Can anyone identify this symbol?

  WhiteTruckMan 00:22 27 Mar 2007

click here

Its part of the control circuit of a boiler. This puts it in context.

click here

I've a feeling I should know what it is, and will kick myself when I find out. From the circuit it appears to provide a load, but it isnt a resistor.

Anyone help? I've been googling electrical symbols but the nearest I have found are inverters, which it isnt.


  Forum Editor 00:33 27 Mar 2007

the gas burner modulator coil?

  wiz-king 05:40 27 Mar 2007

Relay. Looks like the coil of a relay (solenoid operated valve) that is used to turn on/off the gas supply to the burner. On the other hand it might not be, dont know enough about boiler controls to be sure.

  g0slp 07:54 27 Mar 2007

I believe that wiz-king's correct.

Without looking at the whole circuit I can't be absolutely certain, but I think that it is the operating coil for the relay powering the circuit that feeds the burner controls. That's to say, the controls which check that the pilot light is ignited before opening the gas valve as a minimum safety function. (More complex boilers have forced draught fans feeding the burner as well, although these are rare in domestic usage).

If you look at the circuit then there are 2 ways that the coil can be activated. Either the Central heat thermostat operates, if temperature's below the desired level, or the DHW (direct hot water) relay operates if that function is selected. As a safety/control feature, if the DHW setting is selected then there has to be water flow in the hot water system to activate the bottom relay S2.

Hope this helps.


  egapup 08:28 27 Mar 2007

Do a Google search.

  namtas 08:58 27 Mar 2007

Looking at its position in the circuit and the description I would say it serves as a control valve with the purpose to allow the gas valve to fully modulate for load control.

That is to say react to provide exactly the heat input for the required heat output.

Some designs however do not use the mod coil in that way and just have a low and high. These models will not give a DHW output temperature of within +- one degree like the best fully modulating models. I am sure that we will have a boiler expert in the forum who will enlighten us all.

  Bingalau 10:06 27 Mar 2007

egapup. WTM says in his original post that he has been googling for it..

  johndrew 10:39 27 Mar 2007

I`m no boiler expert, but I believe the modulator coil has the function of maintaining a constant flame and hence temperature. It should function to control the flame in a linear manner but faults have been reported where the modulator coil appears to `stick` causing the boiler to cycle.

I don`t know what the problem, if any, that WTM is having but hope this may help.

  Kate B 10:47 27 Mar 2007

Blimey *impressed* You lot are amazing!

  Watchful 12:31 27 Mar 2007

On my electrical diagram (combi-boiler) it is a gas valve modureg regulator. If that makes any sense.

  WhiteTruckMan 14:13 27 Mar 2007

I may have been barking at a completely different kettle full of fish in trying to troubleshoot this pesky thing. Going to study it again, and if I solve it I will post some more detailed explanations here.

(btw, thanks everyone)


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