30/04/2026
Hi folks,
Came across an interesting setup at a client’s property and wanted to get some opinions.
It’s an Open vent system, but the arrangement on the hot outlet caught my attention.
A mixing/thermostatic valve has been installed directly on the outlet from the cylinder, and this same line appears to serve the vent path.
My concern is the following:
In an open vented system, the expansion path from the cylinder to the feed & expansion tank should be completely unobstructed and unrestricted.
Any component that may introduce resistance — such as a mixing valve — could potentially interfere with:
free expansion of heated water
proper venting of the system
pressure relief under certain conditions
Preferred approach (in my opinion):
I would normally install a tee directly off the cylinder outlet:
one branch going straight and unobstructed to the open vent
the second branch feeding the mixing valve / pump circuit
This way, the vent remains completely independent and unrestricted, as intended.
To be clear:
I’m not saying this setup will necessarily fail — many systems like this may operate without obvious issues.
However, from a best practice and safety perspective, I would personally avoid placing any restriction on the vent path.
Interested to hear your thoughts:
Would you consider this acceptable, or would you also prefer a dedicated, unrestricted vent connection?