If your engine is coming up to operating temp ok then unlikely to be main coling system thermostat.

Low coolant could be the cause as not enough hot liquid going through the heater matrix. Not sure about the finer detail of flaps and valves that can stick but i suspect you need to follow the white rabbit (start at the matrix). Might also be the control unit if you are unlucky.

B.A.Nana – Member
As above, only time it happened to me I had virtually no coolant in the car

just to give more details of my experience. It was obvious, not just topping up the res with a bit of fluid, but filling up with a few litres of coolant/water. If you just ‘top up’ the res, it’s not the problem imo. I think the heater matrix has a slight leak as the driver carpet was damp, seems to have sorted it now tho, or it’s just a very slight leak. hth. 2002 mondeo estate.

Fill up the resevoir with 33% antifreeze 66%water mix.

The system took 2.5 litres. Ouch.

It has been running for a whilst now and has not dropped any water?

The thermostat is working I think. The water pump was not engaged when cold. Then when getting up to temperature the water pump engaged. However I cant be completely convinced due to only being able to feel warm pipe versus cold pipe on the inlet and outlet of the water pump.

2.5 litres is a lot to lose.

correction. Duh The water pump always works.

Still no change. The system is holding coolant.

There looks to be red deposits around the thermostat housing. No obvious big leaks which is great. So new housing is needed I think.

Any improvement to heater function now you’ve added 2.5ltrs?

Does sound like youve found the leak…

Right.
Heater function works correctly in all formats.

Being run up to temp twice now and a 20 mile run.

The issue has been long term. There is a powdery residue which is around the thermostat housing. I have looked at the housing, and where the two parts are joined together there is a very small leak when at full pressure. Only slightly but it is a leak.

When I have a spare day I am going to fit a new housing.

Thanks for everyone who has offered help. Much appreciated.