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Is my solar thermal system safe while I'm away on holiday?

Posted by Gabriel Wondrausch on 10 October 2011 at 9:37 am

Q: I’m going away for two months. Is there anything I should do to my solar thermal system while the house is empty? 

A: It’s always important to look at the relevant part in your solar thermal user manual, but the most important advice is to leave your system turned on. It has been designed to endure long periods of low to zero hot water ‘draw off’.

Ideally you don’t want the collector temperature to go above 120-140 degrees as the fluid within will vaporise and ‘stagnate’, which puts the system under additional stresses and strains and reduces the effective life of the solar fluid.

When an installer designs a solar thermal system for a property it is imperative that they take measures to keep ‘stagnation’ to an absolute minimum.  Stagnation occurs when excessive temperature in the collector causes the fluid to evaporate, as the system is under pressure (this usually takes place at 120 degrees C). We know that this is going to occur occasionally during the summer months and holiday periods.  Each time stagnation occurs it reduces the effective lifespan of the solar fluid and causes additional stresses and strains on the system components.

Most solar thermal systems are set to achieve a maximum cylinder temperature of 60 degrees C, so if they generate more heat than this (usually on a very hot summer day or when there is little or no hot water usage), something known as a ‘heat dump bypass’ could be put in place to ‘dump’ the system’s excess heat into a radiator and prevent stagnation ever occurring.

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England.

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

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6 comments - read them below or add one

Paul Hind (Secon)

Paul Hind (Secon)Comment left on: 5 July 2012 at 11:04 pm

Regarding the retro-fit question - a solar thermal controller is a very simple part to change on most systems, (and not too expensive) especially if it uses the same type of temperature sensor as the one it's replacing. (most use what's known as a PT1000 type of sensor, very cheap and reliable).

The Resol BS4 is the most common solar thermal controller available and most good installers will have fitted many of them over the years, contact the Solar Trade Association and use their 'search for an installer' for a list of installers in your area. Similarly, if you know a good electrician or are a competent DIYer it may be worth trying that option. Technical support is only a (UK) phone-call away. It will most likely be me you actually speak to as well!

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Cathy Debenham

Cathy DebenhamComment left on: 2 July 2012 at 9:52 am

Thank you for that Paul. Very interesting. Can it be retrofitted onto existing systems? Of course, an alternative is to do what we're doing for holiday this year and arrange a house swap!

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Paul Hind (Secon)

Paul Hind (Secon)Comment left on: 29 June 2012 at 10:49 pm

Stagnation should not occur in a well designed system, a good installer is always recommended.

However, holidays are the main concern as they are usually taken in the summer (if we ever get one) and no water is consumed from a lovely full cylinder. Heat dumps are an easy way to transfer heat and all Resol solar thermal controllers can do this with a simple program setting. The most popular Resol controller (the BS4) has a 'Holiday ' function - when you go on holiday you set this function to 'On' and whilst you are away it will try to dissipate excess heat from the cylinder by reversing it's normal operation. Normally when the sun comes out and heats the collector, the pump switches on and transfers the heat to the cooler cylinder. With the holiday function activated this still occurs, however, if it sees the reverse and the cylinder is hotter than the collector, it will bring the pump on again..taking the hot water from the cylinder and giving it back to the roof outside...this will run all night if necessary, cooling the cylinder  to a level just as if you were running a bath each night.

 The next day when the sun comes out (hopefully for the rest of us that aren't somewhere as hot as the Caribbean!) the cylinder is cool...the system will run as if you were there using the water. 

No extra cost involved with heat dumps, pipework, valves and radiators! 

Just remember to turn it back to normal when you come home. 

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JohnShrewsComment left on: 14 February 2012 at 9:14 am

Perhaps slightly ungreen from a water point of view but I have cosidered putting an extra thermostat fairly low down on my DHW cylinder which is set at a temperature just below the max temperature at which my solar controller is set. When I am away on holiday I would live up this thermostat with a switch and it would contol a washing machine type valve attached to my now redundat washing machine hot fill valve and would dump a small amount of hot water straight into the washing machine outflow pipe. That should prevent stagnation. Thoughts?

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deeppurpleComment left on: 25 October 2011 at 4:07 pm

It would be a lot cheaper, albeit a bit tiresome, to put a blind over the collectors, assuming that they are easy to reach. Does anyone make a suitable remotely operated blind system the same as is used in conservatories? 

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praxypComment left on: 25 October 2011 at 11:36 am

Hi  Gabriel,

My Solar Thermal installer explained the matter of stagnation and its affects on the fluid. I decided to help prevent my system from going into stagnation by fitting a heat dissipation system to expel excess heat out of the cylinder when the variable thermostat registered a given temperature. When I go on holiday, I set a lower temperature setting -- say 30 degrees; this way as soon as this temperature reached, the heat dissipation system kicks-in and starts to expel as much heat as it can, even when nobody is using any hot water.

It has been a success as my system has not gone into stagnation

for the last two years! A good quality dissipation system will cost just over £1000 pounds (parts).



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