Undoubtedly more knowledgeable folk than me will be coming on the forum later but from what you say I think the solar cylinder route is the way to go. I have such a set-up and it works very well, simple to operate and virtually maintenance free. We have a pressurised 180 litre solar cylinder and during the summer months it provides virtually all the hot water we need - only on very cloudy days we may need to use the immersion heater. (We have a wood pellet boiler which runs from October to about May only which provides both DHW and the C/H). Admittedly there are only two of us in the house. When we have family staying we do need to top up with the immersion heater if everyone decides to have a shower the same morning!

One note of caution though - you say that the roof orientation is no good for PV, I wonder what you mean. Our solar thermal panels are south-southeast facing and the difference between a blue sky sunny day and a cloudy day is huge in terms of the DHW temperature and the "recovery time" after showering etc. I don't know how they would perform on a north facing roof.

Posted By: HoveTomI’m not sure why anyone would have a Thermal Store as a Solar Cylinder would seem to do all I need it to

I have a thermal store because it doesn't need an annual inspection paid for. It just sits there and does it's thing.

I have a thermal store just for DHW. It is fed by a boiler and solar PV powered immersion heater. It works well and perhaps I will add solar thermal at some point, but I think a solar cylinder would be more efficient as it works to lower temperatures.

In my dream world there are two ideal solutions:
1- a solar cylinder that can go up to 80 degrees (with a mixer valve to bring it down to safe levels) to store more energy in a smaller cylinder if/when available, or
2- a TS filled with pellets containing a phase change material so that the temperature does not go over 55 deg C whilst storing enormous amounts of energy.
Anybody know these exist?

Hi HoveTom,

(this over lapped with PiH's note, which makes similar points)

probably need more info to make a correct decision, but from what you've said, I think the following logic applies...

- you wish to receive RHI, therefore you can not input Solar Thermal into anything that would output energy to heating.

- you do NOT have enough southerly roof area for PV (what about ground mounted?)

- you have a gas boiler, and no other heat source to make hot water, so you won't get one of the main benefits of a thermal store, which is a zone into which you can input various heat sources, to be utilised over time. I nearly always design/install using thermal stores, but I don't think there's a driver in your case.

- if you are replacing everything, then you do not have an existing DHW cylinder, so blank page

My suggestion would be to keep it quite traditional up to a point, so gas boiler feeding rads directly, and also into DHW cylinder. That cylinder would have a solar coil as well as the coil from the gas boiler, giving you DHW.

You could be a little more adventurous with the solar coil, and have 2 solar coils, one upper, one lower, and use a solar transfer valve, so that when the solar feed is hot it goes to the top coil first, other wise just to the base, but that's just to squeeze a bit more usefulness out of the solar, and by no means essential.

The key energy issue is how the gas boiler interacts with the DHW cylinder, when it adds energy versus when the ST will input energy. I've seen set ups where the boiler keeps the cylinder roasting hot, so ST almost never inputs energy. That's just about setting the system up correctly, and fine tuning it for your family usage. That's the smart GREEN side of this. Try to keep the gas boiler input at a lowish temp, so that there is always "space" for the ST to input. The gas boiler will be able to heat the DHW cylinder very quickly, if you should need more DHW than expected. Say the boiler is a 12/18 kW (about the smallest boiler avail), that's equiv to 1 or 2 showers instantly, so it will keep up with most demands, even id the DHW cyl was not so hot.

One caveat - get a price from a separate plumber who is NOT on the RHI scheme, as I have found quite often that any RHI payment is eaten up by the accredited installer, and so I don't bother with the RHI route.

Posted By: Peter_in_HungaryI have always thought that the difference between a thermal store (TS) and a solar cylinder was primarily one of size.

No, a solar cylinder is shorthand for an unvented cylinder with a large(solar) coil. i.e. a tank full of potable water that is stored until it is supplied to taps. A thermal store is a bucket of water that just sits there. Aheat exchanger extracts heat to the potable water passing through the other side and which is never stored.

As such, a solar cylinder is a pressurised vessel greater than 15 L and so subject to G3 regulations. It also stores hot potable water so needs legionella disinfection cycles and suchlike. A thermal store is (usually) at atmospheric pressure and does not contain potable water, so has neither concern.

In your situation with a mains gas boiler I would be running the CH from gas and using the solar for DHW.

+1

FWIW I have three showers and a bath and my thermal store is 250 L heated by 4kWp PV and mains immersions. Normally the mains immersion doesn't do much except in winter. There are normally just two people and the thermal store only provides DHW, not space heating.

There are a lot more plumbers around who are familiar with pressurised cylinders than thermal stores.

I dont think a solar cylinder has to be unvented. What we call our solar cylinder is 250 ltrs and vented. Its got separate coils for solar thermal, woodburner and LPG boiler.

Ive always taken a termal store as being a tank large enough to store heat for a days CH which is way above typical DHW tank size of a few hundred ltrs.