So we had a problem with our installation of the new Heatmiser thermostat. The thermostat is now positioned half way along the boat, before the old thermostat has been in probably the coldest part of the boa, much nearer the original heater controls. Now when the batteries are not fully charged the voltage drop on the 1 mm2 cable can to great to make the Eberspacher Diesel heater fire up reliably.
Problem solved by adding a remote relay
To fix this problem we needed to fit a relay where the old thermostat was positioned and use the Heatmiser thermostat ‘Call for heat’ signal to switch the relay, which would intern switch on the diesel heater.
The new wiring looks like the diagram above, with the relay much nearer to the heater there is minimal voltage drop from the relay to the heater, while the Heatmiser thermostat is still happy.
The relays I purchased at 12v automotive spec with a way over the top 30A switch capability. They come with a 30A fuse, this is replaced with a 1A fuse for this installation.