Boiler on when timer is off (3 port valve replaced)

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So recently my hot water was getting excessively hot (was way over the thermostat on the hot water tank setting). Engineer came and replaced the thermostat on the tank.

This had no effect. Still got very hot water. But after he left I found the Radiators were not coming on. Manually setting the 3 port valve to open gave me very hot water and hot radiators. Engineer came back and said the 3 port valve was faulty probably due to sludge and this was not covered by insurance).

I replaced the 3 port valve myself with the identical model (Honeywell V4073A 3 Port Motorised Valve) - being very careful to wire it back in with the same wire scheme. Water that came out of the system was clean when I drained down.

Now, with the timer for both water and radiators set to OFF, when I power up the central heating (by plugging the boiler/ timer etc on) the timer remains off and the boiler ignites and provides hot water. The (new) 3 port valve makes a ticking noise.

Turning the timer for both water and rads to ON results in the the boiler switching off!

I am wondering if then the engineer made his original call and replaced the thermostat the hot tank, he wired it up right - or perhaps the timer controller is faulty. All the radiators (except the bathrooms) have thermostatic valves and there is no "overriding" room thermostat anywhere in the house. I'm not aware of a low temperature thermostat anywhere in the system either.

Any suggestions?
 
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I think i have solved the problem (and a bit annoyed).

I took off the timer panel and with the help of some wiring diagrams of the various units involved (and not helped by the birds nest of wires left by the original installer) I traced the wiring back. When the engineer fitted the new thermostat to the hot tank - he swapped C and 1 wires. So when the tank temp was low wires 1 and 2 were connected and when the temp was hot, 1 and C were connected. I can't still quite get my head around why that caused the symptoms it did - but it was weird!

Once I had that figured out I swapped the wires back - and all seems to be operating normally!

This means I have drained the system and replaced the 3 port valve for no good reason...£100 and my Sunday - gone to the wind...
 
Last edited:
While I was working on your mystery wiring you posted your update, well done Sir!

The correct wiring only has one, but there are more than one mistake varieties on wirings. I also believe the engineer made a wrong re-connection, but haven't worked out which two were swapped.

Anyway, you had a faulty three port, there was nothing wrong with your old cylinder stat, and now you replaced your three port, happy day!
 
oh ok - so you think the 3 port was faulty at the outset and it wasn't the stat that needed changing. They are simple bi-metal strips by the looks of it - so not much to go wrong I suspect. If so that makes me feel slightly better.

While I had the system drained I did add some Sentinel x400 to give the system a clean. I assume I should drain down in a day or so. Do I need to flush out the sentinel x400? If so, is it just a case of filling with water, drain and refill again (with Inhibitor of the second fill) ?
 
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If you say the water was clean then why are going to the bother of adding x400??
 
If you say the water was clean then why are going to the bother of adding x400??

At the start of the day I had taken the guy at his word that the system was probably sludged up and had already bought the stuff when I bought the replacement 3 port valve. Figured I could no harm in using it now I had it.
 
You can leave X400 for up to four weeks (after that it starts to lose its cleaning power and the dirt begins to settle again).

As a DIYer I've used it when I don't know for sure how dirty the system might be, and have been gratified to see how much sludge and sediment it loosens, even when the water looked clean. The cost and effort is very small, so why not.
 

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