Intermittent heating problem

[callmeian]

When my central heating timer triggers, the boiler fires up and provides hot water and central heating as expected. All radiators heat up equally.
However, after about 45-60 minutes, all the radiators start to cool right down.
At this point in time,
-the boiler still sounds as if it's working.
-the pump sounds and vibrates like it's working, but the water going through it is not hot.
-the heating zone valve is still open.
-the heating timer still set to ON.
-the room thermostat is turned up high.
-the boiler thermostat is set to 3 on a scale that is MIN,1,2,3,4,MAX. (setting it any higher gives the same outcome).

After a while, typically 45-90 minutes, all the radiators will get hot again until the next time.
If it was a problem with the room thermostat, e.g triggering when it shouldnt, then surely the zone valve would be closed. To me it looks like the boiler is going through the motions but not actually heating the water despite sounding as if it is.

Here are the system details....

Potterton Netaheat MkII 10-16 boiler (circa 1980)
Grundfoss Selectric Pump (circa 1993, speed set to position 2 of 3)
Honeywell Motorised Zone Valve (set to Auto and positioned between pump and radiator manifold)
Honeywell Cylinder Thermostat (set to 62 degrees).
Drayton Room Thermostat in downstairs hall way
8 x Radiators supplied by microbore 10mm copper pipe. No TRVs.
Programmed by Potterton Mini Minder
The system design allows either just hot water, or hot water and heating. i.e. not just heating alone.
The pump supplies water to hot water cylinder as well as radiators.
All radiators have been fully bled.
Header tank level and water supply OK.

I know that this problem won't be solved until I get a heating engineer in but I wanted to get a better understanding of what it could be beforehand.
Any suggestions ?

 

[callmeian]

An update. It looks like it may well be the boiler.
When I said in the original post that the boiler seems to be working, it turns out that it's just the fan that keeps going. When I look in the spy hole there is no sign of the main flame or even the pilot light.

Something else I noticed, when the system is turned on from cold, the pilot comes on after about 10 seconds but then it takes another 3+ minutes before the main flame comes on.

 

[MANDATE]

From your description I would say the problem is the motorised valve.
The valve has three positions.
Positon 1 is for HW only and you dont have any problem there.
Just note power is fed to the boiler through the cylinder stat

Positon 2 is for both HW and CH. The valve has moved to mid point.
power is fed to the valve to move it to mid point via the room stat but power to light boiler is still through the cylinder stat.
Because valve is at mid point both HW and CH are functioning.

Position 3 is for CH only and this I believe is where your problem is.

It does not matter if you have selected CH direct or CH/HW the valve has to pass through mid point.
To do this, there another power source that takes over at mid point when HW has become satisfied or if HW was never selected.
Sometimes this power source is at fault because it goes through a set of contacts on a micro switch, dirty /burnt contacts stops the flow of electric to the motor so valve cant move beyond mid point when HW has been satisfied.

There is also a second micro switch. When HW is involved boiler gets power from cyilinder stat, but cylinder stat is not relevent with CH only so boiler now gets the power direct from the valves 'orange' wire.
It's the triggering of the second micro switch that allows power out of the valve.
Again valve may correctly move to to CH but faulty micro switch will prevent power leaving valves 'orange'

You should be able to confirm if valve goes to CH position or not.

Apart from faulty wiring, in my opinion a replacment actuater head will cure your problem

 

[callmeian]

Thanks for your input.
Unfortunately, my heating system cannot be set to just CH, if I want CH I have to have HW also. The timer won't allow it and the zone valve doesn't sit on the HW supply pipe anyway.
Therefore, my zone valve doesn't have 3 positions as it only controls the supply to the CH. It has an auto position and a manual open position.

When the system goes wrong, the pump works, the zone valve is open, the boiler fan is on but there is no pilot light or main burner flame.
I've checked the gas meter and can see that no gas is being used as confirmation.

I have wondered what my cylinder thermostat actually achieves on my system because it doesn't have a zone valve. So when it triggers what effect does this have. I can't see how it could turn off the CH as well without shutting of the boiler completely. My system worked fine for 20+ years so it's not a fundamental configuration problem.

Confused of Warwick

 

[Agile]

It certainly sounds to me like the boiler.

Its 34 years old now.

When did you last have it serviced?

Tony




PS Do you go to the Castle Balti ?

 

[callmeian]

It's probably 3 yrs since it was serviced. Too long I accept.


And yes, we do go to Castle Balti occasionaly. My fave Indian actually!

 

[Agile]

I agree that its one of the best. For me because the dishes are good and tasty and they are so large that it makes a breakfast for the dog as well. And of course because of the BYOB policy. Started by the current owner's father!

If you are a member of Saltisford Canal Trust you get 10% discount exc. weekends!

Certainly I find it better than Warwick Spice just up the road although they claim that they are the local favourite. But usually there are twice as many at the Castle!

The place at the top of the parade and the other two further down are rather more upmarket but much more expensive for tiny portions.

In fact I will be at the Castle Balti tomorrow, Tuesday !

Tony

 

[callmeian]

Finally got round to getting an engineer out and it's the gas valve that is malfunctioning. I'm told these parts are no longer available so it looks like maybe a new boiler is needed.
The engineer recommended an Intergas.