Soap dodgers - ASHP quote hot water assumption?

MP1995

I would suggest it is more unusual 4 people having two showers a day than anything else mentioned.

It is known people have become more frugal with energy use, hence the price cap so called average home kwh usage reductions so perhaps that extends to showering as well?

Interesting I average 6-7 showers a week sometimes I go one day a week without a shower.

matt_drummer

michaels said:

Can't quote but they have suggested 300l tank. 

Current tank is 180l (180l at 60C = 300l at 40C) and our 20kw boiler can reheat to 40 degrees in 25 mins so probably effectively limitless hot water.  With the immersion we run the tank to 70C to maximise storage

TBH I don't have an issue with the tank size but do with the assumption on which the annual cost is calculated - I guess it is some sort of MCS standard but on the one hand they are saying the house size/bathroom tank means we need lots of hot water available in a short time (at least 4 showers worth) but on the other hand the annual usage suggests that only 1.5 showers are taken per day.  inconsistent much.

In reality I know that we use about constant 8kw (boiler modulation) to keep the house at 20c when it is 0 outside (50c flow), not sure what the design parameters are for a heat pump but I would be most comfortable if could achieve a slightly higher output at say -5 and of course would also need to support hot water production although against this the heating demand is lower at night when we only need about 15c

Does it matter?

The cost they quote for DHW won't affect what you actually use as you already know what you will use.

As long as you get the correct heat pump at a cost you are happy with I can't see the problem?

In cost comparison terms, you could just continue to use the immersion heater for DHW, you would be no worse off.

If you use the heat pump at night when you need little or no heating then you will be better off than you are now as the heat pump will get a COP of more than 1.

Unless I have missed something it seems like you are just picking holes in the paperwork for the sake of it?

michaels

matt_drummer said:

michaels said:

Can't quote but they have suggested 300l tank. 

Current tank is 180l (180l at 60C = 300l at 40C) and our 20kw boiler can reheat to 40 degrees in 25 mins so probably effectively limitless hot water.  With the immersion we run the tank to 70C to maximise storage

TBH I don't have an issue with the tank size but do with the assumption on which the annual cost is calculated - I guess it is some sort of MCS standard but on the one hand they are saying the house size/bathroom tank means we need lots of hot water available in a short time (at least 4 showers worth) but on the other hand the annual usage suggests that only 1.5 showers are taken per day.  inconsistent much.

In reality I know that we use about constant 8kw (boiler modulation) to keep the house at 20c when it is 0 outside (50c flow), not sure what the design parameters are for a heat pump but I would be most comfortable if could achieve a slightly higher output at say -5 and of course would also need to support hot water production although against this the heating demand is lower at night when we only need about 15c

Does it matter?

The cost they quote for DHW won't affect what you actually use as you already know what you will use.

As long as you get the correct heat pump at a cost you are happy with I can't see the problem?

In cost comparison terms, you could just continue to use the immersion heater for DHW, you would be no worse off.

If you use the heat pump at night when you need little or no heating then you will be better off than you are now as the heat pump will get a COP of more than 1.

Unless I have missed something it seems like you are just picking holes in the paperwork for the sake of it?

I know how much I use but others who get quotes may not and being told they use 2000kwh of hot water per year and how much that will cost might then say 'heat pump is rubbish' when it costs considerably more for a more realistic usage.

BarelySentientAI

michaels said:

matt_drummer said:

michaels said:

Can't quote but they have suggested 300l tank. 

Current tank is 180l (180l at 60C = 300l at 40C) and our 20kw boiler can reheat to 40 degrees in 25 mins so probably effectively limitless hot water.  With the immersion we run the tank to 70C to maximise storage

TBH I don't have an issue with the tank size but do with the assumption on which the annual cost is calculated - I guess it is some sort of MCS standard but on the one hand they are saying the house size/bathroom tank means we need lots of hot water available in a short time (at least 4 showers worth) but on the other hand the annual usage suggests that only 1.5 showers are taken per day.  inconsistent much.

In reality I know that we use about constant 8kw (boiler modulation) to keep the house at 20c when it is 0 outside (50c flow), not sure what the design parameters are for a heat pump but I would be most comfortable if could achieve a slightly higher output at say -5 and of course would also need to support hot water production although against this the heating demand is lower at night when we only need about 15c

Does it matter?

The cost they quote for DHW won't affect what you actually use as you already know what you will use.

As long as you get the correct heat pump at a cost you are happy with I can't see the problem?

In cost comparison terms, you could just continue to use the immersion heater for DHW, you would be no worse off.

If you use the heat pump at night when you need little or no heating then you will be better off than you are now as the heat pump will get a COP of more than 1.

Unless I have missed something it seems like you are just picking holes in the paperwork for the sake of it?

I know how much I use but others who get quotes may not and being told they use 2000kwh of hot water per year and how much that will cost might then say 'heat pump is rubbish' when it costs considerably more for a more realistic usage.

640 litres a day in the shower is not "more realistic usage" for almost all people.

DougMLancs

Plus every kWh you use more than Octopus’s estimate (even high use households of DHW are only classed as using 70-80L of DHW per person per day @ 45C)  is only going to make your savings bigger because the heat pump will always be more efficient overall than just an immersion.

Reed_Richards

My system was built to MCS standards.  My annual hot water demand is estimated to be 2987 kWh, not because we use a lot of hot water but because it is a 4 bedroom property.  And this number was taken directly from my EPC so if you don't like the number you have been given then it's probably the EPC assessment you need to argue about -  which is pointless because the whole thing is generated by feeding numbers into a piece of software.

As @matt_drummer says, the quoted number makes very little difference.  But if you want to heat enough hot water for a whole day with cheap electricity then you will need a much bigger cylinder than you have at present.

And, as I said earlier, if your system spends more time than estimated heating the hot water then it will spend less time heating the house and will have to make the water hotter to achieve the same amount of heat output.

   

smallblueplanet

Isn't a bigger hot water cylinder a better more economical thing, according to some Heat Geek video I watched a while back?

Netexporter

smallblueplanet said:

Isn't a bigger hot water cylinder a better more economical thing, according to some Heat Geek video I watched a while back?

It could be if it allows you to store your DHW needs at a lower temperature and therefore achieve a better COP for water heating. But if you still insist on heating DHW to 65 or 70 degrees at will cost you a lot more.

smallblueplanet

Netexporter said:

smallblueplanet said:

Isn't a bigger hot water cylinder a better more economical thing, according to some Heat Geek video I watched a while back?

It could be if it allows you to store your DHW needs at a lower temperature and therefore achieve a better COP for water heating. But if you still insist on heating DHW to 65 or 70 degrees at will cost you a lot more.

Is there a way to 'vaguely' work out how big is best? I know there's the MSC figures but we've a Octopus survey coming up soon and I'd like to be clued in when they talk about DHW tanks and sizes.

Reed_Richards

The way to vaguely work it out is to estimate how much hot water you use per day and what temperature you use it at (from any mixer tap where you mix it with cold).  Suppose you use 400 litres per day mixed to 45 C.  The most economical way you can achieve that with a heat pump is with a 400 litre cylinder storing water at 45 C.  You might find that that is not possible and you will have to use a smaller cylinder and hotter water but that will cost you more in running costs.