Open vented thermal store pumping over

[Johntheo5]

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 oerercome 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.

You certainly can buy a pump like mine (6M) with variable settings which should enable a 4M version to run with a head of < 0.5M, its a Wilo Yonos Pico.

If you disable the pump by increasing the stat setting to say 90C does the dump rad circulate very hot water and does any (very hot water) reach the thermal store??

 

[baldchris]

So, the conversation on this thread yesterday has been really useful, as it made me observe the behaviour of the system more closely than usual.
I think we are getting closer to the crux of the issue - so thanks for the contributions.
So, behaviour is very much, only using the wood burner causes the cycling of water out of the store vent into the expansion tank - this never occurs with the gas boiler.
@mcmoby69 I do wonder if you are on to something with your normal expansion comments (which should be greater when the stove is running supplying uncontrolled heat, rather than from the boiler).
@oldbutnotdead Similarly - I knew the headroom would be close, so I've measured it - and it is around the 400mm mark! Also - I hadn't looked at this for a long while, as I need a ladder to get up there - but there is clearly some leakage occurring, likely over the top of the expansion tank. I have added photos to illustrate. It looks like it has been occurring for a while given what looks like white mould at the base of the F&E tank. I guess the question is - how can I test this theory that the level in the expansion tank is too high and by lowering it, the cycling over may stop?
@Johntheo5 Magic - thank you for the pump recommendation - that will be really useful, I suspect. The fire isn't currently burning (I'll start it shortly) - so I'll test the behaviour with the stat and the dump and get back to you.

 

[cross thread]

Have you 2 expansion pipes

 

[mcmoby69]

QUOTE.

I can't raise the height of the F&E tank, unfortunately, it is already right up to roof level.

Raising the tank wouldn't make a difference, if anything it would make it worse. IMO. As you would be increasing the volume of water further.

I don't think the expansion tank is big enough for the volume of water tbh. But I'm not the man there.
Someone cleverer than me will have a formula to calculate it. I'm more of a hands on fag packet drawing man.

I've noticed that the balofix valve is almost fully shut. Why is that?

 

[oldbutnotdead]

Expansion tank volume should be about 15% of system volume. Assume 100 l for rads, boiler, UFH etc plus 300l for the store that gives you a minimum of 60 litres which that tank looks not a mile away from.
Couldn't see white mould on the tank pics.
Might be worth dropping the level in the tank a bit (long as level stays above top of highest primary appliance it'll be fine).
Is there any evidence of that tank overflowing (water trails from the overflow pipe- if it was accessible I'd suggest hanging a bucket under it but good odds it splashes on the roof which should leave evidence)
Are there any restrictions or valve gear in the pipework between woodburner and thermal store?
What diameter are the pipes between woodburner and store?
Interesting detail on that pic of the store- the expansion pipe comes out of the side rather than the top. Better for thermal efficiency but could allow an air bubble to form at the top of the store....
Got to say I'd be uncomfortable with the only expansion pipe for the system being on the store, especially with that long horizontal run from the woodburner. Do you get any kettling sounds from the woodburner?
To soothe my paranoia my setup has an expansion pipe direct from the woodburner up to the f & e as well as one from the store. And even when I have run the thing a bit hot (top of the store got to about 90°) no sign or sound of pumpover...

 

[Johntheo5]

A 249L cylinder will expand by 3.13% or 7.8L in heating from 15C to 85C or 4.24%, 10.6L if heated to 100C. Maybe allow another 8L for the secondary circuits = total of 16L expansion.

(From water at 15C)
Water
Temp	Exp
Deg.C	%
50	1.124
60	1.62
65	1.83
70	2.18
75	2.48
80	2.8
85	3.13
90	3.49
95	3.85
100	4.24
105	4.64
110	5.05

The normal tee in for the vent and cold feed looks like this, with the vent and feed no more than 150mm apart.

 

[Johntheo5]

To soothe my paranoia my setup has an expansion pipe direct from the woodburner up to the f & e as well as one from the store. And even when I have run the thing a bit hot (top of the store got to about 90°) no sign or sound of pumpover...

Have you a pump on the woodburner and if so is it on the flow or return?.

 

[oldbutnotdead]

Have you a pump on the woodburner and if so is it on the flow or return?.

It's part of the loading valve and is on the return

 

[baldchris]

Thanks for the comments - I'm away at the moment but back later today, so I'll try to answer all the questions then - I don't want you to think I'm just ignoring them.

 

[baldchris]

Thanks again for the feedback - it is genuinely appreciated and apologies for the delay responding. It also takes me a while to work out the answers to some of the questions!

So... we're starting to push the limits of my knowledge and understanding of the system... but this is what I believe to be true and I've tried to take photos to back that up (or be corrected!)

@cross thread Yes - we have two expansion pipes. One from the thermal store and one from the woodburner feed, just before the pump.

@mcmoby69 You don't know how pleased I am to hear that we don't need to raise the height of the f&e tank
The copper tank was sized at the time - but for the life of me I can't find the spec - from memory, 60 litres sounds about right.
Good spot with the ballofix - no idea why it was nearly closed, but I've opened it now.

@oldbutnotdead It was only a small amount of mould around the base of the f&e tank, on the platform it sits on. From observing today, I suspect it is caused by condensation on the lid of the f&e tank running down the outside of the tank. I've added another photo, but hope this is relatively trivial - solve the 'pumping over' and this goes away.
You put "Might be worth dropping the level in the tank a bit (long as level stays above top of highest primary appliance it'll be fine).". I assume you mean f&e tank - but I'm not sure how to do this to any great extent without changing the ballcock and arm. Is that what you meant, or is there another way to temporarily test the theory?
There's no evidence of the tank overflowing - but it's not easy to spot - it's an internal overflow pipe from f&e tank (see photo - I can't remember the proper term for it!).
Between the woodburner and store, there is one valve - normally closed that I understand opens with the stat on the woodburner (I could be wrong here) (I guess the normally open to the dump also closes as well), and the valve closes when the overheat stat on the store is tripped.
We do have expansion pipe from the woodburner - coming off just before the valve and pump on the feed to the store, so no kettling sounds, which is good.

I've also found the original schematic - that might prove useful. I think we installed true to the schematic.

Thanks.

 

[Johntheo5]

That original schematic doesn't show a cold feed going anywhere from the feed/expansion tank, wonder why not?.

 

[baldchris]

Other than oversight, I don't know - there is definitely a feed out from the f&e tank to the store, as you can see in the pictures.

 

[cross thread]

I think it’s due to 2 expansion pipes ,is it sucking down the solid fuel expansion ?
when you turn the soild fuel pump on.

 

[baldchris]

Neither of the expansion pipes are below the water level in the expansion tank - at the moment (photo attached). The fire hasn't been lit for a couple of days with being away. So, I'll light the fire, and monitor:
1. That the cycling occurs as soon as the woodburner pump kicks in (it definitely stops cycling when the pump stops - I'm just not 100% about the starting point, e.g. it may be when the pump kicks in and the water is hot enough).
2. The height of the water level in the expansion tank when the cycling occurs - and whether it rises above the height of the solid fuel expansion pipe.

Anything else I should check?

 

[baldchris]

@cross thread You said: "...is it sucking down the solid fuel expansion ?". If the water level in the expansion tank rises above the height of the solid fuel expansion pipe - how do I tell if it is sucking? If I put my hand under it, will I be able to tell, or if I hold a piece a string, or something lightweight, will it suck up the pipe?