Dosen't the monovalve prevent the not coolant from entering the heater core and thus prevent the heater from heating...if it were the monovalve then too much heat should be the problem...not too little?
My car has a similar problem and I suspect that it is due to cold air entering from somewhere it shouldn't not the monovalve. I fixed the monovalve in the summmer due to constant heat from the vents.

Your help is greatly appreciated.

Alec.