boiler cuts out via cylinder stat

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I have a vented system with a bog standard Grant external boiler. Basic Drayton timer and room stat. Radiators all come on even if only HW is selected. The boiler cuts out when hot water reaches temp. I've proved this by turning the stat up and boiler fires up.
The Grant approved engineer changed the top of the 3 way valve and the indicator moves correctly but could the actual valve be at fault? If I just press CH on the timer, there is no demand. There is no air in the system. Any advice please?
 
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I take it you have a 3 port valve system? Was valve checked to see if it moves freely when head was off?
As above get engineer back they need to be competent with heating wireing, as has been suggested programmer or even cylinder stat need checking.
 
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I'd get a different engineer back lol.
But yea, as per above.
 
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Thanks for the answers. I had already had the valve electrical bit replaced by another Grant approved engineer because the rads came on when HW selected. We just lived with it for a year. Then the boiler started cutting out and the next Grant bloke said it was air. Consequently Ive spent 6 months trying to bleed the system til I tried the cylinder stat yesterday. I've used 3 local engineers who haven't realised the problem! If I select CH only, there is no demand at the boiler (red led on the on/off switch) does that mean the 3 port valve is a red herring?
 
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Thanks for the answers. I had already had the valve electrical bit replaced by another Grant approved engineer because the rads came on when HW selected. We just lived with it for a year. Then the boiler started cutting out and the next Grant bloke said it was air. Consequently Ive spent 6 months trying to bleed the system til I tried the cylinder stat yesterday. I've used 3 local engineers who haven't realised the problem! If I select CH only, there is no demand at the boiler (red led on the on/off switch) does that mean the 3 port valve is a red herring?

Sounds like a wiring mess.
Perhaps have a electrician look at it.

Or, take some pics and share them here.

Boiler connection, stats, programmer and control valves.

If you are competent, take a pic of the actual wiring connections.
 
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Thanks again. Unfortunately I cant manage to get photos. I will ring an electrician on Monday though. Appreciate everyone's help.
 
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As I said before whoever you get to look at it try to make sure they are competent with heating wireing.
Three port valve systems are not difficult to fault trace but you do need a understanding of how they work.
 
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Stop reaching, could be the programmer, broken wire, faulty thermostat, anything.

Probably a faulty engineer to start with....

Give the DIY bashing a rest.
 

DP

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5C6B7CE5-FAE7-4C41-AA30-D297E0508CC6.png

If I select CH only, there is no demand at the boiler (red led on the on/off switch) does that mean the 3 port valve is a red herring?

Three port midposition valve needs three wires from the programmer.

The boiler runs during HW cycle via cylinder thermostat. The HW port is open at this instant and CH port is closed.
You select CH as well while HW is being heated and has yet to reach temperature.
White wire goes to 240 volts from CH circuit
valve moves to midposition
Eventually HW is up to temperature
Grey wire needs 240 volts. The valve should now close the HW port and ch port stays open as well as orange wire maintains boiler and pump power supply.

For this you could get someone like Stem, or a heating engineer who knows his controls but to get a spark or a plumber or a ‘heating engineer’ you will be wasting your time
 
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DP

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Probably a faulty engineer to start with....

Give the DIY bashing a rest.

Some of the stuff you are quoting or suggesting is way off the mark
An example being replacing a radiator valve on a live system.
Nothing wrong with DIY, but to claim you are the master of all things heating and plumbing, if pigs could fly
 
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Some of the stuff you are quoting or suggesting is way off the mark
An example being replacing a radiator valve on a live system.
Nothing wrong with DIY, but to claim you are the master of all things heating and plumbing, if pigs could fly

You've never replaced a valve on a live system?
Really?
I did this to all my rads when i installed trvs in my house. Swapped both sides as well.
It isn't rocket science. Just because it's beyond your comprehension doesn't mean it's invalid.

I haven't claimed to be something i am not.
Have you?....

For this you could get someone like Stem, or a heating engineer who knows his controls but to get a spark or a plumber or a ‘heating engineer’ you will be wasting your time

Heh.
 
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You've never replaced a valve on a live system?
Really?
I did this to all my rads when i installed trvs in my house. Swapped both sides as well.
It isn't rocket science. Just because it's beyond your comprehension doesn't mean it's invalid.

I haven't claimed to be something i am not.
Have you?....

That is quite amazing, so please explain how you did this so that us mere mortals can comprehend the measure of your skill base
 
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That is quite amazing, so please explain how you did this so that us mere mortals can comprehend the measure of your skill base

I was going to post a smart reply but its getting tiresome.

Just use a bit of common sense. Watch a few YT vids.
 

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