Posted by on July 14, 2012 in Solar Panels, Solar Thermal, Tips and How-To

Solar Hot Water – Is it a good idea?

So why did I do it?

When I invested in Solar PV in 2010 , I looked at installing solar thermal as well. There were good reasons , in addition to the usual ‘green’ ones, for considering this:

  • Cost of scaffolding my house was high so I didn’t want to incur that cost twice if I was going to install solar thermal at some point later
  • The government was proposing a Renewable Heat Incentive (RHI) at the time due to start in October that year that would have produced a similar payback to the solar PV being installed
  • The wholesale cost of gas was rising so it seemed a good hedge against rising gas costs to at least remove much of my hot water consumption from being gas dependent

More things can go wrong with a Thermal Installation!

Looking for a solar thermal installer is not as easy as a pv installer. Installing a hot water system involves installing a new hot water tank and connecting it to the existing plumbing. You need to find both a good solar thermal engineer and a good heating system plumber. They might be one and the same person but I found a specialist in solar thermal who subcontracted the plumbing piece out to a plumber who worked alongside him on the day of installation.

So beware of solar pv installers claiming they can do solar thermal as well because they will probably sub contract to another perosn. It’s a specialist business which seems to a more fiddly job. I think its worth finding a solar thermal specialist, getting them around and deciding if you trust them to do a good job.

I also wanted someone who would be likely to be able to come back in 10-20 years time when no doubt things will start needing attention.

My own installer suggested an annual service visit at £40 to keep on top of things. Well I’ve missed the first one but I’m sure it’s a good idea

What are the things you are buying

A new hot water tank that will have a solar heat exchanger at the bottom, a gas heat exchanger in the middle, and an electric immersion element at the top. This way you are not dependent on the sun shining to stay clean!

In my case it simply replaced the old hot water tank in our airing cupboard but you will need to check that your existing space is large enough for the new tank. So if you have a gas system that has no need for a hot water tank you will need the space for it somewhere relatively convenient and accessible

The panel that goes on the roof

A pump that will probably go in your attic.

Quite a length of flexible hose that runs through your roof into your hot water tank and back. This carries the glycol that heats the water through the heat exchanger.

How does it work?

On my roof is a single panel about 4ft square with 10 tubes containing a form of glycol which heats up very rapidly with warm sun on it . Once the temperature of the glycol is hotter than the water at the bottom of the hot tank then the glycol is pumped around a heat exchanger in the hot tank .

This is called evacuated tube technology. An alternative form is solar plate technology – some people feel this is more aesthetically pleasing on your roof than the black tubes on my roof but they look fine to me.

You should have a little meter next to the hot water tank that tells you what the temperature of the glycol on your roof is (it can get VERY hot), the temp of the water at bottom of your tank, and temp of the water that will come from the top of your tank

Whats the verdict so far?

Let’s get the BAD news out first..

Where’s this proposed RHI payment from the Government?

When I installed all this the UK Feed-In Tariff was Law , but the RHI was in consultation… and that’s the way it’s stayed. Latest delay by HMG means that the earliest any RHI will be paid is summer 2013 which is 3 years after the original proposed date!

If it hadn’t have been for the cost of rescaffolding I might well have waited (and waited… and waited) for the RHI to become law. Still I’m glad I didn’t which is where we get to the GOOD news.

The energy saved is much greater than was estimated!

Instead of the £40p.a saving quoted by my installer I believe its closer to £200 which is more than 20% of the Gas bill. As a rule of thumb, take the Apr-Oct gas bills you get and that should be roughly the saving you make. To completely realise the benefit you will have to switch a to a genuine no-standing charge gas tariff (which resets to zero every 3 months, not every year like most so-called no standing charge tariffs)

In my case the saving was large because I have an inefficient but very much still going gas boiler and a HW tank system. If you have a demand only hw system and efficient gas boiler then I would hope your Apr-Oct gas bills are much lower so possibly solar thermal is not yet for you other than the obvious environmental benefits.

However, if you have limited roof space for panels it may well be best to consider solar thermal before you give in to solar pv and its much greater sales force. The energy saving benefits are bigger in terms of kwh and may provide a big £ benefit particularly for houses without mains gas.

JULY2013 Update!!
It seems the Coalition government is finally ready to recognise the costs of installing solar thermal in a real way.
As from ‘Spring 2014′ they will pay at least 19.2 pence pkwh deemed energy output of your solar thermal system if it was installed after August 2009.
Ok , so its not the 20 years that was originally put forward by the Labour government , but we beggars can’t be choosers and it definitely makes a significant contribution to your heating bills until 2021. Who knows then maybe shale gas will mean dead cheap energy anyway (but then again maybe not…)
For more reading here’s the DECC website link


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1 response to Solar Hot Water – Is it a good idea?

  1. And the good news is…?

    December 3, 2012