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Andrew avatar image
Andrew asked

Airsource heat pump with PV?

Am thinking about the best way to use excess PV to heat the house or at least the water. From basic under-sink electric water heaters, electric hot water taps to electric boilers and now wondering air-source or an auxiliary immersion tank to use excess PV as have a gas combi boiler Ideal Espirit Eco 2 at the moment with no tank. Wasting lots of PV 2/3rds at the moment and then in the winter not enough. I have not purchased to save so much as independence from utilities.


1960s UK, detached house - has new windows, fairly well insulated attic, and has old cavity wall insulation. 3 ppl. Avg use 9 - 14kw electric per day

Q1. Are the air source heat pumps worthwhile? I have been quoted an 8kw


Q2 are they expensive in the winter as i live in the UK my PV was getting just 4kw per day vs 30kw per day now? What sort of cost ratio over the year do they use, i get that now it will rely on my PV and batteries.

Q3 Does anyone have any other ideas on how to heat either the tap water, the heating system or preferably both. I accept I am probably going to have to get a better inverter-charger as the below is not enough for my batteries and the Lux is inflexible compared to Victron, it only charges and discharges when you tell it to as opposed to when batteries needed, nothing like the victron software.

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Main system in East-west 8 x 2 x 370w but looking to change to maybe a quattro 8 or 10kwa as I have possibly the worst system which charges/discharges at set times and lose PV to grid and not on a feed in tarrif so lost solar hybrid coupled growatt 5kw/3.6kw lux

Batteries: mixture of pylontech 3000c x 3, 2 x us3000, 3 x us5000

2nd system (off grid) PV 3.4kw pv 10 x 370w south facing on a pagola with an Easysolar II 48v GX 3Kw 270/50 not completed yet so power in winter unknown.




Hot Water Diversion
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7 Answers
mvas avatar image
mvas answered ·

The efficiency of an air source heat pump drops as the outdoor temperature drops. That will take some research, for you to determine if a heat pump is appropriate. Running an 8kw heat pump is easy, starting the heat pump is much more difficult.

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sharpener avatar image
sharpener answered ·

A good start would be to read up on dedicate heat pump discussion sites e.g. here or here.

Q1. Are the air source heat pumps worthwhile? I have been quoted an 8kw

Sounds on the low side, use this cheat sheet to cross-check

Q2 are they expensive in the winter as i live in the UK my PV was getting just 4kw per day vs 30kw per day now? What sort of cost ratio over the year do they use, i get that now it will rely on my PV and batteries.

Probably not if you choose the right electricity tariff. It is probably not economic to install a big enough battery to avoid using the grid altogether.

Q3 Does anyone have any other ideas on how to heat either the tap water, the heating system or preferably both.

If you do not (yet) have a heat pump then an immersion heater solar diverter is relatively cheap and easy to install. The eddi is part of the myenergi family so easy to integrate their zappi EV charger later.

There are also specialised air to water HPs just for domestic hot water which will have a higher efficiency than a plain immersion heater.

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Andrew avatar image Andrew commented ·
@sharpener - q3 is the way I am inclined to go at the moment. But didnt want to make another mistake as with the so called hybrid - which is grid tied in reality.
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zeroco2 avatar image
zeroco2 answered ·

I have 7.2 kWp of panels (3.3 kW ESE, 1.2 kW SSW, 2.7 kW WNW), 21 kWh of Force L2 and I will rely on a 4 kW Kirigamine heat pump and a 7 kW wood stove (Jotul 85% efficient) next winter... but in Liguria (at 500 m of altitude). Wil be trying to limit the use of wood with the heat pump (~4-5 kWh/day available for it in winter), not using gas anymore to heat the 120 m² house (only to heat the domestic water). Heating water with a heat pump would be well above my PV production (~11 kWh/day at the worst of winter).

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Andrew avatar image Andrew commented ·

@ZeroCO2 thanks for the reply, Liguria. Italy? Is the Italian government trying to get rid of wood burners? The UK is, some of it makes sense - eg the types of wood and how wet it is but I use one with decent cured wood and find it excellent in winter, trouble is I can see them banning them.

What temperatures are you getting approx in winter?

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zeroco2 avatar image zeroco2 Andrew commented ·

No, difficult to get rid of all wood burners (all the more with the prices of gas & electricity skyrocketing), but open fireplaces are forbidden in a number of Italian regions, as well as in France. In Liguria, at the worst of winter, it can get between 0°C and 5°C in altitude... but the energy efficiency of many houses (more than 20 yrs) there is really poor. So next winter we try a combination of wood stove & heat pump (automated on Home Assistant), and if needed next year, more pannels and possibly an insulation from the outside. It is not a national directive, rather a personal will to reduce my carbon footprint.

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Andrew avatar image Andrew zeroco2 commented ·
i think the UK will try and the same exists here in certain smoke free zones. There are not many people in east mids of UK judging by the smoke I can see, 1 in 10 maybe. Certain coal is banned but thats fair as its sulphurous. So do you have a back boiler on your stove?
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Jason - UK avatar image
Jason - UK answered ·

@Andrew Ive a ASHP, ESS battery system and a 5.7kW PV array. The 3 for me work seamlessly and save me a fortune each year in electric costs. Im with Octopus Energy on their Flux tariff of which during the winter, I charge the batteries on the off-peak period as well as run the ASHP for the hot water also during the off-peak period. During the summer, I run my hot water during the expected peak period of my PV and the batteries charge only off solar PV. Being on the Flux tariff, I get paid a decent amount for my export which im on track for that money warned to cover my winter energy costs so in short, zero bills albeit the impending drop in energy prices may upset that idea.

Having a Mitsubishi Ecodan ASHP, it allows my to use what it calls Smart Grid where I can run the ASHP different to peak periods, for example, during off-peak, it ignores the standard weather compensation model and runs at a higher temperature dumping cheap heat into the house and then when I am back on the peak tariff, its just ticking over. Same with the hot water. during the winter, I just see the solar PV generation as a bonus as its never enough for my demands without charger from the grid on off-peak. I have a 8.5kW Mitsubishi Ecodan doing a 5 bed detached house. UFH in the kitchen, rads sized for 45/40*C thought out the rest of the house. Gledhill indirect unvented 250ltr cylinder I heat only once every 24h.

Below is a link to my VRM portal. On there are some photos and also a schematic of my system.

https://vrm.victronenergy.com/installation/171934/share/db6807bb

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Andrew avatar image Andrew commented ·
Looks really good. I didnt like the smart meter with british gas who have been less that honest in billing and still are. What age is your house? Did you change all rads.

I was quoted 12k with 5k gov grant, bit meaningless as they only cost 4k for the 8kw I was quoted for 3 ded detached.

Octopus look good but how energy companies have been i worry about locked in contracts. How long is the Octopus contract?

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sharpener avatar image
sharpener answered ·

Hi @Jason - UK, fascinated 8.5kW HP is enough for your 5-bed det house, could you give me some idea of floor area/date/construction?

Am expecting an HP installer to call tomorrow and it would be v useful comparative background prior to his visit.

Ours is a 4 bed det barn conversion, 200m^2 but I am expecting I will need at least the 11kW Ecodan or equivalent. Good Energy have quoted for a 12kW Midea.

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Jason - UK avatar image Jason - UK commented ·

@sharpener the house is a 1966 4-bed with a 1 bedroom annex on the size. I have a MVHR system serving the house to recover the ventilation losses, no trickle vents. Injected cavity wall insulation bring the main house U value right down. The annex is about 20/25 years old with mineral wool slab insulation. The biggest improvement I have made to the house in bring the infiltration rate down to below 3 and I have split the house into 4 heating zones which is why I can get away with a WM85 Ecodan,. I'm aiming for for an infiltration rate below 1 but that's a year or so away. I'm a mechanical engineer so I've calculated and engineered the crap out of the house as I slowly refurbish the house.

The total floor area of the house is about 160m2. If this had been a normal house, I would have gone for a slightly large heat pump like the 11kW Ecodan but by having a 5kva Multiplus only good for 4.4kW due to the power factor of the ASHP, my WM85 Ecodan sits right on the limit of the Multiplus. This means I can run my heating & hot water system off the batteries in the winter. Any larger and I would need a bigger Multiplus or I use grid energy.

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sharpener avatar image sharpener Jason - UK commented ·

Thanks @Jason - UK, very informative. I can see now how you have reduced the losses to such an extent.

I'm guessing from the zoning (and a few other details) that the design doesn't exactly follow the current MCS standards which require 18/21C in all rooms simultaneously, did you do the install yourself?

I have Honeywell Ecohome wireless TRVs everywhere, which conventional MCS orthodoxy says will have to go but I am reluctant to get rid of. Also have MVHR on the first floor which I don't think MCS rules will give any allowance for.

Armed with this detail I will have to tread carefully with the HP installer tomorrow.



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Jason - UK avatar image Jason - UK Jason - UK commented ·

@sharpener The design temperatures are 21*C for habitable rooms, 22*C for wet rooms, 18*C for everything else. Its not a MCS installation as installed myself but the system would meet MCS on the require design temperatures but based on the final infiltration rate Im refurbishing the house to. I used to have my own MCS installation company but had to give it up due to injury. The only thing that does prevents my installation from complying with MCS is that I also use the Ecodan for cooling in the bedrooms via radiant panels in the ceilings (cooling only) and via a LTHW coil on the supply air to the bed rooms as well for cooling.

When completing a detailed heat loss calculation, you can calculate the difference when not needing trickle vents in the habitable rooms but then a different calculation for supply air on MVHR taking into account the MVHR heat exchanger avg efficiency. The improvement is very small but it all helps out in the final heat loss calculation if you make it more accurate.

You could say I have 7 heating zones & 1 cooling zone. Annex is a single radiator heating zone. 1st floor in the main part of the house is another heating zone with rads for heating & cooling via the as well. On the ground floor I have a manifold with thermal actuators controlled via a programmable room thermostat serving each GF room in the main part of the house. This splits the GF into what is 5 independent heating zones each with independent time & temperature control.

I use Heatmiser Neostat's V2 thought-out, with the 1st Floor being a Neostat HC to control heating or cooling as well as the ventilation rate when cooling via the ventilation system.

I also use a Heatmiser Neostat V2 HW for controlling the off-peak boost on the ASHP tied into my energy tariff

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brade avatar image
brade answered ·

Hi,

Do not oversize a heat pump, do not calculate heat loss for the 5% of the year that it's freezing outside.

Size your heat pump so it makes up the other 95% and use another heat source for the 5% (most heat pumps have backup heaters anyway)

Most important is heat loss calculation and emitter sizing.

Or a heat pump will run on solar/battery's is hard to say, but a smaller heat pump is more likely to do so vs a large heat pump.


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Andrew avatar image Andrew commented ·
Thats interesting, undersizing. Any idea of the average cost of ASHP 8kw without the rads? When i priced up the cost of the parts the 7k without gov grant was nearer 4k the rest labour and profit i assume.
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sharpener avatar image
sharpener answered ·

Sound advice. We also have an AGA cooker and a 6kW woodburning stove.

But under the MCS rules in the UK they are not allowed to take these into account. If the system is not designed in accordance with the rules then you do not qualify for the Boiler Upgrade Scheme government grant. This leads to widespread over-specification of the HP.

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