A new house - a new system, advice please?

[evan]

Hi all,
  I have just bought a small, fairly derelict cottage which has no central heating, and only the most basic hot water provision from a coal fireplace back boiler, with a non-insulated tank and lead pipes!

So, a clean slate.  I want to have a reasonable solar + wood stove setup.  I'd consider UFH for the (straw bale, not built yet) extension, but the main building has wooden floors, so probably radiators there.  It's small, and I'm hoping to get it fairly well insulated, triple glazing and all the rest of it.

I have a 15 tube Navitron panel already, and will add another 20 or 30 tubes to that - roof is SE facing.
I have 2x 120L insulated copper tanks, which do not have coils.   140s, with coils
I also have a 80L 100L DHW tank with a single coil and an immersion heater.
I will buy a decent stove.

I would like to use the larger tanks as a buffer for the heating, so that there is the possibility of a short burst of heating the next morning and perhaps the evening too (especially if it was a sunny day).

Can anyone suggest how they would proceed, preferably using the bits I have already, and without spending too much on fancy gadgets?  Automation, sensors etc I can do myself if needed.

Thanks for any ideas!

[dhaslam]

Although it is good to reuse what you have they are all really too small for the intended use and would end up wasting fittings, space and time.    If you could build in  a  decent sized  heat store  with solar coil  close to the stove  you would get a  better system that may not cost much more in the long run. 

[evan]

Cheers, what sort of capacity would you suggest then?

The problem with a single giant tank is space, unfortunately there just isn't much available at the end where the stove must go.  Normal sized cylinders will happily fit in the loft though (with suitable reinforcing).

[roys]

We have a cottage and we have a log burning stove with back boiler, Charnwood Country 16 I think. Above the woodburner in the attic we have a 210L hot water twin coil cylinder, above the cylinder right in the appex of the roof is the cold water coffin type storage tank.  There is two solar panels feeding the other coil in the cylinder.
When our return water temperature to the fire reaches 50 deg C a pump switches on and puts hot water round the radiators.
The system works well, not brilliant but not bad at all.  You do need to have a good supply of wood for winter time, I reckon we easily use a wheel barrow per day on cold winters day.
The solar is fab in the sunnier days as the fire no longer has to be on, yet we still have the hot water for showers etc.
Cheers Steve

[dhaslam]

I have an 850 litre buffer tank and separate DHW cylinder  250 litre.   The only reason why they are separate is that the  bathrooms are quite a distance from the  stove area.     I originally planed the  buffer tank to go on top of the fuel storage cupboard  but the  space was too small so it is in the attic.    As a result a few KWH of heat goes into the attic.  The stove I have  cannot heat the buffer tank on its own  so the bottom part is heated to a lower temperature by the heat pump.  Given a decent stove I would say  500 litres  of well heated water would probably do the job.  If you could build the buffer tank into a similar enclosure with space for fuel storage underneath  it would be worth sacrificing the room space.  The fuel storage space doesn't need much height, just easy access.

   

[evan]

OK, that could be an option, in a corner.  Are you using a normal vented cylinder then or some kind of fancy thermal store?

The other issue is the price.  A quick look at a 500L tank in the web shop has put me off a bit: £2600?  


I had another look at the cylinders I have - it's actually 2x140 and 1x100, all have  single coils.  If I stack them on top of each other and wrap a load of rockwool round it, have I made a thermal store?  

[Brandon]

Evan,

down the stacking 2 cylinders as a thermal store route, I suggest that you look at this thread, as Stuart has done a lot of thinking, and several of us have chipped in.

You may find some of it helpful.

Brandon

[evan]

Thanks Brandon, lots of reading there.    I wonder how that one worked out in the end...

[evan]

OK, hmm.  Actually constructing a thermal store that works properly, taking into account stratification, heat exchanger rates, all the rest of it is looking a bit challenging.  As well, we're not going to have a mains pressure cold water supply so that removes some of the benefits of the TS.

So, scaling back the ambition a bit, but still using the parts I have;

The 100L cylinder at the top can be used as a normal vented cylinder for DHW.  The wood stove will thermosyphon using the main coil, and I'll add the solar to that tank in one of the available ways.  All normal so far.

Now, I still want to buffer the CH a little, if only to reduce the amount of fussing the stove needs - if I can load it up once or twice an evening and let it burn flat out without much temperature fluctuation, I'll be happy.  If there's enough left over for half an hour of warm radiators in the morning, I'll be very happy    280L should be enough for that, right?

So, the coil of the middle tank can be connected in series with the coil of the DHW tank, so both are heated when the stove is on.  The CH is fed from the (inhibitor containing) water from the lower tanks.  Will the coil transfer enough heat to do that?  We're talking about 4-5kW or so I think.

Next, I want to get a bit of CH from the solar.  The only way I can think of doing this is a thermostat which changes over a couple of motorised valves to divert into the lowest cylinder's coil once the DHW tank is fully up to temperature. 

What do we reckon?  Any big problems apparent?

[dhaslam]

The boiler could probably feed the  bottom cylinder directly as well  on the  way back but with a valve to  bypass   it if the return temperature isn't high enough.   

Oddly enough it looks like a fairly decent system except perhaps that there isn't a lot of capacity for DHW.   

[rob26440]

Next, I want to get a bit of CH from the solar.  The only way I can think of doing this is a thermostat which changes over a couple of motorised valves to divert into the lowest cylinder's coil once the DHW tank is fully up to temperature.

Dependent on how big a panel and how much heat you expect to use, I suggest you will need 2 possibly 3 thermostats - all in series.  2 of them would be "normally open" on the DHW cylinder - one at the top & one at the bottom.

My reasoning: Safety & control.... If you have only one thermostat and it is at the top of the DHW cylinder then once the valves have switched the flow to the lower cylinder they won't switch back to the upper cylinder until virtually all the H/W has been used.  So with a stat at the bottom it will switch back when some of the water has been used and the upper stat will ensure you switch over to the lower cylinder only when the upper cylinder is fully up to heat.

Reason for the third thermostat:  This would be a normally closed one at the top of the lower cylinder so that if it gets up to the max heat setting it will open and keep the solar flow to the upper cylinder.  Otherwise it might overheat.

Then you will need to decide how to get rid of excess solar heated water in the top cylinder... Are you planning for a heat dump?  (I have implemented a 2 DHW cylinder system with 3 stats plus heat dump.  It works fine.  During the warmer months I can get 400 litres at 75C+ out of my 30 x 58mm tubes and it heat dumps several times even when we are at home and using water. )

[rob26440]

Forgot to say that using my method you will also need a sensor at the bottom of the lower cylinder so the temperature difference controller can reference it for turning on the pump when the flow is directed to the lower cylinder.  I used a small relay in the stat circuit to switch between the "S2" sensors at the bottom of each cylinder.  The relay is de-energised until the solar flow is directed to the lower cylinder.
 
I should add that the second cylinder was an upgrade to the existing single cylinder system and by using a relay I didn't have to purchase a more featured controller.  This way I could stick with the Navitron TDC3.   Also, I only used one 3 port valve.
 

[dhaslam]

I assumed that the  return flow to the panel would be diverted to the bottom  coil once the flow reached a  set temperature.   This way the normal difference controller would do the job.   Because the lower cylinders are connected to the radiator circuit it just means  turning on the  pump to the radiators  if the bottom cylinders are too hot.   In both cases a simple adjustable thermostat is needed.   

[rob26440]

Agreed - the normal (TDC3) should be OK.  In my case I have to get the flow back to the first cylinder because that's where the coil is for the radiators/heat dump - which is triggered by S3 max and the controller's R2 relay.

[evan]

Thanks for the comments chaps.

I was thinking about 35 tubes initially - that should be plenty for the hot water.  This is in Fife, and we only get "too much" sunshine for a week or so at best

I was assuming the CH circuit for the heat dump, yes. 


Now the only thing putting me off just going and doing it is the mention of this MCS (or whatever) handout for solar heating.  Which I assume there's no way you can sensibly and affordably DIY it and qualify for that.  I guess that's discussed elsewhere on the forum so will have another read up.

Cheers.