ASHP to heat water for a shower or electric shower?

wittynamegoeshere

Thanks for the info, I stand corrected and embarassed!

Getting back on-topic, it needs maths and depends on the actual real-world COP of the heat pump, what proportion of the hot water you'd typically use in a day and how well insulated it is (i.e. how hot the remaining water would be by the following day).  I don't think there's a simple answer, as there are lots of variables.

One option could be to run the hot water from the tank through an electric heater like this brilliant thing...

https://www.hyco.co.uk/products/hot-water/aquila

This is a lot cleverer than the typical water heater or electric shower, it can modulate its heating power and uses however much power it needs to reach your desired temperature.  If the incoming water is that temperature or higher then it will not heat it at all.

I have one of these feeding the annexe shower room, connected to the cold supply in this case.  I can confirm that, despite Hyco's warnings, this works absolutely perfectly with a thermostatic mixer shower.  But it's no use with a gravity-fed cylinder, it needs an unvented one with mains pressure.

This heater is a long way from where the cylinder will be, so I doubt that feeding hot to it would make sense, as this hot water would need to fill around 40m of pipe before it got there, after use it would be left in the pipe to cool down and waste all that heat.

The main bathroom is closer to the cylinder, but I may consider also adding one there, just as a backup option and/or to use if we decide that heating a tank of water every day doesn't make sense for us.  I'd bypass it for the bath though, as it wouldn't have enough flow rate to fill a bath before it goes cold.

Reed_Richards

If you are considering a Heat Pump you need to think about how much power the wiring to your house can provide and how big your mains fuse is.  Typically you might have a 60 A mains fuse so at 240 V you can nominally consume 14.4 kW in total.  Running just a single 9.6 kW instantaneous water heater on full power together with a Heat Pump (mine is nominally 12 kW) could (at least theoretically) exceed the capacity of your wiring.

My Heat Pump installer applied on my behalf to upgrade my mains fuse to 80 A, which is the most they would give me as a free upgrade.  There was some umming and ahhing while the network provider checked their wiring records for the supply to my house but everything turned out to be okay.  So one day an engineer came, opened up the sealed mains fuse container labelled 60 A and discovered I already had an 80 A fuse!  He told me these mains fuses are very slow blow so would not fail unless their capacity was exceeded for some time but even so this issue should not be overlooked.

   

Verdigris

I wonder if DNOs are going to be instructed to speed up dealing with electric supply modifications for heat pumps, solar and batteries? At the moment it seems to be glacially slow and until you get a decision you can't plan.

What's the betting this half-baked government haven't thought about this aspect of greening the economy?

[Deleted User]

Reed_Richards said:

If you are considering a Heat Pump you need to think about how much power the wiring to your house can provide and how big your mains fuse is.  Typically you might have a 60 A mains fuse so at 240 V you can nominally consume 14.4 kW in total.  Running just a single 9.6 kW instantaneous water heater on full power together with a Heat Pump (mine is nominally 12 kW) could (at least theoretically) exceed the capacity of your wiring.

My Heat Pump installer applied on my behalf to upgrade my mains fuse to 80 A, which is the most they would give me as a free upgrade.  There was some umming and ahhing while the network provider checked their wiring records for the supply to my house but everything turned out to be okay.  So one day an engineer came, opened up the sealed mains fuse container labelled 60 A and discovered I already had an 80 A fuse!  He told me these mains fuses are very slow blow so would not fail unless their capacity was exceeded for some time but even so this issue should not be overlooked.

   

Sorry if I'm being thick but If I read you right, it is the wiring to the meter that might need replacing?  Rather than the house wiring from the meter to all the circuits that you have?  Or is it just the fuse that would need replacing in the circumstances you describe.

Verdigris

It is the supply side of the meter meter. However, the wiring is often adequate, it may just need a larger main cut-out fuse fitted.

coffeehound

Presumably the rating of a HP system in kW is not the power that it requires from the grid but an indication of its output based on a nominal COP?

QrizB

coffeehound said:

Presumably the rating of a HP system in kW is not the power that it requires from the grid but an indication of its output based on a nominal COP?

Yes, the heat pump rating is the nominal output power.

Reed_Richards

coffeehound said:

Presumably the rating of a HP system in kW is not the power that it requires from the grid but an indication of its output based on a nominal COP?

My heat pump is specified to perform down to -15 C.  At that temperature I suspect its actual CoP will be close to 1.  Presumably it will then be using something close to its rated output in electrical power.  But most of the time the power requirement will be much less.

Edit.  It has a dedicated circuit with a 40 A fuse 

coffeehound

QrizB said:

coffeehound said:

Presumably the rating of a HP system in kW is not the power that it requires from the grid but an indication of its output based on a nominal COP?

Yes, the heat pump rating is the nominal output power.

Is it a nominal COP used for every similar system, or a more complex formula perhaps based on testing by the manufacturer, akin to a miles-per-gallon figure for cars?

doodling

Hi,

It is important not to confuse the thermal rating of the heat pump (which is what is normally quoted) with its electrical rating.  A typical 12kW heat pump will draw a maximum of around 24A so around 5.8kW.