Can current flow through a relay's NO channel when it's not energized?
I'm an electronics newbie and I built a circuit where a relay is operated by a Raspberry Pi Zero W. See circuit image. The relay used is a SRU-06VDC-SD-C. It's rated for 6v but I hear it click open and closed using the 5vdc from the Pi.
I would use this to turn my heating on and off at home, the circuit that switches heating on and off is a 110V DC line coming from my heating installation (my AC voltage is 220V). When this circuit is closed, the heating is turned on. This line is wired to the NO contact.
It worked fine for weeks when testing but when actually wired up to the heating it worked for about a week but now the heating is always on, even when I explicitely do not energize the relay. The heating is even turned on when the pi is switched off. When I measure with a multimeter, the meter claims the circuit is not closed. Could someone tell me what I'm doing wrong please?
Thanks in advance!
(updated after advice from Transistor)