Central heating problem

[Fisherman]

I have a Potterton Netaheat central heating boiler which just runs the heating system.
The water tank in the loft that feeds the system is getting hot,
I now notice that hot water is running into the tank from what i beleve to be the vent pipe.
The system appears to be working ok apart from this problem,
I have checked the pump and that is working ok,
Any help on this would be appreciated.

 

[Prof.plumb]

3 things cause that problem. Either a blocked cold feed, the vent pipe doesn't go 18 inch (450,,) over the tank or the pump is set to high.
Firstly check your pump. if it is set too high turn it down.
If this seems normal open a radiator vent and if it fails to thrash water out at you, you have got yourself a blocked cold feed, or a partial blocked cold feed. Eventually your, system will stop working. When the system dries and the water is not replenished the boiler will over heat and cut off.
To clear a blocked cold feed is to a DIYer a fairly time consuming task.
You must firstly isolate the supple to the small tank (the feed and exspansion tank) and bail it out with rags and pots. THis water will be incredibly dirty usually brown.
You need to then trace where the tee from the cold feed from the tank tees into the system. You then have two choices. Either cut above and below the tee leaving enough room to get two sockets in after the work or cut through the tee drain the water away and renew the section of pipe work.
The procedure i follow is drain the whole system, bail out the tank, put a saw nick in the cold feed 6 inch above the tee and let the water thrash into a pan. it will do this for a couple of mins. When it is drained i cut below the tee and to the side. I then clean the pipework out with screwdrivers, brushes etc in a sink. once clear put the tee back in place using compression sockets.
if the vent pipe isn't 450mm you may need to extend this. But, have a look check these things out and best of luck. And don't get that brown sludge on the carpet!

 

[Softus]

...once clear put the tee back in place using compression sockets.

Is that the end of the professional procedure? Or do you refill the system, adding a chemical inhibitor, and vent all rads with the pump off, then turn on boiler and pump and test everything?

 

[Fisherman]

Prof : plumb
I checked the vent pipe dimensions they are ok
Turned the pump down to speed 1 (was on 2)
Then I wondered about what you said on the possible cold feed pipe block
The supply to the tank is only a few inches from the pump isolation valve
So I bailed out the tank to see if there was much sludge it the bottom.
I then drained down the system, took the pump off, and opened the pump isolation valve and poured water in the tank after about 4 pints nothing much had appeared; I was about to put another couple of pints in when sediment and water come through the pipe,
So it appears that I had a partial blockage in the cold water supply.
I have refilled the system bled it and run it up, after a couple of hours there is no sign of the cold water tank getting hot.
Once I am sure everything is ok I will partially drain the system and add corrosion inhibiter (Don’t see the point of wasting £20 by putting it in until I am sure the heating is working ok and won’t need draining down again)
I hope that lot makes sense to you.
Many thanks for your advice

 

[Prof.plumb]

Good thinking. Well done.

 

[croydoncorgi]

If your system had been 'pumping-over' for a while, aerated water will have been running through all the rads, causing corrosion everywhere.
It's also quite likely that the cold feed is still partially blocked - in my experience the crud that causes the initial problem is usually quite hard and stuck pretty firmly onto the pipe walls around where the cold feed joins onto the main Return pipe. (I believe it's caused by the mising of hot water in the Return and cooler aerated water in the cold feed, resulting in some sort of crystallisation process.)

Anyway... The bottoms of your rads are probably still full of brown sludge.
If you add inhibitor, it will just react with the sludge and be mostly 'used up'. Better to get the system clean first, then add inhibitor. See FAQ on flushing.

 

[Fisherman]

Thanks for that the cold water feed is definitely ok.
There were two radiators that were cool in the centre at the bottom I took them off and flushed them out.
Your right they were full of crud the system is back up and running.
There is no hot water running in the cold water tank from the vent pipe now.
But I have noticed that the ambient temperature is slightly higher in the tank is this normal?

 

[broxy]

I wouldn't hurt to just check around the cold feed where it joins the system with a magnet to be absolutly sure there is no blockage.

 

[Fisherman]

Very slight magnetism cheers for that