Hi,
TL/DR? the central heating system connected to a normally-open relay thermostat sometimes still runs even after the thermostat's relay dis-engaged (opened). Why?
Some background: I'm in a larger house which is divided into multiple apartments. Three of them, including mine, share the same gas central heating system. The heating's control protocol (turning it on and off) is based on the simple standard two-wire connection: if those are shorted, the system starts working, if not it doesn't work. Not too long ago, only one apartment had a thermostat (which, like most other thermostats, integrates a latching relay). After a while, the second apartment connected it's own thermostat in parallel with the first one (so anytime at least one of them engages, it starts the heating). Recently I've also connected my own one in parallel with the other two. As for cable length, it's about 5m, 8m, and 12m respectively, all of them originating from the central heating system, so only one electrical node. The issue is that sometimes after my thermostat dis-engages (i.e. disconnects the two wires after the pre-set temperature was reached), the heating system still runs. I've checked:
(1) my thermostat - tried three different ones, all the same behavior,
(2) other apartment's thermostats - when this happens, the line appears as a short (very small voltage between the two cables), but none of them are engaged,
(3) the heating system - at my end, there's around 24V DC voltage potential between the two wires, with a high impedance in respect to live, neutral, or ground (so the internal logic is supplied via a linear/smps transformer); the shunt current is 1mA (around 0.9936mA),
(4) the control signal itself - if I short the wires in a short pulse once or twice after the heating system appears to be blocked on, it will stop - that's why my first "solution" was connecting an normally open button and pressing it a few time when I notice the issue. It also disproves that another thermostat is still on when it happens.
The problem is quite odd, as it will persist for hours at end, up until I apply the mentioned "solution" - otherwise the rooms heats up way over the temperature set in any of the apartments.
The control wires' resistance shouldn't be the problem, as the contact is normally open, i.e. a high resistance should be treated by the heating system as a turn-off signal.
Thus, I would like some of your insights as to what the reason behind this behaviour may it be? Could it be that the heating system has an control input with analog hysteresis? Or some odd voltage and/or current level thresholds?