Yet another F&E tank overflow

[Umbongo]

System: fully pumped/open vented/14 rads/3 stories
Boiler: Gas: Potterton Kingfisher MF CFL 100

New boiler (as above) installed about 5 months ago into 25 year-old small-bore system which was flushed – but not power-flushed – at new installation. System well maintained containing inhibitor from day 1.

About 6 weeks ago F&E tank started to overflow with water coming down vent pipe. fills F&E tank about once every 24 hours (ie have to manually get water out of tank at end of every day to prevent overflow)

Lowered speed of pump – seems to make no difference

Put in Sentinel x400 about 3 weeks ago – seems to make no difference

Water coming from vent is clear and cool – comes out sometimes when system turns off and, sometimes, at start-up.

I’m mystified - any advice gratefully received

Thanks

Umbongo

 

[croydoncorgi]

Cool water coming from a vent on a system which has been running is odd - but may indicate what's happening.

If it's DEFINITELY only when system starts and stops, it's probably linked to motorised valves being closed with the pump running.

Please describe where the zone valves, pump(s) and boiler are located, with respect to each other and to the F&E tank.

 

[Umbongo]

Boiler, pump, motorised valve all in a small outhouse backing on to fireplace at side of house ie the boiler flue uses the old chimney flue. F&E tank at top of house (3 stories up).

Umbongo

 

[Umbongo]

Sorry – been a bit dim – not useful info in my last posting

Flow is boiler => pump => return => valve =>

Valve is 2-zone valve. system default is flow to hot water cylinder. Heating circuit opens when thermostat operates ie hot water always on except when cylinder thermostat closes and boiler thermostat closes down boiler (eventually). Actually system is surprisingly efficient.

Also water comes from vent only on start-up and close-down

Umbongo

 

[croydoncorgi]

If boiler runs on its own thermostat even when roomstat and cylinder stat are both satisfied, system wiring is incorrect.

I assume by '2-zone' valve you mean '3-port': one input from boiler then A-branch leading to rads, B-branch (marked on valve, usually) leading to cylinder.

If so, this is even odder, because a 3-port valve cannot prevent flow completely - one or both ports are always open.

 

[kevplumb]

think he means a C plan croyden

exept the zone valve is on the heating

 

[Umbongo]

Definitely 2-port.

I'm only speculating about operation of boiler thermostat since I don't see how the system shuts down when both room and cylinder thermos satisfied. It may be that system does shut down correctly. But for the last 25 years the system has worked satisfactorily.

 

[croydoncorgi]

Good point...
In which case, with the HW primary circuit permanently open, why is it pumping over when the pump starts / stops???

Also, this can't be right:

Flow is boiler => pump => return => valve =>

Do you mean that the valve (2 or 3-port??) (and maybe the pipe leading to the HW cylinder) are on the output side of the pump? The 'returns' to the boiler would then be from the other sides of the cylinder and rads...

I'm beginning to wonder if unusual pipe layout plus air in a dead leg or radiator means that an 'air spring' momentarily increases pressure in the vent pipe either when the valve closes with the pump running or when the pump starts with the valve shut.... How long does the overflow last? a few seconds, tens of seconds, minutes?

 

[Umbongo]

Overflow lasts about 15 seconds

Again - definitely 2-port

Ahhh - I'm confused (I'm no expert) but it seems that the return is on the output side of pump.

 

[croydoncorgi]

I'm still not getting the full picture of the plumbing. Can you do one and post it?

 

[Umbongo]

I'll try

But while I'm trying to draw the diagram is there anything which might be obvious to you or other experts possibly causing the overflow? and which I as the poor householder would miss.

Meanwhile thanks for your responses so far

 

[Umbongo]

Well I've produced a diagram using Word but I can't download it into the post. As far as I can see the flow is:

Boiler => Pump => Hot Water Cylinder => Return pipe =>Boiler
....................... => Valve => Radiators => Return pipe => Boiler

After the pump there's a 3-way junction but HW is never turned off unless cylinder thermo operates. Room thermo operates to open the valve to the radiator circuit.

 

[croydoncorgi]

So long as vent and cold feed are near the inlet side of the pump, vent first and not too far from cold feed connection, everything should be OK.....

We're missing something here.

 

[Umbongo]

Despite x400 not making a difference, could cause be sludge/deposit partially blocking near the cold feed connection or near the start of the vent pipe (or somewhere else in the system)? Would a re-dosing with x400 then flush be appropriate?

Also I have noticed that the cylinder thermo is very high (at 7 as against a "normal" 3 or 4) - could this be a contributory factor?

 

[fitz1]

is there an aerjec before pump.