Pumped CH Overflow

[LeeK]

Help,

I have a pumped CH system and ever since I turned the boiler temperature down the hot water expansion tank has started to overflow. I had a corgi engineer round who checked the system over and he could not find anything wrong with the boiler. He drained the system down and cleaned and adjusted the ball cock valve. But after 5 days the system overflowed.

I have since changed the ball cock valve for a new high-pressure version, but after 4 days, yes it overflowed. I have noticed that when the CH is on, very hot water enters the expansion tank via a vent pipe from the boiler, is this normal? Where could the extra water be coming from?

Any help would be much appreciated…

 

[oilman]

No, it's not normal. Try turning down the pump speed, or else it might be the thermostat is faulty.

 

[LeeK]

Thanks for the advice.

I checked the pump speed and it was set on the lowest speed. I have now replaced the thermostat but the system still overflows after three days...

What else could I try?

 

[oilman]

It might be a leak in the hot water tank between the coil and the tank. Is the main cold water tank higher than the header tank?

Try turning off the supply to the cold water tank or tieing the ball valve up. and dont let anybody use any taps. If hte level in the tank goes down, you probably have a leak.

 

[LeeK]

Thanks oilman,

I can understand that a leak could put extra water into heating system causing it to overflow, but I can't see how it would cause the water to appear to be pumped into the expansion tank.

It seems to overflow (when the expansion tank is full) about 20minutes after starting the heating when the radiators are warming up, after a while this stops. Could this be a blockage or slow opening radiator valve causing an increase in line pressure, with the excess being dumped into the expansion tank?

 

[masona]

I've noticed you have replaced the thermostat,was it the boiler stat or room stat?

The problems seem to started when you turn the stat down,it does sound like to me the boiler is overheating which expanding the water back to your expansion tank or as Oilman said you may have a leak in the coil.

In simple term the cold water storage tank supply water to the cylinder tank and if you have hole in the coil,it will enter back to your expansion tank depending the level of both tanks.

Another possibility the motorised valve sticking or not fully open and therefore back pressuring to the expansion tank ??? You check this by opening it to manual.

 

[LeeK]

Thanks masona, I'm just about to go and remove the excess water from the expansion tank now, which should last about three days. It was the boiler stat I replaced and both the main water tank and expansion tank are on the same level. I will have a look and check if the water level is the same in each. I presume this should indicate if there is a syphon effect between the two systems.

 

[masona]

I presume this should indicate if there is a syphon effect between the two systems.

Correct ! Shutdown both main water supply to tank and see if the cold storage water level going down providing no one turn the taps on.

 

[LeeK]

As access is difficult to the main cold water tank, I tied up the ball cock of the expansion tank an emptied it to the level of the lowest pipe. I left this all afternoon with no increase in water level. I then turned the hot water on (as this is not pumped) this showed a slight increase in water level, which I presume to be thermal expansion.

I have looked over the system and there is no valve fitted. There is four pipes at the boiler, which looks like two for the hot water and two for the heating system. The pump is fitted on the return of this pair of pipes. Not sure what to try now, any ideas anybody?

 

[oilman]

Let's take this step by step.
You have the header tank overflowing after a few days.
To do this water must be getting in.
I assume the tank is not getting hot. Is it?
If tank is getting hot water will be coming in through the vent pipe.
Ball cock should be closed with water 3 inches below the overflow warning pipe.
The tank is unlikely to overflow from pressure generated by the pump unless it is getting a supply of water from somewhere else.
Have you tried turning the boiler temperature up again?

I'll carry on thinking, but I don't have any idea what's going on yet.

 

[LeeK]

Hi oilman,

I currently have the ball cock tried up with the heating running, the water level has increased about half an inch in about 3 hours, it did not do this when the system was turned off all afternoon, the water temperature is now fully up to temperature. I want to see how much the level drops (if any) when the system is cold.

The expansion tank does get a little warm and I have seen hot water coming from the vent pipe sometimes, although not tonight.

 

[LeeK]

Well the water level did not drop after it had cooled down, in all it rose about one inch. It seems that the water level only rises when the system is in operation, not when off. Any ideas?

 

[LeeK]

Do you think this is the hot water tank?

 

[sifu]

I thought I'd already given this reply a while back, but I notice it isn't showing so here it is again.
Sounds to me like you have sludge in the system causing a restriction on the CH side, so the water takes the easiest route, which is up the expansion pipe. If possible try turning the pump speed to it's lowest setting, if this cures the pumping over then it is most likely sludge in the system. Do you have a fully pumped system, or is it just pumped CH? If fully pumped then you may benefit from close-coupling the system after removing any sludge.

 

[LeeK]

Thanks sifu

It is a pumped CH system only, separate HW supply from the boiler (it appears). As there are four pipes at the boiler, which I presume to be supply/return for the HW and CH, the pump (two speed) is set to the lowest setting and looks like it is on the return side of the CH.

Sludge could explain why water seems to come from the vent pipe at start-up, as the it would have settled during the off period. What is the best method of removing the sludge? would this entail removing the radiators and flushing with a hose? Or is there another way?