Heater tap question

[Bushbasher]

Just before christmas I had a whole new a/c system fitted to the wagon that included compressor, accumulator and condenser plus a full flush and new filter as the old compressor chewed itself up and contaminated the entire system......Aaaannnyyyyway, while the a/c bloke was there I got him to replace the heater tap as well as the old one had died. Now here it my question; Is there a a right way and wrong way to fit the tap, or, can it be put in back to front? The reason I ask is that since it was fitted the heater doesn't want to work when the knob is wound round to the red side of the dial. The tap is connected and there is good vacuum and it is opening when the knob is twisted yet there is no heat forthcoming. It will eventually kick in if you leave it on hot but it can be rather erratic and be 5 minutes or over an hour. I thought there must be a blockage so pulled the hoses off at the tap and where they exit the heater core and blew compressed air through the system and it was clear. As soon as I reassembled the cooling system and started the car the heaters worked like a charm but next time in the car they were back to not working again. I can coax the system into life if I pinch off the LPG converter pipes but that's not a solution either. Prior to the new tap, the heat was on all the time due to the tap failure and as the converter gets hot and everything runs as it should I know there is good flow from the water pump. The engine temp is normal and the thermostat is a regular one, not one with the extra leg so the heater line isn't blocked.

So, back to my question about the tap being the wrong way round; there is no direction arrow on it that I can see but one side has a significantly smaller ID than the other and it would seem to me that there would have to be a pressure variance depending on the direction of flow. If that's the case is the variance causing the coolant to not flow due to back pressure created by the LPG converter return line? I hope that makes sense and if there's any hydraulics expert out there that can confirm or reject my hypothesis I'd be grateful for the input before I put the car back up on ramps and have to crawl under it on my back and wear the contents of the cooling system again.

Cheers
Bushbasher