Weird thermostat behaviour?

[pcmihnea]

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?

[Seekonk]

I'd like to think that I am one of the few people in the world that could understand what you said.  Evidently not.  Are the other thermostats electronic?  The minimal voltage sounds like they are.  That would mean that they derive power from a series connection.  It could be that a parallel short is resetting one of those and it is latching on.  If they are electronic, they will draw power and three in parallel could draw enough current to latch on the furnace control as the control likely has a voltage hysteresis.  Changes in line voltage could make it appear to happen some times and not other times.

[BurningTantalum]

Or it could be a motorized valve acting up.
Am I to understand that the person who sets the highest temp on their stat determines the temperature for the other two apartments? Have you come to blows over this arrangement yet?

[eKretz]

Could be as simple as a damaged/sticking relay in the furnace control also - i.e. damaged contacts welding. That's where I'd look first.

[Brutte]

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?

My guess:
It may be a central heating system with one gas heater and two coils: first coil for domestic water (with boiler and heat exchanger) and second coil for central heating. The switch is made by a servo controlled three-way valve and it is stuck/switches incorrectly. So it already did central heating job and now tries to heat up domestic water but the hot water runs through a wrong coil.

EDIT:

when this happens, the line appears as a short

This puzzles me. Disconnect the wiring from the gas heater and see if it is the wiring or the heater that shortens/disconnects that 24V.