CH misbehaving

[meow2222]

An old Y plan system failing to fire up when CH & HW both called for. Checked programmer output: both lines are indeed live Checked stat output: its live, and switches correctly Checked boiler: no power getting to boiler, or 4v ac to be precise. Checked wiring centre: other than the E&N terminals, ALL terminals except t he pump/boiler one are live.

My conclusion, and I'm no expert on Y plans: I very much suspect a problem with the 3 way valve. However an examination reveals no such valve anywhere ! The funny voltages on terminal 7 (not on terminal 3 which powers the boil er & pump) make it fairly clear that there is a Y plan 3 port somewhere, bu t it has to be either under or behind the HW cylinder AFAICS, or less likel y under floorboards.

But... after failing to fire, while I was searching for the 3 port somethin g went click and the system fired up. Looks like the system is suffering fr om sludging, so perhaps that's causing the 3 way to misbehave. I hope so, a s getting to the 3 way looks likely to be impractical. So I think the first move is to drain, refill with cleaner, redrain refill & reinhibit.

Suggestions/feedback welcome!

NT

[ARW]

My conclusion, and I'm no expert on Y plans: I very much suspect a problem with the 3 way valve. However an examination reveals no such valve anywhere! The funny voltages on terminal 7 (not on terminal 3 which powers the boiler & pump) make it fairly clear that there is a Y plan 3 port somewhere, but it has to be either under or behind the HW cylinder AFAICS, or less likely under floorboards.

But... after failing to fire, while I was searching for the 3 port something went click and the system fired up. Looks like the system is suffering from sludging, so perhaps that's causing the 3 way to misbehave. I hope so, as getting to the 3 way looks likely to be impractical. So I think the first move is to drain, refill with cleaner, redrain refill & reinhibit.

Suggestions/feedback welcome!

Well it's not the valve as the HW ON from the programmer only passes through the cylinder stat to the boiler So recheck the cylinder stat.

[meow2222]

Trying to get my head round this.... how would that stop CH coming on? Y plan does funny things to my brain cells.

NT

[ARW]

My point is that if you are calling for HW and the cylinder stat is in demand then the boiler will have 230V at it regardless of the CH demand.

[meow2222]

Then surely the fault can't be the cylinder stat or the room stat - unless both went faulty, and I know the room stat's ok. AFAICS that only leaves the 3 port valve.

NT

[ARW]

And the cylinder stat.

You can remove the 3 port valve from time and space and the boiler will still have 230V it IF the cylinder stat is demanding heat when your HW is on.

Remove the cylinder stat from time and space and a Y plan setup will never get power to a boiler.

[meow2222]

Oh! In that case I expect it is the cyl stat. Thanks, think I've got one in stock.

NT

[ARW]

I would check that that the existing cylinder stat does not have a loose connection.

[Roger Mills]

Yes, assuming that it *is* a Y-Plan system (and it's strange that you can't find a 3-port valve!) the boiler demand comes direct from the cylinder stat whenever there's a demand for HW-only or HW+CH. Only in the CH-only case does the boiler demand come from the valve. Even then, the valve gets its input from the cylinder stat's HW-satisfied terminal (or from the programmer's HW-off terminal if HW is turned off at the programmer) - so a duff cylinder stat can completely screw things up.

[meow2222]

It is. There has to be a valve because it does HW without CH. The wiring & odd voltages also say Y plan. There's also a diagram from a Y plan valve stuck to the woodwork, albeit no visible valve.

Gotcha. Great, thank you. I know where to focus on now.

NT

[meow2222]

OK, am a bit further forward with this. First the 3 port valve has now been found. Accessing it may prove difficult, but its there, and it is a Y plan type.

2ndly I know better how its misbehaving. The boiler/pump fire up when HW is called for, but not when it isnt. The HW stat is working properly, have tested it. 3rd the 3 way is making a lot of noise at times, buzzing/graunching/whining. Will upload it soon.

So next time the job will be to trace the room stat output.

NT

[ARW]

There is no need to trace the room stat wiring. The mid position valve is f***ed.