If you are lucky enough to have a 991, model years 2012-2015, you have up to 8 of these little change over valves (COV) in your car. They are used to control things like sports exhaust and heater valve. If your heater does not blow warm air or does this intermittently, chances are that this valve has failed. They were poorly designed and subject to a recall no WC-43 but it seems that the improved ones fail as well. They are operated by vacuum and are part of a maze of vacuum pipes in your engine bay. The part number is 7PP906283C and it is inexpensive and not too tricky for a specialist to fit.
So if your 991 heater is not working, there is a prime suspect!