Thermal store advice please

[BenG]

The time has come for a new boiler in the house so I'm looking to make a few more changes as well - basically adding solar panels and using a (vented) thermal store, I'd be very grateful for advice on the following points:

1) The new boiler will be a HE gas boiler (Vaillant?). I am unsure whether to connect this directly or indirectly. Am I right in thinking that a main advantage of indirect (ie via a coil in the store) would be less disturbance of the stratification in the thermal store, if it was direct then the house CH system is in effect an extension of the thermal store and could thus gain from the solar input. What are the pros & cons here?

2) I'm considering putting the thermal store (Navitron 216 or 259 litre?) into the loft:
   a) How much height does the integral top up tank in a Navitron thermal store add?
   b) Is there any insulation between the top of the hot water cylinder and the top up tank - if not surely there's potential for a lot wasted / lost heat here?

3) The main reason for putting the thermal store into the loft is to free up the airing cupboard for use as.. an airing cupboard, putting a small rad in there to disperse overheating (sorry can't remember the proper term) seems a good plan - any comments?

4) I'd like to put the solar panels on the (vertical) gable end of the house, would there be any advantage to me using a flat roof frame on a 'shelf' as opposed to making up my own frame to angle the panel to the correct angle (or do you supply a frame for vertical mounting)?

5) The pipe run from the panels to the cylinder would potentially be the highest point in that circuit, presumably there should be an air vent at the highest point in the circuit (or is the system pressure sufficient to avoid this)?

Thanks in anticipation,
Ben.

[dhaslam]

Putting the store in the attic wouldn't be the first choice normally.  There will be probably be extra pipe runs to all outlets and any heat loss will be to the attic.   On the other hand the heat dump radiator is better in the attic because it is only needed on very hot days in summer.

[kristen]

"Am I right in thinking that a main advantage of indirect (ie via a coil in the store) would be less disturbance of the stratification in the thermal store"

My understanding is that stratification doesn't matter in this case.  My understanding is that you want your boiler to heat the, cold, water in the radiator circuit, and once that is hot to carry on heating the cold water in the Thermal store, so that the boiler can stay in condensing mode as long as possible.  So if its going to heat the whole lot (or everything above the inlet to the thermal store, at least) I don't think the fact that its going to be mixed matters, its all going to be at peak temperature in a few tens of minutes. Then the boiler can shut down until the whole lot is cool enough to be able to resume in condensing mode.

Dunno about the summer when you are using the boiler to heat the thermal store ... but hopefully your solar panels will do that, and you won't be using the boiler at all?!

[BenG]

dhaslam - thanks, I realise that putting the store into the attic isn't ideal but it would give us a useable airing cupboard. I would certainly use max thickness insulation on the cylinder & probably box it in too.
As a further thought, how close would the pump need to be to the store - the pump seems to be regarded as a source of lost heat so I could put that into the airing cupboard (the store in the loft could be more or less above the cupboard).
Kristen - yes, that makes sense. Not intending to use the boiler in the summer of course!

[chickensoup]

Hello Ben,
              Thermal stores are kept between 70-85 degrees, and condensing boilers are usually kept in condensing mode under 55 degrees, so to keep the store topped-up in periods of no or little sun the boiler wouldn't be as economical.
                    If your intending on connecting the heating to the store directly/ indirectly...i wouldn't bother as the heating would zap the heat in minutes, a 200 litre store is really only suitable for hot dhw, to start running heating, either rads or ufh would require an immense size. for example an 500 litre holds approx 25kWh of heat, to run your heating with a load of say 24kWh would empty that in an hour....catch my drift
 
              I personally would fit the store in the roof with a coil for solar, a coil for the boiler ( as i presume the vaillant will be a system boiler, probably a ecotec) and run the heating on the other side of the s-plan. As for the collector you would need approx 46 tubes for a store of 80 degrees, probably more to maximise solar gain for winter....say 50-60 and a great big heat dump..................If you wanted to heat the store purely by solar!

[kristen]

"... the heating would zap the heat in minutes, a 200 litre store ..."

Didn't register that bit, sorry .  My planned system will have a much larger thermal store and I just assumed ... 

[dhaslam]

The smaller thermal store would still be sufficient when used with a gas boiler because all it needs to do is to stop the boiler re-cycling.    The water temperature is a bit of a problem with thermal stores because you loose a few degrees in the heat transfer and more when there is a big draw off.    The condensing boiler can run at a higher temperature but with less efficiency which would offset gains from using a longer burn.   If only 100 litres is available  for the central buffer area there  is only  only a capacity of about 1 KWh with a 10C temperature variation.    Other parts of the system would need to be compatible.   Radiators would need to work at reasonably low temperatures and shower mixers would need to be able to use almost all hot water.  There is still an advantage  in the thermal store system if a good area of solar panels is used because the system is more integrated.   

[Loir]

The pipe run from the panels to the cylinder would potentially be the highest point in that circuit, presumably there should be an air vent at the highest point in the circuit (or is the system pressure sufficient to avoid this)?

If you can locate the panels below the cylinder then it will thermosyphon. This will save the electricity for the pump and save the cost of a solar control system. Oh yes, and the cost of a non-return valve. You still have the choice of a pressurised system or an open F&E tank.

Loir 

[BenG]

Loir,
Thanks for this suggestion - How much height differential would be necessary for efficient thermosyphoning? the run from panel to tank would be approx 5m, would it ideally have to be inclined all the way up to the tank or would a level section be OK? Would a larger bore pipe encourage thermosyphoning?

Many thanks for all the other advice too - spoke to a heating engineer about the boiler yesterday so the plan is coming together 

[dhaslam]

Loir,
Thanks for this suggestion - How much height differential would be necessary for efficient thermosyphoning? the run from panel to tank would be approx 5m, would it ideally have to be inclined all the way up to the tank or would a level section be OK? Would a larger bore pipe encourage thermosyphoning?

A small incline is OK but pipes shouldn't be level at any point.  Pipe size will need to be bigger.  If you have 60 tubes it might be safer to use 22mm, very well insulated.   The return pipe from the store needs to be higher than the outlet from the collector.  You could use a gravity heat dump as well.  The antifreeze mix will allow the temperature to go higher than 100C even without the high pressure system.   

[BenG]

I think giving the pipes an incline over the full 5m would be difficult in terms of loft space (having to lift the store a bit, & lots of other "stuff" stored in the loft!) so I'm inclined (oops sorry, no pun intended!) to use a pump for flexibility.

I was looking at the heat dump in the airing cupboard (below the store, so would have to be pumped)  but perhaps a gravity one above the store would be better - more food for thought!
Thanks again,
Ben.

[chickensoup]

A small incline is OK but pipes shouldn't be level at any point.  Pipe size will need to be bigger.  If you have 60 tubes it might be safer to use 22mm, very well insulated.   The return pipe from the store needs to be higher than the outlet from the collector.  You could use a gravity heat dump as well.  The antifreeze mix will allow the temperature to go higher than 100C even without the high pressure system.   
[/quote] 
 
           I beg to differ regards the pipes not running level Mr Haslam, there are thousands upon thousands of examples of level pipework in homes across the u.k that use to have gravity h/w from bbu to first floor cylinder.....they worked o.k!.............fairplay they may of been a swine to remove air at times, but a 5m run in the loft will be fine Ben, try and get a 1" rise to help remove air if your'e going to do gravity(thermosyphon).


    Chicken

[lightfoot]

Ben,

If your new gas boiler can modulate it's output to match the needs of the heating system, ie reduce it's output during times of part load (which may account for much of the heating season), then there is not much point in using a thermal store as a buffer for the CH, unless you wish to combine additional heat sources, such as a wood stove/range etc.  However, if you wish to go ahead, then when used in conjunction with a (on demand) oil/gas boiler, I suggest you allow a additional 50 to 100 litres buffer for the CH, over and above the DHW requirements (more for wood stoves etc).  Also as others have already mentioned, to get the most out of a thermal store, it is better to use over-sized (lower temp) rads or preferably UFH if possible.

.....and make sure it's well insulated, ie 75mm PU foam or more, especially if it's going in the loft.


Good luck,

Lightfoot.

[BenG]

Thanks Lightfoot,
Yes I think I've come to the conclusion that using the store as a buffer for the CH isn't really practical. The boiler will modulate & I'll stick with the current rads.
I'll certainly go for max insulation on the store, still wondering about the integrated top-up tank being a liability for heat loss.

Ben.

[lightfoot]

Ben,

I assume then your only reason for using a thermal store is because you would like mains pressure DHW and don't wish to use a unvented DHW storage cylinder ?

If you used a unvented DHW cylinder and if the boiler was suitable, you could use a sealed boiler/CH circuit and not have to worry about the F&E tank, integrated or otherwise .  But you wouldn't be able to directly connect a WBS etc at a later date if that's your plan...plus the other potential issues associated with unvented DHW storage cylinders?


Lightfoot.