Heat pump quote

NedS

70sbudgie said:

@Reed_Richards I am going to apologise here as well for missing your comment. I think I took too long to do my calculations and refreshed out of synch.

NedS said:

70sbudgie said:

michaels said:

The 11.2 probably makes sense.  If you have a modulating boiler currently and a smart meter you should be able to see your energy usage on a historic cold day check weather underground for a local station if you don't have your own 'weather station'.  From this I was able to see that on a sub-zero day our gas boiler was running full time modulated down to 8kw, assuming an efficiency of about 90% this gave a constant heat demand of about 7.2kw.  I am a bit surprised yours is more than that given the A EPC and a similar floor area to us.  With the Heat Pump you want to go a bit larger because (1) the heat pump also needs to do meet the hot water demand (2) the heat pump will run defrost cycles when the weather conditions lead to icing which reduces the proportion of time it is heating and (3) you may well want a night set back and recovery to full temp in the morning that can take a very long time if your max heat output is only really enough to maintain temp rather than increase.

But I thought you weren't supposed to oversize a HP for best efficiency? 

The lowest temp we've had recently was -8°C. That did feel a bit cooler than comfortable, but not enough for me to go around and change all the heating settings. And I am currently running the gas boiler at ~50°C flow temp. 

10 Jan this year, Octopus Watch says the temp was -4.9°C. Octopus app usage data says the boiler peaked at 6.6kWh/half hour, but mostly ran at what appears to be the minimum of 3kWh/half hour. At 92% efficiency that suggests to me a peak of ~12kW and a minimum of 5.5kW. But at -4.9°C, that is rather cooler than my design temp. 

A couple days earlier, it was -2.2°C with a peak of 9.5kW and the same minimum. 

Are these peaks the numbers that suggest the 11.2kW unit?

It's difficult, and see @Reed_Richards reply above.

Rather than considering individual half hour usage, try looking at the total heat output/usage for the whole day (24h period), adjust it for your 92% efficient boiler, and divide by 24 to give you your kWh heat loss for that day. If you do this for the coldest day of the year, that is a good indication of the size of heat pump you will need.

Using this method, I get 6kW! Surely that can't be right?

And whether that is -4.9°C or -8°C (the former temp coming from Octopus Watch, the latter being what I understood was the reported temp at the time), wouldn't this be within the 36hrs /year of insufficient heating? 

Taking on board the extra capacity for defrosting, DHW etc, if the 6kW is correct, is 2.5kW enough headroom?

Do you know the outside design temperature for your postcode? I.e, at what outside temperature have they designed to? This is often stated in the design documents. For example, it might say designed at a flow temperature of 50C for an outside temperature of -2C. This is the temperature that historical weather data says is the coldest it gets for 99.6% of the time.

If you know the design temp (or reasonably assume it to be somewhere between -2 to -4C, depending where you live), then pick some days when the temperature was in that range, and double check your figures. If you have daily gas usage figures, it should be easy to do. Or just pick the 5 days with the highest gas usage throughout winter (excluding days where you've just come back from holiday to a cold house where the heating was turned off that would use more to reheat the fabric of the house).

6kW does not seem unreasonable, especially if you like to keep your house a little cooler than the MCS design temperature of 21C.

QrizB

70sbudgie said:

Using this method, I get 6kW! Surely that can't be right?

It certainly could be right.

I've got a model of my house in the MCS spreadsheet and it calculates a 7.38kW space heating load, but the most gas I've used on any day in the past winter was ~94kWh, and that ws the day we came back from a week away when the heating was turned right down and we had to re-heat the building structure from ~10C. 94/24 is near-as-dash-it 4kW. And that's consistent with the amount of gas my boiler burns when maintaining a steady temperature.

Here's my Octopus gas smart meter chart for that day:

These are 30-minute slices, so double the shown numbers for kW output.

My boiler worked pretty hard for the first hour, achieving its 18kW rated output for a little while - it was heating water as well as getting the circulating system up to temperature - but once over the initial hump it pootled along at about 3-4kW output for most of the day before picking up again late in the evening when the hot water came on again for bathtime.

If it had been running from midnight, an 8kW heat pump with a 75% duty cycle (allowing time for defrost etc.) with 6kW effective output could have easily met this heat demand, IMHO.

Have you got access to similar data? How does yours look?

michaels

70sbudgie said:

@Reed_Richards I am going to apologise here as well for missing your comment. I think I took too long to do my calculations and refreshed out of synch.

NedS said:

70sbudgie said:

michaels said:

The 11.2 probably makes sense.  If you have a modulating boiler currently and a smart meter you should be able to see your energy usage on a historic cold day check weather underground for a local station if you don't have your own 'weather station'.  From this I was able to see that on a sub-zero day our gas boiler was running full time modulated down to 8kw, assuming an efficiency of about 90% this gave a constant heat demand of about 7.2kw.  I am a bit surprised yours is more than that given the A EPC and a similar floor area to us.  With the Heat Pump you want to go a bit larger because (1) the heat pump also needs to do meet the hot water demand (2) the heat pump will run defrost cycles when the weather conditions lead to icing which reduces the proportion of time it is heating and (3) you may well want a night set back and recovery to full temp in the morning that can take a very long time if your max heat output is only really enough to maintain temp rather than increase.

But I thought you weren't supposed to oversize a HP for best efficiency? 

The lowest temp we've had recently was -8°C. That did feel a bit cooler than comfortable, but not enough for me to go around and change all the heating settings. And I am currently running the gas boiler at ~50°C flow temp. 

10 Jan this year, Octopus Watch says the temp was -4.9°C. Octopus app usage data says the boiler peaked at 6.6kWh/half hour, but mostly ran at what appears to be the minimum of 3kWh/half hour. At 92% efficiency that suggests to me a peak of ~12kW and a minimum of 5.5kW. But at -4.9°C, that is rather cooler than my design temp. 

A couple days earlier, it was -2.2°C with a peak of 9.5kW and the same minimum. 

Are these peaks the numbers that suggest the 11.2kW unit?

It's difficult, and see @Reed_Richards reply above.

Rather than considering individual half hour usage, try looking at the total heat output/usage for the whole day (24h period), adjust it for your 92% efficient boiler, and divide by 24 to give you your kWh heat loss for that day. If you do this for the coldest day of the year, that is a good indication of the size of heat pump you will need.

Using this method, I get 6kW! Surely that can't be right?

And whether that is -4.9°C or -8°C (the former temp coming from Octopus Watch, the latter being what I understood was the reported temp at the time), wouldn't this be within the 36hrs /year of insufficient heating? 

Taking on board the extra capacity for defrosting, DHW etc, if the 6kW is correct, is 2.5kW enough headroom?

I am sorry if it seems like I am going round in circles, there is a lot of information. I appreciate that I might be getting distracted by red herrings. (The installer sales initially mentioned an 8.5kW unit, but the designer hasn't actually put my house heat loss in writing, just the individual rooms. Getting written information out of them is extremely hard work. The quote I've had is for an Ecodan, which seems better than the Daikin that Octopus included for in their quick quote, which is much cheaper, but I will need to pay for another heat loss survey to get their actual design)

Also, other half doesn't really want a HP as he thinks there's plenty more life left in the gas boiler, so I'm trying to work out running costs that aren't a complete fabrication. I am aware that the MCS calculation is pretty rigid and I don't think it reflects our atypical house. Which we also run cooler (18-19°C in living areas, 16°C at night).

I also suspect (but haven't yet managed to verify) that the Ecodan 8.5kW and 11.2kW are different models, not just throttled versions of the same model.

It doesn't surprise me.  Octopus said I need a 10kw heat pump based on the heat loss calculations even though I have the clear evidence that gas input of 7.5kw (and no doubt less than 100% efficient) is enough to maintain my house temperature when it was as cold or slightly colder than the design temp (-2c here).

As you note, it is pretty likely that the 11.2 will be the larger form factor, noisier, higher minimum output range and the 8.5 will be he largest for the smaller quieter range.  But check if you can whether the 8.2 can actually output 8.2 at -5/45 (outside temp/flow temp).

matelodave

The calculations are pretty specific to enable an MCS certificate to be issued. If you dont get a certificate you dont get the £7500 grant towards the heatpump. Likewise if someone like Octopus is subsidising the installation then you get what they have to offer.

As has been said before, the calculations are based on the property rather than how you personally use the property. This is to take into account that you may not be living there until it falls down. The system has to be able to do what its specified to do whether that happens to be the way you use it or not.

The way round this is to specify your own system, pay the going rate and forgo the grant, however you might end up with a problem in the future when selling if the installation doesn't have an MCS certificate.

 You pays your money and takes your choice

70sbudgie

matelodave said:

The calculations are pretty specific to enable an MCS certificate to be issued. If you dont get a certificate you dont get the £7500 grant towards the heatpump. Likewise if someone like Octopus is subsidising the installation then you get what they have to offer.

As has been said before, the calculations are based on the property rather than how you personally use the property. This is to take into account that you may not be living there until it falls down. The system has to be able to do what its specified to do whether that happens to be the way you use it or not.

The way round this is to specify your own system, pay the going rate and forgo the grant, however you might end up with a problem in the future when selling if the installation doesn't have an MCS certificate.

 You pays your money and takes your choice

Indeed. So there are 2 things I'm trying to do - get the best value from my £8k (after grant) quote and understand how I can operate that design so it doesn't cost me more than what it is replacing.

Because £8k is not insignificant to me.

70sbudgie

QrizB said:

70sbudgie said:

Using this method, I get 6kW! Surely that can't be right?

It certainly could be right.

I've got a model of my house in the MCS spreadsheet and it calculates a 7.38kW space heating load, but the most gas I've used on any day in the past winter was ~94kWh, and that ws the day we came back from a week away when the heating was turned right down and we had to re-heat the building structure from ~10C. 94/24 is near-as-dash-it 4kW. And that's consistent with the amount of gas my boiler burns when maintaining a steady temperature.

Here's my Octopus gas smart meter chart for that day:

These are 30-minute slices, so double the shown numbers for kW output.

My boiler worked pretty hard for the first hour, achieving its 18kW rated output for a little while - it was heating water as well as getting the circulating system up to temperature - but once over the initial hump it pootled along at about 3-4kW output for most of the day before picking up again late in the evening when the hot water came on again for bathtime.

If it had been running from midnight, an 8kW heat pump with a 75% duty cycle (allowing time for defrost etc.) with 6kW effective output could have easily met this heat demand, IMHO.

Have you got access to similar data? How does yours look?

Yes, I do have access to similar data. I will try to add an image to this conversation (which will take me a bit of time)

In the meantime, my graph is similar in shape to yours, but peaks at 6.6kWh (at 3:30am), settles to 3kWh in the middle of the day and has a total consumption of 159kWh. According to Octopus Watch (the only source of historical day temps I know) it was -4.9°C.

The design temp from my potential installer is -3°C and the design flow temp is 45°C. Takes a bit longer to dig the right data out of the Ecodan databook. I will post it later.

matelodave

70sbudgie said:

matelodave said:

The calculations are pretty specific to enable an MCS certificate to be issued. If you dont get a certificate you dont get the £7500 grant towards the heatpump. Likewise if someone like Octopus is subsidising the installation then you get what they have to offer.

As has been said before, the calculations are based on the property rather than how you personally use the property. This is to take into account that you may not be living there until it falls down. The system has to be able to do what its specified to do whether that happens to be the way you use it or not.

The way round this is to specify your own system, pay the going rate and forgo the grant, however you might end up with a problem in the future when selling if the installation doesn't have an MCS certificate.

 You pays your money and takes your choice

Indeed. So there are 2 things I'm trying to do - get the best value from my £8k (after grant) quote and understand how I can operate that design so it doesn't cost me more than what it is replacing.

Because £8k is not insignificant to me.

Totally agree - my system set me back £14k nearly fifteen years ago and I also had the slight dilemma of an 8kw or 11kw unit although my heat loss was worked out at around 9.5kwh - I could have plumped for the 8kw unit with an expected use of the 6kw backup heater on around fifteen days a year or the 11kw unit which I can turn down if it gets too hot indoors. We did get around £5k back with RHI payments spread over seven years.

We do get days when it flogs its heart out but most of the time it idles away almost silently. I suspect that a unit that's on the cusp might be thrashing away for a lot longer and may be doing a lot more defrosting.

You may also find that it struggles and needs a bit of supplementary heating if it gets very cold. We've been quite cozy when the temp dropped to around -14 a few years ago. This year we've had several days when it dropped to -5 to -8 and although the unit works a bit harder it still keeps the place at a comfy 20+degrees and we dont need or use supplementary heater.

We have decent insulation in the loft, 30 year old double glazing and some sort of rockwall insulation in the walls (installed when it was built in 1986) so by no means anywhere close to modern standards.

michaels

70sbudgie said:

QrizB said:

70sbudgie said:

Using this method, I get 6kW! Surely that can't be right?

It certainly could be right.

I've got a model of my house in the MCS spreadsheet and it calculates a 7.38kW space heating load, but the most gas I've used on any day in the past winter was ~94kWh, and that ws the day we came back from a week away when the heating was turned right down and we had to re-heat the building structure from ~10C. 94/24 is near-as-dash-it 4kW. And that's consistent with the amount of gas my boiler burns when maintaining a steady temperature.

Here's my Octopus gas smart meter chart for that day:

These are 30-minute slices, so double the shown numbers for kW output.

My boiler worked pretty hard for the first hour, achieving its 18kW rated output for a little while - it was heating water as well as getting the circulating system up to temperature - but once over the initial hump it pootled along at about 3-4kW output for most of the day before picking up again late in the evening when the hot water came on again for bathtime.

If it had been running from midnight, an 8kW heat pump with a 75% duty cycle (allowing time for defrost etc.) with 6kW effective output could have easily met this heat demand, IMHO.

Have you got access to similar data? How does yours look?

Yes, I do have access to similar data. I will try to add an image to this conversation (which will take me a bit of time)

In the meantime, my graph is similar in shape to yours, but peaks at 6.6kWh (at 3:30am), settles to 3kWh in the middle of the day and has a total consumption of 159kWh. According to Octopus Watch (the only source of historical day temps I know) it was -4.9°C.

The design temp from my potential installer is -3°C and the design flow temp is 45°C. Takes a bit longer to dig the right data out of the Ecodan databook. I will post it later.

Are you sure re the 159kwh as that is 24 x 6.625kwh which doesn't fit with your max hourly usage being 6.6kwh and the min being 3kwh.

NedS

michaels said:

70sbudgie said:

QrizB said:

70sbudgie said:

Using this method, I get 6kW! Surely that can't be right?

It certainly could be right.

I've got a model of my house in the MCS spreadsheet and it calculates a 7.38kW space heating load, but the most gas I've used on any day in the past winter was ~94kWh, and that ws the day we came back from a week away when the heating was turned right down and we had to re-heat the building structure from ~10C. 94/24 is near-as-dash-it 4kW. And that's consistent with the amount of gas my boiler burns when maintaining a steady temperature.

Here's my Octopus gas smart meter chart for that day:

These are 30-minute slices, so double the shown numbers for kW output.

My boiler worked pretty hard for the first hour, achieving its 18kW rated output for a little while - it was heating water as well as getting the circulating system up to temperature - but once over the initial hump it pootled along at about 3-4kW output for most of the day before picking up again late in the evening when the hot water came on again for bathtime.

If it had been running from midnight, an 8kW heat pump with a 75% duty cycle (allowing time for defrost etc.) with 6kW effective output could have easily met this heat demand, IMHO.

Have you got access to similar data? How does yours look?

Yes, I do have access to similar data. I will try to add an image to this conversation (which will take me a bit of time)

In the meantime, my graph is similar in shape to yours, but peaks at 6.6kWh (at 3:30am), settles to 3kWh in the middle of the day and has a total consumption of 159kWh. According to Octopus Watch (the only source of historical day temps I know) it was -4.9°C.

The design temp from my potential installer is -3°C and the design flow temp is 45°C. Takes a bit longer to dig the right data out of the Ecodan databook. I will post it later.

Are you sure re the 159 as that is 24 x 6.625

Or allowing for the 92% efficiency of the gas boiler, more like 6.0-6.1kW heat loss. I assume the gas also heats the hot water, so we don't need to worry about correcting for that.

I think that's a really good data point to present to Octopus when considering the size of heat pump. But as @matelodave says, they may choose to ignore it as it doesn't address the point that you may have only had the heating at 17C on that particular day, and they need to be able to demonstrate viable calculations for the heat loss that meet MCS guidelines to gain the MCS certification (and various grants).

I made similar arguments during the planning of my installation, and the installers simply would not budge from the larger sized unit based on their MCS calculations.

barker77

QrizB said:

70sbudgie said:

Using this method, I get 6kW! Surely that can't be right?

It certainly could be right.

I've got a model of my house in the MCS spreadsheet and it calculates a 7.38kW space heating load, but the most gas I've used on any day in the past winter was ~94kWh, and that ws the day we came back from a week away when the heating was turned right down and we had to re-heat the building structure from ~10C. 94/24 is near-as-dash-it 4kW. And that's consistent with the amount of gas my boiler burns when maintaining a steady temperature.

Here's my Octopus gas smart meter chart for that day:

These are 30-minute slices, so double the shown numbers for kW output.

My boiler worked pretty hard for the first hour, achieving its 18kW rated output for a little while - it was heating water as well as getting the circulating system up to temperature - but once over the initial hump it pootled along at about 3-4kW output for most of the day before picking up again late in the evening when the hot water came on again for bathtime.

If it had been running from midnight, an 8kW heat pump with a 75% duty cycle (allowing time for defrost etc.) with 6kW effective output could have easily met this heat demand, IMHO.

Have you got access to similar data? How does yours look?

@QrizB how can I get that type of data from my octopus smart meter please ?