Quote for 7kw Vaillant - surely not enough?

[Michael_S]

So we had the £250 Heat Geek survey.  They have come back with a proposal for a 7kw Vaillant heat pump (apparently they only do Vaillant under this offer)

 

Their heat loss calc says that at the reference temp (-2.7 here) the heat loss is 7.83kw.  I know this is about accurate and our rads support this at a flow temp of about 45C

 

WE heat our hot water by immersion and use of order 24kwh per day.  There is no proposal to change the tank so lets assume that at the -2.7C day the cop on hot water is only 2. That means another 0.5kw of heat energy needed on average taking it to 8.23kw.  And that is before there is any thought about recovering an overnight setback or other unexpected heat loss (power cut, windows open for painting or whatever).

 

Too me the 7kw sounds under powered, are there any charts for its output at different air and flow temps to suggest that it would be powerful enough? 

 

What would people think would be the correct size heat pump for this heat requirement - I think we need a unit that provides 9 or 10kw at -2.7 air / 45 flow.

 

Thanks

[JohnMo]

First heat pump name plate ratings are meaningless. They all take a different datum point for the rating. Vaillant tend to use a low ambient.

 

Worst thing you can do for efficiency is oversize you heat oump. On the design day and they don't happen often you can always flick on the immersion, but doubt you will need too.

[Dillsue]

Our calculated heat loss is 8.5kw at -2. We put in a 7kw LG Therma V and it's kept us warm all through this winter but it's worked very hard for the few days it's been freezing.

 

I was wary of oversizing for the reasons JohnMo mentions and was wanting to install an 8.5kw Ecodan to exactly match our heat loss. The LG came up on ebay secondhand for a few hundred £ so went for that with a plan to run a fan heater or light the woodburner if the smaller LG unit couldn't cope. No fan heater or woodburner needed as the smaller LG has coped throughout and I'm very pleased with how things have worked out.

 

We'll hopefully have benefitted from efficiency gains in running a smaller unit but the older Therma V doesn't calculate COP or energy produced so I'll never know for sure.

[ProDave]

Most heat pumps only heat the house OR the hot water, never both at the same time.  So if you use 24kWh of hot water in a day and want the HP to heat it, the heat pump will spend just over 3 hours heating the HW leaving only 21 hours available to heat the house.

 

So your 7kW HP will over those 21 hours add 147kWh to the house.  That's an average of 6.125 kW over the whole day.

 

Not sure where you are but designing for -2.7C is not very cold.  Here I designed for -10, a very real winter temperature.

[SimonD]

15 hours ago, Michael_S said:

So we had the £250 Heat Geek survey.  They have come back with a proposal for a 7kw Vaillant heat pump (apparently they only do Vaillant under this offer)

 

Their heat loss calc says that at the reference temp (-2.7 here) the heat loss is 7.83kw.  I know this is about accurate and our rads support this at a flow temp of about 45C

 

WE heat our hot water by immersion and use of order 24kwh per day.  There is no proposal to change the tank so lets assume that at the -2.7C day the cop on hot water is only 2. That means another 0.5kw of heat energy needed on average taking it to 8.23kw.  And that is before there is any thought about recovering an overnight setback or other unexpected heat loss (power cut, windows open for painting or whatever).

 

Too me the 7kw sounds under powered, are there any charts for its output at different air and flow temps to suggest that it would be powerful enough? 

 

What would people think would be the correct size heat pump for this heat requirement - I think we need a unit that provides 9 or 10kw at -2.7 air / 45 flow.

 

Thanks

 

The Aerotherm 7kW catalogue output at -2 is 9kW and the heat loss calc is probably on the conservative side, so in theory, it's just about right. The only question to run by them is the known percentage of under performance by Vaillant units which could put you very close to the edge. You can have a geeky look at the data  here: https://energy-stats.uk/vaillant-arotherm-performance-data/
  

But generally, you can trust Heat Geek designs especially if you're going through them and the performance assured route. If it doesn't perform, they will sort it out.

 

 

[Michael_S]

Thanks for the link to the Czech tables, have looked at them before - at 45c for the heating and -2 the output looks sufficient.  What worries me is the hot water in addition, I know we use lots and with no tank upgrade we are talking a high flow temp to get the tank to a high enough temperature (happy to aim for 45-50c and use an immersion to do legionella cycle) but that still means a flow of 55-65, possibly towards the higher end due to having to limit tank heating time due to the space heating demand.  At -2 / 55 the COP also falls off a cliff, again suggesting that a non-negligible part of the day will be needed to heat the water and thus wont be heating the house.

 

Also these numbers don't include any allowance for defrost cycles....

 

I can see it might almost work if there was a slight overnight setback during which the hot water tank was heated and then perhaps in the middle of the day when it is warmest there could be another hot water window but this is -2.7.  Most winters we have days that average this level or slightly below and we often have -8 coldest nights.

[JohnMo]

2 minutes ago, Michael_S said:

Most winters we have days that average this level or slightly below and we often have -8 coldest nights

Where are you located - your profile dies show it? Your design case (outside temp) is something like to cover 99.7% of the time, so your -8 is very likely not going to be applied.

 

5 minutes ago, Michael_S said:

and with no tank upgrade

Why, do you have a heat pump coil or are they adding an external plate heat exchanger?

[Michael_S]

7 minutes ago, JohnMo said:

Where are you located - your profile dies show it? Your design case (outside temp) is something like to cover 99.7% of the time, so your -8 is very likely not going to be applied.

 

Why, do you have a heat pump coil or are they adding an external plate heat exchanger?

AL4 postcode district - I know the design temp is -2.7 for here but I also know what weather we get.  I also currently have a heat meter so know that the heat loss calc is pretty much bang on the money with a 45 flow temp.

 

The quote is using the existing hot water cylinder.

[JohnMo]

53 minutes ago, Michael_S said:

The quote is using the existing hot water cylinder

So is your existing cylinder a typical gas cylinder? Or a direct cylinder (just immersion)?

[Michael_S]

1 hour ago, JohnMo said:

So is your existing cylinder a typical gas cylinder? Or a direct cylinder (just immersion)?

Indirect 'gas' cylinder Viessmann 180l, about 15 years old

[JohnMo]

Going to be less than efficient doing DHW, via a heat pump, as you will have higher flow temperature to get the same cylinder temperature. It also sounds a very small cylinder for the high usage of water you seem to have. There are only two of us and we have 210L and wouldn't want any smaller.

[SimonD]

Are you going for Zero disrupt? How many people is this cylinder serving and do you have any data yourself on hot water usage? 

[Michael_S]

Yeah, currently we do immersion set to 70c heated overnight at 7p per unit which gives us 5 showers, at say 45C we would need to reheat during the morning shower period but should be doable at say 7kw reheating.  [Say cold water is 10c, 70c x 180l gives us 360l at 40c whereas 180l at 45c only gives 210l so I will then need another 5.25kwh - say 45 mins - to heat the remaining 150l of 40C water needed each morning]

 

Either way we also then need to reheat for evening showers.

Zero disrupt - I have already changed enough rads that I know we stay warm with a flow temp of 45 (it went a bit higher when we were -5 and below outside but not over 47C)  Plan is also to not change the DHW tank and accept the hit to COP.

Edited 2 hours ago by Michael_S

[SimonD]

2 minutes ago, Michael_S said:

Plan is also to not change the DHW tank and accept the hit to COP.

 

Are they proposing to plate load the cylinder? Even then, as an MCS installer myself (with the full Heat Geek training) I'd be seriously suggesting a cylinder upgrade, especially given the existing one is 15 years old and having lived with high storage temps, and given you've got them in on BUS Grant and zero VAT now. 

[JohnMo]

17 minutes ago, Michael_S said:

Plan is also to not change the DHW tank and accept the hit to COP

Not the best decision, but yours to make, as long as the size of cylinder complies with MCS regs - which it may not unless your in a 2 bed property - your worst case is short cycling doing DHW. Plate load as a minimum, ideal skip it and a suitable sized heat pump cylinder.

[Michael_S]

15 minutes ago, JohnMo said:

Not the best decision, but yours to make, as long as the size of cylinder complies with MCS regs - which it may not unless your in a 2 bed property - your worst case is short cycling doing DHW. Plate load as a minimum, ideal skip it and a suitable sized heat pump cylinder.

Yeh - strange on the cylinder, with 5 beds I thought MCS would insist on bigger but the optional cylinder upgrades are only 175l or 210l ??  There is no specific mention of whether they would be using a plate heat exchanger rather than the existing coil.  Our loft hatch won't fit a standard cylinder so keeping the current one has advantages.....

[JohnMo]

21 minutes ago, Michael_S said:

beds I thought MCS would insist on bigger but the optional cylinder upgrades are only 175l or 210l

They do. Min size should be, based on number of beds would be 270L.

 

Section 3.5, would say 175 and 210L are way too small for a 5 bed.

 

https://mcscertified.com/wp-content/uploads/2025/02/MIS-3005-D-2025-V1.0.pdf