Hi folks. I’d like to pick your brains on a system for our small 2 bed croft house. We currently have an old Rayburn we are switching for a more efficient boiler cook stove. It’s our main cooker so on every day in winter but not so much in summer. We have 4kW PV but soon will have 7kW with a battery and diverter to immersion. The new stove will output 9kW to water. Currently got a bath and will add a shower. We just need 2 rads.

I can’t decide between a thermal store or vented cylinder. We can only fit about 150L cylinder and have very little head for pressure on a vented system. No radiators yet but planning either 2 radiators or low temperature in-wall heating (like underfloor but in the wall!). the benefit of the thermal store is that the PV could contribute to running the heating, which would be great, and be a good dump for summer days (we still need heating in summer). Plus nice to have mains pressure HW. Vented system would be simpler for plumbing, I’d probably use standard rads, and could add an electric radiator as a summer dump if needed. Shower would probably need a pump.

The stove is rated to work with an unvented system too so that is a possibility, just a bit put off by the fact it needs a registered installer and annual service.

Thermal store would be gledhill or advance appliances. Leaning towards AA as it works in a power cut, which we do get quite regularly in winter. Heard a few negatives on thermal stores needing to be maintained at high temp and dropping quickly with bath or heating. I don’t want to be topping it up from the grid if I can help it.

Thanks in advance!

Thanks, that's very helpful. I realise it's a lot of info to cram in! The back story is that I was set on a thermal store until a few recent conversations with friends have put me off!

The vented cylinder I was considering was an indirect combination type, probably 150L with boiler through coil, and immersion via dump. I'm not worried about "wasting" PV as I'll have a second dump for when the tank is hot which wold happen if the stove was running. I've installed these systems before on rayburns and boiler stoves so familiar with them (gravity feed though, not pumped). The downside is that the PV can't contribute to heating in summer, but perhaps a dump to a simple storage heater is a simpler concept.

The concern on thermal stores is that I've heard they don't give hot water unless the tank is up to 70+ deg? Whereas our current vented system can still just about fill the bath if it's at 50deg.

Improving insulation is definitely on the plan too, a massive part of it! We have a solid wall croft house that has been rendered in and out with concrete!! We are removing this and will use an insulated lime plaster inside (no space for more than that), in-roof insulation will be improved too. As you can imagine this is quite a large job so taking it a bit at a time. The current rayburn is in need of an overhaul so instead of that we're planning to switch it for something far more efficient - we coppice all our own firewood so something that can burn wood more efficiently and take longer billets is going to help massively.

The current system is with the rayburn to a very badly sited 112L DHW indirect cylinder (hanging over the edge of the bath downstairs!). Heat leak rad in bathroom but no others. The only space for a new cylinder is the upstairs floor, and max height of about 1500mm. No loft, so the header tank has to be included in that too. Very low hot water pressure as you'd imagine just now. No shower at the moment so definitely want to add one so we don't have to fill a huge cast iron bath every time! I'm not bothered about amazing high pressure showers, but a reasonable flow is obvously needed. The shower will be directly underneath the DHW cylinder so "rain" type might be ok with a small booster pump (used a 12v inline type before that worked ok on a similar system.)

The temptation for an unvented DHW system is that there are a lot of nearly new tanks available at the moment as folk are getting free ASHPs, so I can get a tank for very cheap. The stove has the cold quench system you mention built in so technically can be installed on an unvented system, but not a DIY install so there's cost there. A mate is a ex-plumber though so maybe he'd commission it for me...

That’s really good to hear too, thanks! I was a bit suspicious of taking too much notice as I couldn’t see any other people saying that and I’m sure a lot of folk have TS systems. Does yours do heating too or just HW? I wonder if there is a difference between the gledhill heat exchanger type vs. Internal coil exchanger. I’d preferred the internal coil as it’s not dependant on power.

<blockquote><cite>Posted By: djh</cite>That's nonsense. We have a 250 L Gledhill thermal store exclusively heated by electricity. In winter the store is heated to 60 °C from mains electricity, and only half the store at that. There is plenty of water for our two showers.</blockquote>

I agree. We have a 350 l Gledhill with solar thermal coil. It is also vented but hot water is at mains pressure as the contents of the store is used to heat the mains water on demand via the heat exchanger. (So fresh hot water you can drink)The store also provides the water for our u/f heating so with the solar even in winter some heat even if taking the chill from the water contributes, Store can run up to 79C in summer but only very occasionally. We have a propane gas boiler which we use in winter to heat the store to 55C optimum for the boiler efficiency. There are tappings for solid fuel back boiler but we did not use that due to the complexity of the plumbing and the need for a safety assessment every year. The ufh is run at 40C and takes what it needs from the store which the boiler being self modulating just trickle heats the store. We have 6kw solar plus 10 kw battery but not found it economical to divert solar generation to the immersion particularly as having solar thermal as if heated with electricity it might be too hot for the solar thermal to kick in. However that could change this winter as having just got an EV and signed up to Octopus GO will be able to heat the store at night for 7.5p per kwh as well as the battery. The solar excess we export we get 15p per kwh so is better to export solar and buy back at night at a cheaper rate.

We’re on-grid but can’t get paid for export as it’s a diy PV install. Good to hear that the gledhill stores work ok for folk. I could fit a 150L but not more than that. There would be a heat leak rad to take excess and rads would also happily take more heat (no room thermostats so can’t switch off)

At the moment we have to run the Rayburn 12hrs a day in winter. I’m hoping to reduce this need a bit with the new cooker which will be more efficient. In summer it’s only on to cook so 3 hrs or so. We have an electric hob that runs off the PV battery.

Very low tech really, but trying to reduce our grid usage to zero is the aim.

If you keep your hot water separate from heating, using a small pressurised cylinder as you describe, you might be able to put some storage heaters around to cheaply allow a bit of excess solar PV to add to the heating in spring/autumn.

But if you're then using wet radiators to distribute the solid fuel heating it's going to be asking for a lot of stuff bolted to walls!

In which case, especially as the house sounds pretty compact, you could attempt to keep the solid fuel heat distribution by airflow, ducted and maybe via/incorporating mvhr to keep the space fresh.