ASHP cylinder size?

[gc100]

All the quotes I'm getting for our 3 bed 4 people family ASHP install are quoting about 180 to 200 litre cylinder. I've read in some other thread people recommending at least 300 due to the fact the water temp is lower.

However I challenged one of the installers about this and they said it doesn't really work like that as it will take much longer to heat the water up. They are all respected installers in my county yet they all quote around the same size - are they all wrong??

 

Anyone install a 200 litre with a family of 4??

Thanks as always.

[ProDave]

What size ASHP?

 

It might have been one of my comments.  We only have a 5KW ASHP and with that (just by the laws of physics) 5KW will heat water a lot slower than even a small gas boiler, so it will take longer to heat hot water than many people will be used to.  So I treat DHW as a "stored" medium and don't expect any useful real time re heating while I am using it.

 

I store hot water at 48 degrees, so that will reduce the amount of usable stored hot water for a given volume.  What temperature are your installers suggesting?

[gc100]

Most of them are for 8Kw system. They haven't suggested a temp on the quotes

Edited September 9, 2019 by gc100

[PeterW]

Ok so you can’t go to 65c (standard cylinder temp) with a standard ASHP without secondary heating or a HT ASHP.  Their calculation will be based on a standard high recovery cylinder that is heated by a gas or oil boiler. An ASHP can’t keep up with the usage that a family will give a 200 litre tank, and a 300 would be much better. They are correct that it will take longer to heat but it will also mean you could heat it overnight on E7 and not have to recharge at peak rate. 

[joe90]

I have a 300 Ltr cylinder and a 5 Kw ASHP, like @ProDave water temp 48’, usually 2 people but 2 visitors as well and it copes well, not run out of hot water yet. I am thinking of E7 or E10 next year to run the ASHP.

[SteamyTea]

DHW storage is an odd thing.  The cylinder rarely goes below 30^o C.

If you then heat to 48^o C, that is only an 18^o C lift.

Assuming a mean temperature in the cylinder of less than 48^o C, say 42^o C, then that is an even smaller lift.

200 litres of water will require 2.8 kWh.  Which is about a baths worth.

I found that my 200 lt cylinder, heater to 50^o C, was adequate for 2 people.  When I had guest, I just turned the top element on, adding in an extra 1.5 kWh per person seemed fine.

So my guests cost me 60 pence a day.