Worchester combi oil boiler 'goes to sleep'

[griff22]

I have a Worchester Heatslave combi oil boiler linked to CH system. Each radiator has a TRV. Towel rail has no TRV. No room thermostat. I found that after firing for a while and presumably reaching the set water temperature the firing stops and the pump switches off. With the pump off, the water in rads goes cold and because there is no circulation the system 'goes to sleep'. To try to solve this I have fitted a room thermostat which, as it is wired to control the pump, can be set high to keep the pump running all the time the system is on.
Despite this I now find that the system still goes to sleep. Although the room thermostat is set high and therefore 'calling', the pump switches off and the boiler does not restart.
What am I doing wrong? What should I do to fix this strange performanc?

 

[Agile]

It sounds like the PCB but a competent engineer would be checking everything else first!

Tony

 

[oilhead]

Could be the boiler thermostat is faulty. Do you know for sure the pump is not working, it may be circulating cool water. There are terminals on the PCB to connect the room thermostat. How have you connected it to the pump?

 

[griff22]

Thanks oilhead.
Room thermostat is connected to the appropriate terminals on the PCB, as per manual. Pump is working. With system 'on', for no apparent reason, pump sometimes stops a while after boiler has stopped firing and water in radiators does not circulate and gets cold. Turning the room thermostat down then up will wake it up, pump restarts and after a few seconds (when cool water has travelled back to boiler), boiler fires up. Before I fitted the room thermostat I had to use the 'advance' button to advance to 'off' then back to 'on' to wake it up.
I fitted the room thermostat to solve the problem, theory was as follows: With the room thermostat set high (say 30!), therefore constantly calling, the pump should run continuously. Heating level will then be controlled by the water temp setting on the boiler and by the TRV settings for the individual radiators.
This problem is particularly annoying as the system is in a holiday cottage where it is important that I provide reliable heating - I cannot expect (or want) guests to fiddle with the controls!

 

[Burnerman]

Can you hear the CH water temp stat clicking on the boiler front, as you turn it?
Also, has the boiler overheat stat ever tripped? Presumably the DHW is OK.
John

 

[Agile]

I am not familiar with the PCB used on that boiler but I have encountered several with that PCb fault on their gas boilers.

I have enquired but so far no body has ever confirmed they have found the same fault. Its seems to be associated with their particular PCB design which is similar on most of their models from that era.

The fault is that once it has reached the set temp and the burner has gone off then it just stays like that for ever.

There are two ways to restart it, disconnect power for a few seconds or disconnect the CH temperature sensor for a few seconds.

But those PCBs use an NTC sensor to monitor the flow temperature whereas Burnerman suggests its a thermostat on your boiler.

Tony

 

[griff22]

Thanks to Burnerman and to Agile.
Burnerman, some answers - yes I can hear the water temp stat click when I turn it. Boiler has never overheated to trip stat. DHW is fine.
Agile - As I am not a pro I am not sure of the significance of your point about NTC sensors.
I was really hoping it was a known problem and that I might get a known solution from you experienced guys out there!
Anyone got any last minute suggestions before I call someone in?

 

[Agile]

Simple way to identify if the fault is simply the boiler stat.

Wait until it goes off and wait say 30 minutes to cool and then turn UP the stat and see if it clicks and starts the boiler. From what you have said so far its not that.

You could also monitor the voltage across the contacts. Obviously no voltage if the contacts are closed.

They might be "closed" click wise but actually open circuit resistance wise. But again not what your comments have suggested.

To repair boilers you eliminate the simple/cheap possibilities first. Thats why females are so good at diagnosing boiler faults.

Tony

 

[Burnerman]

I may be way off beam here but the Heatslave has a 3 port valve, which is energised open when the demand for CH is there...if it has stuck shut then DHW will be fine, but no circulation to the rads. I don't know if this controls the pump directly though.
Check out the Worcester website - you can download info for obsolete boilers which may have fault finding diagrams.
John

 

[underwurlde]

I have had something VERY similar happen to my Baxi Boiler....

But first, there are 2 heat sensors used on my boiler.

1 is a thermistor - this is an NTC resistor (Negative Temperature Curve), the resistance of which is related to its environmental temperature. It is usually located on the heated water output pipe from the boiler. This temperature variable resistor's voltage is compared to the heat setting pot voltage on the front of the control PCB and this is how the heat of the water in the boiler is regulated - it's a closed loop system if you like.

2 is a thermostat. This is a normally closed heat dependant switch... it is an oooo-gorrr-blimely, things-are-way-too-hot-so-go-open-circuit switch. It may or may not have mains voltages on it. If there is a fault in the boiler whereby the water in the boiler is being constantly heated with no regulation or circulation then the water temperature may get to boiling point and beyond! The purpose of the thermostat is to shut the system down into a latched off condition if this happens. The thermostat is located right next to the thermistor and they look very similar.

Of course you have to have water in the pipes for the thermistor and the thermostat to work correctly so watch out for air locks etc!

--------

Now the fault with my system was neither the control PCB, the thermostat or the thermistor... there was another safety system used on my boiler which may also be used on your boiler - the spark electrode for the pilot light is also used as a flame detector (a flame will give off negative ions which can be used to detect the presence of a flame).

The fault with my boiler was that the electrode and, over time, become bent out of shape and it got to the point where it would intermittently work. Sometimes the system would be fine, responding to heat demand but then it would get into this funny 'I'm not bloody working' state and the house / water would drop to 1C. After bending it back into shape.... viola system working....

Finally another thing to be wary of is that for my system there is a fan which is used to blow the exhaust gasses out of the flue. Associated with this is an air pressure switch which is used to detect the change in air pressure.... With all this windy wet & damp crappy weather lately these kind of conditions have been known to effect the air pressure switch.

GL

Andy

 

[oilhead]

The temperature control of both DHW and CH is by conventional thermostats. Is the presure up in the boiler? Is the boiler at the highest point of the system? If you have low pressure, and the system pressure is low, it can sometimes happen that the sensor of the heatstore is not in water, so you have a constant DHW demand, which precludes CH.
Turn your DHW thermostat to off, and see if the problem persists, if not, then change the DHW stat. There could be a problem with one of the relays on the PCB, which means a new PCB unless you are good with a soldering iron.

 

[casius66]

If your HW is OK then it sounds like the cartridge in the 3 port valve is getting stuck - I take it the programmer lights up as if the CH is on but nothing happens?

 

[Agile]

I think that some of you have not read the original posting!

The problem is that the boiler runs on CH and when it gets up to temperature it cycles off but never comes back on!

Typical of a failed CH flow thermostst in the boiler!

But the OP is AWOL at the moment.

Tony

 

[griff22]

Hi all you helpful guys!
Sorry been away for a while as Agile noticed!
Unfortunately the problem has not gone away.

Very grateful for all your thoughts but as Agile says the conversation has wandered away from what I think is a fairly basic set of symptoms.
The problem is that the boiler runs on CH and when water in system gets up to temperature it cycles to off and the pump stops running even though the room thermostat is still demanding heat. Because pump is not circulating the water, the CH thermostat does not sense the cooling water in radiators and the boiler never comes back on.

At present when this happens and system 'goes to sleep' all I have to do to waken it, is turn room thermostat down past 'click' and up again. This sets pump running and boiler fires soon after. So it seems to me that the CH thermostat is working.

 

[Agile]

No I did not mean your room stat! I meant the temperature measurement INSIDE the boiler which measures the flow temperature and calls for heat as required.

If your boiler uses an NTC sensor then a common fault I have encountered on several different Worcester boilers is this going to sleep when the flow pipe gets up to the set temperature. I usually find that disconnecting the sensor for a second will reinstate normal operation until the next time.

Oddly no one else ever seems to have reported this.

As it only seems to happen with boilers about 10 years old I am wondering if the anticycle delay is failing to come out of its delay. Since we dont get information or the circuit diagrams I dont have enough information to work out how it performs this delay. It may just be a leaky electrolytic cap.

Tony