Problem with open vented system: boiler keeps cutting out when WATER ONLY

[Jugal]

Hi All,

I hope someone can help me in diagnosing what's wrong with my system.

I have an open vented system - boiler, hot tank, cold tank, expansion tank, pump, rads. Earlier this year, I got a power flush done, it needed it. I've had problems with Water heating subsequently.

Just changed hot water tank (old one was circa 20 years old) and drained and re-filled the system. After this,heating (radiators) works fine. Whenever I put ONLY WATER on, problems. Boiler and pump fire up and then gurgling/bubbling noises follow. In a couple of mins, water spurts out into the expansion tank and boiler cuts out. This doesn't happen when I put Heating+Water on, but then water does not get heated either.

Is this a blockage somewhere? Where should I be looking? Appreciate your advice.

With powerflush and subsequent 'try to fix it', I've spent over £1700 on this. It's getting expensive and annoying.

Thanks, Jugal.

 

[Neil K]

Did plumber who did job put a new valve on the cold feed from the expansion tank that fills the heating. If they did, you'll find it either in the airing cupboard or the loft, make sure it's open.

 

[Jugal]

Did plumber who did job put a new valve on the cold feed from the expansion tank that fills the heating. If they did, you'll find it either in the airing cupboard or the loft, make sure it's open.

Thanks Neil K. Yes the cold feed valve is open and has a ballcock attached. I saw this being suggested on another thread. Slight variation in the level and sure enough water seems to fill in OK. I will check again shortly. I don't understand why this would arise when it's a ONLY WATER ON situation. Why does the problem not happen when we have HEATING ON? Does WATER ONLY draw down a lot of water from the Expansion tank?

 

[Neil K]

Thanks Neil K. Yes the cold feed valve is open and has a ballcock attached. I saw this being suggested on another thread. Slight variation in the level and sure enough water seems to fill in OK. I will check again shortly. I don't understand why this would arise when it's a ONLY WATER ON situation. Why does the problem not happen when we have HEATING ON? Does WATER ONLY draw down a lot of water from the Expansion tank?

Your reply suggests that you are talking about the cold feed TO the tank, ie. what fills it which is cold water mains. I'm talking about the cold feed FROM the tank that fills the heating system, check that for a valve

 

[PhoenixServices]

Sounds to me like the cold feed tee is blocked/ restricted so pumping over on hot water only.

Try a magnet on it. If your not sure which pipe I mean it's the outlet pipe from the heating header tank, trace it down till you find the tee.

 

[JCplumb]

Blockage or closed valve somewhere after the zone valve, between the valve and the coil or the return from coil.
When on heating and hot water the flow goes through the rad circuit, but when on hot water only the zone valve closes the rad circuit and the water isn't getting from boiler to coil and back to boiler so it over heats, goes up expansion and boiler trips.
Possible that the power flush only cleared the radiator circuit and left a blockage in the hot water only part of the run, but as it is a new problem it's more likely to be a valve as stated above.

 

[Neil K]

As above. Check all valves are open then follow advice as above

 

[Pipes Plumbing]

Airlock somewhere, probably the coil... if all valves are open, system has filled and the water level in the f&e changes as the system heats / cools I would say the cold feed is clear.

If gurgling starts as soon as the pump spins up on hw only, but it takes a few minutes for water to leave the vent it sounds like you have no circulation due to air and the boiler is overheating and blowing out the vent.

Have you tried venting the coil on the cylinder?

 

[Jugal]

Your reply suggests that you are talking about the cold feed TO the tank, ie. what fills it which is cold water mains. I'm talking about the cold feed FROM the tank that fills the heating system, check that for a valve

Hi NeilK,

Thanks for the dumbed down explanation. I've just done this and it's now purring like a cat!! Definitely was the cold water feed tap mostly closed.

Thank you so much for the diagnosis and advice.

Regards, Jugal.

 

[Jugal]

Hi NeilK,

Thanks for the dumbed down explanation. I've just done this and it's now purring like a cat!! Definitely was the cold water feed tap mostly closed.

Thank you so much for the diagnosis and advice.

Regards, Jugal.

Damn, damn!

worked great for 2 x water only runs. Then tried
a combo water and heating this AM, boiler
keeps cutting out. Latterly tried water only, repeat of
splash back into expansion tanks and boiler cuts out.

i will follow the other suggestions made earlier and try these one by one and report back. Watch this space.

 

[Whyme]

when it was power flushed did they remove the pump and do it from there ?? if so did they put the pump back the right way round (sucking from vent)..?

 

[Neil K]

when it was power flushed did they remove the pump and do it from there ?? if so did they put the pump back the right way round (sucking from vent)..?

Good thinking Batman

 

[Jugal]

Good thinking Batman

Power flush kit was attached to lowest radiator, not pump. After flush was done, true to form, remnant debris screwed the pump up, had to replace the pump. Now you've got me thinking..

If pump was fitted wrong way, would heating work? Would hot water have worked initially (after replacing hot water tank)??

 

[Jugal]

Power flush kit was attached to lowest radiator, not pump. After flush was done, true to form, remnant debris screwed the pump up, had to replace the pump. Now you've got me thinking..

If pump was fitted wrong way, would heating work? Would hot water have worked initially (after replacing hot water tank)??

Update at 11am on 26/08:

Pump fitted correctly. Arrow pointing towards the motorised valves for water and heating.

Problem has evolved. Now when switching on any of: WATER only, or HEATING only, or both, boiler fires up for 30 secs or so and then stops. No feedback into expansion tank. Pump continues. After a few mins, boiler starts again for 30 secs then stops.

Suggestions around tracing the cold feed from the expansion tank to the tee and doing the magnet test will require prising up floorboards. Before I do this, is there anything else I could do/test? Note, the boiler when firing up does also make an unusual 'struggling' sound.