cold feed filling tank from cylinder

i tried changing all the mixer taps but its still the same

 

The cold feed from tank to cylinder is getting warm and back filling cwst

 

Vented ,indirect massive system feeds 20 plus taps

 

sorry not the coil checked it ,its ok

 

Yes i thought that after i found coil was ok so changed the tmv.the return also was getting cold so was sure it was that.have you ever come accross a blocked vent?

 

Aggghhh now the hot flow to new taps is very poor i was going to change some old gate valves for ball valves.So this could cure the fault?am i on the right track?

 

Another plumber has been doing that for 5 years.But it didn't used to back feed up the cold.I have put air valve at top of cylinder.I think it's had lots of faults and I am working my way through them.Do you not thing it could be the gate valves cruded up ?

 

It's a clinic had ten mixer valves I changed them for mixer taps.The old taps where spray type anyway so had to go.mains not pumped comes in 3 inch pipe.cylinder hot has a pump on the return.So not equal pressure the cylinder is gravity.yes cold back filling tank it is also warming tank up.When water not used at night it's much worse

 

Could the flow be nearly blocked?

 

The taps are reliance TMV 3 Blender taps.We use them cause there rated for this application £280 each come with check valves are they not fit for purpose?