Boiler/Pump not firing up when HW is ON.

[frankmce]

1) Boiler make / model (gas or oil) – Gas boiler system 5 yrs old, system has a 3 port motorised valve PLUS a 2 port motorized valve on the pipe leading from the 3 port valve to the cylinder (the plumber I had in could not explain why there was this seemingly extra 2 port valve).

2) What is / is not happening

Pump & boiler are not firing when the HW comes on – this happens whether or not the CH is also on. This is an intermittent problem, it works sometimes, and sometimes not (dodgy wiring somewhere??? – but where?)

When I switch on the CH either with the timer or the manual override – pump & boiler fire up OK – when I then switch the HW on as well the pump & boiler go off while the three way valve switches over – but they never come back on, unless I go back to CH only.

I have been getting around this problem by manually switching the HW off waiting a minute while the system fires up on CH only then switching the HW on again. Most of the time this works, but on odd days (like today) it will not work at all. So immersion needed for HW.

When its not working the boiler & pump do not come on, on HW only either.

3) Has ANYTHING else been done / changed recently

I got a plumber in who diagnosed a sticking 3 port valve which he replaced, this has not however solved the problem. Looking at the new 3 port valve it is moving between the tree positions as it should do, the 2 port valve also seems to be working when the HW comes on (it makes a humming noise, but there is no indicator to show that it has actually switched across)

4) When did it (problem) start

This started in the autumn when we started using CH again – had been using the boiler all summer on HW only (by timer twice a day plus regular manual overrides for top-ups) with no problems.
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[kevplumb]

extra 2 port valve unvented cyl

 

[frankmce]

yep - unvented h/w system

 

[oilman]

2 port valve is a safety device and is part of the required system. It will shut if the power to it fails, and so close off the heating to the cylinder. If the 3 port fails, it will just stay where it was when it failed.

 

[frankmce]

thanks Kevplumb & oilman - seems like the 2 port valve is not part of the problem then - if it was not working I guess then the boiler and pump would come on - I'd just not get any heat in the cylinder.

I saw a posting response to an earlier problem (see below) I was wondering if this means I should look for a wiring fault around the cylinder thermostat? - my problem is that I do not know how the system is supposed to work - what what wires should be live at each stage ?

"...If your system is a Y-plan (has a mid position 3-port motorised valve) then, as ArtfulBodger mentioned, it is normal for the boiler and pump to shut off when the cylinder stat stops calling for heat.

While there is a requirement for hot water, the boiler/pump are powered directly from the cylinder stat. When the cylinder stat reaches its set temperature, it switches the power from the boiler/pump to the motor of the 3-port valve. This causes the valve to move to the central-heating-only position. As the valve reaches this position it operates a micro-switch that connects the output from the room thermostat to the boiler/pump. Hence, while the valve is moving the boiler/pump are briefly not powered.

If the bang is from the boiler, and occurs as the main burner ignites (explosive ignition), then the usual cause is the pilot light assembly - it is partially blocked (resulting in a pilot flame that is too small) or it is badly positioned (relative to the main burner)...."