No hot water when set for Heating & Water

[marconi]

I would be very grateful for any suggestions to cure a problem in a
domestic CH system located in the home of a relative of mine. The
system is gravity water, pumped heating. Boiler is Potterton Netaheat Mk2.
It has in the loft, an open vented copper cylinder with an integral ballcock
cold feed tank at the top. The flow and return pipes for the internal coil are
28mm, directly from and to the boiler located downstairs. There are no
venting plugs for these pipes upstairs or downstairs. No Share/zone valves
are used. The only valve (Drayton thermal) is in the horrizontal return pipe
near the cylinder .

About 3yrs ago, the cylinder had to be renewed because of pinhole damage
around the immersion heater flange, caused by forced removal of an open
circuit immersion heater element which had been CEMENTED in rock hard.
Prior to this, there had been no problem heating the water from the boiler .

Since the installation of the new cylinder, and for the past 3 years they have
relied upon the immersion heater to provide hot water because of no gravity
circulation through the heater coil in the cylinder. the installers had attended
twice, drained a little water out of the primaries and got it running again but f
or only a day or two. The user then gave it up and settled for the immersion
heater for their hot water and have since depended on this for 3 yrs.
The householder states that with control panel set for "Water Only", circulation
is ok and water reaches high temperature BUT set for "Water & Heating", only
the radiators get hot - no hot water!
This would seem to be an electrical problem but the flow and return from the
boiler is GRAVITY circulated, not pumped and seperate to the pumped radiator
circuit with no share valves deployed. Why would the gravity circuit fail when
the radiator circuit is running? Could there be a partial blockage between the
boiler and cylinder, reducing the flow/return and if so, how can it be cleared?

I have attended this week to renew the immersion heater which was badly
scaled, corroded and open circuit. To do this, I had to drain the cylinder to
change the immersion heater which is bottom side entry, 11 inch 3KW.
This is now OK but of course has made no difference to the lack of gravity
circulation through boiler and cylinder coil. I think that there must be debris
or air trapped in the pipes causing blockage ever since the cylinder was installed.
Would dearly love to solve this problem so that the immersion heater can be
used for backup and have the hot water primarily provided by the gas boiler.

Any suggestions would be very welcome. Thanks in advance.

 

[45yearsagasman]

Lots of detail in your post, but the boiler make and model would help.
It sounds like a Drayton tapstat on the return, so check that the pin is free to move (like a TRV).
Obviously check water level in exp tank and that the cold feed is clear.
I have known this to happen when the spreader in the boiler return has disintegrated. Use a magnet to check for any magnetite build up in any part of the circulating pipework.

 

[marconi]

Boiler make and model (Pottererton Netaheat Mk2) is already mentioned
in my post. If the Drayton Thermostatic valve was in any way to blame,
why would it not be always at fault and impede the primary circulation when the heating control box is set to "Water only", when the gravity flows and the cylinder coil heats the water normally? The gravity circuit only fails when the control is set for "water & Heating" This is not a valve problem, the answer lies elsewhere.

 

[45yearsagasman]

Missed the boiler name Have you actually checked the tapstat is working correctly by removing the sensor head? If it is proved to be 100% opening then fair enough. When the pump is on, then it is reducing the effectiveness of the gravity circulation to the cylinder, and you will have to do a bit of work to find exactly the where the problem is. Have you checked for magnetite blockages? Have you disconnected any pipework to check for restrictions? How fast is the gravity circ when on HW only. You state that it works OK with pump off. Still sounds like a restriction to me.

 

[JohnD]

edit

ignore that

I was thinking of a Cyltrol, not a Tapstat.

 

[marconi]

I haven't yet disconnected any pipework, there is hardly any room behind the boiler to get at the flow/return pipe unions which would make it difficult to drain anything off downstairs. All the other joints, apart from entry/exit to the cylinder coil are soldered.
The valve upstairs in the horrizontal run which you are calling as tapstat is a straight through valve with setting marks on the head and is stamped "Drayton 6904/2100". I have searched the web for info on this device, including the Drayton website and cannot find any recognition.
Would this valve be temperature or pressure operated?
I will pay another visit to the premises soon and have another good look at the layout, paying special attention to the cold fill side of the primaries
and the state of the FE tank before deciding on a course of action. Not keen on following some suggestions, such as running the system with the boilerstat disconnected to overheat and force any blockages out.
Can I disconnect the flow at the cylinder coil and force mains pressure cold down through the coil, through the return, through the boiler and back up through the flow, then out through a hose to attempt a blockage clearance? That may sound a bit daft but I am thinking as a sparks, not an experienced plumber.

 

[JohnD]

I'd really like to see a photo of this valve

//www.diynot.com/forums/viewtopic.php?t=129539

 

[marconi]

Thanks JohnD.
Don't know when I will visit the site but will bear that in mind.