Heat Pump Pricing versus current costs

NedS

Reed_Richards said:

matt_drummer said:

To heat my dhw to 50c I need a flow temperature of 56c

At 8c outside my COP at 56c was 2.0, my heating at 30c had a COP of 5.20

I find this a little surprising.  I was promised an SCOP of 2.95 for hot water at 50 C (with a flow temperature of 55 C).  Unlike @matt_drummer I have no means of measuring what COP I actually achieve but if I sometimes got a COP of 2 I would need a fair few days of a COP higher than 3 to compensate.  Fortunately, now I am on an EV tariff I can do all my hot water heating at 6.9p per kWh. 

I'm currently running my DHW cycle once per day, at the warmest time of the day (typically around 2:30-3pm), and I'm seeing daily COPs of around 3. Not sure how realistic a SCOP of 2.95 would be.

Admittedly I am reheating from cold as it's summer and the heating is obviously turned off, so in the winter it may be better as the circulating water will already be at 35-40C depending how cold it is, but on the downside the COP would be lower because it's colder outside, but that 55C heated water can then be diverted into the heating at the end of the DHW cycle. My system has only just been installed, so it's early days and I'm still gathering data. I'm not able to measure COP separately for heating / DHW, other than if only one is running like now.

matt_drummer

NedS said:

Reed_Richards said:

matt_drummer said:

To heat my dhw to 50c I need a flow temperature of 56c

At 8c outside my COP at 56c was 2.0, my heating at 30c had a COP of 5.20

I find this a little surprising.  I was promised an SCOP of 2.95 for hot water at 50 C (with a flow temperature of 55 C).  Unlike @matt_drummer I have no means of measuring what COP I actually achieve but if I sometimes got a COP of 2 I would need a fair few days of a COP higher than 3 to compensate.  Fortunately, now I am on an EV tariff I can do all my hot water heating at 6.9p per kWh. 

I'm currently running my DHW cycle once per day, at the warmest time of the day (typically around 2:30-3pm), and I'm seeing daily COPs of around 3. Not sure how realistic a SCOP of 2.95 would be.

Admittedly I am reheating from cold as it's summer and the heating is obviously turned off, so in the winter it may be better as the circulating water will already be at 35-40C depending how cold it is, but on the downside the COP would be lower because it's colder outside, but that 55C heated water can then be diverted into the heating at the end of the DHW cycle. My system has only just been installed, so it's early days and I'm still gathering data. I'm not able to measure COP separately for heating / DHW, other than if only one is running like now.

I get a COP of 3.5 to 4 on dhw heating at 15.00 every day to 47c at the moment

You will find that heating your dhw from cooler tank temperatures gives a better COP for the dhw cycle. Heating the dhw tank when it is already hot means that the whole cycle will be at the higher flow temperatures and lower COP.

Your dhw tank is heated by water flowing through a coil inside the tank. There's not much water in that coil but it does give a very short burst of heat to the heating system as the three port valve switching from dhw to heating.

When you are heating and switch to dhw the fact that your heating water is already hot has no bearing on the dhw if the tank temperature is much colder. As the three port valve switches to dhw any water flowing is pretty much immediately cooled to the temperature of the water stored in the dhw tank. There is no appreciable gain from the water already being up to temperature, it isn't for very long as the dhw sucks all the heat difference away in seconds.

NedS

matt_drummer said:

NedS said:

Reed_Richards said:

matt_drummer said:

To heat my dhw to 50c I need a flow temperature of 56c

At 8c outside my COP at 56c was 2.0, my heating at 30c had a COP of 5.20

I find this a little surprising.  I was promised an SCOP of 2.95 for hot water at 50 C (with a flow temperature of 55 C).  Unlike @matt_drummer I have no means of measuring what COP I actually achieve but if I sometimes got a COP of 2 I would need a fair few days of a COP higher than 3 to compensate.  Fortunately, now I am on an EV tariff I can do all my hot water heating at 6.9p per kWh. 

I'm currently running my DHW cycle once per day, at the warmest time of the day (typically around 2:30-3pm), and I'm seeing daily COPs of around 3. Not sure how realistic a SCOP of 2.95 would be.

Admittedly I am reheating from cold as it's summer and the heating is obviously turned off, so in the winter it may be better as the circulating water will already be at 35-40C depending how cold it is, but on the downside the COP would be lower because it's colder outside, but that 55C heated water can then be diverted into the heating at the end of the DHW cycle. My system has only just been installed, so it's early days and I'm still gathering data. I'm not able to measure COP separately for heating / DHW, other than if only one is running like now.

I get a COP of 3.5 to 4 on dhw heating at 15.00 every day to 47c at the moment

You will find that heating your dhw from cooler tank temperatures gives a better COP for the dhw cycle. Heating the dhw tank when it is already hot means that the whole cycle will be at the higher flow temperatures and lower COP.

Your dhw tank is heated by water flowing through a coil inside the tank. There's not much water in that coil but it does give a very short burst of heat to the heating system as the three port valve switching from dhw to heating.

When you are heating and switch to dhw the fact that your heating water is already hot has no bearing on the dhw if the tank temperature is much colder. As the three port valve switches to dhw any water flowing is pretty much immediately cooled to the temperature of the water stored in the dhw tank. There is no appreciable gain from the water already being up to temperature, it isn't for very long as the dhw sucks all the heat difference away in seconds.

I am heating from cold, as tank is being reheated once daily and temps have dropped significantly from the set point (also 47C).

We also have a 50L volumiser which is fitted in the primary return flow, so must also be heated for a DHW cycle (urgh!), so the first 15mins of the DHW cycle really does little other than heat the volumiser (volume of water in the circuit), and often pumping that cooler water around the circuit has the effect of cooling the DHW tank to the temperature of the cooler circulating water (I think yesterday the tank started at around 32C, and cooled rapidly to 26C before it started warming).

During winter, when the volumiser/circulating water is already at ~35C and closer to the DHW tank starting temp, we should significantly reduce the DHW cycle time, and then benefit from that 50L slug of 55C hot water passing back into the heating circuit from the volumiser once the DHW cycle has finished and the system switches back to heating. Having that additional 50L volume means our circulating volume is far larger in respect to the DHW tank volume (158L actual volume) we are trying to heat than in your setup where your circulating volume is much smaller.

But for now, the COP for DHW reheat only in summertime is around 3. I will see if I can measure how long into the cycle it takes for the RWT to reach the starting temp the DHW cylinder (i.e, how long it takes to heat the volumiser circuit up to the starting cylinder temp)

matt_drummer

NedS said:

matt_drummer said:

NedS said:

Reed_Richards said:

matt_drummer said:

To heat my dhw to 50c I need a flow temperature of 56c

At 8c outside my COP at 56c was 2.0, my heating at 30c had a COP of 5.20

I find this a little surprising.  I was promised an SCOP of 2.95 for hot water at 50 C (with a flow temperature of 55 C).  Unlike @matt_drummer I have no means of measuring what COP I actually achieve but if I sometimes got a COP of 2 I would need a fair few days of a COP higher than 3 to compensate.  Fortunately, now I am on an EV tariff I can do all my hot water heating at 6.9p per kWh. 

I'm currently running my DHW cycle once per day, at the warmest time of the day (typically around 2:30-3pm), and I'm seeing daily COPs of around 3. Not sure how realistic a SCOP of 2.95 would be.

Admittedly I am reheating from cold as it's summer and the heating is obviously turned off, so in the winter it may be better as the circulating water will already be at 35-40C depending how cold it is, but on the downside the COP would be lower because it's colder outside, but that 55C heated water can then be diverted into the heating at the end of the DHW cycle. My system has only just been installed, so it's early days and I'm still gathering data. I'm not able to measure COP separately for heating / DHW, other than if only one is running like now.

I get a COP of 3.5 to 4 on dhw heating at 15.00 every day to 47c at the moment

You will find that heating your dhw from cooler tank temperatures gives a better COP for the dhw cycle. Heating the dhw tank when it is already hot means that the whole cycle will be at the higher flow temperatures and lower COP.

Your dhw tank is heated by water flowing through a coil inside the tank. There's not much water in that coil but it does give a very short burst of heat to the heating system as the three port valve switching from dhw to heating.

When you are heating and switch to dhw the fact that your heating water is already hot has no bearing on the dhw if the tank temperature is much colder. As the three port valve switches to dhw any water flowing is pretty much immediately cooled to the temperature of the water stored in the dhw tank. There is no appreciable gain from the water already being up to temperature, it isn't for very long as the dhw sucks all the heat difference away in seconds.

I am heating from cold, as tank is being reheated once daily and temps have dropped significantly from the set point (also 47C).

We also have a 50L volumiser which is fitted in the primary return flow, so must also be heated for a DHW cycle (urgh!), so the first 15mins of the DHW cycle really does little other than heat the volumiser (volume of water in the circuit), and often pumping that cooler water around the circuit has the effect of cooling the DHW tank to the temperature of the cooler circulating water (I think yesterday the tank started at around 32C, and cooled rapidly to 26C before it started warming).

During winter, when the volumiser/circulating water is already at ~35C and closer to the DHW tank starting temp, we should significantly reduce the DHW cycle time, and then benefit from that 50L slug of 55C hot water passing back into the heating circuit from the volumiser once the DHW cycle has finished and the system switches back to heating. Having that additional 50L volume means our circulating volume is far larger in respect to the DHW tank volume (158L actual volume) we are trying to heat than in your setup where your circulating volume is much smaller.

But for now, the COP for DHW reheat only in summertime is around 3. I will see if I can measure how long into the cycle it takes for the RWT to reach the starting temp the DHW cylinder (i.e, how long it takes to heat the volumiser circuit up to the starting cylinder temp)

I also have a volumiser, a larger tank and enormous radiators, my volume of circulating water is quite large.

None of it holds much energy and it all evens out very quickly.

The volumiser is not receiving or taking heat assuming it is properly insulated. The heat all goes to your radiators or DHW tank. The water entering and leaving the volumiser are practically the same temperature so nothing has been heated up and no heat has been lost.

The same volume of water in the volumiser flows around the system whatever happens and the heat delivered is the difference between the flow and return temperatures at the flow rate. The amount of water you heat up depends entirely upon the flow rate, the total volume of water in the system is irrelevant.

If the flow is 10lpm then that is what the heat pump is heating, it doesn't matter if that 10l is the same 10l or part of 1,000l

DHW is much less efficient the colder it gets and the heating being on previously is of no consequence.

I have MID certified heat and electricity meters and very detailed data so I can see exactly what is going on second by second.

You do get a very brief slug of heat on the switch from DHW back to heating but it is over in a second or two.

Anyway, it is what it is.

wrf12345

Great explanation about defrosting cycles versus temperature, thanks. However, somewhat quaint assumptions that the house would be heated to the same level as when using gas before prices went crazy, whereas I would be aiming for lot lower levels of heat (and wearing thermals and other layers of clothes) and surely the heat pump would need to run for less time if running at a higher temperature, off-setting the need to defrost (and send the house's heat back outside)? Running a low temp heat pump for 24 hours versus running a high temp heat pump for say three one hour slots when electric is cheap.

To get back to the original point, where a high temperature heat pump is fitted to an existing system, surely the £7500 grant should cover the cost now that such (decent brand of) heat pumps are available for well under three grand, rather than trying to justify current prices by adding unnecessary complexity.

matt_drummer

Higher temperature doesn't mean more heat.

A 5kW heat pump can only deliver 5kW of heat and it doesn't matter if it does that at 65c or 30c flow temperatures.

If your house needs 5kW of heat to maintain the temperature you require running 24/7 then you will need 15kW if you only want to run once every three hours.

You'll need a much bigger heat pump.

My house needs 4kW of heat at worst. I can run continuously or do it by running at 8kW every other hour. You may think that would use the same amount of energy but it will use more because I have 12 compressor starts instead of one and I am more likely to need to defrost.

Heat pumps running at higher flow temperatures ice up really quickly, mine took 20 minutes before it needed to defrost.

You won't get a grant by designing a system where you need to wear thermal underwear and multiple layers of clothing.

Minimum room temperatures need to be met.

As you have been told before, the grant is for the home, its present and future owners.

You already can get a heat pump fitted for a close to the grant from Octopus or BG

MP1995

This is wherea tariff like Agile doesn't help scop. So we go when the electricity is cheapest so today that was 6am and less than 5p kwh and getting a scop of 3.25 heating the hot water.

I would suggest if we went for when the day was hottest 3pm we woukd hit 4 this time of year.

Scop is not the be akka and end all when considering cost per kwh and that's where heatoumpmojotor and other sites give a twisted view imo.

FreeBear

matt_drummer said: You already can get a heat pump fitted for a close to the grant from Octopus or BG

BG were quoting silly money for a replacement gas boiler last year (£4K+), whilst the like of Boxt were around £2500.

Octopus quoted me £2700 (subject to survey) for an ASHP earlier in the year - If the £7500 grant plus relaxation of EPC recommended upgrades rules had both been on the table, I would have probably gone for the heat pump.

Having fitted a heat meter to the new gas boiler, I can monitor just how much energy is needed to keep the house warm. When the time comes, I can spec a heat pump based on real data - Hopefully, a detailed assessment would come up with a similar HP size.

matt_drummer

MP1995 said:

This is wherea tariff like Agile doesn't help scop. So we go when the electricity is cheapest so today that was 6am and less than 5p kwh and getting a scop of 3.25 heating the hot water.

I would suggest if we went for when the day was hottest 3pm we woukd hit 4 this time of year.

Scop is not the be akka and end all when considering cost per kwh and that's where heatoumpmojotor and other sites give a twisted view imo.

You are correct, COP is not everything.

You can view the data there in many different ways and compare all sorts of data including cost of electricity.

That site is useful to compare heat pumps and see how they perform.

How one person chooses to run their heating is up to them and many do it with reference to their electricity cost at different times of the day.

An efficient installation is still an efficient installation and that gives options when it comes to operating times and electricity tariffs.

I am sure it is pretty clear though that within all of those monitored systems none of the best performers run at high flow temperatures.

matt_drummer

wrf12345 said:

Great explanation about defrosting cycles versus temperature, thanks. However, somewhat quaint assumptions that the house would be heated to the same level as when using gas before prices went crazy, whereas I would be aiming for lot lower levels of heat (and wearing thermals and other layers of clothes) and surely the heat pump would need to run for less time if running at a higher temperature, off-setting the need to defrost (and send the house's heat back outside)? Running a low temp heat pump for 24 hours versus running a high temp heat pump for say three one hour slots when electric is cheap.

To get back to the original point, where a high temperature heat pump is fitted to an existing system, surely the £7500 grant should cover the cost now that such (decent brand of) heat pumps are available for well under three grand, rather than trying to justify current prices by adding unnecessary complexity.

I still don't think you understand the fundamental difference between a heating system using a heat pump compared to a gas or oil boiler.

Gas and oil boilers are usually, if not always, oversized. My house had a 24kW gas boiler with a 5kW heat loss at -3c

That excess power means you can choose to do what you like within the constraints of the hardware, either cooler water for longer or short bursts of really hot water. You always have enough heat to cover the heat loss of the property.

Heat pump systems aren't and cannot be designed like this.

A heat pump will be chosen where the maximum output pretty much matches the heat loss of the house at the lowest expected outdoor temperatures.

You cannot run them hotter to get more heat, that is not how they work.

The heat delivered by a heat pump is not  limited by the flow temperature chosen. Within hardware limits they can produce their maximum amount of heat at their highest and lowest flow temperatures, it just depends on the flow rate of water.

Of course, to deliver the maximum amount of heat at the lowest possible flow temperature then the heat pump must be connected to emitters that can supply that heat to the property.

But as the heat pump has been designed to be only just big enough at the coldest outside temperatures, you wont be able to run it for only one third of the day and maintain the desired room temperatures.

If you need a 5kW heat pump because that is your heat loss, then to run for only one hour in three you will need a 15kW heat pump.

That will be a disaster on all but the coldest days as for most of the time it will be too big.

It will cost more to run despite only being on one third of the time using cheap electricity slots.

Finally, because the heat output of your chosen heat pump is limited, it is always going to be cheaper to operate if you generate that heat at the lowest possible flow temperature. Running at 65c will not get you more heat, it will just save the expense of changing radiators.