Cold feed and header tank get hot!

[bancanus]

Hi,

I have an ongoing problem of hot water seesawing into the header tank
when the central heating pump is either switched on or off (not a problem
whilst it is actually on!)

This is causing a build up of sludge in the header tank which in turn is
damaging the boiler heat exchange.

Initially I supposed that the header tank was not high enough above the
pump (1.9m) - however, I then got to thinking why the problem only
occurs when the pump switches off or on? Surely if the static head isn't
great enough then the problem would occur all the time the pump runs?


the pipe arrange looks ok to me (see below), the pump is on minimum
(no. I), the system is microbore.

Has anyone any ideas - I have called several plumbers, none are interested
in small jobs and won't even come out to look.

I am more than happy to change some pipe work - problem is knowing
what to change! lift header tank, change pump, change pump position
if so where to, or perhaps all the above.

 

[Bahco]

You could try increasing the height of the vent pipe so that it is 18 ins above the cistern.

 

[gas_man_gaz]

sounds like the configeration is about normal! what ya could do is to have the system flushed and fit a air sep where the "H" is (this area is common for blocking up) or dependin on what boiler ya got combine the cold feed and open vent

 

[bancanus]

Thanks for the quick replies:

The vent is already 20 inches above above cistern and the system has been flushed, twice at great expense. Plus there are two air seps either side of the 3 port valve. Still the hot water backs up the cold feed.

Its almost as if the water is circulating and then when the pump is switched off it backs up the cold feed. I thought about putting a one-way valve on the cold feed to stop the hot water backing up? Any suggestions

 

[gas_man_gaz]

why's it got two air seps? ya dont mean auto air vents do ya? who did the flush and how long did it take?

 

[warrens1964]

Have you checked to make sure the openvent has not slipped below the the water level, i have seen this many times when this happens the vent begins to syphoon water into the tank and heats the tank up

 

[bancanus]

Hi, thanks for the continued advice

The vent pipe isn't dipping into the water - indeed if I get my wife to switch on and off the pump its only the cold feed that gets warm!

Sorry gas_man_gaz, I meant to say two bottle air vents NOT seps. Where exactly on the H is the best place to fit a seperator?

The system has been power flushed once (took most of the day and cost a fortune) and a second non-power flush by me using Fernox restorer. I left it in the system for a week at normal operating temp.

cheers

 

[Agile]

A simple and bound to work solution would be to convert to a combined feed and vent.

Its not surprising you have to keep having it power flushed because whenever water circulates through the vent it brings new oxygen into the system and that rusts the inside of your rads. The next stage is for them to leak!

Tony

 

[bancanus]

Thanks Tony,
Do you know of any info online which gives guidance on converting my H to a combined with separator?

Do I, for example, simply T the 15mm cold feed into the 22mm vent and then T the combined T into the main flow? Where best to site the separat.?

cheers

 

[gas_man_gaz]

if ya convert to a combined cold feed and open vent the boiler needs to have a overheat stat

 

[bancanus]

Its a potterton 24 HE, which I think already has an overheat stat - indeed in the manual a combined vent-feed is mentioned as an option

 

[gas_man_gaz]

best way to combine the cold feed and open vent is to change the 15mm cold feed from the f+e tank to 22mm (t in behind the pump) and get rid of the other pipework altogether[/img]

 

[bancanus]

thanks gas_man_gaz, did you post an image? I can see the IMG tag but no image

 

[gas_man_gaz]

i tried but iam crap at using the tools on this site

 

[bancanus]

Thanks to everyone for their advice,

I have put together the following picture of a possible solution - raises two questions, see piccy. Any ideas?

[/img]