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How do I know if my solar thermal panels are working correctly?

Posted by Cathy Debenham on 3 September 2013 at 9:55 am

Q: We have had solar PV and solar thermal installed in 2010. While I'm very happy with the solar PV, the solar thermal has never worked particularly well. But as the boiler makes sure there is hot water mornings and evenings when we need it, we haven't really looked into it until now.

After observing the readings during July when the heat wave started I have noticed that the temperature at the solar panel never goes above 45C, not even during a whole day of uninterrupted sunshine. So surprisingly, the water never gets hotter than 42C without using the boiler.

I have been given projections for the performance of the solar PV, but not the solar thermal. So I don't know if the system is not working correctly, or if my expectations are unrealistic. I was expecting to generate enough to get by without needing to use the boiler during the summer months.

I have emailed my readings to my installers who said they would look into it, but I have heard nothing back from them. Can you advise, please?

A:  I think you should get straight back onto your installer, as your system is definitely not working as it should. Your expectations are realistic, but sadly do not seem to be being met.

We have a solar thermal system at home, and generally turn the boiler off sometime in April, and don't turn it on again until near the end of September. During that period we will generally have to give it the odd boost when it's been cloudy, but very rarely have to put it back onto the regular timer.

I checked our readings at the weekend: on Saturday at 2.15pm the collector (solar panel) was at 75C, the temperature at the bottom of the cylinder was 66C and the top was 67C.

On Sunday, I checked at 6pm, just after I'd had a bath. The collector had cooled to 49C, and the bottom of the cylinder was at 45C, but the top was still at 69.6C.

You say that you weren't given projections for the performance of the solar thermal. If you look at your MCS certificate, there should be an expected output in kWh on that, which you can check against your total generation figure on the meter.

To get the best out of your system (once you have got it working properly), it's best to adjust the timer on your boiler so that it's not heating the water in the morning, as that restricts the amount of heating available to be done by the solar panels. What suits you best will depend on the demand for hot water in your house, and how much you want it automated.

There are just two of us, and we're happy to keep an eye on the weather, and manually boost the boiler for an hour if a reading shows that there's not enough hot water. We've got quite good at it, and there's only been one morning this summer that I've got into the shower and found no hot water, but I know that approach doesn't work for everyone.

From October through to April, we have the boiler timed to just go on once, in the evening. This allows plenty of time for the sun to do its bit during the day, but also ensures that there is hot water for morning showers.

Having the boiler water heating come on automatically in the morning is fine if you're then going to use most of it in the morning, leaving the cylinder needing more heat from the sun during the day.

More information about solar water heating on YouGen

YouGen guide to solar thermal

Estimated RHI payments for solar thermal

7 things to check before you install solar thermal

How user control improves performance of solar thermal

Photo: szczel


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1 comments - read them below or add one

Plumb Solar LTD

Plumb Solar LTDComment left on: 3 September 2013 at 12:15 pm

We can help you, if you are within the east midlands area-ish

Most of this summer we have spent repairing and  fixing poorly installed solar thermal systems-usually installed by PV companies 'adding solar thermal' at the same time.....

Kind regards



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