Thanks Jazz, I suspected as much.

There is an additional note of caution. I shall use 24v DC relays as the AC versions rely more on the inductance of the coil to limit current. I have recently encountered a relay coil burn out, in a control unit for a furnace, which had been on continuously for at least six months. The coil was energised with 24v DC, but the relay in the socket was an AC version.