Central Heating Problem - Advice Please

[keithmdw]

Noticed tonight that the central heating isn't working. We have two 'sliders' - one turns on hot water, the other central heating.

Usually just turn on CH and set the thermostat to the temerature we want (you hear the thermostat click when it passes the current temperature and the boiler fires up).

Any way, what is happening now, is that the thermostat still clicks when you turn it past current tempertaure, but the boiler doesn't fire up.

However, if you turn on hot water (as well as central heating), the boiler fires up and the hot water gets hot, and more importantly the heating system gets hot.

I assume from this that (a) the boiler works, (b) the central heating pump works as the hot water in the tank gets pumped through the heating.

I thought maybe it was the thermostat not telling the boiler to fire up.

Boiler is a Potterton Promax HE (means nothing to me). The controller with the timer and CH/HW sliders on is in the kitchen under the boiler and the thermostat dial is at the foot of the stairs. Sure this isn't relevant but gives people an idea what it is that is malfunctioning.

What does everyone else think?

We are in rented accommodation (so the landlord is going to get someone to fix the problem - I already called him), but I am curious as to whether it is likely to be something simple or not and what others think.

Thanks

 

[keego]

Is the programmer a Potterton? if it is then it's the slider switch. New programmer required.

 

[Soggy_weetabix]

motorised valve

 

[keithmdw]

Is the programmer a Potterton? if it is then it's the slider switch. New programmer required.

Yes, all parts are Potterton. Sliding the CH slider to ON does make the red ON light come on, it just doesn't fire the boiler.

 

[Agile]

I would place my bet on the motorised valve. It must be opening if the rads get hot but perhaps its not operating the microswitch?

Depending on the wiring, the thermostat could also be faulty but they are usually quite reliable.

Tony

 

[keithmdw]

Had a good poke around with the controller this morning and it looks like the slider is damaged. Got it clicked into an on position eventually and all is fine.

Not sure why it was turning the red 'on' light on if it wasn't making contact, but oh well!

Called our landlord and cancelled the plumber until a more convenient time seen as it's working.

 

[keithmdw]

Also, could someone answer me a question to help me understand how the heating system works (I like to know these things ).

This is based on a working system.

If I set the heating to, let's say, 20C, the boiler fires and runs for a while, then goes off (even though the temp is < 20C). The pump still runs. Then a while later the boiler fires up again and goes for a while. This can happen a few times during the night.

Is this, as I suspect, that the water has reached the correct temperature (I assume this is the 60C set by a small knob on the tank) and the pump is keeping the hot water running through the radiators. When it cools a little, the boiler fires again? Does this make sense? Maybe I am completely off as I understand nothing about these things

 

[Agile]

Not quite right but half way there!

The boiler heats the system water until it reaches say 70*C as the setting on the boiler's thermostat. Then the boiler shuts down until the flow falls to say 65*C then it switches on again.

The hot water cylinder allows the system water to heat it until the cylinder water reaches the stat setting of say 55*C then the motor valve closes.

The rads continue heating the property with a flow temperature of say 70*C until the room stat set temperature is reached and then the heating motor valve closes and the boiler shuts down.

Tony

 

[ACOperson]

Motorised valves(s) - 99% certain

 

[Softus]

It seems that you might have two faults:

1. Programmer slider switch.
If you got the CH to work without DHW being on, by manipulating the slider switch, then it seems likely that the switch is faulty. It might be highly resistive in its faulty position, which could be enough to light the LED but not enough to power the MZV, pump and boiler.

However, it might turn out that it isn't faulty, and it was just a coincidence that the system starting working when you were working the switch.

2. MZV microswitch.
Even if the programmer switch is faulty, it would appear that the CH MZV is stuck open, because when you moved the DHW slider the heating operated as well as the water heating.

One cursory test for this is to switch CH off and DHW on - your rads should stay cold. If they get hot, then locate the MZV and see if it appears to open or shut. It could be shut, in which case it's probably partially open owing to sludge, or it could be fully open, in which case it probably has a faulty synchron motor, or jammed mechanics, or is jammed open owing to sludge.

That's my opinion anyway.