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Author Topic: Small store or Loading unit...  (Read 3941 times)
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« on: May 08, 2012, 11:19:41 PM »

I have been procrastinating for a while on this one and can’t decide on a small store or a Loading Unit…
The WBS we are looking at has a 8kW to water output, which will be enough to heat the 300ltr Triple Coil DHWC and provide a bit of heat to the house and workshop. Occasional usage during the week if I have the energy when I get home from work, but light on a Friday night and keep it going until Monday morning, Autumn through to Spring.

The Store option is shown below.
Hopefully fairly self explanatory. Gravity circuit up to the small store and the heat is pumped away. In the event of a power failure, the store absorbs the heat until such time as the stove damper has calmed it down. The gravity return could be twin tapped off the store, normally running off the higher tapping and a zone valve closing the lower, which power fails open, increasing the available store volume.
Advantages:- Easy to understand and should we wish to put in some underfloor heating later on it’s fairly future proof.
Disadvantage:- I’ve got to shoe-horn the cylinder into a small cupboard and we loose the cupboard space. There is also the question of return temperatures and back end rot, but that can be got around.

Then there is the Loading Unit option
Advantages:- We get to keep most of the cupboard and controlled return temperature to the boiler…
Disadvantages:- Not so future proof. Power fail requires a heat leak rad, which would have to go in the loft (bedrooms above WBS). Shortage of space downstairs so LU would have to go in the cupboard in the Nursery above (boxed in). Maintenance required.

Either way, the costs work out about the same, so that doesn’t help the decision either.

So, questions:-
1)   Is return temperature a problem with a gravity circuit, or is it self regulating?
2)   Is there any point double tapping the return from the store, or is this overcomplicating it?
3)   Gravity or Loading Unit?
4)   Esbe or Laddomat?
5)   If the Loading Unit, is the heat leak rad freezing a credable risk and will having 15mm connections “bridging” the 28mm risers be sufficient?

There have been many and varied threads along these lines, but I’m struggling to come up with a definitive answer myself, so HELP!!!!

The plumbing for the DHWC, rads, etc is almost done, I just need some way of heating it now…


« Last Edit: May 08, 2012, 11:21:48 PM by Stochengberge » Logged

On the North Downs of Kent with 3.2kWp facing 12' west of south @ 33', 36 x 58mm Thermal tubes on an east / west split, 300ltr triple coil DHWC and an 8kW to water WBS.
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Backache stuff!!

« Reply #1 on: May 09, 2012, 09:49:54 AM »

Hallo SB, I ummed and ahhed exactly the same issues as you, and ended up opting for the small store, 190 litre fed via a gravity circuit, i also fitted a pump to boost the flow should that be needed but in three years now it has not. My main reasons were as you say to keep it simple and fail-safe in the event of a power cut, and in our situation fitting a loading valve would have required the lifting of a lot of flooring and modifying a lot of the existing heating system.
My system uses either the store or the Gasser to supply heat to the heating system under the control of a changeover stat on the store, it is really simple, automatic and so far works really well. During the heating season I burn the stove every night for at least 5-6 hours, so the store never drops below 45-50 degrees C, this seems to reduce the back end rot problem for me. If you like I can post you plumbing and wiring diagrammes.


still a crazy old duffer!
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« Reply #2 on: May 09, 2012, 10:19:59 AM »

I have used four different systems, boiler heating cylinder through coil,  boiler directly heating  store by gravity with and without thermostatic valve  on return and  pumped circuit with  return valve.     The big difference between the gravity feed  with and without valve  is that the former  allows the top of the tank to heat to a good temperature first.     I don't think the pump adds much  but what it does do is  keep the temperature  very constant, within one degree,  but part of that is the more advanced mixing valve  that only works with  pumped circuits.      For the gravity sytem I used a normally open  motorized valve that allowed  the return temperature to fluctuate by about ten degrees.   

If cost is  a factor then gravity feed  with a normally open valve  controlling the return temperature is quite a good and inexpensive option.   Also the bigger store that doesnt need a  heat leak radiator would be better and in any case you should leave the option for solar  panels later.   


DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
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« Reply #3 on: May 10, 2012, 10:22:48 PM »

Desperate - if you could post plumbing & wiring diagrams, that would be appreciated.

dhaslam - I think I'm reading it right in that you are suggesting the small store would be my better option? I would point out that we already have a roof covered in solar thermal & pv so there isn't at this stage any likelihood of there being a further solar connection on to the WBS store.
Would I be right in saying that the zone valve system you refer to is the one I have seen on here somewhere bridging the flow and return from the boiler that is controlled via a flue stat and a pipe stat?
Fire lit and flue gasses come up to temperature;
flue stat energises zone valve open short-circuiting the gravity loop;
return temp comes up to temp (55+?), pipe stat breaks the circuit and the zone valve closes.



On the North Downs of Kent with 3.2kWp facing 12' west of south @ 33', 36 x 58mm Thermal tubes on an east / west split, 300ltr triple coil DHWC and an 8kW to water WBS.
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« Reply #4 on: May 25, 2012, 12:21:38 AM »

loading unit, and ESBE at that,

LTC 141 55 degree Rp11/4 £255 delivered inc vat.

Drop me a line.
« Last Edit: May 25, 2012, 12:59:21 AM by Brandon » Logged

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