In a 58 plate FH last night and night heater struggled to get hot and blew cold air before switching off. Would cut back in but never really got hot all night.
I thought I was going to park in layby so didn’t fill up. Was just about in red. Would that affect it ?
The supply pipe from the diesel tank to the night heater is usually shorter than the main supply to the engine so that having the night heater on won’t cause you to run out of fuel. So yes the fact that you were low on fuel could definitely stop the night heater from getting warm.
I had same problem with a scania. Fitter that fixed it said that because I ran low on fuel the fuel had gone back from night heater back to tank. Even after refuelling, fitter had to do something to get fuel back into night heater. I won’t run that low again 
I moved some 62 plate volvos for Tesco a couple of years ago and they were run on fumes but heater still worked. Did they change design
I had the same prob one night - 12 outside NFT Middleton,couldn’t,t believe how easy it was sorted out.On the inside of the drives side chassis leg just above the tank at the back of the cab is a small dosing pump about the size of a large C battery,you will see a small fuel line coming from the tank to it and leading on the heater .You can trace it back from the heater as well.It is a dosing pump ,this means it only clicks a very small amount of fuel at a time.Switch on the heater and listen for the clicks on starting,if it does not click it is stuck closed over the summer.On starting give it a tap with some thing metal,this worked for me
Thanks will try that.
Did you turn the dial to the right?
I know someone who was cold all night cos his thermostat was turned to cold.
Yup