no hot water from cylinder

[ausblue]

Problem - No hot water from cylinder except when using electric immersion heater.
Setup - System corresponds to fully pumped with two way split after pump to a) Drayton Tempus 7 controller controlled valve to rads b) Drayton 2 way tapstat valve on the cylinder inlet with sensor mounted on the bottom of the cylinder
Symptoms - What is baffling is a) The tapstat appears jammed in the closed position (pin down) but the pipe is steaming hot all the way into the cylinder and b) The outlet (manual valve open) is stone cold
Question - If I am getting hot water flowing in but no return flow where is it going? I would prefer minimum interference with the system as it has worked for donkeys, and the tapstat obviously needs replacing but what is with the cylinder?
Ideas would be appreciated

 

[doitall]

The Tapstat should be on the return and not the flow.

I would suspect an air lock in the coil/return

 

[45yearsagasman]

Tapstat is ok on the flow as long as its after open vent connection. Drayton tapstats are prone to siezed pins and best bet is to replace IMO.
Flow is hot only because the flow is restricted to a small flow through stat and has cooled by the time it exits the cylinder.

 

[ausblue]

Tapstat is ok on the flow as long as its after open vent connection. Drayton tapstats are prone to siezed pins and best bet is to replace IMO.
Flow is hot only because the flow is restricted to a small flow through stat and has cooled by the time it exits the cylinder.

Yes I noted on the Potterton boiler installation instructions that there is a circuit (b-zone valves) where an 'hw valve' is on the inlet whereas for cicuit (c - thermostatic valves) the thermostatic is on the outlet.
The tapstat is obviously siezed (pin only moves about 1 mm c.f roughly 5mm travel on the adjuster) and the restricted flow explanation obviously makes sense. Also the possibility of an airlock makes sense as we recently had to partially drain the system to replace a radiator as well as that I cannot find an air vent pipe on the hw inlet as shown in the recommended circuits.
As an AGA legacy system this would appear to be a real 'committee camel'
Many thanks, will check the above points
Ausblue