Central Heating Problems... Diagnosis Required!

[A1Dan75]

Hi All,

Got a bit of an intermitent problem with our central heating - having read up quite a bit on here and elsewhere I keep changing to mind as to the likely culprit, so thought I'd see if any of the eperts in here could advise...

The heating seems to work fine every morning - comes on, along with the HW before I get up, and all the radiators are good and hot, and plenty of HW.

However, in the evenings all does not go so well - HW seems to be fine, but the CH tends not to come on properly. Typically (what I presume are) the first couple of radiators in the circuit (upstairs bathroom next to HW tank / airing cupboard and nedroom next to that) heat up, but other upstairs ones and all the downstairs do not.

When the CH should be on, but isn't, the boiler will sometimes flame for a while and then go out - I'm not sure if this is just for the HW. Even when the flame is down though, the pump keeps on going.

My suspicion has been that it is to do with the Honeywell 3-port valve which is after the pump and splits the flow to the HW tank and the radiators. The manual open switch on it is totally loose, whereas I have read that it should be spring loaded, and you have to push it firmly to the open position.
However, even when the CH is not working, both pipes out of the valve are hot, so maybe this is a red herring.

I can't for the life of me think what would cause it to work in the morning and not the evening!

Any suggestions of more things to check etc welcome!

 

[jackthom]

The system seems to work properly when both central heating and DHW are required. it's fairly common for problems to occur when only CH should be on.

When only central heating is required the orange wire on the 3-port valve should have 240V on it. This tells the boiler to fire up and runs the pump.

It's possible the microswitch inside the valve head is only switching this intermittently, enough to occasionally get the boiler going and the pump may just be on overrun for a couple of minutes following this.

 

[A1Dan75]

When only central heating is required the orange wire on the 3-port valve should have 240V on it. This tells the boiler to fire up and runs the pump.

It's possible the microswitch inside the valve head is only switching this intermittently, enough to occasionally get the boiler going and the pump may just be on overrun for a couple of minutes following this.

Would that mean the valve will need replacing?

 

[jackthom]

Yes, you can buy the actuator head and replace it without disturbing the plumbing.

If you have a multimeter you can check the orange lead to be certain that's the problem.

 

[A1Dan75]

Thanks JackThom, will do just that. Glad to know that you can replace the actuator head without having to start messing with the plumbing (which would be beyond me!)

 

[jackthom]

Before spending any money I'd use a meter or at least a mains tester screwdriver to check that is the problem.

 

[A1Dan75]

Yep - that's what I meant... will check the voltage!

 

[D_Hailsham]

Yep - that's what I meant... will check the voltage!

You need to check the voltage on the orange wire when it is disconnected.
Turn the HW OFF at the programmer and turn the cylinder stat to MIN.
Turn CH ON at programmer and turn room stat to max.

You should then have:

240v on the grey wire;
240V on the white wire;
240v on the disconnected orange wire.

Boiler should notlight as the orange is disconnected. If it does, there is something else wrong.

 

[A1Dan75]

Right, checked the voltages (with orange disconnected), and got the following... still not totally sure what it tells me though!:

HW Only
Orange 0v
White 0v
Grey 0v

CH Only
Orange 240v
White 240v
Grey 240v

CH & HW
Orange 150v
White 240v
Grey 80v

Neither
Orange 90v
White 90v
Grey 240v


These all basically fit with what I'd expect from my understanding of the Y-Plan wiring diagram. I'm not sure about those partial voltages of 80-150v but there's nothing that I would have thought would lead to the problem (eg 150v on orange for both on shouldn't matter, as the cylinder thermostat will be telling the boiler to fire anyway).

One thing to note - I got home and the CH was once again not working, but by the time I had my tea and went to look at it, the radiators had finally started working - So it could be that the readings above are not representative of the problem! I'm going to check it as soon as I notice it not working next time...


I'm still not too sure about the implications of what I find. Presumably one option is that when not functioning the orange has no voltage when CH is on, in which case the actuator head needs changing. If this is not the case, though, what would it imply may be wrong?

 

[D_Hailsham]

HW Only
Orange 0v
White 0v
Grey 0v

That's correct. The valve plays no part for HW only as the boiler is controlled by the HW thermostat

CH Only
Orange 240v
White 240v
Grey 240v

That's correct. The boiler is fed from the orange wire, so it should fire up.

CH & HW
Orange 150v
White 240v
Grey 80v

That's wrong. There should not be any voltage on the grey wire.

Neither
Orange 90v
White 90v
Grey 240v

Not sure what you mean by neither. Have you just turned the CH and HW off at the programmer?
If so, I would expect 240v on grey as it comes from the HW OFF terminal of the programmer. But there should not be any voltage on either orange or white.

Have you checked if the valve is sticking?

Turn off the power (setting programmer to OFF is NOT enough). This will reset the valve to HW only.
Move the lever from Auto to Man
You should feel a resistance as the valve is being opened to CH against a spring. When you release the lever it should return to HW only position.

If you feel no resistance - lever is floppy - the valve is sticking.

Remove the actuator (box on top) to reveal the valve spindle. This should turn easily by hand , but only 10-15°. If it is stiff, try a little WD40 or similar to free it.

 

[A1Dan75]

Thanks D_Hailsham...

Yes, by "Neither" I meant just turning off both CH and HW on the control panel. I'll try following your suggestions when I get home - I do know that the lever is floppy pretty much all the time (if not all!), so maybe this is the issue.

Do you know what those "partial" voltages could be a symptom of? I presume that under normal conditions, all the zero voltages should actually be zero, not some in-between voltage?

 

[D_Hailsham]

Do you know what those "partial" voltages could be a symptom of? I presume that under normal conditions, all the zero voltages should actually be zero, not some in-between voltage?

The only partial voltage I am aware of is that, when the valve has been in the CH only position and CH is satisfied (room stat clicks off), there will be 50-150volts on the orange wire. This voltage is too low for the boiler gas valve to operate so the boiler goes off. Turning HW ON or turning power off will reset the valve and this voltage goes away.

 

[A1Dan75]

The only partial voltage I am aware of is that, when the valve has been in the CH only position and CH is satisfied (room stat clicks off), there will be 50-150volts on the orange wire.

OK, so that's about what I'm getting (plus the same on white for some reason!). Why is there any voltage in the orange under these circumstances?

 

[D_Hailsham]

The only partial voltage I am aware of is that, when the valve has been in the CH only position and CH is satisfied (room stat clicks off), there will be 50-150volts on the orange wire.

OK, so that's about what I'm getting (plus the same on white for some reason!). Why is there any voltage in the orange under these circumstances?

Are you sure?

If HW OFF (either at programmer or HW thermostat), CH ON and room thermostat turned up, the valve should be in CH only position.

Voltages should be:
Grey - 240V (from HW OFF or HW stat satisfied)
Orange - 240V
White - 240V

When the CH reaches temperature, the room stat will open and you should get the following voltages:

Grey - 240V (from HW OFF or HW stat satisfied)
Orange - 50-150V (residual voltage via resistors and diode in actuator)
White - 0V ( the room thermostat is open)

 

[A1Dan75]

I'll check it again this evening.

What you say makes sense, I guess the only way there could be voltage in the white would be if it is somehow linked to the orange (identical voltages)... I guess that would be a sign of a knackered actuator?