Sleepy Central Heating - Lazy Boiler?

[jockinafrock]

Hi All,
Having a bit of a problem with my CH system whereas it starts up as programmed at 6pm then is due to go off at 10pm - only problem is that after about 1-2 hours all rads go cold. The boiler switches back to pilot only and the circulating pump stops. After about another hour it'll all start up again! The hot water is fine - the pump runs ok & the 3 way switch "switches"! My system has been installed since the mid 80s but the boiler has been regularly serviced annually. The Salus valve was changed in Jan 09 as was the HW thermostat & the Grundfos pump changed about 3 years ago. The hot water system is fine.

The system components are:
Gloworm Fuelsaver 50B Mk2 boiler
Salus 3 Port Motorised Valve (MV322)
Grundfos Selectric Pump/Circulator
Potterton EP2000 Programmer
Stelrad Accord Radiators with Danfoss "S" TRVs to all except the bathroom rad

I was thinking that the boiler thermostat was faulty but feel that I need an expert opinion before I start changing various components. Any advice will be very welcome.
Thanks
Jock

 

[D_Hailsham]

TRV's on all rads except the bathroom? Do you have a room thermostat - you don't mention it.

The hot water is fine - the pump runs ok & the 3 way switch "switches"

Which 3-way valve do you have - make and model? On most systems with a 3 way valve, the valve plays no part when just hot water is on; it only works for HW and CH, or CH only.

 

[kevplumb]

just a wild guess

Salus 3 Port Motorised Valve (MV322)

 

[D_Hailsham]

just a wild guess

Salus 3 Port Motorised Valve (MV322)

Woops! Missed that bit

Still need to know if the OP has a room stat.

Also useful to know which way round the valve has been installed: the normal way is Port A to Heating, Port B to Hot Water cylinder.

 

[rojaca]

i'd start with establishing whether or not you have voltage present at the switch live of the boiler when the boiler is not firing but you are expecting it to. e.g programmer on, room stat calling.
This at least will tell you if its a control or boiler fault

 

[jockinafrock]

Sorry all - should have mentioned that there is no room stat. Everything has been working fine until this past few weeks! Including the 3 way switch for HW and/or CH. Thanks
Jock

 

[D_Hailsham]

Sorry all - should have mentioned that there is no room stat. Everything has been working fine until this past few weeks! Including the 3 way switch for HW and/or CH.

I assume that the HW thermostat you mentioned is on the side of the HW cylinder.

You have such an unusual and non standard system that it will be difficult to advise what to do. As rojaca says you need to the voltages on the circuit.

If you have a multimeter, you can check the voltage at the 3-way switch.

When in the HW position there should be 240v coming in from the timer HW terminal and going out to the boiler.

When in the CH and HW position and HW is still calling for heat, there should also be 240V on the connection to the valve white wire and 0V on the grey wire.

When the HW reaches temperature, the white wire should still have 240v but the grey wire will now have 240V.

You can check this by turning the HW stat to max (0v on grey), then to min (240v on grey)

When in CH only position, there should be 240v on the white and grey wires.

If these voltages are OK, the problem is elsewhere.

 

[jockinafrock]

Checked all voltages at system start - all fine and but now, 1 hour later everything stopped but voltages at 3 way valve still as per readings kindly supplied by D Hailsham. Boiler sitting with pilot lit but nothing happening - programmer showing HW & CH ON! Still have an inkling that the boiler thermostat is shutting the system down - or am i barking up the wrong tree?
Regards to all
Jock

 

[bellairaphon]

sealed or open vented system?

 

[jockinafrock]

Not too sure! There are header tanks in the loft with cistern type ballcock valves if that helps.

 

[bellairaphon]

u have a open vented system then.

1) water in the tank, how much normally quater full.
2) water colour clean/dirty
3) when you have central heating on does any water come back into the tank in the loft from the pipe that comes up an over?

check them 3 then get back and tell us what the answers are

Nath

 

[jockinafrock]

Hi,
Water in header tank 1/4 full
Water clean
No water coming through tube when CH on.
Thanks
Jock

 

[bellairaphon]

when the timer is on constant does it work correctly?

Nath

 

[jockinafrock]

No! Both HW & CH are showing "on" at the programmer but pump is stopped & boiler is on pilot.
Jock

 

[bellairaphon]

ok so are all stats turned up? do u have a electrician neon screwdriver. or even better a multi meter?.if not turn the programmer off then on to ch only can u hear the valve do anything?

Nath

just seen that u dont have a room stat boiler stat turned up!!!