Boiler problems.

[John Brown]

Also, after thinking about things half the night to come up with my theory as to the wiring, I've now found a Big Clive video that explains everything totally clearly. It's a small comfort to know that I was right in my conjecture..

 

[Spectric]

By the way, is the fourth orientation a bad idea because of possible leakage?

Yes it is a potential if the seal goes, in the first three positions any leaks should drip clear of the electric's.

 

[Spectric]

come up with my theory as to the wiring

The basis of boiler wiring is that the boiler should only operate when there is a demand from a controller that opens a zone valve and there is a flow path, the zone valve has to be fully open to close the contacts to operate the boiler. These measures ensure the boiler cannot boil the water in the heat exchanger which in modern boilers is not a great lump of cast iron. Maybe this diagram of my boiler wiring may help

Ignore the mid position valve and assume the orange wires from the zone valves goes direct to your boiler, the CM907 is the controller that sends power to the motor in the zone valve when there is demand. The valve will open and close the contacts which then sends power to turn your boiler ON. The same logic applies to the hot water, the ST9100C controller will operate the zone valve via the cylinder stats, one is for over temperature protection and the other sets the cylinder temperature. Again the zone valve will open and close the contacts which in turn turns your boiler on.

For those interested the mid position valve is my bypass, when there is no demands from the controllers it is held in the bypass position by the internal spring and boiler flow is round a bypass loop for pump overun. When there is a demand this powers the mid position valve to it's other fully open position which opens flow to the zone valves and closes the bypass. The reason for this is I use a smart pump which senses pressure and regulates flow accordingly, so with a normal bypass valve it would not open.

 

[John Brown]

I finally managed to change the valve head, although I had to improvise an ultrashort screwdriver, and do the job mostly by feel. I left short ends of the old cable, as suggested by some kind soul earlier in this thread, only to find that the new valve head had more wires! All working OK now.
The wiring is a total mess(not done by me...), so when the warmer weather arrives I might just invest in a Wago box and rewire according to the S plan convention.
Thanks, as always, for the help and advice.

P.S. Thanks to our local Screwfix for having the replacement head in stock!

 

[John Brown]

One final(?) question - our system uses a combi boiler(Vaillant Ecomax) but the hot water side is not connected, the heating side does the CH and the DHW tank. This is probably because the solar was added to an existing system(long before we moved in). However, I was wondering what happens when the CH demand stops, the motorised valve returns to closed, but the boiler goes into pump overrun mode.

 

[Spectric]

our system uses a combi boiler(Vaillant Ecomax) but the hot water side is not connected

First are you sure it is a combi boiler and not just a system boiler which are also condensing? It would be very strange to fit a combi knowing it was connected to an S plan with hot water cylinder. If it is a combi then some do have an internal bypass but depends on model whilst others will use the heating circuit but this would not work if you have a zone valve in the way.

 

[John Brown]

Definitely combi. As I say, the previous owners had solar water heating and PV installed, so they isolated the DHW side of the boiler.

 

[Agent_zed]

One final(?) question - our system uses a combi boiler(Vaillant Ecomax) but the hot water side is not connected, the heating side does the CH and the DHW tank. This is probably because the solar was added to an existing system(long before we moved in). However, I was wondering what happens when the CH demand stops, the motorised valve returns to closed, but the boiler goes into pump overrun mode.

My understanding is that there needs to be a call to the boiler. CH will have thermostat(s) that call for heat, this will open the zone valve which will then trigger the boiler to run.
Hot water will have a tank thermostat which again will normally open a zone valve and trigger the boiler. As long as the trigger wire is not connected to the boiler the zone valve can open and close as it likes and the boiler will do nothing as it doesn't know or care about an unconnected zone valve.

 

[John Brown]

My understanding is that there needs to be a call to the boiler. CH will have thermostat(s) that call for heat, this will open the zone valve which will then trigger the boiler to run.
Hot water will have a tank thermostat which again will normally open a zone valve and trigger the boiler. As long as the trigger wire is not connected to the boiler the zone valve can open and close as it likes and the boiler will do nothing as it doesn't know or care about an unconnected zone valve.

Forget about the hot water. The boiler has a pump overrun mode, which keeps the pump running for a short period after the burners have gone out. This may be specific to combis, as opposed to conventional system boilers, I don't know... During this overrun period, the motorised valve will be closed. I guess there must be some sort of pressure operated bypass?

 

[Agent_zed]

Forget about the hot water. The boiler has a pump overrun mode, which keeps the pump running for a short period after the burners have gone out. This may be specific to combis, as opposed to conventional system boilers, I don't know... During this overrun period, the motorised valve will be closed. I guess there must be some sort of pressure operated bypass?

ah ok. My S plan system has a separate circulation pump that shuts off when the valve closes with no pump in the boiler. There is also a pressure bypass in the system incase there is a blockage or radiators are all turned off. looks like this Honeywell Home Straight Auto Bypass Valve 22mm - Screwfix

 

[Spectric]

This may be specific to combis, as opposed to conventional system boilers, I don't know

No the pump over run feature is normal for modern boilers due to the lightweight construction of the heat exchanger and lower water content, without it the boiler would kettle, boil and make lots of noise and be detrimental to the life expectancy of the exchanger. In days of old they had larger cast iron heat exchangers and unlike today where one person can hang a boiler it used to take a lot more effort due to weight.

My S plan system has a separate circulation pump that shuts off when the valve closes

The pump should be controlled by the boiler and not any zone valves unless it is a very old system with something like an old Baxi where the pump would be wired from the junction box and more likely be a Y plan.

 

[John Brown]

@Spectric
As far as I remember, in a combi boiler used as a combi, the default position of the diverter valve would be CH, so pump overrun would work as designed. My combi, having been coerced to function as a system boiler, could be pump overrunning against a closed zone valve. I guess that either:
A) This is no problem, as the zone valve is spring return, and will pass some water if the pressure is sufficient (sounds dodgy to me).
B) There is a bypass circuit that I haven't seen(which would not be surprising, given how difficult it is to see anything behind the giant Oso tank).
But this is just me being curious. The system has worked reliably for many years, I just like to understand stuff so that I can troubleshoot future problems.

 

[Spectric]

There is a bypass circuit that I haven't seen(

That is possible, a take off before the zone valve you cannot see, have you tried a mirror to get a better view?

 

[John Brown]

That is possible, a take off before the zone valve you cannot see, have you tried a mirror to get a better view?

Yes, I couldn't have effected the valve head replacement without a mirror.
I'll have another look the next time something goes wrong. It's all working now and I have other things to worry about.
Thanks again for all the help.

 

[OldGreyDog]

I know Honeywell type valves have a lever that you can override the valve and lock open, Assuming it's a motor problem rather than a microswitch problem I would have thought this might fire the boiler.

I don't know what make you have fitted but it might be worth your daughter having a look as its quite easy to talk through how to do it over the phone.

(I did this with my girlfriend just before Christmas, after delicately telling her, that it wasn't helpful having the camera pointing towards her face, try pointing it at the pipework so I can talk her through it )

Exactly right. Our Honeywell ‘Y’ Plan valve has a lockout lever.