Funnily enough, and quite relevant to this conversation, my central heating zone valve failed just yesterday, discovered when I woke up to a cold house....
It's a Honeywell V4043 and is less than 3 years old which I installed myself. The motor has gone open circuit, so not terribly impressed by that given the price of the Honeywell valves vs others... hopefully the control head is still available separately so I can do a dry swap.
As a workaround I have had to latch it into the fill position so effectively I have no heating zone valve now.
And how does that affect the system performance ? As my custom wiring scheme kept the heating zone valve open continously when the pump was running and hot water was NOT heating, no change outside of hot water reheating, however the difference during hot water reheat is stark.
I was sitting in the dining room just now at a comfortable 21C with the radiators cold (due to warm weather) and noticed the radiators creaking, went over to touch them and the previously cold radiators were now blasting at full heat, in the 10 minutes it took the hot water to reheat the room has now overshot to 22.5C and is uncomfortably hot. And I don't even boost the flow temperature for hot water reheat like some systems do.
If the heating zone valve was working, in this low heat demand scenario it would have maintained a very low duty cycle (old firmware) or stayed off (beta firmware) and the radiators in the room would not have heated at all.
So a heating zone valve is definitely beneficial in my scenario. My boiler is perfectly capable of getting radiators up to full temperature during a hot water reheat so this needs to be controlled by a heating zone valve to prevent rooms overshooting during hot water reheat.
With the old firmware with TPI control of the heating zone valve during hot water reheat I did not experience overshoots related to hot water reheating.