Heating is excessive: Honeywell Y Plan diverter failure?

[jimblywee]

Hi all, hoping you can help.

I was away on Xmas hols for the last week, and when I got back the house was an astonishing 28.5C. But there was no hot water.

Due to the cold weather, before I left I had set the room thermostat to 5C frost protection only, with the boiler activated. Normal behaviour would be that the hot water would keep being heated but the CH would only activate if the house got excessively cold.

The thermostat has a live switch direct to the white wire on the Honeywell Sundial Y Plan 3 Position Diverter Valve.

My theory: something in the valve failed while I was away, locking it in the "A", central heating only position, activating the CH pump and gas boiler, which just churned out CH heat for days (the woman feeding my cat says it has been unbearably hot in the house since Xmas day - heaven alone knows how much money this has cost me).

The valve switch has no physical resistance, and when I took off the cover there seemed to be no connection between moving the slider to "Manual" position, and any of the inner workings; nothing is turning either when it's on or off and there's no spring action at all. I tried to take the motor unit off the valve via the backplate, but water started squirting everywhere.

One thing I don't get: with the power switched off, there is no electrical resistance between the two terminals coming into the thermostat (Live and Live Switch), or at the valve end. Could this indicate a short circuit in the valve perhaps?

So my question is: is my theory likely to be correct? If so, is there any way to fix it without a) draining the system; b) calling out a heating engineer?

If it isn't the valve that's failed, does anyone have any ideas what might be the problem?

Thanks very much for your help with this.

 

[doitall]

Sounds like you have the early valve without a dimple on the silver case, and if so the actuator is not removable with the system loaded.

Could be the valve has stuck but you should check the controls first. (turn the power off and back on) and see what happens.

If you have a multimeter and know how to use it, you can the stats, programmer etc.

 

[D_Hailsham]

Read Mid-position valve working and Diagnostics

One thing I don't get: with the power switched off, there is no electrical resistance between the two terminals coming into the thermostat (Live and Live Switch), or at the valve end. Could this indicate a short circuit in the valve perhaps?

This bit does not make sense, can you elaborate?

The valve has stuck in the Heating only position; that's why there is no resistance when you move the lever from Auto to Man.

The boiler has been lit by the thermostat on the side of the HW cylinder, but the rads have been heated instead of the cylinder.

If water comes out of the valve when you try to remove the motor etc, you have a very old valve. You can separate the actuator (metal box) from the valve body on current valves, which have a small bump on the top of the cover.

 

[jimblywee]

Thanks for your replies!

Yes, it's the very old version unfortunately. Is there a manual override I can perform with a screwdriver or something? The lever does nothing: it doesn't seem to be connected to anything in the valve unit any more.

What I mean about the resistance is:

1. There is a live wire going into the thermostat, at +240V when the power is on.
2. The thermostat has a live switch wire coming out of it that goes to the valve. When the thermostat switches to 'on', this wire is connected to the +240V.
3. I would have expected there to be infinite resistance between these two wires with the thermostat in the 'off' position and the power off. But there isn't any electrical resistance at all.

The above may be a red herring. I have drawn a circuit diagram deciphered from the junction box, and see that the orange wire from the valve is on the live side of the AC, so that may be accounting for the lack of resistance. Just thought it may have some significance.

 

[doitall]

If the valve is stuck with the power off, you need a new valve.

If you have valves on all ports, ok, otherwise I would recommend you drain down.

 

[D_Hailsham]

The lever does nothing: it doesn't seem to be connected to anything in the valve unit any more.

The lever is not physically connected to the valve; it just pushes against the valve to open it. Because the valve is stuck in the CH position, the lever will feel loose.

Have you turned off the power at the main connection to the heating system and then checked the valve lever? Just turning off at the timer is not enough.

Which thermostat do you have and which wire (live and switched live) is connected to which terminal?

Do you mean that the meter reads 0 ohms between live and switched live when the power is off and stat off? If not 0 ohms, what is it reading?

 

[jimblywee]

Have you turned off the power at the main connection to the heating system and then checked the valve lever? Just turning off at the timer is not enough.

Yes, it behaves the same whether the main power is on or off.

Which thermostat do you have and which wire (live and switched live) is connected to which terminal?

I have a Honeywell CM907 digital timed thermostat. Live is connected to terminal A, and Switch Live is connected to terminal B. Terminal C is not connected. When the thermostat clicks on, it completes the circuit between terminals A and B. I have tested this unit in isolation using the multimeter and it behaves exactly as expected. This is per Honeywell's installation diagram for heating systems using a zone valve. It was working fine before I went on hols.

Do you mean that the meter reads 0 ohms between live and switched live when the power is off and stat off? If not 0 ohms, what is it reading?

The wire coming from terminal B on the themostat connects directly to the white wire going into the valve.

However, with the power off, at the valve end, the red/live wire that goes into the thermostat, and the white wire that goes into the valve, show 0 ohms resistance even when the thermostat is switched off. Like I say, it may be a red herring, but it doesn't seem to be intuitively correct.

 

[jimblywee]

I've posted my circuit schematic below in case it's any help. The wiring is circa 1980s and is a bit of a dog's breakfast, but this is as logical as I can get it...

Untitled

• jimblywee
• 31 Dec 2010
• 1

 

[D_Hailsham]

I have tested this unit in isolation using the multimeter and it behaves exactly as expected.

I assume you mean that it reads open circuit between A and B when stat is off and 0 ohms when stat is on.

The wire coming from terminal B on the thermostat connects directly to the white wire going into the valve.

However, with the power off, at the valve end, the red/live wire that goes into the thermostat, and the white wire that goes into the valve, show 0 ohms resistance even when the thermostat is switched off. Like I say, it may be a red herring, but it doesn't seem to be intuitively correct.

I agree it may be a red herring, but your description of where you are measuring is very confusing. Where exactly are you placing the two prods from your meter? Are the wires connected to the thermostat A and B terminals when you measure the resistance?

 

[jimblywee]

I assume you mean that it reads open circuit between A and B when stat is off and 0 ohms when stat is on.

Yes, that is correct.

I agree it may be a red herring, but your description of where you are measuring is very confusing. Where exactly are you placing the two prods from your meter? Are the wires connected to the thermostat A and B terminals when you measure the resistance?

Sorry for being unclear, and thanks very much for persisting with me!

Here's what I did: I turned the power off at the mains. The thermostat (it's battery powered so was still functional) was connected but set to "off" - i.e. an open circuit across terminals A and B. I measured the resistance between the red wire going to terminal A of the thermostat (marked "red" at the "Thermostat" piece on the diagram above) and the white wire that goes into the valve (marked "white" at "Valve" on the diagram above) and the resistance was 0 ohms. I would have expected an open circuit. (But then again I don't know what goes on electrically within the Valve unit - or it is indicative of a short circuit.) Hope that is clearer.

 

[D_Hailsham]

Here's what I did: I turned the power off at the mains. The thermostat (it's battery powered so was still functional) was connected but set to "off" - i.e. an open circuit across terminals A and B. I measured the resistance between the red wire going to terminal A of the thermostat (marked "red" at the "Thermostat" piece on the diagram above) and the white wire that goes into the valve (marked "white" at "Valve" on the diagram above) and the resistance was 0 ohms. I would have expected an open circuit. (But then again I don't know what goes on electrically within the Valve unit - or it is indicative of a short circuit.) Hope that is clearer.

Thanks for this info. I have just looked at your wiring diagram, which is very unconventional and has raised several questions.

1. What make/model is the timer?
2. What does the timer control?
3. What is the make and exact model of the boiler?
4. Which boiler terminals do Boiler 1 and Boiler 2 connect to?
5. Is the "immersion heater" the thermostat on the side of the HW cylinder? It may look similar to this:

View media item 10596
6. Are you sure the "immersion heater" has only two wires?

 

[jimblywee]

Well after all that... I abandoned the entire situation and spent New Year's Eve at my girlfriend's house where the heating and hot water are fine.

When I got home this morning the house was cold so I cranked on the full-blast heating. It was fine for about 2 hours, but then the whole system just cut out. No boiler, no pump, nothing. I got a sinking feeling and started to suspect that the problem was now systemic rather than the valve issue I'd isolated.

Then I noticed something - the feeder pipe from the valve to the hot water tank was hot all of a sudden. Its lack of heat had been one of my diagnostic criteria.

Then I had a brainwave... I had turned the thermostat off yesterday. So I turned it back on and ran back to the valve, where I heard it go click-click-click-whirr and everything started up. I turned the thermostat off again, and the heating stopped again.

The entire system is now functioning as intended. I suspect that my partially dismantling the valve freed up whatever electrical/mechanical fault that had jammed it up.

I will still get it replaced with a much more modern unit, but it's no longer dreadfully urgent to do so.

Thanks very much to everyone who contributed to this thread and helped me put my thoughts in order. And a very happy new year to you all!