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.