Hot water system design - have I got it wrong?

[marshian]

8 minutes ago, knobblycats said:

 

That makes sense, thanks!

 

You've really helped me to get to the flaws of my thinking and I've adjusted my spreadsheet to account.

 

Looks like the payback time for an ASHP would be around 5 to 8 years depending on how it was used. Makes more sense now!

 

 

If I hadn't done the math  @SteamyTea would have but he'd have used Joules or some other unit of measure 😉 

[Marvin]

Hi @knobblycats

 

Sorry if I missed this earlier...

 

Have you tested to cold water flow rate?

 

I only say this because, as I understand it, there may three showers in use at once. (ignoring everything else in the building that uses water like the loos etc)

 

Say they each run at 12 litres a minute that's 36 litres per minute. will you have a big enough pipe to supply that in the house?

[JohnMo]

28 minutes ago, knobblycats said:

Can you still get that heat pump for £1300?

 

This took about 10 seconds to find

 

https://coolenergyshop.com/collections/air-source-heat-pumps/products/proinverter-air-source-heat-pump-6kw

[Marvin]

Hi @knobblycats

 

Again sorry if I missed it....

 

Another thought is the recovery rate of the hot water tank.

I would have a tank with at least one immersion heater positions.

We heat our water via an ASHP which only goes up to 50C and at that temperature in cold winter gives a quite low COP.

 

If the storage tank is cold how many kWh's would it take to heat - worth calculating before you go to far - 

 

[knobblycats]

2 hours ago, Marvin said:

Say they each run at 12 litres a minute that's 36 litres per minute. will you have a big enough pipe to supply that in the house?

 

Good question @Marvin I tested it a little while back and our current supply is delivering 26 litres per minute at about 5-6 bar. 

[Iceverge]

4 hours ago, knobblycats said:

 

 

That's fantastic! Thanks for sharing - are you happy with your system?

 

As much as I can be, the UVC is too far from the kitchen tap in our house though. I think maybe 13-16m? 

 

I would use 10mm to the kitchen tap again as 6l/min is fine and it really speeds the delivery time. It also stops the Mrs pouring too much hot water directly down the drain. We get 10l/min at the tap if needed just by blending it down a little with cold. I did the same setup in my parent's house. Speed of delivery trumps flow rate in my view. 

 

We too have an A2A unit as the main heating so no water heating available . If we had an A2W ASHP I would certainly use that but the prices we were quoted would never have made sense given our small heat load. PV and divert is the eventual plan but the ~€400/ year (3500kWh)  we spend on hot water on a TOU tarriff won't break us. At a guess you'll be double this usage for your house.

 

The money for the PV is making far more use of itself as AVCs in my pension at the moment anyway.

 

 

 

 

 

 

[Iceverge]

4 hours ago, JohnMo said:

This took about 10 seconds to find

 

https://coolenergyshop.com/collections/air-source-heat-pumps/products/proinverter-air-source-heat-pump-6kw

 

Great value. I'd do this if I was to new build again.