Open vented thermal store pumping over

[baldchris]

I'm looking for help with an open vented central heating system that is pumping over. We have a thermal store that has two heat sources - gas central heating, and a wood burner - both feeds are pumped with a Wilo Smart A 25/4-130 circulation (one pump per feed). When the wood burner kicks in (when water is 75 degrees), the water pumped into the thermal store is pumped over into the feed and expansion tank. We don't get the problem with the gas central heating feed.
The wood burner pump is set to the lowest control mode - the gas to the highest. I can't raise the F&E tank higher, nor the vent into the F&E tank.
The only option, I think, is to install a 'weaker' pump for the wood burner. The only reason this has a pump at all, is because there is quite a long, near horizontal feed into the thermal store, so there was a fear that gravity alone would be insufficient. So, my thoughts are that if I more gently pump the water into the store, then it is less likely to pump over (or this could expose my poor grasp of physics!).
So the questions are - do I have any other options - and if I change the pump, are there any recommendations? As you can tell from my explanation, I am not qualified in this area, but I have a vague grasp of the issue!
Thanks for any advice!

 

[oldbutnotdead]

Where are the feed & expansion pipes relative to the store, woodburner and gas boiler? (A schematic would be useful)
As a slight aside, have you thought about a loading valve for the woodburner (rather than your current presumed pipe stat on the woodburner pump)? Is the pump in the feed from woodburner or the return?

 

[baldchris]

Thanks for the reply - I'll try and draw a quick schematic. I haven't previously considered a loading valve - I'll have to check out that option. You're right though, we do have a pipe stat on the woodburner pump. The pump is also in the feed - not the return (also something I hadn't considered!

 

[Johntheo5]

How smart is the Wilo pump, smart A rated pumps generally have 3 modes of operation, constant Curve, constant pressure & proportional pressure, CC,CP&PP?

 

[baldchris]

@oldbutnotdead Here's the schematic - if you let me know what else needs to be added, I can do that. The central heating is missing - I don't know if that is important. But basically, there are two manifolds that are fed from the thermal store. One manifold is next to the thermal store and feeds radiators, the other is on the ground floor and feeds UFH to both the ground floor and cellar.

@Johntheo5 I'm struggling to answer your question. It's an older model, purchased in 2013. This, I believe is the link to the discontinued product on the Wilo website: https://wilo.com/gb/en/Support/Sele...-guide/en/wilo/wilo-wilo-smart/smart-25-4-130
It looks like this: https://www.tradepumps.com/smart-25-4-130
I've also checked the manual, and I'm not sure which of the three modes you list is used by this pump - https://www.manualslib.com/manual/1670989/Wilo-Wilo-Smart-25-4.html?page=15#manual

Thanks both for your help!

 

[Johntheo5]

If its this one ans set to the "smallest house" then probably only pumping at a 1M head so can't see any reason for pump over as there should be very little resistance in the wood burner heat exchanger, have you checked that the pipestat is actuallu starting the pump at 75C and that the temperature isn't rising to say 90 to 95c and causing venting??, may need a higher setting depending on the size of the fire.
Check also with a cold stove if you are getting pump over.

 

[mcmoby69]

Is this a new installation or one you have added to? Ie the thermal store or the UFH. Zones.

 

[oldbutnotdead]

Mmm. You have to be careful putting things in the pipework from an uncontrolled heat source. Who designed/installed the system? - there should be a heatloss rad on the primary circuit from the woodburner. What volume is the store?
The pump in the woodburner circuit- is it the right way round (as in is it pumping into the store)? (Should be an arrow cast into the casing, don't be fooled by the orientation of the control head, that can be changed)

 

[baldchris]

@Johntheo5 As far as I can tell, the stat is opening the valve and starting the pump at 75 degrees - the thermal cut out in the store is 85 degrees (we never get that hot at the top of the store to see if that needs adjusting higher, due to the pumping over). This evening I have tried the 'medium house' and the 'large house' settings and pumping over continues, but with increasing vigour. The gas boiler was not being called for heat and so the boiler pump was not running (so it's not an issue with the combination of two pumps running). This issue only occurs with the woodburner - which baffles me.

@mcmoby69 This is a new system - it has always pumped over, but this year we are using the woodburner a lot more, it is more noticeable and frustrating (it stops the thermal store from really heating up, due to the heat loss caused by pumping over).


@oldbutnotdead You're right, there is a dump radiator - I've added that to the diagram and re-attached. It was designed by a company called 'Tailored Heat' - who seemed competent at the time and we've not had any issue other than this. The store itself is a Gledhill Greenheat OV and has a capacity of approx 249 litres. And yes, the pump is the right way around on the woodburner circuit (I've double-checked - just in case - paranoia is creeping in!).

Thanks

 

[MeldrewsMate]

I can see a scenario whereby air in the top floor dump radiator will expand as the woodburner pump starts-up. This would give rise to pumping over in your configuration. Will it be easy to move the dump rad's flow pipe to be between the pump output and the heatstore inlet?
If not try bleeding this top floor rad with the pump OFF, then see if pump-over occurs.

 

[oldbutnotdead]

Is that 'always open' valve (for the heatdump) actually always open or is it motorised fail open?
I'm assuming there's a return from that rad as well...
Best get the installers back, a properly designed system shouldn't pump over.
While they are there, talk to them about a loading valve. It's not so important if you're running the woodburner full time, where it really helps is preventing condensation in the woodburner when it's warming up.
The types of loading valve with built-in pump might be useful- the pump is on the return and near the woodburner. I have a similar setup to yours, zero pump over from gas boiler or from woodburner .

 

[baldchris]

Yep - there is a return from the dump rad - I've put this on the diagram as well (I think this is correctly drawn). I've just checked the dump and bled it - there was no air trapped. There's probably a 3m near horizontal run to the dump radiator (flow out from woodburner is on one side of the chimney, dump is on the other side, one floor up from the woodburner).
The gas boiler has also kicked in tonight, and I checked, we still get pumping over from woodburner, regardless of whether gas boiler contributing or not.
I'll certainly get in touch with the installers, but I'm not expecting a great outcome from that conversation. I like the look of the loading valve - and on the return. I take it that no-one thinks a reduced 'power' circulation pump is a good way forward (the logic being that I'm just giving gravity a small nudge to overcome a longer than desirable horizontal run - and that that would not be enough to cause the pump over)? I thought if I could buy one with variable settings, I could manually reduce the 'power' until the issue stopped. As @Johntheo5 mentioned, there are several modes of operation for a circulation pump - so which I should be looking out for, and adjusting, I have no idea.

 

[mcmoby69]

Is it pumping over or is it simply a case of not enough expansion available due to the volume of water involved.?
Is the tank big enough? Is it plastic or metal, is the correct ballvalve fitted, is the water level too high in the tank?.
Because the wood burner heats the water hotter than the boiler suggests it to me that the water is expanding and increases the volume ofcwatwr more.

 

[baldchris]

@mcmoby69 Thanks for your reply - as additional info, the recommended temperature of the store, at the control stat is 70-75 degrees. The affect of the 'pumping over' is that it appears to cool the top of the store, so that the store overheat stat at the top of the store will often record a temperature between 50 - 60 degrees (even though the max temp is actually set at 85 - we never get close to that temperature, even with the woodburner running all day). So, the boiler sends water to the store at around the 70 degree mark, the woodburner kicks in at 75 degrees - not a great deal of difference between the two.
So - in answer to the questions - I don't know whether there is enough expansion available in the store - I assume so and it doesn't appear to be something I can control. The flow of water from the vent to the F&E tank (the 'pumping over') is continuous for a large portion of the time that the pump runs (I don't know if it is a perfect match). The tank seems to be sized according to the amount of hot water required based upon no. of rads and bathrooms. It is appropriate for that and I have added a couple of images that give a summary view of how it and my solution works (taken from their manual). I don't know how to tell if the correct ball valve is fitted, beyond it being made of brass to handle high temperatures. The water level is low in the f&e tank - but above the feed level, obviously. Is there something I can do here to check?
I can't raise the height of the F&E tank, unfortunately, it is already right up to roof level.
Cheers for you help!

 

[oldbutnotdead]

The long horizontal from woodburner to store isn't ideal but the big vertical before it will establish decent syphonic flow. Just to check- you aren't getting any output from the expansion pipe while woodburner is burning/woodburner pump not running, as soon as the woodburner pump kicks in you get more or less continuous pumpover.
Have you got the recommended 500mm headroom on the expansion pipe? (ie the top of the bend should be 500mm above f & e water level)
The f & e tank for the primary water should be sized for the volume of radiators, pipework and the store which will make it considerably bigger than an f & e tank for a standard vented system. Number of bathrooms is not relevant to the primary circuit, it is to cold water storage (if you have any).