Heat inside the cabin was good - 70 C coolant still provides a fair amount of warmth, particularly when the outside is only -6 C.

I recognize that the interior heater provides additional engine cooling. However, I don't think the interior heater would be large enough to meet the engines complete cooling requirements with an ambient of -6 C. I would have expected that the thermostat would have closed up to restrict flow to the radiator to restore the temperature.

The Volvo Service Manual provides no information on the direction of coolant flow through the heater system. From the external construction of the coolant pump and the location of the heater valve, I deduce that the direction of coolant flow in the heater circuit is out the back of the head, through the heater valve and heater core and then back to the pump. If that is correct then I would expect that the thermostat should be able to close down enough to keep the temperature at the front of the head around 82 C and the back of the head is normally a little hotter than the front (I have a B20 E and can read the old D jet sensor at the front of the block). If coolant flow is in the opposite direction and going through the heater core before it enters the back of the head, then the coolant entering the back of the head could be below 70 C and is mixing to give the dash sensor the approx 70 C reading that I am getting. If that is the case, then this is a non issue.