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Last time I looked at this subject it was not possible to have Solar panels connected to a condensing boiler. They had to be linked to a heat store and then the heat store fed to the boiler but the boiler manufacturers would not guarantee it etc.

Anyone know if there has been developments in this area?

Depends what you want to do with the energy.

aFAIK, you can connect solar thermal to a normal hot water cylinder if you just want solar heated domestic hot water. You only need a (much larger) heat store if you also want the solar to contribute to the central heating. A heat store is basically a very large, very well insulated, hot water cylinder.

You'd need a solar coil in your cylinder, to keep the water in the solar circuit separate from the domestic hot water, since it contains antifreeze. The only exception to this I'm aware of is the "Solartwin" system which doesn't use antifreeze, has a solar powered pump and puts the heated water straight into the hot water cylinder.

Not sure why you'd want to feed the water from the heat store into the condensing boiler (unless you mean a combi boiler). Common systems would have all the various heat sources feeding into the heat store (or hw cylinder). If you have a combi boiler, which heats the hot water as it passes through the boiler, then your suggested method makes sense. Not sure what impact it would have for the boiler to be receiving "unexpectedly warm" water though.
HTH

stumuzz wrote:Last time I looked at this subject it was not possible to have Solar panels connected to a condensing boiler. They had to be linked to a heat store and then the heat store fed to the boiler but the boiler manufacturers would not guarantee it etc.

Anyone know if there has been developments in this area?

When I was involved with them, this wouldn't work with most boilers, as they shut down if fed with hot water. No idea if they've seen the light yet.

You need an indirect heating loop powered by the boiler -- although, like the yeti an bigfoot, though rumoured to exist I've never seen one for gas CH, only for solid fuel back boilers, Rayburns etc.

As the local gas installer explained to me: The theory goes that you have a hot water tank with two heating loops. The one at the base heats from the solar thermal; the one in the middle heats from the gas boiler. The problem is that because the loop from the boiler has such a small volume it can overheat pretty quickly if you use a conventional 15-25kW boiler that runs DHW and CH. That means using a small, 5-10kW rated boiler, and it has to have a specially designed mechanism for switching the changeover valve from CH to the indirect heating loop -- and it's that part that isn't common.

Why not just go for a woodburner with a a back boiler? E.g., if we can ever move home and find the ultimate doomstead, this is going to be my house warming present to myself (no pun intended ). Doesn't cost that much more than a new gas CH installation, and Esse's "afterburner" technology is OK for smokeless zones.

Have you looked at a Gledhill Thermal Store cylinder. We've got one although the boiler loop is, or will be, connected to our generator cooling system rather than a gas boiler. Works well for us but make sure your solar panel feed and return both drop to the bottom of the cylinder before connecting at each end to stop thermo-syphoning off your hot water back to the panel at night.

Thanks for the replies.

A bit more info may untangle my muddled thinking.

The house is a semi detached 1910 job. I have put 300mm insulation in the loft, cavity wall insulated, double glazed all windows. Last year a Worcester greenstar AAA gas condensing boiler was installed to replace the old G rated one.

The young mum living there (2 children) has approached me about the green deal.

She wants to install PV panels and get the cash. However, I doubt that would pass the golden rule test.

I thought that installing some solar thermal and heating up an insulated tank of water and feeding the greenstar boiler with pre heated water would save more cash as she uses a lot of hot water.

As most of Britain has gas boilers it seemed such a simple idea. Maybe not.

Running a condensing boiler into a thermal store is a good way of using it as the boiler gets to run for a longer session less often and it's more likely to be in condensing mode most of the time. You get the benefit of the solar preheating as well. When the solar is working the boiler won't run for as much time as it would without. It works the same as preheating the water.

It's best to fit the thermal store, the Gledhill one has very good Rockwool insulation but, like most cylinders, it's not thick enough, and then box around it and fill with broken up polystyrene packaging. If you think about it, your house would have a 300mm of insulation (PassivHaus standard) for a 21C temperature difference whereas your cylinder has 75mm for a 40c temperature difference. Up to 300mm extra wold make a big difference to the heat loss especially if you use scrap insulation that doesn't cost anything.

Thanks Ken. Good idea about the insulation on the tank.