Draining central heating

[Adrian]

Thanks for that thought A while ago, think it must have been when I fitted the new float-valve, I deliberately bent the arm on the valve so as to reduce the water level in the tank at cut-off.

However - the darn thing just keeps filling up. Just been up there - and the water level's back up to the overflow....

Hmmmm

Baled out the tank again, and tied the ball-valve shut with a bungy strap. Be interesting to see if it can manage to refill the darn f&e tank with the ball-c*ck tied shut ! This is starting to get personal !!

The only three theories I can think of for the refilling / overflowing thing are

1) Natural expansion of the volume of water in the system. But that wouldn't explain why the tank continues to overflow... surely it'd just expand as far as it needed, maybe overflowing, and then stop. If the system then cooled (as in overnight) then the water would 'contract' and the level would drop again....

2) Primary-to-secondary 'leak' somewhere in the system. Can't be between the ground loop of the heat pump and the circulating rad system because the ground loop water is luminous green, thanks to the glycol anti-freeze, also the ground loop is pressurised and it's holding pressure

3) Some weird condition that causes the water level in the f&e tank to drop (where can the water go to ??), the float valve to open, and then, once whatever odd condition goes away, for the water that had gone 'somewhere' to reappear back in the f&e tank, causing an overflow.

Just in case it's #3, I've tied up the ballvalve... and will see what happens now....

I know nothing about thermal stores. Alongside the heatpump we have a dirty great blue insulated cylinder. The circulating water comes back from rads or dhw cylinder, through this thermal store. Would you expect it to be completely full of water ?? I ask because I can hear what sounds like water 'trickling' in the thermal store - which suggests that there's some air in the store.... Maybe this air is expanding & contracting, and causing this odd effect..... - there seems to be no provision for expelling the air from the dhw circuit - other than an auto-air-release valve on the flow side of the dhw pump.

Most puzzling.......

Adrian

[John Evans]

I'm having the same problem with our system (basic CH without thermal store). Some of the radiators made a noise similar to the one you described. I partially drained the system last week and topped up with inhibitor, refilled and bled it. I've had it running with the float valve tied up and there has been very little expansion. The float valve has now been released and I'm keeping fingers crossed. Previously it started overflowing after a few days - not a lot but a steady drip.

Just a thought, and I don't know it it has already been mentioned, but is there a possibility of air being trapped in your thermal store?

[Adrian]

Doesn't seem to be any way of geting the air out. Can't really see too much of the construction of the store as it's heavily lagged in yellow expanding foam.... but it is making this trickling sound - which suggests that it's not completely full of water....

Adrian

[Roger Mills]

Is there any way you can draw a schematic diagram of your whole system, and upload it somewhere where we can look at it?

My understanding of thermal stores is that they are normally heated directly by the boiler, and have a heat exchanger inside which indirectly heats the CH water as it passes through, with no mixing of the water - so the boiler will only fire when the store needs topping up, and the boiler to store and CH circuit will have separate, independent, pumps. Is your system like that? Does the store have its own F&E tank? If so, where is it, and is it higher than the CH's F&E tank? If so, a leak in this heat exchanger could explain your symptoms.

[Adrian]

I'll try & sketch something out later tomorrow....

I'm pretty sure that this one only has two connections - I think it's nothing cleverer than a big drum of (well insulated) water - but I'll double-check tomorrow

As I understand it (and I'll check tomorrow) the return from the rads & the DHW tank coil passes into the heat store. I believe the plan is that the circulating pumps can run (to dump the heat from the heat-store) after the heatpump itself (= boiler in a conventional system) has shut off.

Nope - I believe it's in the same circuit as the water that's passed through the rads & cylinder coil...

I'll check later tomorrow. If the weather's anything like half-decent I'll be out at the local Market tomorrow morning.... By later tomorrow I'll also know if tying up the ballvalve in the f&e tank has had any effect....

Thanks for the continuing assistance..... Adrian

[Dave Baker]

Ignoring the expansion of the various water holding components themselves, rads, pipes, tanks etc, which will take up some of the slack, the maximum allowance a system is going to need for thermal expansion of the water in it is about 2.5% or 2.5 litres per 100. A small F&E tank of approx 18" x 12" is going to have a capacity of about 3.3 litres per inch so with an overflow 3 to 4 inches above the starting level as previously mentioned that's 10 to

13.5 litres expansion capability which should be enough for a system capacity of 400 to 540 litres.

That should be plenty unless there's air in the system somewhere which will expand much more than the water and drive water into the F&E tank. So I'd start by setting the ballcock level in that as low as possible and then bleeding every part of the system thoroughly while it's cold. If any major parts of the system such as the thermal store or immersion cylinder don't have proper facility for bleeding the air out of them then the problem could easily lie there. The pipe runs will also need examining to make sure they don't have loops to trap air.

[Adrian]

That's about the same ballpark that I was working in... I think

Ballcock is already about as low as it can go.. Don't understand the significance of bleeding the system whilst it's cold..... can you explain please ?

Well - the thermal store doesn't seem to have any way of bleeding the air out. There is what looks like a draincock - but it's about 1/3 of the way up from the bottom - which is kind of an odd place to put an air bleed valve . The trickling noise of water in the store makes me wonder.... I'd have thought it should be 'full' of water, though the bosses are on the side of the cylinder rather than one on the top, but for the trickling sound to occur I'd have thought that meant there was a considerable amount of air trapped at the top of the store.... odd....

As I say - the overflow problem existed before the heatpump and the thermal store were installed - so maybe they're not part of the problem....?

By bleeding the immersion cylinder, do you mean the heating coil or the cylinder itself ? The feed from the top of the cylinder to the hot taps has no facility for bleeding air, I guess it'd just get swept away & exit via the taps ?

It's all in that new-fangled plastic stuff - the pipework I can see all looks reasonable (nice sweeping curves and all) , but who knows what's going on with any hidden pipes, under floors etc......?

Thanks for the comments.....

Adrian

[Dave Baker]

Only that if you do this you have a known starting point for the water level in the F&E tank. Once it's hot you may already have some expansion into that.

If the thermal store doesn't have an outlet at the top I don't see how it can work.

The coil and associated pipework. The main body of water in it isn't part of the CH system anyway.

[Adrian]

Ah - OK - I see Thanks....

As I say - hard to see exactly what the configuration of the thing is

- there's a couple of bosses where the pipework connects, and some careful poking about in the foam insulation with a wooden barbecue skewer (!) suggests that the top boss (which is connected to the return flow from rads & dhw) is about at the top of the side of the cylinder. The store seems to have a slightly domed top - so there's the potential for a small amount of air to get trapped in there, above the water... The lower connection (at the bottom of the store) goes back to the boiler return connection.

I guess 'how it works' is that the water in the store will tend to stratify, so's the coolest possible water finds its way back to the heatpump . The heatpump is happiest if the return water is as cool as possible, apparently. Under certain conditions the heatpump compressor turns off, and the system contines to circulate the water, dumping the heat from the store out through the radiators....

Hard to know exactly what the thing's 'thinking' - gotta computer in it, innit

Right. There's no direct provision for bleeding the dhw coil - although there is an auto-air vent on top of the circulating pump for the dhw circuit.

All very curious.....

All this chat about the system has made me (finally) send off for one of those 'wireless power monitor' thingies. Last two months whole-house electricity bill was 650 euro / 500 quid - so I'll be interested to see where that went...

..but we do have a couple of PCs on 24/7, plus all the usual electrical / electronic odds & ends, and a medium-sized kiln for melting glass (which was fairly busy at the time) - it'd still be nice to be able to watch specific items and see how much juice they're using.

Thanks for the continuing discussion. I have emailed the Manufacturers in Germany and asked if they have another local expert (as the guy who fitted it isn't responding to phone, text or email) - maybe we can get 'a man' to come see it...

Adrian

[Roger Mills]

Yes, thinking about it, ISTR that a CH system typically holds about 100 litres, which expands by 2 or 3% from cold to hot. So, taking the worst case of 3%, and adding 200 litres for your store, the expansion should be no more than 9 litres - which is well within the capability of your F&E tank if it starts low enough - so we can probably discount that.

It got to be worth investigating! If your plumbing skills are ok, it shouldn't be too difficult to re-pipe the overflows such that the one from the F&E tank goes downwards on its own for a bit before combining with the one from the cold tank. That way, anything overflowing from the cold tank shouldn't end up in the F&E tank.

[Adrian]

Also if you add in the fact that the problem predates the introduction of the heatstore..... - makes it even less likely that this is the reason...

Yes - certainly do-able. In fact, if I could add an elbow, then I could probably arrange that the overflow pipe from the cold tank ran 'below' the overflow tapping from the f&e tank. Turn the existing 'tee' through 90 degrees, so it points upwards, and then use an elbow to turn this back into the f&e tank overflow....

I'll have to take a look at the levels in the cold tank - a bit of poly foam taped to the bottom of the float might drop the level a couple of inches - just to prove the principle....

I'll have a look tomorrow & report back

Adrian

[Adrian]

Hi All - An UPDATE - 'cos I know you're all desperate to know

Regular readers will recall the saga of the heat-pump / heatstore ch / dhw system that suffers from an overflowing f&e tank...

Some digging around on the 'net found me another Irish specialist who supplies the same make of heatpump.

He kindly gave me a few minutes 'consultancy' over the phone - and commented that I should not be able to hear trickling water from the heat-store - and that this was indicative of there being air trapped at the top of the store....

Apparently there should be an air bleed valve at the top of the store

- but my plumber seems not to have bothered to fit one....

However, it was possible, by loosening the plumbed connection to the store and leaning carefully on the plastic pipework, to encourage the air out. Took ages - I'm guessing that the tank was at least 1/3-full of air. Store is now running silently - for the first time ever...

(Side-benefit of this - I'll be able to spend longer in the glass workshop without comfort breaks ! Have you ever tried to concentrate on something with the sound of running water close by ??! )

Anyway - my theory, and fervent hope, is that the air that was trapped in the top of the store was expanding / contracting (much more than water) and giving rise to the fluctuations in water level and overflowing of the f&e tank....

Time will tell whether this is true ! Think I'll add an auto-air-vent to the heat-store feed pipe when I do the draining-down exercise....

Ain't life fun ?

Adrian