HW overflow into CH header tank

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Hi everyone, I have a standard traditional potterton boiler in 5 bed house and recently noticed that when the CH and HW are both on hot water is overflowing into the header tank.

1. If I turn the HW off either on the programmer or by turning the cylinder stat down then the pumping over stops immediately even with the pump on speed 3.

2. I tied up the ballcock in the header tank and ran both HW and CH and the level of water in the header did not rise.

3. I have opened up all the lockshields and TRV valves on all 12 radiators to make sure there was no restriction in the CH circuit thereby isolating it was a HW problem.

4. About 2 months ago i fully cleaned the pipework with X400 and then refilled and added Fernox. The plumber cut away a bad blockage in the 15 mm overflow just above the pump and everything was working fine.

Any ideas on why the CH should cause the overflow I assume there must be a restriction in that circuit somewhere??
 
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Most likely your problem is a result of the water within the boiler overheating. This will be caused by poor circulation. How old is your pump? Although spinning it may have lost its 'oomph'. Or it could be a blockage.
 
Pumping over as you describe is almost always caused by restrictions in the pipework around the feed and vent connections. Post pics of your pipework that is near the pump etc.
 
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Just thought I would add that I have a heated towel rail running off the HW system in the upstairs bathroom which is on all year and this is the highest point in the pipe system so is good for bleeding.
 
I would first check how high the bend at the top of the vent pipe sits above the water level in the F&E tank. (should be 450mm min)

The cold feed and vent pipes look a little far apart where they tee in above the pump and it may not take much of a restriction inside the horizontal pipe to cause a pressure difference between them.

When hot water is on there is likely to be much greater flow around that pipework and a greater pressure difference. There may be a gate valve in the hot water circuit and partially closing that down could be a temporary fix.
 
Thanks for the reply. I have checked the height and it is above 450mm and will investigate if there is a gate valve to tweak.

I have added the X400 and already noticed that the radiators on one side of the house are taking longer to heat up than on the other side so perhaps I do have lots of sludge still in the system which is causing this restriction.

Would a power flush on a system this old create more problems than it may cure?
 
if you can do basic plumbing, and can afford £100 or so for a system filter, you can achieve quite a lot with chemical cleaners. By monitoring how much sediment comes out the filter when you empty it, you will get an idea of how the sludge is reducing.

If you take off a radiator and hose it through in the garden, you will see if there is much hardened sediment.
 
Also i just put a magnet on the horizontal pipe and the 22mm below it to the pump and it leapt on it and stuck. Can i tap these pipes with anything to disturb?
 
It appear that section is not getting washed away with your X400.

most likely you will have to cut out that section, and either replace with new, or scrape it out. Often the sediment goes hard where it combines with limescale.

there are other chemicals that will dissolve limescale, but unless you use a harsh acid (may cause other problems) it takes a long time.
 
Thanks JohnD i am in agreement with you and it follows that my plumber had to clear a blockage from the 15 mm just above the pump leads to more potential hard scale in that area.

Must give my plumber a rest over xmas though you guys work long and hard hours :D
 
:D :D Just thought I would let everyone know I had the pipework where I thought the blockage was cut away and replaced and these are the pictures of the offending pipes.


As we anticipated quite a blockage and the pump overrun is fine now even on speed 3. Many thanks for everyones expertise.
 

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