Open vented thermal store pumping over

[Johntheo5]

I didn't start the fire yesterday and it was too cold to get the pump running - it's a high temp stat, so only runs in the range 50 - 95 degrees. I've started it today and even at 50 degrees, the cycling occurs. For now, I've set the stat to 85 degrees simply to delay/reduce the flow to the thermal store, and I'm dumping to the radiator.
I did ring the company that put the schematic together, and they essentially just followed the Gledhill schematic and recommendations - so I'll give Gledhill a ring on the 3rd Jan, and see if I can get someone in to investigate the issue. He did suggest, that it might be worth reducing the diameter of the vent pipe from store to f&e tank from 22mm to 15mm. So if I did this, and perhaps changed the pump, and perhaps re-instated the cold feed, then perhaps the cycling may be prevented?? I don't know how you guys do your job - it would drive me crazy, lol!

Dumping to the rad but no indication of flow into the T.store (pump off) ??. Would be inclined to change the pump to a smart 4M type like mine (Wilo Yonos Pico) but still doesn't explain riddle of no pump over from the boiler through the T.store vent.
Even with a flow/teturn temperature of 75/20c then the circulating head is only 0.118M or 4.66ins assuming a circulating height of 5M. (gravity circulation)

Temp/density	20	998.23
Temp/density	75	974.87
Circ Height	5	M
Circ Force	0.118	M	4.66	ins

 

[Johntheo5]

He did suggest, that it might be worth reducing the diameter of the vent pipe from store to f&e tank from 22mm to 15mm.

I wouldn't dream of reducing the vent to 15mm as that's your safety valve, if the HW coil fractured then the 15mm may not keep the store pressure from reaching dangerous levels.

 

[oldbutnotdead]

Don't reduce the vent pipe. The feeds from woodburner to store should be 28mm not 22.
Been looking at the schematics (Gledhills generic and your as installed), couple of thoughts;
1 Gledhill generic does not feature a pump between woodburner and store.
2 The motorised valves on the woodburner to store schematic- I'm hoping they are both fail-open types. If not then in a power fail scenario the limited volume of water in the burner/heat dump could boil rather quickly- not catastrophic but not ideal.
The pumping over is very intriguing- my theory (similar to @MeldrewsMate ) is that there is some sort of restriction in the return to the woodburner. But that would give low pressure in the woodburner expansion pipe and you've checked for that. Besides which, for continuous pumpover the water has to come from somewhere to sustain it.

Does the heatloss rad stay hot when the woodburner pump has been running for a while? That motorised fail open valve on the heat dump rad- is it closing when the pump starts running?

 

[baldchris]

I wouldn't dream of reducing the vent to 15mm as that's your safety valve, if the HW coil fractured then the 15mm may not keep the store pressure from reaching dangerous levels.

Ok - as this was just a speculative suggestion on his part, I hold off from this.

 

[baldchris]

@oldbutnotdead You're right about the pump - it was recommended that it was added when the plumber installed the system as there was such a long horizontal run after the initial rise from the woodburner. That is a good spot - but I would be nervous to remove the pump altogether.
The valve is normally open to the dump radiator only, as the fail safe - it is a large rad which I remember we sized appropriately at the time, but I'd have to dig around to get the figures to confirm that.
The heat dump rad does go cold when the normally open valve closes and the store pump kicks in. I can here the valve close (and open) if we play with the stat, so I'm confident that's working as intended.

I forgot to add - the feeds from the woodburner to store are indeed 28mm

 

[baldchris]

Would be inclined to change the pump to a smart 4M type like mine (Wilo Yonos Pico)

By process of elimination, I've tried to work out exactly which pump you're recommending - I've filtered here: https://www.tradepumps.com/manufact...ze=2315&inline_length=379&product_family=2964
and of the 2 pumps returned, only 1 is 4M - so is this appropriate: https://www.tradepumps.com/wilo-yonos-pico-15-1-4
In my head - I'd like a pump that I can control and through trial and error, simply dial it down until the problem stopped occurring. Excuse my ignorance, but can I do that with this pump?

 

[Johntheo5]

Yes, the 4M yonos Pico above .

 

[oldbutnotdead]

Mmm. Good news on the 28mm primaries. I'm rather appalled that the original installers consider continuous pumpover to be acceptable.
I'm also not that comfortable with that standard pump on the flow but you have got that fail- open valve on the heatloss
If you've got the budget for a pumped loading valve I'd go for that- it puts the pump on the return and will do a much more efficient job preventing condensation in the firebox than your current setup (loading valves are cute. There's a short loop between flow and return and a thermostatic valve. Temp <70- water circulates in the short loop. Temp > 70- water starts circulating to store. Modulates continually so woodburner jacket doesn't get a sudden dump of cold water)

 

[baldchris]

loading valves are cute

I like neat, simple solutions - maybe it's a sign of age. I will look into this - it's not something I can do straight away, as access is now awkward, but we are looking to move the heatloss radiator soon, to a better location (removing almost all horizontal travel), so that will be the prime time to do this for us.

Thanks to everyone who has contributed to this thread - it's really appreciated. The next thing I'm going to try is to simply replace the pump, as suggested, as that's the proverbial 'path of least resistance' (it's a quick and easy job) - as well as contact Gledhill for advice. If either of these lead to a resolution, I'll come back to this forum and let you know.

 

[baldchris]

I'm trying to purchase a variable speed pump to replace my current Wilo Smart 25/4-130 pump (here). If I was to stick with Wilo (which seem to be reliable and good value), then I seem to have a couple of choices - either the Wilo Yonos Pico 25/1-5 (around £100), or the Wilo 25/1-4 (around £240). I made a mistake and the one I highlighted yesterday to @Johntheo5 only had a 1" threaded connection, so wouldn't fit. Obviously, I'd prefer to spend £100, rather than £240, but not if it is no better than my current pump.
This also begs the question about whether a 15mm or 25mm bore matters in my case? Or, as I am interested in reducing the speed of the pump to a bare minimum, then I just need to focus on the minimum specs and ensure that the pump speed has a variable setting? If 15mm bore, with a 1 1/2" connection is also viable, then I may have more options (although I suspect not many).

Thanks for any recommendations!

 

[Johntheo5]

Go for the 25/1-5, my 25/1-6, (6M) can be regulated down to 0.5M so the 1-5 will/should be the same and you won't do any better than that IMO so give it a lash.

 

[baldchris]

Go for the 25/1-5, my 25/1-6, (6M) can be regulated down to 0.5M so the 1-5 will/should be the same and you won't do any better than that IMO so give it a lash.

Brilliant - thanks

 

[baldchris]

I just thought that I would follow up on this issue, as I received a lot of help and I always like to know the outcome. I did replace the pump, I used a was a 'Circulating Pumps CP50 High Efficiency Central Heating Pump', because it allowed me to run the pump at a low constant speed. All of the other pumps we discussed are no longer available with the manual regulation capability (I believe some regs have changed - but don't ask me which!).
This almost solved the problem - and the overflow was down to a very low trickle. I've managed to push up the overflow pipe about 1-2 inches higher into the roof space, and now we're are down to the occasional drop of water overflowing. I think that that is probably good enough - but if it does frustrate me enough, I could extend the current overflow pipe a little further towards the apex of the roof, to gain another couple of inches, which should completely resolve the issue. But for now, I'm enjoying the fact the system is (broadly) working as designed.
Thanks again for your help - much appreciated

 

[Johntheo5]

Thanks for posting back, I'm surprised that you now can't change the CP & PP settings (incrementally, in 0.1M steps), on the Wilos, however your CP50 should fit the bill even though it hasn't got a CP mode.
You say you can run the pump on a low constant speed, are you now running it at C1?, if so by chenging to PP mode you can certainly get a lower head which may stop that pump over completely?.

For example if in CC setting C1 and assume pumping 1.0m3/hr at 1.1M then changing to 1.0M PP mode will flow 0.78m3/hr but at a substantially lower head of 0.68M and maybe no pump over?.

 

[baldchris]

Thanks John - but this is exactly the issue that I found. The pump you post can't be purchased anymore - in spite of my best efforts. I know someone with the pump you posted, and I tried to buy the same. However, that is the old version CP50.1. All that is available now is the CP50.2 and that's what turned up - I've added a photo. Most websites still show CP50.1, and most technical spec sheets are also for CP50.1. I can't find the specs for CP50.2. This pattern is essentially the same for Wilo too (I took two of them back as they sold the new version, not the old as advertised).
I tried all 3 modes of operation on the CP50.2 at the lowest setting (variable differential pressure, constant differential pressure and constant speed). Only constant speed on the minimum setting more or less resolves the issue.