Boiler firing up at random. Please assist

[BenWBell]

Hi there. I have a Baxi Solo boiler connected to a Honeywell timer control system. The control system consists of a ST699 time controller, a T6360B Room Thermostat, a L641A Cylinder Thermostat and a V4073A Motorised Mid-position valve. The entire system is installed about 7-8 years I believe.

Right. The problem I have is that the boiler is firing up randomly when it shouldn't be on. The timer says no, but the boiler fires up, stays on sometimes a few seconds and sometimes an hour or two, then shuts down again. Sometimes it will then come on again within seconds or a few minutes. I've replaced the room thermostat and the time controller to no avail. At one point the heating portion wouldn't work at all but setting the lever on the mid-position valve to manual enabled the heating when the hot water was due to be on. Now the lever on that keeps shifting itself back towards the auto position, not sure if this is anything to do with it.

This is becoming a real pain as the boiler is in a cupboard in the bedroom and is very annoying if it comes on at 3 in the morning. It generally is fine when the timer says it should be on, and I can get it to work on demand, but it's the randomly coming on at other times bit that is the issue.

Any thoughts on this? Please? I'll be your bestest buddy if someone can help me with this. I don't want to call a heating engineer yet as the expense with not knowing quite what is wrong is worrying, when I mentioned this at the service a few weeks ago they didn't know what it could be and I generally trust that company.

[Roger Mills]

Has it ever worked properly while you've owned the house, or has it always done this?

It's very difficult to envisage a fault which could develop on a correctly installed Y-Plan system to make it come on when the programmer is off. [S-Plan systems, on the other hand, *do* have possible failure modes which can make this happen].

With Y-Plan (which is what you've got) the only thing which should be live [1] when the programmer is off is the HW-OFF terminal on the programmer - which is connected to terminal 2 of the cylinder stat and to the grey wire of the mid position valve. See Y-Plan in

for a wiring diagram. Your system should be wired exactly like this except that the pump is almost certainly controlled by the boiler - so the boiler will have a permanent live feed in addition to the switched live shown in the diagram, and the pump will be connected to the boiler's pump terminals rather than just wired in parallel with it.

If your system has *never* worked properly, I suspect that the cylinder stat may be wired incorrectly. It's vital that it's wired exactly as per the diagram. If any of the wires are crossed, all sorts of funny things will happen - possibly explaining your symptoms, although I can't quite get my head round the exact mechanism.

[1] except for the permanent live to the boiler, if it controls the pump - but this won't make it fire.

[BenWBell]

in

a wiring diagram. Your

Yes the system worked perfectly fine, absolutely no problems until about a month and a half ago. We've been in the house 6 and a half years and no problems before this point with anything to do with the heating system.

I believe that is how the system is wired, but tomorrow I shall open up what I can and check all the wiring that I can see. It was odd that the problem I had for a short period a couple of weeks ago was solved by moving the mid-position valve lever to manual, but now the heating still operates when it is set to automatic (a mode the switch seems to want to move itself back into somehow). Perhaps the valve in there is sticky or something. Would some stray electrical wiring or an electrical fault in the mid-position valve possibly send a fire command through to the boiler? If not what components are capable of doing this and I can pay specific attention to those.

I am not a plumber or electrician, but have a physics degree and can find my way around wiring and electricals given the time, though I have a plumber and heating engineer in the family but they live in another country unfortunately.

Ben.

[Roger Mills]

OK, if it *has* worked in the past, and no changes have been made, it's very unlikely that the tank stat is wired wrongly - so disregard my previous suggestion.

Having ruled that out, the prime suspect has to be the 3-port valve or - more particularly - its actuator (the electrical part). This has a strategic role in the control of Y-Plan systems and is more often than not the culprit when malfunctions occur. An explanation of what this does can be found at

get your symptoms, you'd need an intermittent short between the white and grey wires - which could perhaps happen if one of the microswitches in the actuator is faulty. Given that you have already had problems with this, I would be inclined to replace the actuator, since that will probably cure the problem. Unless your valve is *very* old (which I don't think it is) you can unbolt the actuator from the 'wet' part of the valve without needing to drain the system. Make a careful note of the wiring before disconnecting the electrics in order to connect the new one in exactly the same way.

Incidentally, the manual setting simply physically moves the valve to the mid position, and is useful when filling the system from scratch since it opens the HW and CH circuits at the same time and makes it easier to get the air out. It's not intended for running the system in anger. In an emergency (like a failed actuator motor) it gives you *some* heating - but only while the HW is being heated. As soon as the HW demand is satisfied, the heating goes off! In Auto mode, the valve automatically goes to one of 3 positions [HW, Mid (both) or CH] depending on the combination of demands presented to it.

[BenWBell]

at

> To get your symptoms, you'd need an intermittent short between the white and

Okay, thank you very much for your assistance, I guess my best bet is to try and replace the top of the valve (it does look like I can just unscrew the top bit of it). I'll see about getting a replacement part and I'll let you know. If you can think of anything else then please let me know.

Many thanks.

Ben.