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Five key ways to get the best from your solar hot water system

Posted by Cathy Debenham on 19 October 2011 at 1:17 am

Solar hot water systems work best when the user takes control of them, yet only 36 per cent of the householders taking part in the Energy Saving Trust's solar hot water field trial said that their installer gave them information on how they might make best use of their system.  

When they did give advice, most of it related to advice on modifying the time of day people use hot water (have showers, baths or use appliances), and only a quarter to advice on hot water system settings and integration - which can make much more of a difference.

The key ways to improve the performance of a solar water heating system identified in the trial are:

1.  Use boiler timers and/or solar controllers to ensure that water is only heated by the back-up heating source (ie the boiler or immersion) after the water has been heated to the maximum extent possible by the sun. This is likely to be after the sun has left the panels (solar collector). When you use hot water during the evening and early morning hot water, you will ensure that there is plenty of cold water in the cylinder for the sun to heat the next day. Using back-up heat in the morning means less opportunity for the sun to contribute.(We tend to turn our boiler off altogether from around mid April to mid September, just using it to boost the cylinder when necessary).

2. Have an adequately-sized section of the hot water storage vessel that can only be heated by the solar water heating system. This will generally be a twin coil cylinder where one section is only heated by the solar collector, or a thermal store. If a dedicated solar volume is not used, for example in systems that do not require the existing cylinder to be changed, the timing of back-up heating has a even more important impact on performance.

3. Adequate insulation of cylinder and pipes is vital. Systems with poorly insulated cylinders can suffer from inadequate hot water provision in the mornings.

4. Use of electric showers, which heat water themselves rather than using the solar-heated water, will reduce overall hot water use, meaning that you won't get maximum benefit from having a solar water heating system.

5. Allow the hot water temperature to vary. If you do not need high temperatures all the time, you will have less need for back-up heating. You will also reduce heat loss. However, it is important to make sure your cylinder reaches more than 60 degrees centrigrade at least once a week to avoid risk of Legionella.

MCS standards require installers to communicate the how the timing of back-up energy sources can impact overall system performance to householders. Let us know if you got the best advice from your installer, by commenting below.


If you have a question about anything in the above blog, please ask it in the comments section below.

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18 comments - read them below or add one


timbukComment left on: 7 June 2016 at 10:00 am

Hello Cathy.

Do you hold any information on solar thermal systems using the SOLiC 200??

Thank you.

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Cathy Debenham

Cathy DebenhamComment left on: 4 November 2013 at 10:53 am

Hi Stewart

You're probably right, but it's a pity, and not good for business. If installers take the time to explain, they will have happier customers, getting better performance, who are then more likely to recommend both the solar thermal system and the installer to friends and neighbours.

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stewartyardleyComment left on: 4 November 2013 at 4:51 am

Some installers just cannot be bothered to inform their customers of better performing tips because they think it is not part of their job to give advises. Therefore, customers are the ones who are at the losing end who only get a hot water solar system that helps to reduce their bills by a small percentage and not fully beneficial to them like how it is supposed to. End customers are encouraged to find out on their own the tips that they can take to reap the full benefits of the solar water system setup.

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Cathy Debenham

Cathy DebenhamComment left on: 24 September 2012 at 8:43 am

Hi attar

I've just had a word with the engineer behind the calculator. The running costs would be around £20-30 a year for electricity on the pump. The rest is for the maintenance costs of having it serviced regularly and the anti-freeze replaced every 5 years or so.

This calculator is only a rough guide, as it doesn't take into account the size of your property and how great your demand for hot water is. The economics of solar thermal are better for larger families off the gas grid. It you currently heat the water with oil or gas, you can work out how much you are using for hot water by looking at your summer bills. Generally it's quite a small proportion of your overall use, and if you are swapping from mains gas, then it probably won't be a cost effective move.

However, if your goal is to reduce carbon emissions, solar thermal is effective - and speaking from personal experience, it is extremely satisfying to get your hot water from the sun.

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attarComment left on: 12 September 2012 at 12:12 pm

Thanks Cathy - I'll await news anon...

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Cathy Debenham

Cathy DebenhamComment left on: 4 September 2012 at 2:32 pm

Hi attar

I'm not ignoring your enquiry, I'm just waiting for the company that designed the calculator to give me an answer.


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attarComment left on: 30 August 2012 at 10:57 pm

As a newbie I must admit I am getting a little confused by the costs and efficiency of solar hot water.  I have just used the site's calculator and it came back with a £167 annual saving with a running cost of £96.  This leaves a benefit of £71 per year!  Is it unusual to pay more to run the system than you make from it?  Also I read on a website (and I sadly can't find it again) that you can run air through evacuated tube systems thereby saving the maintenance costs and risks of leaking (and freezing?) of liquid running through the pipes.  Is that possible, or have I misunderstood it?

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Cathy Debenham

Cathy DebenhamComment left on: 27 October 2011 at 4:02 am

Hi jrobson - yes, you are right. You can read more detail about when your installation counts as eligible here.

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jrobsonComment left on: 26 October 2011 at 10:29 am

Hi Cathy,I'm considering installing a 4KW system of solar pv.Am I right in thinking that if it is installed and working before the FIT is changed, the 43.3 p rate will not change.i.e. the new FIT's apply only to systems installed after the change.?

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Cathy Debenham

Cathy DebenhamComment left on: 25 October 2011 at 11:04 pm

@doubledevondream You'll find people's experiences of how their energy company deals with FIT applications at these blogs
FIT start date and Which energy company is most helpful

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doubledevondreamComment left on: 25 October 2011 at 6:17 pm

Thanks Cathy. The reason for the 8.00 am kick-in is that by then there is a reasonable amount of daylight which I assume is producing enough P.V. electricity to power the immersion. Agreed I could boost it towards the end of the daylight period but by then there`s usually other demands on the supply.

The rads I have seen are not expensive ( £200-400 each) , made in this country, and widely available at plumbers merchants. I haven`t seen many others yet, but I feel that as our only source of heat at the moment is two wood-burners,these could be a cheap automated addition even if it means importing from the grid on occasions.

By the way ,has anyone else had problems with there utility company not coping with the FIT demand? Ours has admitted that we can`t expect our first payment till January 2012 having submitted the application at the end of July!    

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Plumb Solar LTD

Plumb Solar LTDComment left on: 25 October 2011 at 9:22 am

Hi Tayviewer, in response to your question about adding more panels to a 500 litre thermal store. You will always get more heat from the sun with more panels (and this makes it attractive in spring autumn and even winter), but will you be able to use that heat?

In mid summer, with lots of panels working you need to put that heat somewhere or you risk damaging your heat transfer fluid and solar panels. A good idea if you are going to oversize the panels on the roof is to use a heat dump system to lose the excess heat in summer. A heat dump could be a set of towel rails off the thermal store on separate controls, or even dumping the heat into the ground to use in winter with a ground source heat pump.

Hope this helps


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Cathy Debenham

Cathy DebenhamComment left on: 24 October 2011 at 8:35 pm

Hi doubledevondream. Interesting that you have the timer kicking in at 8am. Doesn't that mean you get less benefit from the sun, as you've already heated half the cylinder, leaving less for the sun to do. I do ours at 9pm, then shower in the morning so there's plenty for the solar thermal to do.

I'm afraid i can't answer your question about the stand alone heaters, but will see if I can crowd source some advice from twitter.

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doubledevondreamComment left on: 24 October 2011 at 7:18 pm

We have 1 solar thermal panel and 10 P.V.s. I like the involvement I have with our system! We have a timer on the immersion which kicks in at 8.00 am ,unless there is enough hot water left over from the previous day ,in which case I override it. A 1hr. boost is usually enough for us two.

I`m currently thinking about replacing the old storage heaters with quick heat-up " stand-alone" ones to make the most of our P.V. leccy. Does anyone have any thoughts on those products? 

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@adrianenactComment left on: 21 October 2011 at 9:51 am

The number of panels that you can install will depend on the size of your hot water cylinder.  Page 64 of the document below states that the dedicated solar element of your cylinder should be a minimum of 25 litres per net M2 of panel. Hopefully one of the solar specialists will comment if this figure has been superseded?  Also I am not sure if you are just using for hot water or also for heating?  Would recommend talking to a specialist installer about your options but with the RHI it may be beneficial to go larger.

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tayviewerComment left on: 20 October 2011 at 11:37 am

I already have solar thermal panels on my roof and am thinking about adding some more. Is there a point of diminishing returns, when adding panels will not produce enough hot water to make them worthwhile? I have a 500litre thermal store.

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Cathy Debenham

Cathy DebenhamComment left on: 19 October 2011 at 8:33 pm

Hi Nick

Thank you for making that important point. After we had our solar thermal installed I noticed just that problem, and we had a new timer clock installed which allows us to programme the central heating separately from the hot water. I'm afraid I can't remember how much it cost, but the one we've got allows us to have two separate zones in the house for heating, and have the hot water separate. This extra level of control is great. Even in winter we only have the hot water turned on for an hour in the evening.

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Nick Hanna

Nick HannaComment left on: 19 October 2011 at 11:23 am

Hi Cathy, I read this (and the EST report) with interest...Your first point about using boiler timers to maximise heat from solar thermal  panels is clearly sensible BUT whilst this may be possible in  'shoulder' months (Spring and Autumn say) when you only require hot water back-up from your boiler, how feasible is it during the winter when you require central heating?

Since most of us need a bit of heat when we get up in the morning, and the CH function cannot be separated from the water heating function, that means in the winter your water is going to be pre-heated by default before the solar thermal kicks in.

In other words, the contribution of solar thermal in the winter will be negligible not because the sun isn't shining but because the water has already been heated by the boiler providing CH.

Maybe I'll turn my boiler off now and let you know later in the day if the solar thermal has added anything to the cylinder....

All the best


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