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Solar hot water: It's the best thing I ever did

Posted by Cathy Debenham on 31 August 2010 at 9:53 am

Solar thermal panels are a no-brainer for John Wood of East Devon, as he explains in this video:


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

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7 comments - read them below or add one


PorterComment left on: 8 May 2018 at 12:40 pm


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PorterComment left on: 8 May 2018 at 12:38 pm

My slave cylinder system works perfectly for hot water and then dumping excess heat to radiators. 

What is the simplest PV panel s / Ac/DC water heater system I can install to the slave cylinder to make best use of any Sun&

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MaxadolfComment left on: 8 May 2018 at 10:42 am

John’s assessment appears strongly in support of thermal solar panels. But sadly, he and others ignore issues that plague owners of panels and that detract from their true benefits. 

1.  During hot (cloud-free) weather in summer, the collector temperature can get to 165 C   When the cylinder water gets to 85 C the system automatically switches off which removes the heat sink, resulting in excessively high panel temperatures that cause the glycol to boil with vapour escaping via the relief valves. This means occasional topping up with glycol/water mixture, a process that is far from straight-forward because the fluid has to be injected under high pressure and requires the installer to service the system with a total replacement of heat-exchange fluid. Typically this can cost £150  (and rising) and needs to be done every 4 or 5 years  

2.  The high cylinder temperature risks serious scalding.  And if the cut-off temperature is reduced to say, 70 C cylinder temperature, then the run-away panel temperature occurs earlier in the heating cycle.  Glycol doesn’t like sustained extreme temperatures!

3.  Since installation a few years ago, I have had two Drayton drive motors and one diverter valve replacements costing me £800.

I also have pv panels that have never given problems in the 5 years since installation and genuinely regret having used up valuable roof space with the thermal solar panels.  I should have used all of the available space with pv panels that, so far, have not incurred any maintenance costs.  I do not qualify for any kind of government grant for the thermal panels but continue to receive generous FIT payments for the pv panels.  Much of the cost of the  “free energy” from the thermal panels has been lost on maintenance charges. 

Sadly, the installers of the thermal panels don’t come clean during their sales pitch on the true maintenance costs! I expect a payback of at least 15 years for the thermal panels and 10 years for the pv panels.  I understand that the payback of thermal panels has been increased further by “needing” to include a water softening system for hard-water areas.  


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paulgarbComment left on: 12 May 2011 at 8:27 am

It is a pity that John's simple and effective testimony should be balanced by overly pedantic comments. Solar Thermal alone will never solve the energy problems but it does make a contribution and 'every little helps' as they say. 

I'll be going for it very soon - not for money, not for 'carbon footprint' reduction (I do plenty of other stuff for that) but for the very simple reason that it makes sense. I agree with John it is a no-brainer. 

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Cathy Debenham

Cathy DebenhamComment left on: 12 May 2011 at 8:21 am

Good point paulgarb. I'm slightly bemused by the obsession with payback whenever renewable energy is mentioned. No other appliance you buy for a house pays back. They all depreciate. Yes, they are more expensive than the alternative, but if it was all about price no one would ever buy a BMW. I love my solar thermal panels. They, more than anything else, have brought us closer to our energy use. It's now normal for us to notice how sunny it's been, check the thermostat and boost the cylinder a bit before having a shower. Before we didn't give it a thought. It's very satisfying to know that we've barely used any gas for water heating since the beginning of April.

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Cathy Debenham

Cathy DebenhamComment left on: 6 September 2010 at 1:23 pm

Hi Spa639

Payback varies enormously, depending on both how complex the installation is, how people use the system, hot water usage within the household and whether all hot water comes from the cylinder or is heated electrically.

If, like John, you switch off the boiler for five months of the year and don't boost the cylinder on cloudy days, you'll get faster payback that if you have the boiler coming on every day as backup.

I agree, solar thermal is not the most cost effective way of reducing your carbon footprint, or making your house more energy efficient. However, given the right circumstances, it is a great technology once you've done the easy wins like insulation and draught proofing.

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spa639Comment left on: 5 September 2010 at 2:36 pm

This is a bit of propaganda.  I've done this and I would say

- Payback is much much more than 5 years.  I estimate that I am saving about £200 max per year on gas bills after installing the panel.  Payback under current gas prices would be 22 years.  (I didn't do this project for the payback and you should do your own calculations carefully if you want financial gain).

- The disruption is not minimal.  My hot water tank had to be changed for a dual coil tank and new piping laid from the roof to the tank.  Not a minor task.  We were without heating and hot water for two days during the installation period.  That could be reduced to one day I believe.

 So why did I do it?

I was having a loft conversion done and losing the old hot water tank anyway so it made sense in my situation to do this.


Many plumbers hate greeny types like me and were very negative about me doing this.  They're new to this stuff and made mistakes in the installation.  The big thing to watch out for is that the solar company installs the solar loop but usually a plumber rips out the old tank and puts in a new one.  When something goes wrong (as it did in my case) each side blames the other.  That's a biggy.

Was it worth it?

At the moment it seems not.  Longer term I think it will be.  I expect energy and climate issues to rack up during this decade.  

A physicist friend of mine and I calculated that if you're doing this to save on CO2 then you're better off doing 700 miles less per annum in your car.  Either don't do those journeys or use public transport for them.  That will save more personal CO2 than solar water heating for 4-5 months of the year.

As with these things there are lots of things to balance out and the most CO2 saving impact can be made by reducing heating, eating less animal products and using public transport.  Solar panels are way down the list.

Shame I only worked that out afterwards.

 I haven't stopped though. I've worked out lots of ways of reducing my car miles and boy I've learnt a new way of travelling.  Brompton and buses and trains.  I was always a cyclist though so it's no big deal for me.  It's a great way to get a round.

I don't see myself as greeny but others do.  So expect to get labelled if you get panels or you start cycling.  Didn't bother me. 

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