CH Thermostat indicator lit when turned off

[joebloggs2022]

Hello

I have a central heating system, with a Y plan layout.

The neon indicator on the thermostat seems to be sometimes dimly lit / flickering even when the thermostat is turned off. I measure approx 50V AC across terminals 2&3 of the thermostat when it is happening.

I have narrowed it down to the following circumstances:

• With the hot water off but thermostat set low, the indidator is dimly lit
• When the hot water is switched on, the indicator goes out
• When the hot water is switched back off, the indicator remains out until the thermostat is turned up high enough and for long enough for the heating to kick in (the indicator fully lights as it should).
• Once this happens and the thermostat is turned back down, the indicator is again dimly lit.

My first instinct is that this could be a problem with the mid position valve when a connection is not being completely broken. However my hesitation with this is that the mid position valve motor and wiring assembly was only replaced only a few months ago.

Does anyone have any thoughts on what might be causing this ?

Many thanks in advance
Joe

 

[Lucien Nunes]

Does the system work correctly?

I don't think this is a fault as such, but an incompatibility between the thermostat with indicator and the mid-position 3-port valve. The reason the dim glow only occurs after a certain sequence of operations is a result of the internal construction of the valve. What I think is happening is that the demagnetisation resistor in the valve is backfeeding a trickle of current to the room stat output terminal (causing the indicator to glow) when the valve is parked in position A (CH).

When the system is idle with neither CH nor HW calling for heat, the valve can be in one of two states.

If the last demand was for HW, the valve will be de-energised and resting in the spring-return position B (HW). The boiler will have gone off because the cylinder stat or timer HW demand output (paralleled with the valve orange output wire) is off. The 'HW satisfied' input to the valve (grey wire) will now be live, but the internal microswitches will have completely isolated the grey wire in this position, thus, no backfeed and no indicator glow.

If the last demand was for CH, the valve will have driven to position A (CH) and will now be held there with the motor energised from the grey wire, even after the CH demand via the white wire has been removed. In this position, not only is the motor winding energised, but there is a pathway of some 270 kilohms via the demagnetisation resistor to the orange output, which will be presenting a voltage to the boiler input, but it's too low for the boiler to respond. There is however another circuit through the valve completed in position A via the second microswitch, from the white CH demand input to the orange output, to enable the room stat to fire the boiler. This is allowing current from the demag resistor to continue through to the white input and causing the room stat indicator to glow.

You might be able to solve it by connecing a small load, such as an RC snubber of say 0.1μF+120Ω from the room stat output to neutral (i.e. in parallel with the indicator.) This will soak up the backfeed current without causing any measurable power consumption or heat dissipation. I don't know what the indicator voltage threshold is, so some experimentation might be needed. What type of stat is it? If bimetal, there will be an accelerator resistor in parallel also, but as it is of the same order of resistance as the demag resistor, it won't hold the output terminal below 100V or so.