to get back to 21c), needs a higher flow temperature to do it, is less efficient and uses as much electricity as just leaving it it on at a lower output for longer.

NedS

matt_drummer said:

<snip>

My COP is 5.2 and his is only 4.2, the heat pump has to work harder to recover the house temperature (it took 4 hours to get back to 21c), needs a higher flow temperature to do it, is less efficient and uses as much electricity as just leaving it it on at a lower output for longer.

Absolutely correct. Where his strategy may become more effective is with TOU tariffs rather than comparing flat rates. For example, we turn the heating off overnight between midnight and 4am (otherwise charged at 26.22p per kWh) and turn it back on at 4am where the rate is 12.86p per kWh for the next 3 hours. Overall we use less electricity as we've saved 4h of running. We do use a more for the first hour as the system comes back up to temp, but it's half price so that only enhances our savings. Overall there is no loss of comfort as we want the house cooler overnight to sleep, and by the time we get up, the house is back up to temp so it's a win-win.