Discuss 25 year old boiler wiring in the Central Heating Systems area at ElectriciansForums.net

Andy5678

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Been looking at changing a faulty timer on an oil boiler.

the current timer switches between DHW or DHW and HTG.

I cannot find any motorised valves, it looks like it’s just controlled with a room stat connected to a pump for heating and some other stat I can’t find for the water.

had a look on google and can’t find any diagrams that would suit this and the model of programmer that’s installed just now is not available.

anyone had a similar problem?
 
Sounds like a gravity fed heating system, comprising a 2 channel programmer, pump,room stat and a hot water cylinder stat. Most 2 channel programmers are capable of this but there is normally a small dip switch on the back of the programmer which needs setting to gravity fed system

EBAA4BD4-9A94-4099-9C66-6BCFAE9A2000.png
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From memory these are referred to as hot water priority systems
 

Andy5678

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Cheers, that looks like the one I need. I have just got no idea where the cylinder stat is... will go back and hunt a bit more.

the pump was down stairs under the shoe cupboard the boiler further along in utility room and the hot water cylinder upstairs with no sign of cylinder stat.

I have the dip switch setting on the new programmer.
 

Andy5678

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The cable that disappears is a twin and earth so may not be exactly the same as that diagram
 

snowhead

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Is the oil boiler in an outhouse, if so the other stat could be a frost stat.?

And it could be wired the traditional;
Hot water demand from programmer = Boiler only on (hot water temp is whatever the boiler is set to)
Heating (and hot water) demand from programmer via room stat = Boiler and pump on.

No Motorised valves used.
 

Andy5678

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The boiler is in the house.

starting to make a bit more sense of it know, will just have to track down this rouge stat
 
Are you sure there is a second stat? When I moved into the house I'm in many years ago all that regulated the water and house temperature was the temp setting on the back boiler (Baxi). I soon fitted a tank stat and room stat. The previous owners said it was always lovely and hot in winter. I hate to think what the gas bills were like. I currently use a Drayon LP522 timer. It's got the DIP switch on the reverse for gravity feed systems.
 

davesparks

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Sounds like a gravity fed heating system, comprising a 2 channel programmer, pump,room stat and a hot water cylinder stat. Most 2 channel programmers are capable of this but there is normally a small dip switch on the back of the programmer which needs setting to gravity fed system

View attachment 54743
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From memory these are referred to as hot water priority systems
A gravity system does not use a cylinder stat and it is not a hot water priority system.

The programmer DHW channel directly fires the boiler, the CH channel feeds the room stat which feeds the pump.
 

Andy5678

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I think it is a frost stat as I tested continuity across it and it was open circuit. When cold this would close and switch on heating. makes sense when following wiring on site
 
C-Plan_old2.jpgThe old C Plan relies on the time controller energising DHW at the same time as CH you can see there is no connection from room stat to boiler, it only runs pump.
With controllers where you have access to the com in the contacts and it is not fixed to the line then another possibility.C-Plan_basic_Nest.jpgNote with this the com goes to boiler, a modified form of this is what my home has, modified as the granny flat and house uses same boiler. The DHW is always running when the CH is running and the only control of the DHW is time.
 

Timbo

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Another possibility is to fit a 28mm 2 port zone valve to the hot water circuit, controlled by a cylinder stat. The boiler is then wired through the changeover microswitch to operate with both ch and hw controls, giving full temperature control.

Used to do a lot of these in the 80's.
 
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