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PostPosted: Mon Jan 03, 2011 9:30 pm 
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Evening,

Not too sure whether this is a plumbing or electrical question.

I've got a conventional LPG boiler connected to an external pump and two motorised valves (one for hot water and one for heating). I believe the wiring is of the "S-Plan" convention, but using a combined programmer/ thermostat.

Prior to going on holiday I safeguard the house from frost conditions. I set the thermostat to holiday mode (fourteen degrees). I closed the mains water stop valve and opened the lowest cold water tap to drain the header tanks empty. This is something I routinely do prior to going away.

I got home from holiday to a house that was roasting hot. The boiler had been running all the time. Despite this, the thermostat was off and the Hot Water / Heating Motorised valves were indicated as being closed. I dialled in a higher room temp and I could see the motorised valves opening up once the thermostat clicked in.

Once I de-isolated the mains water supply, the boiler switched off, and it's now controlling to whatever desired temperature the thermostat dictates.

Having a look at this diagram, it looks like the only way the boiler can fire up is if the valve position switches register the motorised valves being open.
http://www.gasman.fsbusiness.co.uk/images/spl.gif

So, any experts out there? How can my boiler be running with both valves closed? I've never seen this problem before, and now it's back to being OK again.

Cheers

Anthony


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PostPosted: Mon Jan 03, 2011 11:55 pm 
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Is there a frost stat keeping the boiler on.

Where is the boiler.

What is the make/model.


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PostPosted: Tue Jan 04, 2011 1:22 am 
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It would be so much more helpful if this site encouraged posters to add their location!

I agree the obvious answer would be a boiler in the garage with a frost stat and in a colder area like parts of Scotland.

Tony


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PostPosted: Tue Jan 04, 2011 10:14 am 
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It's an Ideal Mexico 2 boiler, located in my utility room, so it's not in an "outside cold" location. The last week hasn't been particularly cold in Aberdeenshire, at most just a slight frost while I've been on holiday.

Looking at the boiler wiring diagram, there's no frost stat within the boiler itself. There's just the external room thermostat / programmer which carries out that function.


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PostPosted: Tue Jan 04, 2011 2:00 pm 
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anthony_d wrote:

Looking at the boiler wiring diagram, there's no frost stat within the boiler itself. There's just the external room thermostat / programmer which carries out that function.


So you do have a frost stat :roll: assuming the external room stat is a frost stat, what is it set on.


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PostPosted: Tue Jan 04, 2011 2:56 pm 
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I've got frost protection set to 14 degrees (which is the same thing as holiday mode in this instance).

When I got home from holiday, room temp was 23 degrees.


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PostPosted: Tue Jan 04, 2011 3:20 pm 
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anthony_d wrote:
I've got frost protection set to 14 degrees (which is the same thing as holiday mode in this instance).

When I got home from holiday, room temp was 23 degrees.


One would expect the room temp to be off the scale.

What was the temperature where the frost stat is fixed.

Holiday and frost are very different, the frost stat should be set 5-7c, which brings the system on when the recorded temperature is below set point, however the space that houses the frost stat could be minus 10 for weeks, meaning the system will never shut down, to overcome this small problem we install a pipe stat inline with the frost stat, what happen then is the frost stat fires the system up, and the pipe stat turns it off again when it records say 20c in the return pipe, so, all the system is protected, without wasting bucket loads of cash.


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PostPosted: Tue Jan 04, 2011 3:53 pm 
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This is the Programmable thermostat that I have:
http://www.heatmisershop.co.uk/products ... DHWTS.html

I don't have a separate frost stat anywhere in the wiring circuit (or at least none that I've seen).

The frost setting on the programmable thermostat is adjustable. By default holiday mode and frost stat settings are the same temperature (12 degrees), but I raise the holiday temp setting to 14 degrees to cover for really cold nights.

The Heating and HW valves are responding correctly to thermostat commands, but with the header tanks drained, the boiler was running constantly when both valves were closed. According to the S-Plan diagrams, the boiler can only get 230V via the motorised valves. So with both valves closed, I am rather curious as to where the boiler gets its power from to open up the gas valve.

The boiler's been running fine according to thermostat commands since I've re-filled the header tanks.


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PostPosted: Tue Jan 04, 2011 4:35 pm 
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Drain the heating header is not something I would recommend, it would seem to have affected the working some how, but right out of ideas as to how or why, unless one of the micro switches stuck, and was picking up a call signal.


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PostPosted: Tue Jan 04, 2011 4:57 pm 
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I drained the Cold Water Header Tank by closing the mains stop valve and opening up the lowest cold water tap in the house. Unless the central heating header tank is coupled to the cold water header tank somehow, then I wouldn't have thought I'd have drained the central heating system.

How do the boiler contacts normally work on the motorised valves? Is it a a relay contact that is closed when the valve is energised? Or is it a microswitche which closes when the valve is in the open position?


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PostPosted: Tue Jan 04, 2011 4:59 pm 
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You have a power wire which opens the motor, and a switch wire from the stat that puts power on the boiler/pump via the miroswitch.

With an S plan the two orange wires are common.

Much depends on the valves and how many wires you have.

Could be.

brown.

white.

Gray.

Blue.

Earth.


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