Using PV energy to run a heat pump?

ABrass

Roseybug said:

Hi, I'm unfamiliar with all the technology but exploring options to update my house and replace a last-century combi gas boiler (not condensing) with ASHP + HW tank. 

Will also need a re-roof (+ insulation) as currently between my bed and the stars there is a layer of plasterboard, 10cm-ish of air, then some slate. (Rooms in roof, uninsulated). External wall insulation also planned, subject to affordability.

I have a south-facing roof, not huge because it has dormers in it, but think I could fit about 2 kW of PV onto it, which according to PVGIS modeller would give me c. 6-7 kWh per day for April-Sept, less in the winter.

In order to use summer PV power cost-effectively - most of the stuff I've read seems to suggest using what you can in the day and shunting any excess to a hot water tank via an immersion heater, with any surplus sold back to the grid (or stored in a battery). I currently use about 4 kWh/day of gas for hot water heating. Current daily electricity use is c. 4 kWh in summer. Doesn't look like I will generate sufficient excess for a battery to be economic.

My question - if my normal hot water heating source is the ASHP, wouldn't it be more efficient to get the ASHP to use the PV electricity to heat the water at a COP of 3ish, rather than an immersion? Or are there voltage/technical reasons that make this impossible?

Thanks for all the discussion threads so far; I've read many of them

Why are you doing this? It is unlikely to save you money if I'm reading it right.

DougMLancs

70sbudgie said:

Have you considered PV roof tiles? They are more expensive, but might not be if you need to replace your roof anyway. I don't know what the grants are, but it might bring you roof replacemei VAT down?

Only thing is you said you have slate tiles, so you might need to check planning. I think a PV roof is considered more permanent than panels. 

Wise recommendation. GBSol will offer you a free quote for solar slate tiles which will be much cheaper than PV + new tiles. And much better looking! 

My quote for 4kW GB Sol solar tiles came out well over double the price of an equivalent in-roof panel system on GSE mounting. In-roof panels look great still; especially on something like a slate roof and if you’re reroofing anyway then install is easier and you still save on tiling costs. 

ABrass

Right now you have a dated boiler, a virtually uninsulated roof and no hot water tank(?).

You're thinking of getting an ASHP, tank, insulation and small solar array.

The cost of an ASHP is far higher than the BUS, you'll have to pay at least a few £, I don't have good numbers but I've seen £7,000 to £17,000 quoted.

If you are thinking of installing a hot water cylinder those cost money too, that and the diverter costs are unlikely to break even. If you want a cylinder anyway then that's another matter.

The solar array will do virtually nothing to offset the energy use. With a Dormer you're probably going to experience shading and with 2kWp you're going to be generating a very small fraction of your shoulder months power needs and negligible over winter. You'll probably benefit nearly as much without the ASHP if you work from home.

ASHPs are fractionally more expensive to run than gas boilers under historic pricing. It will not save you any money.

The flue for your boiler doesn't have to go through the roof so that doesn't need to be a factor in your roof planning if you were to go gas again.

AFAIK Solar tiles are extortionate and give lower outputs than normal panels for the same areas.

Insulating the roof will be worth every penny you spend, but to get the thickness you should be aiming for (~120-200mm celotex) you'll need to drop your ceiling heights. Technically if you're reroofing and insulating you should be contacting building control. 

Pile_o_stone

Reed_Richards said:

Most heat pumps have a maximum output water temperature of 55 C.  Some go higher for a price premium but why would you want to?  The only reason is for a one hour a week Legionella cycle of your hot water cylinder.  Instead use the immersion heater to raise the temperature from 50 C to 60 C.

Alternatively, you could fit a hot water tank that has a heat exchange for hot water, so you get the benefits of never having to worry about Legionella, you get drinkable mains pressure hot water and you don't have to use additional energy just to heat the tank each week to above 60C. If you also have a heat exchange with a mixer valve (as we do), then you can have a tank of water at 80C powered by summer PV without any risks of scalding at the taps.

Reed_Richards

Screwdriva said:

I purchased a Viessmann V-200W gas boiler (not a Vaillant), as it is the most efficient boiler sold. I did so for two reasons: 1) the installation cost for the boiler was £6K lower than the cost for a heat pump install and projected heat pump savings weren't enough to recover this cost over the following decade. 2) The supply of Arotherm pumps were extremely limited - I was quoted a 3 month wait. 

Apologies for confusing Viessmann and Vaillant, both begin with V and are not manufactured in the UK (both German?).  And perfectly reasonable to install a gas boiler instead of a heat pump if you have that option.  In these turbulent times who knows how long the cost recovery time will be.  

70sbudgie

Pile_o_stone said:

Reed_Richards said:

Most heat pumps have a maximum output water temperature of 55 C.  Some go higher for a price premium but why would you want to?  The only reason is for a one hour a week Legionella cycle of your hot water cylinder.  Instead use the immersion heater to raise the temperature from 50 C to 60 C.

Alternatively, you could fit a hot water tank that has a heat exchange for hot water, so you get the benefits of never having to worry about Legionella, you get drinkable mains pressure hot water and you don't have to use additional energy just to heat the tank each week to above 60C. If you also have a heat exchange with a mixer valve (as we do), then you can have a tank of water at 80C powered by summer PV without any risks of scalding at the taps.

How does a hot water tank with heat exchange differ from a standard hot water tank? I have a tank with dual coils (though only currently using one) and they are described as heat exchanger coils. 

I keep wondering if it could all be plumbed in a different way to improve efficiency. That said, one of the reasons for replacing our combi boiler with a tank is that we are less dependent on the water mains pressure (which is rubbish. We are at the back of the estate and everything is at the bottom of the allowable range!)

Reed_Richards

There are two types of hot water tank

1. You use the hot water in the tank.
2. You don't use the hot water in the tank but use it to heat cold water to give you hot water out of the taps and possibly also to reheat your central heating water.

Type 2 is called a Thermal Store and if that is what you mean it is clearer (to me) if you use that terminology.

Pile_o_stone

70sbudgie said:

How does a hot water tank with heat exchange differ from a standard hot water tank? I have a tank with dual coils (though only currently using one) and they are described as heat exchanger coils. 

I keep wondering if it could all be plumbed in a different way to improve efficiency. That said, one of the reasons for replacing our combi boiler with a tank is that we are less dependent on the water mains pressure (which is rubbish. We are at the back of the estate and everything is at the bottom of the allowable range!)

As Reed_Richards said, there are two types of cylinders, those where the hot water comes directly from inside the cylinder and those where the heat in the cylinder is exchanged with mains water. With these systems, there is either a coil inside the cylinder that mains water is pumped thru to heat up, or those with a plate heat exchanger on the side of the cylinder and heats up mains water. With both methods, the mains water never mixes with the hot water in the cylinder, so any bacteria in the cylinder water cannot get into the hot water you use in the shower.

As to which heat exchange methods is most efficient, that's up for debate. We have a plate heat exchanger which are supposed to be more efficient than a coil, but they pump water from the top of the tank thru the heat exchange and return the water to the middle of the tank, which can ruin stratification (I'm not sure if it actually does). What I do like is that when we turn on the gas boiler for hot water, the output from the gas boiler into the cylinder is very close to the input of the heat exchange plate and so we get hot water really quickly without having to  heat up all the water in the cylinder.

Roseybug

Thanks for all of the responses. Lots of food for thought and ideas for me to work through. 

The key question raised by ABrass: why am I doing this?

Apologies, I could have been clearer - despite posting on MSE, money-saving is not the motivation, it's much more complex. Maybe the money-saving thing to do would be to sell this house and buy a newer energy-efficient one instead - but I want to stay living here - the fruit trees I planted are doing great! After nearly a decade of 1-year part-time contracts and childcare costs, my head is finally above water - one child has gone to uni, and I've worked my way towards a much better paid job with practically full-time hours on a permanent contract in a recession-proof field.

During the last decade, I've put off investing in the house because I was too busy working/earning/parenting/studying, and didn't know whether an appropriate job would come up locally - there was always a possibility I would have to move away. So now I have a decade's backlog of maintenance, plus the ability to remortgage to fund improvements and future-proof my existence here. This is going to include a modest single storey extension so that I can have a downstairs toilet and shower room. I'd like the flexibility to have others living here if needed, e.g. post-uni return of child, or care of elderly relative.

So - what NEEDS doing?
1. Gas combi (non-condensing) boiler aged ?25y needs replacing with something - on its 9th life already and probably very inefficient
2. Windows need replacing - single glazed, wooden frames that are rotten in several places.
3. One dormer roof leaked, got lead patched 8 years ago, the plasterboard inside is still cracked though
4. Roof needs insulating; it haemorrhages heat in winter, and is unbearably hot in summer.
5. Various bits of lath-and-plaster crumbling off 

The rest is aspirational - I would like to end up with a comfortable house that is as eco-friendly as feasible. I'm not too fussed about payback periods - I wouldn't ask myself what the payback period is to replace a worn-out sofa, I would just ask whether I could afford to pay for it, and whether it met our needs. When my current (petrol) car becomes unreliable or uneconomic, the next one will be electric, so I'm considering solar panels in advance as there will be scaffolding and a roofer needed anyway. The heat pump seems like the right thing to do for environmental reasons - and I'm not great at organising these things so would rather have all the upheaval in one go than replace a gas boiler again in 10-15 years.

Roseybug

In response to specific Qs:

@70sbudgie - yes have looked at PV roof tile brochures (GB sol) but not got quotes yet - they would look good with the slates but I'm equally open to roof-integrated panels. Not keen on sticky-out-mounted PV panels, would be afraid they'd take off in a stormy gust. I'm likely to need planning anyway as the roof insulation done from the outside (between and over rafters) is going to need an increase in roof height.

Unfortunately insulating a roof during re-roofing is not VAT-free like other forms of insulation, because you are deemed to be doing a home improvement rather than an insulation only task. This applies even when the architecture of the house makes insulating from the inside impossible/suboptimal. 

@Reed_Richard - that's a useful warning, I had assumed that all heat pump installers would be MCS certified, but maybe not. Will look at feasibility for underfloor heating to keep the flow temps as low as possible, but for now it would only be in the rooms that need upheaval anyway due to the new shower room. To be honest, I currently run the house very much on the cold side and have got good at "heating the human" over the last year. Our gas use was 27000 kWh/a during 2020-1 (heated to comfortable temp by teen child who was "studying" at home during pandemic while I was at work), down to 15,000 in 2021-2 (but I overdid the draught-proofing, underheated and ended up with a few condensation-related mouldy patches). I think for now we'd be comfortable enough with a low flow temp and a coolish house, then as I age I can consider room-by-room renovation and adding more underfloor heating.