Back boiler tripping immersion safety cut-out


Postby Skids » Tue Aug 05, 2008 11:11 am

Back boiler tripping immersion safety cut-out

Hi all,

Problem: ‘back-boiler’ heats HWC to above 95°c, which trips the immersion heater safety cut out.

Solution: fit thermostat the HWC/primary flow, so that when temperature reaches (say) 75°c, thermostat tells the CH pump to ‘turn on’ and divert the hot water from the HWC to CH.

Question: how to wire the thermostat, with a permanent live connection so that in the event of a overheat situation, the pump is activated, irrespective of whether it is turned on at the ‘double- pole switch’, thus dissipating excess heat quickly through the radiator circuit and prevent boiling the WHC.

There are three terminals on the thermostat wiring:

1. Common inlet.
2. Outlet (opens the circuit with increasing temperature).
3. Outlet (closes the circuit with increasing temperature).

Where does wire 1 connect to? (on the double- pole switch)
Where does wire 2 connect to? (on the double- pole switch)
Where does wire 3 connect to? (on the double- pole switch)

So the question is: where do all the wires need to come from and go to?

Before you ask, there is no heat sink rad.

Regards

Skids
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Postby ericmark » Tue Aug 05, 2008 5:36 pm

I can see the problem and yes it on the face of things seems a good solution. But there are quite a few different plans used on central heating and one would need to know which plan you are using.
For example with the Honeywell Y plan you would need to power the motorised valve which would normally power both the pump and boiler the latter you would not want to run and even if you relay the boiler so it does not run it would pump water to boiler then to radiators and I am not at all sure if it would cool the water enough.
In old days with side boiler on solid fuel cooker we would manually run off water to cool it.
We did have a damper to direct where the heat was directed but it would not completely stop the heating.
Is the back boiler an extra to main boiler or the only central heating? Some back boilers were really a separate unit to main fire and so where part of it and before anyone can advise on wiring one would need to know a lot more about the system being worked on.
Eric
ericmark

Postby Skids » Tue Aug 05, 2008 9:33 pm

Hi ericmark,

The back boiler is the main heat source with no controls, put another log on for hotter, if you know what I mean

Thermostat is a simple Tower pipe/cylinder stat.
The only other control is a pole switch which turns on the CH pump manually.

I need to wire the ‘stat’ into the pole switch to override the pump

Regards

Skids
Skids
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Postby ericmark » Wed Aug 06, 2008 2:31 am

I should think you only need to wire tank stat across switch.
You say Pole Switch if this means it switches both line and neutral then you will need to bridge neutral wires also so only lines are switched.
I think you may have a problem switching off as once started you may find thermo siphon will keep radiators running.
You are running C plan which normally to stop overheating of the hot water would include a motorized valve in your case this would burn out boiler so can't be fitted.
Normally you would have a junction box to connect all control items together and you may want to look at standard wiring http://www.ericmark.talktalk.net/boiler.html
I added the motorized valve to my parents system because of domestic water overheating last year I used a radio controlled thermostat to save having to run wires and it has worked.
All best Eric
ericmark

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