Any Ideas?-Hot water venting problem

[Baffled Bas]

Fully pumped Gas central heating system. HW OK but when central heating is switched on, after about twenty minutes hot water pours from the vertical overflow pipe onto roof of house. I'd be very grateful for any possible causes/solutions.
Bas.

 

[justlead1]

Hi Bas. Its difficult to determine the set up from your description.
Is it an overflow pipe that runs to the outside of the building that discharging or the vertical vent pipe that terminates of a small cistern in the loft space?

 

[bobpape]

The fact that the expansion pipe has been extended through the roof suggests that
there are ongoing problems,
I have witnessed similar situations one was on an upside down system, (the boiler was
located on the top floor as was the pump), In that case I fitted another pump on the ground floor and disconected the top floor pump, I left the disconected pump in, in case the new pump position did not work,

I would check that your pump is on the return pipe and is pumping in the correct
direction,
depending on how the system is piped I might try turning the pump around,
I would also check the pump speed setting,
and check the temperatures as it could be thermo syphoning due to a faulty thermostat.

 

[Baffled Bas]

Thanks for both replies. Yes it is the vent pipe that would normally terminate in the cistern and yes, there have been problems in the past that led to it being extended skywards. Any further extension and it should make a great lightening conductor! Pump is currently set to setting 1. Not sure yet if it is pumping in correct position and direction since system (mother-in-law's) is in Northern Ireland and I am in scotland. Could you please explain further how thermo syphoning might be a possibility. Thanks.
Come to think of it, is there anybody within range of Newcastle, County Down who fancies a challenge......?

 

[justlead1]

Hi, The position of the pump in relation to cold feed and vent is the likely problem. A local heating engineer would be able to sort it by altering pipe work. And make sure the vent pipe work gets put back into the building, in the event of freezing it could cause major problems. Good Luck

 

[bobpape]

Hi, Thermo syphoning is a phenomena (for want of a better word) that occures when
primary or domestic water circuits are over heated,

What actually happens is the over heated water expands up the expansion pipe and flows into the cistern (a cistern is an open topped tank) then the space left in the
cylinder by the expanding water is replaced with water from the cistern, it will continue untill either the heat source it turned off or hot water is run off,

In the worst scenario it can cause a plastic cistern to deform and collapse or disconect its self from the tank conectors, either way it is messy,

Being an old plumber I have been aware of this potential problem since plastic tanks
first came to the market, having worked on coal fired heating systems that were without thermostat control, when they got hot they could shake the house which
provoked much laughter and some one rushing to damp down the boiler and run off some water,

I have experienced it on several occasions and once it caused me some injury,


It was on a Harcopak or Horton unit (or some such item) situated in a cupboard, in this case the overflow was running which prompted the call out,

I reached up intending to shake the ball valve but my hand passed into what was a super heated steam atmosphere and I was scalded, the steam had collected in the upper unvented cupboard space, when I shone a torch I could see tiny particles of steam seemingly swimming about in the space,

Oh the joys of being a plumber,

 

[Bernie2]

Those where the days! Up at five in the morning, set and light the boiler, stoke it, and get warmth from the rads just in time for lunch at 12.

 

[Baffled Bas]

Thanks for further info Bob. Water temp doesn't seem excessive when overflow occurs so doubt it is thermo syphoning but will check it out.
I like your optimism Mr Justlead- " A local heating engineer would be able to sort it by altering pipe work" So far on heating engineer number three with no luck. Will check out pump position and if all else fails resort to arson.

 

[justlead1]

Hi Bas We live in hope. Check the pipes out your self, trace the pipe that exits near the bottom of the small cistern in roof space. It will join into the heating system (probably on the suction side of the pump) The vent to the system is likely to be on the delivery side of the pump. So its sucking out the cistern and discharging out the vent pipe. If this is the sonario move the cold feed to within 6" of where the vent branches off. I do not like giving advise when not surveying the system but it cant be any worst than the situation as is. Good luck (As a test of the theory push a cork in the cold feed, the cistern end and run the system up, moniter results. But DO NOT LEAVE it there for ever)

 

[bobpape]

It seems then, that it is a pipe work problem, but you would probably have to take the
building apart to find it,
Have you considered changing it into a sealed system, all you would need is a pressure
vessle and filling loop and pressure guage and connect the feed and expansion pipes
probably in the loft space,

Addendum to that last message, you would also need to install some type of low water pressure inhibitor switch, perhaps connected to an interupted thermocouple which would
prevent the boiler firing up in the event of a major water loss,