heating expansion tank float valve overflowing again

gamston

hi folks, a couple months ago the central heating expansion tank started overflowing, so I replaced the float valve with a fluidmaster float valve (https://www.toolstation.com/fluidmaster-12-brass-shank-valve/p92220) worked ok overflow stop, after about 3 weeks it started to overflow again, when I checked the expansion tank it was warm (returning hot water from the central heating expansion pipe) I thought maybe the water gets too hot for the plastic fluidmaster float to work correctly, so I got a new brass float valve pt2 ( https://www.toolstation.com/float-valve-part-2/p55538 ) that worked ok for a few weeks then yesterday the overflow started leaking again, the water in the expansion tank was warm (returning hot water from CH expansion pipe) I turned off the water supply to the tank and drained down the water level, turned on the water tank filled and the float valve shut off the water, this morning the overflow is leaking again, 
the CH/hot water has been off for 8 hrs, 
the water is coming out fast not dripping
this is the style of tank i'm using  https://www.screwfix.com/p/polytank-cold-water-tank-4gallon-uk-480-x-250-x-320mm/10011
yesterday I turned the CH pump down to speed 1 ( was on speed 3, its a big system 16 rads most TRV shut down when heat wave started)
anyone got any advise please

gamston

update with the water turned off to the expansion tank the overflow has stopped leaking, the boiler came on as normal and now the overflow is running again ? I've turned the boiler off and its down to a drip from the overflow,
the CH has motorised valves fitted to both the central heating & to the hot water cylinder.

update Expansion tank 1/2 full, filler pipe stopcock turned off not water now going into tank from the feed side, hot water only now ON on the boiler/pump, hopefully to prove if extra water is coming in from the hot water side,  if ok I will try again with CH & hot water ON, (can't just run CH with my system)

Mutton_Geoff

The water is being pumped up from your CH system, what speed is your pump set to? Is the water from your hot taps dirty? (Leak in HW heating coil). Do you have a bypass pipe between the main flow and return circuit on the heating? Are there thermostatic valves on all your rads or is one left open, eg bathroom rad. If so, has that been closed off leaving no radiator circuit for the CH water to flow through.

If your CH and HW have their own motorised valves, that sounds like an S Plan system, then why can't you have CH on it's own? Is it just a function of the programmer that forces this option? If so, you could investigate changing that (although that won't be the cause of the overflow issue).

The header tank should never contain hot water or be overflowing but a little expansion may cause it to overflow if the level has been set too high (ie just below the overflow pipe) when the tank doesn't need to hold that much water.

gamston

pump speed is 1  (did that yesterday)
hot water in taps is clean 
no bypass fitted
got a heat sink rad, working ok
I will check again regards CH working on it own, 
I did have the pump speed at 3, (as I could not get all rads hot enough for me with pump speed 1 or 2,) but I should have turned it down as I have turned TRV's settings down to 1 or less 
having looked again at the expansion tank it looks like the float arm "could" hit the CH system expansion pipe/return
the concern is why is the expansion tank full of warm water yesterday and other times in past (think that may be because pimp speed too high when rads TRV's turned to low setting

Mutton_Geoff

gamston said:

pump speed is 1  (did that yesterday)
hot water in taps is clean 
no bypass fitted
got a heat sink rad, working ok
I will check again regards CH working on it own, 
I did have the pump speed at 3, (as I could not get all rads hot enough for me with pump speed 1 or 2,) but I should have turned it down as I have turned TRV's settings down to 1 or less 
having looked again at the expansion tank it looks like the float arm "could" hit the CH system expansion pipe/return
the concern is why is the expansion tank full of warm water yesterday and other times in past (think that may be because pimp speed too high when rads TRV's turned to low setting

If all the radiators are not heating up properly, you need to look at balancing the system (to achieve a c 12C drop across each radiator and the boiler in/out flow) to maximise system efficiency.

gamston

all rads off (other than heat sink one) 
with just the hot water ON from the boiler everything is fine no water returning to expansion tank via the expansion pipe
with heating & hot water ON from boiler (NO rads on other than heatsink) hot water is returning to the expansion tank from the expansion pipe, so water in the expansion tank/header tank is now hot, ??
checked pipework temp at boiler 60c, boiler cutout temp so that is working , but getting hot water flowing back to expansion tank

heatsink rad heats up when I have just hot water ON, and when CH is ON, the heatsink pipework flows into pipe that connects to expansion pipe, cannot understand why hot water does not come out of expansion pipe when "hot water" is ON, but does come out when hot water & CH are ON, the only extra bits are rads (that are turned off) and some pipework, pump speed the same

thorganby

A common cause of this, is a blockage where the vent and cold feed pipes from the expansion tank join the main pipework, usually close together commonly in the airing cupboard below.

The blockage causes hot water to go up the vent into the F&E tank and overflow.

The vent pipe should be teed into the system where the cold feed joins the system.

Try using a magnet around this area to find out if it sticks where the pipe is blocked.

Hot water in a plastic F&E tank can be dangerous and needs fixing ASAP.

The simplest solution is cut the blocked portion of pipework out and replace it.

If you can't then get in a plumber who can and recharge the system with inhibitor afterwards.

gamston

thanks, I'll take a look at the pipework late, at present I'll turn off the CH side of the programmer
looks like there are 2 problems,  1 the overflowing,  2 the hot water feeding back into the F&E tank
I think I've sorted the overflow problem the new ball !!!!!! I put on 3 weeks ago had a slight hole in it and was filled with water, new ballcock fitted this morning looks ok
so its now problem 2 that needs sorting

gamston

been thinking about this problem, on my heating system side (Central Heating) I have 2 circuits (up stairs & Dn Stairs)both controlled by MOS's on the dn stairs circuit I don't have a heat sink rad just TRV's which are off/lowest setting, I wondering if that is the problem the water in the dn stairs circuit has no where to go except up the expansion pipe ?

royalmike

Normal best practice is to have 10% of the system not controlled by trv radiator valves normally the ones close to room thermostats, so remove a couple trv heads one upstairs one downstairs.
You do require a by-pass some boilers do incorporate a built in one. 
But as mentioned by Mutton_Geoff this problem is often caused by sludge in around the vent cold feed location

gamston

I've drained down the header tank a few times in the last few months (had to change the float valve & moved tank out/in to insulate the roof) drain it from downstairs radiator, water drop quickly tried the magnet on the Feed pipe that was ok, so think the feed in is OK,
not tested system yet with one TRV opened on each heating circuit, maybe able to do it today, if that solves the problem I with have to put a standard valve on each circuit