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Discuss Solar hot water in the Renewable Energy area at PlumbersForums.net

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billybongo

Hi all , I have moved into a new build with solar hot water and would like to know how to fit it in with my boiler , at the moment the water is heated for an hour in the morning and an hour at night , does this cancel out my free heat ! I have tried solar only and have occasions where we run out of hot water due to cloud or whatever , basically what is the recognised way to use solar for hot water .
many thanks
Mark.
 
Hi Mark
Try just having the hot water on for an hour early in the morning say between 5 & 6am (if you tend to get up around 6.30 - 7am) with nothing in the evening.

Moden cylinder are very well insulated & I take it you have a twin coil cylinder which have a bit more capacity so your hot water should last through out the day.

If you are a typical family where most of the hot water usage is in the morning with washing & showers etc, so by the time any sun starts to hit the collectors there is cold water in the system to be heated (free heating). You may use a bit during the day / evening but with any luck it will last you right through to the morning heating time again.

Make sure the pipes are well insulated around the cylinder to retain as much as possible.

PS
if it is a twin coil solar cylinder the hot water which is heated via the boiler in a smallish amount at the top with the solar coil at the bottom to heat the rest if it is available from the sun.
Have any heating for the radiators timed to come on after at least 40 mins from the start of the hot water heating as this will help the boiler with the loads when it is very cold & with the efficiency.
 
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billybongo

Thanks Chris that explains perfectly what I want .
Mark
 
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billybongo

Chris , couple of things on solar , i guess the stat at the bottom of cylinder brings solar pump on ! does the liquid in solar pipes have to be certain temp before pump circulates ! and when temp is reached in the cylinder does the solar circulation stand still as i guess it can get very hot without disipation .
I am just trying to understand the hole system .
Thanks
Mark.
 
Hi Mark
It does depend on what system you have installed but most have a controller (brain) which works with a number of stats or sensors.

So yes the lower stat will bring the pump on but as you say only when there is useful heat to collect are the panel or tubes otherwise it could be removing heat from the cylinder.

The major technical challenge is when the hot water cylinder is up to temperature & the sun is still shining, it is know as stagnation. As you have worked out the pump must stop otherwise the cylinder would quickly overheat but any liquid in the collect would quickly boil if there were no circulation to carry it away.

What to do ?
If you have flat panels then the panel drains of any liquid, into a sump, as soon as the pump turns off, the panels get very hot but no liquid = no boiling.
If you have tubes then there are a couple of ways they do it but Heat Pipes use heat exchange, with a liquid to vapour method to get heat from the tube collector up to the heat exchanger manifold at the top. As the temperature inside this rises as the circulation has been stopped the pressure builds & all the liquid turns to a vapour (boils) it then just sits there getting very hot inside its little copper tube until the temperature drops & it can start over. It is all safe inside its glass tube & the main transfer liquid in the system does not over heat.
 
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billybongo

Thanks Chris , my panels are flat with the roof , I know feel I have a good understanding of how it works with your explanation , it's hard to find info like that .
Cheers mark
 
Ideally there must be a backup boiler thermostat in the top quarter of the tank, which switches the boiler on (backup heat source), when it cools down to ~37-42 degrees or so, so you always have some warm water.
Having hot water on a timer was needed with old uninsulated cylinders, modern ones don't loose much heat, so just have a cylinder stat on a standby on a low setting in the top of the cylinder.

PS: The correct configuration of a solar controller is very important for maximising the efficiency, ideally you want it to start puming, when the solar colector is warmer by at least 5-10 degrees than the bottom quarter of the tank.
 
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Not so sure I can agree with your first statement Bronze, stored domestic hot water must always be heated to a minimum of 55-60deg C & Building Regs still required time as well as temperature control.
 
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