Room stat not turning on heating

[Doryman]

hi. would appreciate some advice on an intermittent problem that's developed on my central heating system over the last week or so. the programmer (honeywell st1507) seems to be working ok however, if I turn up the temperature on the room stat (honeywell t6360B 1028) the boiler doesn't always kick in. the only way I can get it to start is by turning the power to the programmer off and then back on.

the programmer is approx. 15 years old. I changed the room stat yesterday but it didn't fix the problem.

any advice would be appreciated.

 

[seco services]

Have you checked the stat is getting power from the programmer when fault occurs ?

 

[petertheplumber12]

On the room stat check which two wires are the live in and switch live out, turn of electrics and link these two together, if that cures the problem, ie heating stays on, the you need a new room stat, if it doesn't cure the problem, then you need a new programmer.javascript:emoticon('')

 

[doitall]

What on earth is a Honeywell ST 1507

 

[Doryman]

thanks for the prompt response guy's.

I will do the recommended checks this evening. I've already replaced the room stat, problem still exists. shall I still do this check?

I can't find a replacement Honeywell ST1507 programmer so suspect that it has been discontinued, the only one I can find that doesn't require the existing backplate to be rewired is the Horstmann Channelplus H27 XL.

I did some research on the internet last night. look like my system is a Y plan system and has a Honeywell V4073A motorised mid-position valve fitted. a new valve, pump and auto air vent were fitted in march this year.

 

[seco services]

shall I still do this check?

Yes to find if the programmers giving power to the stat.

 

[stem]

I have come across this exact fault twice. Both times were with a 'Y' plan system with a three port motorised valve.

It took me some time the first time to find the fault, the second time it didn't Here's what I found.

If heating only is required, the motorised valve travels to the full extent of its swing to close off the supply to the hot water circuit. When it reaches the full end of its travel, it operates an internal microswitch that switches on the boiler for the radiators. If the microswitch gets worn, the valve doesn't make contact with it every time, so doesn't turn on the boiler. If the hot water circuit is satisfied (live on the grey wire to the valve) then turning the room thermostat up and down doesn't move the valve, it just removes and re-applies the live supply to the microswitch, so the situation persists.

However, when you turn off the supply to the programmer, and thus remove the 'hot water satisfied' signal the valve returns under spring load to the opposite side. Then when power is reapplied it swings back across and makes contact with the microswitch and the heating comes on.

The fault usually occurs when the valve moves from its central position (hot water & heating) to heating only. My guess would be that this is probably because it has less momentum than for a full swing.

It's easy to test. When the problem occurs test for a live on the white wire to the motorised valve and also the orange wire. If the orange one is missing and the white one is present. Then the microswitch is not operating.

 

[petertheplumber12]

Good call stem that sounds like the answer, those motorised valves made by Honeywell are very good, but the motor is often fails, as do the micro-switches, that is why they replaceable heads, it is also a good idea to make the valve head last longer, have the hot water on at a different time to heating, the head lasts longer that way. the heating works better. it's all pluses.javascript:emoticon('')

 

[MANDATE]

There are two micro switches, each with two sets of contacts.
Switch (a) power from white wire goes through contacts(1) to drive motor. switch is triggered at mid position. contacts (1) break and contacts(2) make.
Power from grey wire if live goes through contacts (2) to drive motor from mid position to CH. This switch has two inputs but just the 1 output.
Burnt/ dirty contacts can prevent the motor operating even though the motor itself is OK. Contacts have a habit of not making or not breaking.

Switch (b) This switch is wired different. 1 input and 1 of 2 outputs.
The input is fed from the white wire and output 1 is through contacts (3) to the PCB that produces the modified voltage for holding the valve in mid position.(when grey is not live)
When grey is live and valves moves on from mid position the switch is triggered and power diverted through contacts (4) which connects to the orange wire to light boiler.
I have had the problem of the second switch contacts (4) burnt. It allows CH along with HW while in mid position only, but not CH only. This gives the impression of being intermittent.

 

[petertheplumber12]

considering how cheap the replacement heads are and `30 minutes to change one, get a replacement head.

 

[MANDATE]

Couldn't agree more!

 

[Doryman]

replacement head fitted & now working ok. many thanks guy's.

 

[stem]

Thanks for taking the time to post the outcome. It's very helpful to have a completion to a thread for those that follow later with a similar problem.

I was surprised that a Honeywell motorised valve that was replaced in March has failed already though. IMHO they are one of, if not the most reliable valve. My mother has an original two port valve still working in her bungalow that was installed when it was built in 1980, and one of mine is 10 years plus.