A2A + DHW separate from heating - but all eventually on electric rather than oil

[RichardL]

Currently I am trialing A2A heating  making the most of the heat pump COP for the timber frame part of the house.
Cooling in summer via solar PV, heating in winter mostly offset battery off-peak power.

IF that works out - i.e. is useable in terms of comfort and fundable in terms of electric vs. oil, potentially extend to the rest of the house - bedrooms ducted version etc.

 

Hot water is the challenge.

I sort of like the idea of separating hot water generation from heating - I think they may only the same thing by tradition of having hot water in pipes - and if you got back to first principles of energy/electric they aren't necessarily the same system.

So - sunamp/chemical heat battery looks like its on the decline as a concept - but perhaps a smaller air source for hot water directly - i.e. as its own unit and DHW as its only purpose is an option?

I've seen these aroStor (and similar) setups for example - its one and only task to heat a tank of water daily for hot taps and bath etc.

In the summer hot water is via resistive heat PV diversion - but the rest of the year and on cloudy days...

Sanity check
Am I barking up completely the wrong tree to think about hot water separately from heating?
Even if the rest of the house ended up with A2W or A2A/Ducted - is having a separate hot water system a daft idea?

Context
 I'm not particularly into 'pays for itself' ..    more focused on investment in the property and getting off oil
Capital investment is ok within reason provided running costs are equitable with the current electric/oil mix

That doesn't mean waste money - just means capital vs. running costs sort of mindset.

Stream of consciousness/thinking out loud rather than specific questions - I know.

[JohnMo]

Exhaust air heat pumps, i.e. those attached to a tank for heating DHW, seem to do a good job, get best CoP from taking internal air instead of external air.  Some are coated tanks with require an anode changed annually 😥 but can cost the same a duplex stainless cylinders.  A normal UVC and a smallish fixed duty heat pump would possibly give better reheat times. 

 

If your not careful you will have a house covered in external heat pumps.

 

7 hours ago, RichardL said:

summer hot water is via resistive heat PV

 

Any sort of heat pump is better use of PV than resistance heating as the CoP will always be better.

 

 

[RichardL]

2 hours ago, JohnMo said:

 

If your not careful you will have a house covered in external heat pumps.

Yes - this is a major consideration -   the DHW one will be in the utility room with the tank,   

One already outside for the front room/office test

Potentially I can obscure the one for the other end of the house in a roofless outhouse currently only used to store junk - or at least convert its roof to more a pergola effect for free airflow.

[JohnMo]

Or just get a suitable size for hot water and fan coils, you have to run ducts for a ducted system, so just as easy to run pipes.

[ReedRichards]

2 hours ago, JohnMo said:

Any sort of heat pump is better use of PV than resistance heating as the CoP will always be better.

 

Of course that's true, but my heat pump draws up to 6 kW when heating the hot water and that's more than my solar panels can provide.  So my choice is between a 3 kW immersion heater which is completely free to use because I have solar electricity to spare, or using the heat pump solar-assisted for which I still have to import electricity.

[JohnMo]

Very true, with a large HP, but in the context of a small exhaust air HP, the OP was discussing, which should only draw 100s of W, maybe not.

[PhilT]

18 minutes ago, ReedRichards said:

Of course that's true, but my heat pump draws up to 6 kW when heating the hot water

Do you mean output power? Mine never draws more than 2kW and heats to my set temp in around 30 min, using an 11.2kW HP

[ReedRichards]

1 hour ago, PhilT said:

Do you mean output power? Mine never draws more than 2kW and heats to my set temp in around 30 min, using an 11.2kW HP

 

I mean electrical power consumed, input power.  My tank is heated to 50 C and to achieve this the output water from the heat pump is heated to 55 C (12 kW HP).  It can recharge the tank in under 30 minutes but is consuming around 6 kW by the time the output water reaches its maximum temperature.  I guess it tries to heat the hot water as fast as possible without regard to economy of operation.

Edited December 24, 2022 by ReedRichards

[PhilT]

2 minutes ago, ReedRichards said:

 

I mean electrical power consumed, input power.  My tank is heated to 50 C and to achieve this the output water from the heat pump is heated to 55 C (12 kW HP).  It can recharge the tank in under 30 minutes but is consuming around 6 kW by the time the output water reaches its maximum temperature.  I guess it tries to heat the hot water as fast as possible without regard to economy of operation.

Ah yes I see I suppose mine's doing similar but to a much lower tank temp and the average works out at a rate of 2kW

[Iceverge]

12 hours ago, RichardL said:

sort of like the idea of separating hot water generation from heating - I think they may only the same thing by tradition of having hot water in pipes - and if you got back to first principles of energy/electric they aren't necessarily the same system.

 

Correct.

 

Combining the two is fraught with compromises. 

 

A2A for space heating, ESHP for DHW and ventilation and solar PV to assist I think is a good combo. 

 

Low capital investment, excellent COP for both as each is doing a dedicated job. You get the energy recovery aspect from the ESHP so you might achieve passivhaus running costs even without the same level of airtightness. Occupant comfort might not be as good however. 

 

[Nick Laslett]

14 hours ago, RichardL said:

So - sunamp/chemical heat battery looks like its on the decline as a concept

The decline in sentiment on this forum would appear to be due to initial equipment cost outlay, and cost and hassle of repairs. A UVC with ASHP will be cheaper to buy and maintain. The Sunamp only has a size advantage compared to UVC. 
 

 

[Iceverge]

39 minutes ago, Nick Laslett said:

The Sunamp only has a size advantage compared to UVC. 
 

 

I think there's no G3 requirement to service it annually either saving a few quid per year. 

 

Also the heat loss from sunamps are less than the equivalent UVC. 

 

The balance of sums still seems to be in the UVCs favour however like you say. 

[Luke1]

5 hours ago, ReedRichards said:

 

I mean electrical power consumed, input power.  My tank is heated to 50 C and to achieve this the output water from the heat pump is heated to 55 C (12 kW HP).  It can recharge the tank in under 30 minutes but is consuming around 6 kW by the time the output water reaches its maximum temperature.  I guess it tries to heat the hot water as fast as possible without regard to economy of operation.

If I remember correctly you have a NIBE?

 

I have recently found you can adjust the compressor frequency curve compared with outdoor temperature separately for heating, hot water and cooling. Perhaps you could utilise this to slow down the heat pump if you want to use excess PV via the HP?

 

There is also a module and settings I believe that can optimise the HP based on current solar generation. 

[ReedRichards]

I have an LG Therma V, not a NIBE.  Since I am only paid for deemed export from my PV, there is little virtue in being economical with the PV electricity I use in-house.