Advice needed

[madmas]

Evening folks,
First post (and limited knowledge)- so be gentle

My situation is :- Live in Dordogne,SW France,  existing set–up is, Oil fired burner which provides Hot water and Heating. Switching options for this are, either hot water or hot water and heating.  Hot water is contained within an integral 100 litre tank. I have looked at the manual and basically the HW tank is within the Heating tank, hence why have to have HW at same time as heating – not one OR other.
Burner is below which heats the heating water which heats the Hot water.
This is an unvented mains pressure system over here.
Have existing bathroom with switching tap fed shower, and will be converting the barn into a bedroom with ensuite shower and basin.
There are 50weeks 2+2 people here.
Distance from panels to tank would be about 7 metres vertically

I would like to install solar hot water and want to know
1.  Would I need to have, say, a 200 litre tank because of overheating in the summer (temps reach about 37 centigrade).
2. Would I still need a heat dump if I have the 200 litre tank.
3. How would I plumb? to utilize the Oil burner as back-up in winter. Would I need to pump the water from the new cylinder to the existing integral (or vise a verser)? Or would thermo siphon be working?  With shut off valve as not needed in summer? this giving me 300 litres of hot water, even though we run ok on the existing 100
4. Or. Could I use a smaller tank, say 120 litres linked to the existing 100 litre tank?
5. which diameter tubes would I need 47 or 58?

Many thanks

Ian

[dhaslam]

It doesn't seem to be all that easy to integrate the two  cylinders, even though  the one attached to the boiler  has some storage capacity it still may need to be regarded like a combi boiler.  To do this   you would have a mixing valve   on the outlet from the new cylinder that opens  at a set temperature to allow hot water from the  boiler heated source to mix and bring it up to a minimum temperature.  In summer  you probably could just turn off  the oil boiler  and in  winter have it heat the water  as at present so that it is always available to mix with the solar heated water.   

There are a few safety details , the solar cylinder would also need to  have the normal temperature  limiting mixer valve on its outlet, a heat dump  to avoid excessive tempeatures and a way to  increase the  water temperature to 60C once each week.     

When you are doing the job it is as well to make it a little bigger than you think you need at present.  The smaller 47mm tubes are  easier to transport  but you need a few more of them.   Something like forty 47mm tubes for a 200 litre cylinder, i.e. five litres storage per tube.         

[micko]

Does it freeze ever where you are, if not you could let the system stagnate and overheat. Stagnation affects the anti freeze if you were just running water I can not see a problem with this.

[madmas]

Micko, yes it does freeze. Last year average Jan/Feb was -6 hitting -9 a couple of days!
this year only gone to -1 so far, average +3/4.

Any further knowledge on my questions?

Could I use a smaller (say)130 litre twin coil tank in conjunction with the existing 100 litre?

thanks

Ian.

[micko]

Ian,

What you are looking for is beyond my knowledge, however I can answer a few of your questions.

Tank size depends on what you want out of the system. A small tank will give you hotter water than a big tank using an identical solar tube setup. With a large tank you will be throwing less heat away via a heat dump, with a small tank you may not need to use your boiler as much to top your heat up.

It’s all on how much water you use, the Navitron panels can cope with stagnation but the ant freeze starts to break down if it gets to hot. You can set the system up to dump the heat into your radiator system by getting the central heating to run without the boiler firing up but again this is beyond my knowledge, I use a radiator in the solar loop. In hindsight I should not have bothered as my system in the UK with a 300L tank and 30 X 58mm tubes never gets near a dump temperature.
 

Mick.