Central heating doesn't work but hot water is ok

[Sausage]

I'm sorry if this is something that has already been answered.
I have a Potterton Suprima 80 boiler.
Lately my central heating wont come on but the hot water works ok. The thermostat in the hall comes on but the boiler doesn't fire up.
Sometimes when the hot water and central heating is on the radiators warm up. But only when the hot water is on.
I've checked out the PCB but all the solder connections look ok
I've also noticed a steady drip from an overflow pipe out the back (don't know where it is coming from). It looks like it drips all night but stops in the day. don't know whether it is related to the heating problem.
Would be grateful if anyone has any advice.

 

[EliteHeat]

Check your motorised valve(s) first. Almost certainly duff.

 

[Softus]

I've also noticed a steady drip from an overflow pipe out the back (don't know where it is coming from). It looks like it drips all night but stops in the day. don't know whether it is related to the heating problem.

Well it isn't a good sign.

Have you checked the system pressure?

What does it read when the system is cold?

What does it read when the system is hot?

 

[garrymum]

Check your motorised valve(s) first. Almost certainly duff.

Maybe the motor in the motorised valve. I had the same problem years ago, water fine, CH not working. Replaced the motor in the valve and that cured it. Worth trying, it's only a cheap part ( £13 from screwfix)

http://www.screwfix.com/prods/28670/Plumbing/Central-Heating-Controls/Drayton-Synchronous-Motor

 

[Softus]

Would any of the people who think it's a MZV care to explain how that accounts for water dripping out an overflow pipe (sic.)?

Diagnosis consists of more than just listing various parts you've replaced in the past.

 

[uselessFCUK]

I've also noticed a steady drip from an overflow pipe out the back (don't know where it is coming from). It looks like it drips all night but stops in the day. don't know whether it is related to the heating problem.

Quite likely your cold water storage cistern float valve passing... water used in the day keeps the level at or around desired level, yet when not in use the valve passes enough water to creep the level to the overflow point. Depending on type of valve; replace seal.



Regards
UF

 

[Sausage]

I'll get a new motor a replace.
Just to check. The boiler will only fire-up when the motorised valve has moved to the correct position. Is there some feedback to the boiler?

 

[Softus]

I'll get a new motor a replace.

Oh. My. God.

Why are you going to do that? Is the existing motor faulty?

Just to check. The boiler will only fire-up when the motorised valve has moved to the correct position. Is there some feedback to the boiler?

It isn't "feedback". The boiler is provided with an electrical supply termed a 'demand for heat' via the programmer, a thermostat, and the switch in the motorised valve.

Have you carried out any tests, at all, of any sort?

 

[Sausage]

No I haven't carried out any checks.
If you can list a few I will have a go.

Currently in order to get the heating on I have pushed the manual overide slider into a locked position (bottom of motorised valve). When the hot water is then switched on the water is also pumped into the radiators.

The water coming out of the overflow pipe out the back has been happening for a few months. The lack of CH has only just happened.

If there are any checks which I can carry out before changing the motor please let me know.

 

[MANDATE]

If the valve does not move from the HW position when power is applied, it is most likely the motor at fault. But first make sure power is being applied.
The programmer supplies power to the room stat, then the room stat ( when turned up) provides power to the MZV.
If power is there on the relevent wire you know room stat is functioning and prolem is within valve.
Have you tried removing the actuator head off the valve spindle to see if the spindle is too stiff. You will struggle if you don't get more familier with your MZV and how it functions and the wiring.
There's a lot been written on here on 3 port valves and related problems

 

[Sausage]

Thanks for the information.
I have taken some measurements from the junction box.

There are 9 terminals being used.
Below is the relevant measurements from each of the terminals.

OFF HW ON CH ON
1 240V 240V 240V
2 0.3V 0.3V 240V
3 Gnd Gnd Gnd
4 Gnd Gnd Gnd
5 240V 180V 240V
6 27.2V 240V 23V
7 0V 240V 0V PUMP
8 23.4V 240V 23.4V
9 Earth Earth Earth

I hope someone can confirm the fault by these measurements.
One more thing, there is also a buzzing noise coming from the motorised valve. Perhaps another indicator that something is up with it.

 

[MANDATE]

without knowing what is connected to what at the terminals your measurements don't mean much.
Beside any earth and neutral wires your MZV should have 3 other wires.
These are usually white, grey and orange wires (if its a modern 3 port valve). The room stat, when turned up should make the white wire live and confirms everything ok up to the MZV.
It's the white that drives the motor over the first half of the full range
If you set the programmer to HW only all power to the motor is removed and the motor unwinds to the rest position by the return spring.
If you now put programmer to CH only the motor should move and you should be able to hear it move or if you have the head removed you should be able to see it move.
When the valve reaches mid way, power is switched from white wire to grey wire. motor then continues along second half of range.
During the second half a micro switch is operated and allows power out of the MZV through the orange wire to light boiler.
Question is? is the motor moving or not. and dont forget a stiff spindle could also be the cause.
So its, power at the white wire? remove head stiff spindle? does motor move to CH position? if not replace motor if it does micro switch faulty replace actuator head.
I would not worry about the buzzing sound. In the CH only position the motor comes up against a stop so although it's not turning it is energised to keep it there.
Likewise in the mid position a modified voltage holds the motor against the return spring.

 

[Sausage]

Sorry for very late reply.

Just to let you know, changed the motor in the motorised valve and everything is working great.

Thanks for the advice.

 

[Agile]

Would any of the people who think it's a MZV care to explain how that accounts for water dripping out an overflow pipe (sic.)?

Well he claims its all OK now so perhaps it is?

 

[spacethegas]

guaranteed to sort out the overflow leak.