No hot water when set for Heating & Water

[blanco]

I would be very grateful for any suggestions to enable remedy for 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 port valves are present and the HW cicuit appears to be seperate from the pumped radiator circuit. There is no Cylinder Thermostat.
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 system.
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 for 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
the ciculation is ok and water reaches high temperature BUT set
for Water & Heating, only the radiators get hot - no hot water!
Sounds like an electrical problem but I don't see how when the water is gravity circulated with no share valves deployed.
I have been there 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, as should be the case and have the hot water primarily provided by the gas boiler.
Any suggestions would be very welcome and thanks in advance.

 

[Simon F]

Hello blanco,
how long does it take to heat the water when the radiators are not being used?

 

[dontknowitall]

I reckon there will be a zone valve somewhere, probably a 3-port. It's a puzzle (at the moment) as to why the heating is working but not the hot water.) It seems, from the problem described, that the zone valve has stuck on to the heating. This can happen when the boiler is used for heating and the immersion is used to heat the hot water.

If you trace the flow pipe from the boiler it normally goes to the pump and near this part will be the zone valve. Sometimes they're hidden under the floor or in a little nook or cranny.

 

[blanco]

The water Will Not heat at all on the "Water and Heating" setting, only on "Water Only" setting, As long as the roomstat is calling, the pumped circuit will heat the rads but there is no water heating. I am told that there has been no hot water after a six hour period, which is why they resort to the immersion heater.
Thanks for your quick reply.

 

[dancinplumba]

i reckon there is partial blockage on primary return, allows enough heat to cyl when ch not on, but not enough when ch on ???

 

[blanco]

Quote: If you trace the flow pipe from the boiler it normally goes to the pump and near this part will be the zone valve.
The water side is not pumped, it is gravity and no zone valve

 

[blanco]

Dancinplumba: That sounds more like the cause I had in mind. Question is , How to purge
the circuit of the blockage? I'm warm on electrics but cold on plumbing (excuse the pun).
I guess the blockage could even be in the coil. If debris, it prossibly got into the circuit when the cylinder was replaced.

 

[dancinplumba]

you could put in a chemical cleanser first, perhaps ??