ASHP + electric boiler instead of water cylinder?

[Ben Weston]

Hi all,

 

We're currently building a 4 bed house with high insulation levels, MVHR, triple glazing, wet underfloor heating throughout, etc. We're trying to be as environmentally-friendly/sustainable as we possibly can be. We'll be using an ASHP for the space heating but are considering whether an electric boiler might be a better fit for our hot water needs than a convention water cylinder with immersion. We could probably squeeze a water cylinder in somewhere but, to be honest, would rather have the space. The house will have solar and batteries so, theoretically, there's the potential the electric boiler could provide 'free' hot water for showers/sinks/baths. We don't want gas; in fact we had the gas line removed.

 

Has anyone else had this configuration and is there any reason why a water cylinder might be a better choice here?

 

Our water pressure generally is pretty poor so I don't know if a cylinder might give us more scope to improve this over an electric boiler?

Edited May 19, 2023 by Ben Weston

[ProDave]

An "electric boiler" can be 2 types, one is a hot water storage tank with a low power heater, and the other is a high power heater with no storage it just heats the water as it passes through.

 

Either will take up some space.  So why not make space for a proper unvented hot water cylinder.  This can be heated by the ASHP at a COP of at least 2, usually better, halving the energy needed to heat your hot water.  And a solar PV diverter can connect to the tanks immersion heater giving you free hot water when the sun is out using up all the surplus PV not being used by something else.

 

Is the water pressure really poor or is the flow rate poor?  That is a separate problem to address and an accumulator may be the answer to that one.

[Ben Weston]

Thanks ProDave!

 

I was looking specifically at a high power electric boiler with no storage. I.e. like a combi boiler but not gas and not used for space heating.

 

We can fit a water cylinder in but we'd lose an airing cupboard upstairs or an appliance space in the utility room. Neither are end of the world if it's a better solution.

 

Mains water into the house (from meter) is 32mm but it's actually the flow rate that's poor. I probably need to get it measured but it's a problem for all the properties nearby.

 

Given that space is at a bit of a premium for this type of stuff (we've over-concentrated too much on living space in our design - live and learn), I wonder if the space would be better used for an accumulator and continue the electric combi boiler route for hot water?

[ProDave]

To be any use, an instant heater has to be at least 10KW and that will only give you a mediocre shower flow compared to an UVC.  The chance of getting much "free" electricity from that with your solar PV is slim.  You would be doing all your showering and washing at about mid day to get maximum sun.

 

An UVC with a diverter just sits there with whatever spare PV energy going into the tank, and then just sitting there waiting to be used, day or night.

 

It sounds like an accumulator is what you want for the water supply issue, but be prepared for another cupboard space to be used up by another big tank.

[Ben Weston]

We'll have 19kW-ish of batteries too so, hopefully, whatever we went for, we'd have the capacity to utilise the solar at any time (or off-peak rate electricity).

 

I take onboard everything else you say, thanks. I presume an accumulator couldn't sit before an electric instant heater to boost the pressure/flow for showers, then?

 

Also hadn't realised it'd need to be 10kW+ but makes sense. I was looking at this sort of thing: https://www.screwfix.com/p/strom-sbsp15c-single-phase-electric-combi-boiler/655hr

[ProDave]

It will be the inverter capacity that limits how much of your stored energy can be used.  Will your proposed PV and battery system be able to supply 14kW?  If not it will never all be provided by "free" electricity.

[Ben Weston]

That's an excellent point. I think the inverter we're looking at is rated at 4 kW max continuous (peak 10 kW).

[joth]

37 minutes ago, Ben Weston said:

We'll have 19kW-ish of batteries

 I expect you mean you'll have 19kWh of batteries. I.e. energy capacity is 19kWh, enough for 1.9 hours of showers at 10kW. However batteries have a max discharge current that typically limit them to 5kW output power. This is only half what you need for aforementioned electric boiler hence the other half would likely come from grid. 

 

[Ben Weston]

Thanks Joth; yes, I did mean 19 kWh (despite having an EV, I still get my kW and kWh mixed up )

 

You and ProDave are both right on the discharge rates and inverter. I guess I need to do some sums. I imagine, on our general use, a 'boiler' might be on for baths/showers for a total of 45 mins - 1 hr day. That's potentially £4.75/day if it all came from the grid (worst case scenario) on our current tariff! 

[JohnMo]

A slight left of field suggestion

 

Very small cylinder say 50l. Heat to around 60 degrees via heat pump - cop of 2. Or via immersion and solar. Not a  big cylinder with an outside diameter of 450mm dia and 600mm high. A cylinder such as an Alpha Flowsmart 50 has a DHW coil inside and has heating system water inside the cylinder.  Have a thermostatic valve on the outlet of the DHW coil and set to about 30 degrees. This would then provide loads of preheated water to an inline electric heater.  The lower heating requirements of decreased delta across the inline heater (no longer with an 8 deg inlet temp) will give a considerable flow increase or allow a smaller kW input for the same flow.

[Ben Weston]

Very interesting, thanks JohnMo. Will look into the Alpha Flowsmart 50. Seems like it could be a potential solution for our setup.

[JohnMo]

The best place to buy is eBay, they come up quite often, usually for a good price.

 

Here is a manual, just replace boiler with electric inline heater. 

 

 

 

New FlowSmart Inst and serv instructions 2-12.pdf

Edited May 19, 2023 by JohnMo
Wrong file attached

[TonyT]

I would have a 3 kW immersion rather than a 10kW boiler…..

 

solar diverter for a start 

2 immersions giving back up.