Have Solar PV and a Combi Boiler..... what else could I do? heat-pump? ASHP? Storage heater?

I have 5.1kWh PV setup across various aspects/roofs generating about 4300kW per year. (only been installed 10 months)

DHW and heating in our 3 bed semi comes from a 3 year old Vailant Ecotec Combi boiler across 8 radiators on 3 levels (ground, first and loft conversion). The boiler has a 10 year warranty… so I don’t expect any problems any time soon.

Currently have no EV or battery, so i’ve got tons of PV going spare… I currently export about 75% of what I generate.

I’m wondering what long term and financially viable options I have around heating and DHW?

First off I thought about a ASHP in the dining room to provide a little heat and air con… wife ruled out having an ugly office-type aircon thing on the wall!! :frowning:

So maybe go down the storage heater / heat-pump route?
But the problem here is I have no internal space in the house to fit a large header tank.

Only place I could remotely think is at the side of the extension, outside. So build a brick surround for it. But will it be insulated enough to reduce heat loss and as its totally the opposite side to where the combi is, will running 10m of piping cause issues?

As I write this I just get the feeling i’m looking for a project for a projects sake?
I’m really not sure it will be cost effective to replace a 3 year old combi with something else just to make use of some spare PV.

Unless you can persuade me otherwise? What’s header tank / heat pump going to cost me? Couple of grand?

The other thing to consider is that the PV isn’t really free is it? Because if i’m not exporting it, that’s 5.5p per unit per kWh that i’m not getting back in export payments (i’m moving to Octopus Outgoing soon so all exports will be worth money to me, not just deemed 50%).

Batteries are too expensive for the payback too at the moment.

Maybe I just sit tight and wait for EVs to come down in price and put my money and spare PV in to that?

Any thoughts would be appreciated. Thanks.

Sounds like you’ve got your kW and kWh swapped. :wink: :grin:

1 Like

Even after nearly a year of Solar geekery I can never get this right!!! :laughing:

No worries. Many people get those two confused. :grinning:

1 Like

It does sound a bit like it, TBH. The most economical way to store PV energy in a domestic setting is as hot water, but that requires having a tank to store it in.
You’d need to add a lot of insulation to a new tank, or a new building conatining the tank (think 200 mm or more) plus you’d need a lot of insulation around 10 m of pipe, plus suffer delays waiting for hot water to run through.

So I suspect the best thing is to collect your export payments and feel good that you’re helping to increase the proportion of renewable energy on the grid.

1 Like

I would agree with Dave. A hot water cylinder and a solar diverter to power up the immersion element is probably the best use of excess energy for the lowest outlay, providing you had a DHW tank in the first place. As you have a pretty new combi, it would be a waste to chuck it out and like you say, there’s little space for a DHW tank without building an outhouse for it.

If you were looking to rip out the combi in the future then yes, plan to use a well insulated DHW tank & PV diversion. The solar PV will help offset a heat pump too to an extent, but do not expect to be able to command a heat pump to rapidly switch on/off according to solar availability. Heat pumps have 1 goal placed higher than looking after space heating or DHW, and that is looking after itself, ensuring min run times, starts per hour etc. You can’t flick the compressor on and off at will, like a light.

Another factor for heat pumps in general is that the flow temperatures will often be lower than most gas combi boilers, and lower flow temperatures = better COP figures! Are your existing radiators sized correctly for lower flow temperatures? Are the pipe sizes sufficient to maintain the flow rates required? Most ASHPs need a buffer tank too, to reduce number of starts per hour and to ensure energy is available for the defrost cycle. So that is potentially two tanks you need to house.

One thing I will say regarding storage heaters. They do not make the best bed-fellows with solar PV. In summer, there’s an abundance of PV energy, and your storage heaters will likely be switched off. In winter, there is likely to be insufficient PV energy to charge all your storage heaters. Not only that, modern storage heaters use electronic controls and cannot be safely fed from an Immersun like device which chops the AC waveform.

Save the pennies, add more insulation to the home where you can to reduce burning gas. Put the savings towards a nice electric car :grin: That is exactly what I am doing right now.


Thank you both… confirmed my thoughts… pennies towards an electric car it is then. :grinning:

just pointing out there almost always room to install DHW buffer tank within the home-- outside the home not the best option unless you live in a warm climate where heating not really a consideration… if I base on modern house design

several locations for consideration:
between your joists: you can get long narrow horizontal hot water tanks that you can fit within your joist structure you loose 6in inches of ceiling height where they are placed if it a floating ceiling general no difference then its normal insulation . but I would think you can also place somewhere the horizontal DHW above or below where you current boiler location as you can mount them on the wall, floor, ceiling, crawlspace or in the attic ( little bit of caution require there if installed in the attic in cold climates)

the crawlspace: - you can place your storage tank in there. if it an insulated crawl space- you can use both water storage and earthen storage. water heats easier and faster whereas earthern heats much slower but also releases the heat slower so you can bank months of summer over production to some benefit in the cooler months to the home… ie once your DWH reaches 60-70C - diverts to the earthen storage with in the insulated crawlspace. a large earthen mass heated takes forever to cool to ambient which provides you a benefit - ( you loose 15 - 20% of your heat to the floor the longer you you can keep the crawl space ambient temp closer to internal temp the more energy you save)

“pretty new combi, it would be a waste to chuck it”
just hook the tank in as preheat buffer tank . most tankless stuff can handle this type of configuration. and since it a combi for heating and hotwater I think is a distinct possiblity it will supports that…

1 Like