Domestic plumbing query

[graduate_owner]

Hi,
I wonder if there are any experienced plumbers who might be able to advise.
My daughter had her oil boiler replaced a few months ago. The system is a conventional open vented one with two tanks in the loft. The plumber disconnected the central heating header tank and fitted a pressure vessel in the airing cupboard.

Everything seems to be working fine, but I was asked to replace some radiators so I turned off a valve at the pressure vessel and draned down the system ( yesterday). The upstairs rads have drained, and I have replaced 2 of them, intending to continue tomorrow when manual help will be available.
However my daughter telephoned to say that there is still water draining out of the drain valve downstairs. This can't be radiator contents but I don't understand where it is coming from.
Could a defective heat exchanger cylinder coil be the cause?
I don't really understand what is going on with the pressure vessel anyway, I thought they were used to pressurise the domestic hot water in a sealed system, but this is, or was, open vented.

 

[MikeJhn]

Not a Plumber, but installed a couple of systems, Perhaps the DHW tank has been converted to mains pressure, hence the need for the pressure vessel, the boiler can still be open vented because the circuit is separate from the HW tank by the internal coil, this does not equate to the CH header tank being disconnected are you sure it was that tank that was cut off.

 

[Doug B]

Is the ballvalve running in the remaining header tank when no water has been drawn off from the hot water taps, if yes this would suggest the coil in the cylinder is faulty & water from the cylinder is leaking into the central heating water via the coil.

If the plumber has removed the small header tank & put an expansion vessel in it would suggest he has converted the system to a sealed system, if this is the case there should be a filling loop & pressure gauge for filling the system, this is usually situated near or on the boiler but is not always the case.j

 

[graduate_owner]

There is a filling loop and pressure gauge, located in the airing cupboard by the pressure vessel. The boiler is an external one.
I will look into the remaining header tank issue.
Thanks.

 

[Doug B]

If the coil is faulty when the sealed system is filled the pressure will probably be greater than that provided by the header tank so the leak will be the other way from the sealed system into the cylinder.
This will be detectable as the system won’t hold pressure which will be noticeable on the pressure gauge.

 

[Sheffield Tony]

I'm not a plumber, but I'm confused as to why, if you have a sealed system, it would be necessary to turn off any valve at the pressure vessel before draining the system ?

If the filling loop isn't connected, there are limited ways the water can be coming through. If the filling loop is connected, those isolator valves have been known to fail ...

 

[HOJ]

I presume that the primary heating system has been converted to a sealed system, whilst retaining gravity hot water, not sure why you still have water coming out of a drain valve, if it is shut down, is it the pressure release valve letting through, by any chance.


Warning: If the primary coil has gone, the pressure gauge will show a lower value (back to static pressure from the header tank) which, will also be serving hot & cold taps to baths and upstairs taps, so possibly the central heating water is now contaminating it.


When converting an old gravity fed systems to a pressurised system, particularly with old radiators and pipes and also retaining the old cylinder, the system should have been pressure checked, to help to establish, although not guarantee, whether the system can function under increased pressure, old pipe-work quite often reveals weaknesses that are not evident under low-pressure conditions, possibly going up from 0.5/0.8bar working pressure (depending on the height of the header tank) to 1.5 bar or even more when the system is running hot.

Even having done a pressure test, we would always carry out a thorough power flush, and/or change radiators etc prior to fitting a new boiler, as part of the Boiler guarantee requirements, power flushing also has its own risk of exposing weaknesses as well.