The house we bought has UF heating fed by a Brötje - Heizung oil boiler which we hope to replace with a pompe chaleur. In the meantime I’m adding some smart control to the UF heating. Currently there’s just one seimens thermostat. I pulled the cover off and checked for mains voltage on the contacts (contactless sensor tool and neon). No power observed. I came back a few minutes later and the contact/s were live. I suspected the boiler was energising it intermittently. Asked Chat about it -
It told me - Quite right — and a very common one with older continental heating systems.
People are used to the idea that a thermostat line is either:
- permanently live,
- or permanently low-voltage.
But with boilers like your Brötje, the control feed is only energised when the boiler’s logic decides it should be. So someone testing the thermostat line can easily think:
- “Ah, this isn’t live — safe to work on,”
and then - a few minutes later the boiler runs a pump cycle, sends 230 V up the line, and suddenly the wire is live.
Fortunately I wouldn’t risk touching anything with the boiler still plugged in but I imagine some people have had a nasty surprise. I thought I’d mention it here as I’d never heard of this type of operation.
if it goes wrong. Here endeth the lesson.