Posted by: @mattc

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Posted by: @judith

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Are you in a colder part of the country?

Not particularly. Cambridgeshire

I lived in Cambridge for a while.  That wind from Siberia really got to me.

@mattc

Sorry, I should have made it a general question to the forum.

@derek-m 

No problem.  Do those prices sounds high to you?  The original set of pre-survey quotes ranged from £2700 (bare system from Octopus) to £7500 (Greener Living, including cylinder but without any radiator upgrades).

Posted by: @mattc

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@derek-m 

No problem.  Do those prices sounds high to you?  The original set of pre-survey quotes ranged from £2700 (bare system from Octopus) to £7500 (Greener Living, including cylinder but without any radiator upgrades).

I assume that the above prices are after the £7500 grant has been subtracted. It would be interesting to see if many of the installers increased their prices after the grant was increased from £5000 to £7500.

Wouldn't be surprised.  It's hard to know what to going rate when you're coming to something like this for the first time.

I'm interested in following this thread as your ahead of me in the ASHP process and I couldn't find the option to get notifications about a thread without posting in it.

Hi again

I think I'm very close to making a choice and it's looking like it will be (drum-roll) ...

Supplier 2 - Puraflow
Total heat loss: 8150 W
ASHP: 11.2kW Mitsubishi
Cost: £7000 after BUS grant

Based on my observations of gas usage over the last few months, the current heating uses around 10kW in the first hour (or less when the weather is milder), then drops down to less than half this for the rest of the day.  This is with the heating on between 7am and midnight, with 50C flow temp and main stat at 20C.  Daily average gas usage over the last 3 months is about 60kWh, max 140kWh. Readings are from 1-hour smart meter data.

To me, this suggests that the theoretical heat loss figure above (the lowest of the three by far - as already reported) is rather high, but I'm assuming that this is because they are using an external design temperature of -3C, which is quite a bit lower than we've seen over the last few months.

So, here are my questions:

1. Does the total cost quoted, which includes a 200ltr cylinder and multiple radiator replacements, sound excessive - bearing in mind its the lowest of the three I've been offered?

2. From earlier comments, it looks like some folks on here feel that the 11.2kW Mitsubishi may be oversized, apart from a few of the coldest days in the year.  How difficult would it be to achieve a decent COP when the weather is milder and it would only be running at, say 5kW for most of the day?  To match the running cost of the current gas system, I'd need to get this up to at least 3.5 on average.

3. Our current pattern of usage is to turn the heating off for 7 hours overnight in the winter and off completely from around May-September.  If we keep up the same pattern, would this have a negative impact on the system?  If so, what would be a better strategy?  We have very cheap electricity for 5 hours at night, so I'd plan to run the system during these hours to heat a tank of water, and only turn this on during the day on rare occasions if we have a house full. 

Thanks in advance for any comments.

Posted by: @mattc

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...

Our current pattern of usage is to turn the heating off for 7 hours overnight in the winter and off completely from around May-September.  If we keep up the same pattern, would this have a negative impact on the system?  If so, what would be a better strategy?  We have very cheap electricity for 5 hours at night, so I'd plan to run the system during these hours to heat a tank of water, and only turn this on during the day on rare occasions if we have a house full.

...

We used to do something similar when we had an oil boiler but that's certainly not playing to the heat pump's strengths. What we do instead these days is to have the thermostats set to, around bedtime, let the room temperature drop to about 15 degC and then have a stepped increase during the day; up to 17 degC at about 6:30 (a little before when we have to get up) since we're likely to be more active during the daytime and then up again to 19 degC ready for the evening when we're expecting to be more sedentary and relaxing. There are other threads discussing the benefits and drawbacks of temperature setbacks so do have a bit more of a search around the forum but this particular strategy is the one that works for us in our house.

As for heating the water tank, it makes a lot of sense to do this when it's cheapest. However, do bear in mind that this will be a relatively minor component during the cold months when the ASHP has to work for its living. In our house, water heating tends to bounce about between 1.5-4 kWh (yes it can vary more but that's the normal range for us) and that's pretty constant throughout the year - unsurprising since we don't choose to stop showering and start stinking just because it's warm outside. However, in the summer that's about all the ASHP will be doing whilst in the winter it can bounce about between 15-45 kWh (once again yes it can vary more but that's the typical) so is potentially ten times what it takes to heat the water tank.

Given we got a solar PV system around the same time as the ASHP, we do things slightly differently. We do heat the water in the evening so that more than one person can have a hot shower but after a cut-off point we specifically avoid heating the water during the night. If it's hot when we go to bed then it'll still be warm enough for a shower in the morning but we'll only let the ASHP do its stuff mid to late morning when the half-hourly prices are out of the expensive times and there's a high likelihood that the water will be heated using home-grown (free) leccy. We'll also force the water heating if we see a plunge price and are therefore being paid to use electricity. From that you can obviously see that a successful strategy is hugely dependent on the home, the usage patterns of the occupants and the energy tariff you're on.

Thanks Major

I see you have an 8.5kW Ecodan.  Do you know what the total heat loss figures are supposed to be for your home? It would be interesting to compare, as I think that's the next size down from the one we've been recommended.

How does the stepping-up/down of the target temperature through the day impact on your average COP?  Presumably it means that the system is running well below its rated output for quite a lot of the time.

From what you say, I assume you're on the Octpus Agile tariff or something similar.  We've been with Ecotricity for years and, up to now, our priority has been a cheap overnight rate for charging the EV, which is by far the largest portion of our usage.  Once we have ASHP heating on through the day, then that dynamic will undoubtedly change.  Adding solar PV would change the picture again, but that's for the future.

Posted by: @mattc

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Thanks Major

I see you have an 8.5kW Ecodan.  Do you know what the total heat loss figures are supposed to be for your home? It would be interesting to compare, as I think that's the next size down from the one we've been recommended.

How does the stepping-up/down of the target temperature through the day impact on your average COP?  Presumably it means that the system is running well below its rated output for quite a lot of the time.

From what you say, I assume you're on the Octpus Agile tariff or something similar.  We've been with Ecotricity for years and, up to now, our priority has been a cheap overnight rate for charging the EV, which is by far the largest portion of our usage.  Once we have ASHP heating on through the day, then that dynamic will undoubtedly change.  Adding solar PV would change the picture again, but that's for the future.

I do indeed have an 8.5kW Ecodan and it has been absolutely rock solid from the start, no doubt largely thanks to being specified and installed properly at the beginning.

The heat loss calculations the installer carried out gave us a total yearly requirement of about 15,000 kWh for heating and about 2,700 for hot water which, once they crunched the numbers, gave a theoretical requirement of a heat pump capable of 6.63kW output. In fairness, they initially specified a 7kW Vaillant Arotherm (one of their R290 ones) but supply problems made that difficult and they had a Mitsi 8.5kW unit in stock. They were working on the general idea of a COP of 3.39. Although I don't pay that much attention to the minor fluctuations that estimate has been proven pretty accurate over time. I would also say the greatest consumption we have ever seen to date was on 16th December 2022 when it hit 62.5kWh - must have been a real cold snap then.

As for the tariff, you're right; we are on Octopus Agile both for import and export. Combining those two tariffs with some automation of the inverter and battery courtesy of Home Assistant (to fill the battery when it's cheap and there isn't enough solar PV being generated, or to take advantage of times of negative prices, but either way in order to ensure the battery has enough juice to avoid us having to import during the expensive 4pm-7pm stretch) means we have lately been seeing average prices per kWh of 16p for import and 7p for export after separating from the standing charge. 

@majordennisbloodnok 

Thanks again.  Was the high of 62.5kWh the total for a single day? 

I assume that's the electricity consumption not the heat output.  Do you have data on the min/max range in terms of heat output per hour/day? 

Posted by: @mattc

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@majordennisbloodnok 

Thanks again.  Was the high of 62.5kWh the total for a single day? 

I assume that's the electricity consumption not the heat output.  Do you have data on the min/max range in terms of heat output per hour/day? 

Yes, that was for a single day as a total and yes that was the electricity consumption. I've pm'd you a copy of my stats which are daily figures but I don't keep track of min/max consumption per hour. That said, once Melcloud is back up and running I could probably take a look for you and see; no guarantees but quite possible.