DIY Doctor

Solar Heating Systems used in tandem with a Combi Gas Boiler

Postby House renovator » Sun Aug 03, 2008 8:00 pm

I have a technical question concerning the planned installation of an Active Solar Heating and Photovoltaics System to an existing property that is subject to a complete renovation.

The issue here and which I would appreciate any advice users of this forum are able to give, is that I already have installed into this large 3 bedroom property, a new combination gas boiler (a Worcester Bosch Greenstar Highflow 440 to exact) which currently provides hot water at 20 ltr/min and heating. This is more than adequate for my needs and succeeds in heating up the house satisfactorily. However, because I'm environmentally minded and I would like to reduce my fuel bills wherever possible, what I would like to do is install an Active Solar Heating System that would work in tandem with the Gas Boiler. My question therefore is: Is it possible to install such a system and if so, how would it work?

The problem I have of course, and which I would like advice about, is that I do not have a hot water cylinder (if I did, I wouldn't have installed a combination boiler). Although the house is relatively large, there is really no scope for installing a hot water cylinder anywhere and given that I already have a perfectly good nearly new combination boiler, I don't want to have to get rid of it and replace it with a new conventional boiler just so that I can install an aditional new active solar system.

As I understand it (and please bear in mind that I am a just lay member of the public and [b]NOT[/b] a plumber or Gas engineeer), in order to make use of an active solar system, it is essential to have a hot water system. With such a system, an electronic controller constantly compares the temperature of the solar collectors with the temperature of the water in the cylinder. Whenever the collectors are hotter than the cylinder, the controller switches on the system's circulating pump. A mixture of antifreeze and water is then circulated through the collectors and the cylinder's heat exchanger, heating the cylinder in just the same way as a central heating boiler. However whenever the collectors are cooler than cylinder, the controller switches over control to the gas boiler and the boiler then heats water in the cylinder in the conventional manner.

The whole point of acquiring a combi boiler was because I didn't want to use a conventional boiler and I don't have the space for a hot water cylinder, but by the looks of things, in order to achieve the result I want, I am going to have to have one. I don't know of any existing systems that are designed to work with combi boilers or how they might in pratice work. Therefore any advice you would be able to give would be gratefully appreciated.

As things stand, I suspect the only benefit I am going to get from Solar Cells is to have them generate my electricity.
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Postby ericmark » Mon Aug 04, 2008 1:19 pm

To use solar water heaters is as far as I can see dependent on some form of storage. With electric solar cells there are a number of ways they can be used again normally some form of storage is required but some radiators i.e. towel rails will allow duel source and there are electric boilers so in theory it could be combined. A modern boiler does not switch off and on but gives a variable output and the control required to combine both systems would be very complex so I would think having independent heating devices in the same room would be the answer and allow the thermostat radiator valves to do their job and reduce the boiler output when heat is supplied from another source. As you say solar water heating has to be carefully controlled to prevent it exporting heat but the electric system does not have this problem. In the main DC is produced which is then turned to AC with an inverter and some method is required to maintain the load and often a heat sink or battery is required. Very dangerous things to work on as they can’t be switched off and not really a DIY job.
Solar seems to be better than wind. On the Falklands many farmers needed to power their electric fences and either a 5 amp solar panel and battery or a 15 amp wind charger and battery were offered as options. In spite of the Falklands being know as the windy islands by Spanish speakers it was found solar was more reliable as very rare for snow to stick on sloping service for long enough for battery to go flat but quite common to not have enough wind for long enough to get flat battery. Alternative technology centre was interesting to see how many water biased systems failed so quickly in fact longest running one last time I was there was a standard radiator painted black under double glazed window and as with voltage systems they need cooling and power cut can cause water to boil so all sorts of backup is required. I think simple idea is to have a large units big enough to be monitored 24/7 supplying a national system so when the natural power is abundant in one area it can be supplied to another and all controlled by a central national grid who in turn sell us the power when required. I can’t see any way the expensive equipment required to produce power on a house by house basis will ever be viable I saw many houses in my younger days with small generators and even cellars full of batteries all by time I saw them redundant as a national system was so much cheaper by time one included maintenance costs and I can’t see solar or wind being any different in this country. May work in Spain and other countries near to equator but not UK.

Postby sparx » Mon Aug 11, 2008 7:51 pm

Hi HR,
there are so many ? on these forums about alternative energy we may need a seperate forum soon [Mr. Moderator please!]
I have had solar water heating here for last 3-4 years, we are all electric with no gas readily available.
You have answered your own question I think since the panels produce smallish volumes of VERY hot water for quite short times per day control is the big problem.
Without storage the only gain to be had would be to use a direct flow through flat panel type collector to go in line with cold feed to boiler to give some preheating, however there is a limit to the temp. allowed into a combi [or any other boiler] so you would need to have some sort of bypass/mixing valve arrangement, also collector could freeze in winter!
We have a large indirect cylinder with 2 coils in, one for a sealed glycol mix loop through an evacuated tube type collector.
The second one is intended to be connected to a boiler back up circuit for winter.
we don't have one, so there is an immersion heater also fitted, which we can use on night rate to top up if [when] required.
We are on the South East coast 26M from france so can't get better solar location in UK.
Biggest lie going is "solar doesn't need sun, only daylight", B@ll#X!
Other end of scale is a sunny day, cylinder full of max temp water 65*C by midday, temp on collector rises as pump turned off to 150*C! (steam).
missus does a load of washing, has abath etc. tank goes cold, system can't pump steam so no solar gain until sun goes off abt 6PM, temp in collector drops slowly to 100*C, pump starts & within minutes temp on collector down to 40*C or less N.B.G! I come home from work to cold shower! thank heavens for me hot tub!!!
anyway sorry abt length of reply but In you scenario Forget it!
regards SPARX
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Postby avenir.jim » Tue Nov 18, 2008 7:50 am

Whether you are converting your whole home to run on solar power, or simply looking to save some money and help the environment at the same time, you should seriously consider using solar power to heat your water. These system can either completely replace or supplement you existing hot water system.
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Postby avenir.jim » Tue Nov 18, 2008 7:51 am

Whether you are converting your whole home to run on solar power, or simply looking to save some money and help the environment at the same time, you should seriously consider using solar power to heat your water. These system can either completely replace or supplement you existing hot water system.

Here is an information that might be useful:
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