"BruceB" wrote in message
...

"Doctor Drivel" wrote in message
...

Old wives tales. A heat bank (using a plate heat exchanger) can operate a
store temperature only 5C above DHW tap temperature. If you set the taps
to 50C, you can have 55C store temp. You need to a have an oversized
plate heat exchanger, say 150kW, to extract as much heat as possible from
the stored water. This does not add much to the cost. Using an internal
DHW coil (thermal store) you need to have higher store temperatures. The
larger plate heat exchanger makes all the difference and transforms
thermal stores into a heat bank which is something special in DHW
delivery.

I assume the heat pump is air sourced? The Mitsubishi is about the best.
Ground sourced heat pumps perform much better and generally will give a
60C plus store temps no problem.


My heat pump is water sourced. And although you are right that my heat
pump can deliver 60 degrees (in fact my NIBE Fighter 1330 can deliver
water at up to 65 degrees) the efficiency drops markedly the higher you
go.
The NIBE figures that I have to hand a
output 35 deg efficiency 481%
output 50 deg efficiency 342%

output 65 deg - does not say, but no doubt falling fast!

We are also in the country and at the moment when the power fails - a few
times a year - then we have no hot water because there is no power for our
current plate heat exchanger.
No combination is perfect in every scenario, but I have made my choice
now.
Regards
Bruce

Best you re-assess. A heat bank can operate on two temperatures. The top
for DHW can be 55-60C, the bottom can be dictated by a weather compensator -
variable temperature.

A flow to the top of the cylinder (DHW) and one to the bottom section (CH).
Priority DHW. When the top calls for DHW the heat pump comes in and heat
the top section. When satisfied, it diverts to the bottom weather
compensated controlled variable temperature section, which most of the time
will be on a lower temperature than the top. More like 40 to 50C. So, the
heat pump on average will be running efficiently.

Then if you are on cheap overnight electricity, you can store as much heat
as possible by having larger thermal storage - bigger cylinder.

Don't make decision flippantly. Look into it properly. Heat pumps and heat
banks go together very well.