Heat dump possible option - using woodburner boiler circuit?

[desperate]

Hi sleepybubble

I,ve never seen a plastic impellor on a pump before, was it really spec,ed for a heating system? It might be ok on a fishtank  

IMHO it,s bordering on   territory.

Desperate

[sleepybubble]

Hi sleepybubble

I,ve never seen a plastic impeller on a pump before, was it really spec,ed for a heating system? It might be ok on a fishtank   

IMHO it,s bordering on  territory.

Desperate

Not being funny but have you ever serviced a pump before, the particular pump in question was a DAB, but I just unjammed a Wilo Gold today and that had a plastic impeller, I actually bust the impeller  after I had unjammed the pump when reassembling... I could post a photo of it if you want 
To be honest I think all the major brands of heating circulators use plastic impellers, presumably because it is a part that can fail without damaging the motor and is therefore cheaply replaceable, bit like the plastic drive dogs between the motor and oil pump on pressure jet oil burners.

and before you respond with 'ah but they are just cheap import rubbish'.... IIRC Grundfos circulators use plastic impellers too....

[desperate]

Hi Sleepybubble

Well I,ve learned something today, I,ve just been out to my garage and checked out 3 old grundfos pumps, 2 have plastic and 1 has a metal impellor, I couldn,t tell you how old they are, but not ancient. I think in light of this information I would still place the pump on the flow pipe because the risk of an over heat is less than the risk of air entrainment into the pipework, after all a pump is cheap compared to radiators.

Desperate

[sleepybubble]

Hi Sleepybubble

Well I,ve learned something today, I,ve just been out to my garage and checked out 3 old grundfos pumps, 2 have plastic and 1 has a metal impellor, I couldn,t tell you how old they are, but not ancient. I think in light of this information I would still place the pump on the flow pipe because the risk of an over heat is less than the risk of air entrainment into the pipework, after all a pump is cheap compared to radiators.

Desperate

Lol, love the fact that you still didn't believe me, but glad I dont have to arse around with taking a photo.
I would generally agree with you, and if the pump is located at some distance from the WBS then fine but, if it is close to the WBS, ie, within about 3m pipewise I really wouldn't do it. Of course the other thing to consider is how a pump on his WBS circuit is going to affect the gravity circulation during a power cut. However having established that the WBS already has its own heat dump provision, which presumably would be before the pump is connected it kinda implies the pump is going to be in the Cylinder closet anyways.

I just wanted to discourage any follow on readers from banging a pump in on the Flow from a WBS which would most likely result in early failure.

If of course the sequencing was correct, Heat source/Vent/F&E/pump then the pump would of course be protected from steam anyhow.... when I first read the OP's thread I had presumed it was an existing WBS circuit which was being utilised and without establishing how the pipework exactly went I choose the easy option, which is put the pump on the return.

[Eco Andrew]

Interesting discussion between you two guys.  Just to clarify, in my case, I intend to have the pump close to the thermal store, in the loft, two storeys above the woodburner.  So to have the pump on the flow pipe at this position sounds OK - heat damage is unlikely? 

Also the heat dump radiator connection on the WBS boiler circuit will be before the pump.  The heat dump for the WBS boiler circuit will be switched by a Drayton Tapstat, controlled by a thermostat sited on the thermal store set to, say 70 degrees.  This will continue to operate by gravity circulation (no pump).  The pump will only be used to enable a heat dump (through the WBS boiler circuit) when the solar panels cause excess heat in the thermal store.  The controller will be set to operate this heat dump function at a higher temperature, say 85 degrees.

My builder is using copper near the WBS boiler then plastic pipe up to the thermal store.  Does this sound OK?

Regarding the pump affecting gravity flow, my understanding is that pump's impellers are generally freely-rotating when the pump is not operating so shouldn't impede the gravity flow significantly.   The thermal store is almost directly above the woodburner and I have specified a circuit largely free from horizontal lengths of pipe, so the gravity flow should be quite efficient.

You might want to refer to the original diagram I attached with my original post for clarity (although this does not include the pump position, nor the feed/expansion connections from the spare boiler tappings in the light of advice given by Desperate) .

[sleepybubble]

Andrew,

sorry about our sidetrack there, happens a lot on forums.

I've seen your diagram now, missed it the first time around, particularly like the idea of pre-heating water using a compost heap, that will take a whole other bunch of thinking about to avoid acidic corrosion. maybe a PEX coil.

Anyways on your diagram the vent is on the return from the boiler coil and the f&e is on the flow to the coil. I'd put them the other way around....(as I think Desperate already pointed out) but looking at your diagram closely it states an integral header tank on the thermal store, is that correct? In which case why not connect the WBS direct and do away with the extra header tank...

You can still position the pump on the flow if you wish too  and connect your heat dump across the flow and return as per your diagram, but something more like the image attached... admitting it doesn't have all the extra coils for the solar etc. You'd get much better gravity circulation this way.

Although conflicting arguments, might be that you'll get much cooler return water to the stove which is not so good for its efficiency.

[Eco Andrew]

Sleepybubble, the compost pre-heating is a future project, still entirely in my head.  Thanks for the PEX coil suggestion though.  Are there ready-made connections suitable for linking to copper pipe?

Oops, yes the vent and feed pipes are the wrong way round.  I had just changed the flow and return labels over when I realised that the flow should go in the top of the coil (I had drawn it in the bottom originally), but I forgot to change over the vent and feeds.

Is your diagram mainly to illustrate a direct boiler connection to the Thermal store rather than through a coil?  This would result in a better gravity flow, would it?

My reasons for planning a coil connection was that, with the position of the coil, it wouldn't need to heat the whole store before having usable hot water and to maintain the heat stratification (a direct flow would destroy this).  I'm also anxious not to take too much heat out of the fire as the main purpose of the woodburner is to heat the house.

I had originally intended to use the integral header tank for the woodburner circuit too but read some months ago on this forum (can't remember where now) that it is best practice to have separate header tanks for each system to avoid mixing of the fluids between systems.  However it would simplify things considerably, especially since we're having trouble finding a header tank that will fit in the very limited roof space above the new loft ceiling.  Any thoughts on whether a single header tank might cause problems?

I've heard another option is to use an expansion vessel instead of a header tank.  Could the solar system expansion vessel be utilised in this way, being used for both systems?  Or is it inadvisable because the solar system would be pressurised?

[Eco Andrew]

To answer my own question, my builder reckons that a woodburner boiler can only be run on a sealed system if the manufacturer specifies this for their stove.  My Stovax is not specified for a sealed system.

I'd still be interested to hear anyone's thoughts about whether the integrated header tank on my thermal store could be utilised both for the woodburner boiler plumbing and for the solar.

Another query related to the thermal store; I can't see an overflow pipe connection on the integrated header tank - is this normal for a thermal store?