Alpha cd28c pressure not increasing when heating is on

The Alpha CD28C doesnt have an actuator, it is Diaphragm controlled

It has an actuator
Part 30:

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Ian, I've run HW only and the return pipe for the central heating below the pump( NOT the flow) got hot . I don't know if that's normal as there will be hot water in the pump but i think it means the diverter is ok.
no you have a faulty diverter valve, you can get a repair kit and also a service kit, repair kit would probably do for yours, very easy repair or you could change the complete valve but a more difficult job to change,neither pipe should get hot while the HW only is running, you could try as already advised, remove the actuator no need to drain anything, its just a clip, and try moving the pin back and forth with some long nosed pliers , for all the time it takes you have nothing to lose
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The pin moves freely against the spring. I can feel it hit it's stop when pushed in and springs back ok. It was dry when i first looked at it then after depressing the pin a few times there was one small drop of water so I've ordered a repair kit.
if the gland is passing water there is a good chance that it has damaged the actuator and that is what is causing your problem, remove the actuator and switch the heating on , note the position of the piston then run a hot tap and see if the piston moves, if it doesnt you need an actuator too
The actuator is dry but i will check it. Having a problem undoing the drain valve, see pic. It won't budge. It should just be 1/4 turn shouldn't it ? I'm now thinking i might damage seals in it if i try too hard. should i just drain down at radiator drain cock ?


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The actuator didn't move when hot water running but heating outlet was getting hot. So i turned the wall stat up and left it running. All rads now getting hot. I can push the actuator pin in, it's got a strong spring (i thought it would be just motor driven in or out) when the heating switched off the pin pushed against me and when flush with it's body, went click. Can't push it back in now. So it could be sticking.The strong spring maybe a problem when i look inside. I've got the repair kit for the diverter valve which is just the end cover/seal, rod and disk with two seals on it. I'm going to do this in situ. Hope i don't loose any bits inside when i pull the pin out. So the problem was/is the diverter valve or actuator and not the pump or anything else.
Just an update. I bought a aftermarket actuator that didn't fit. It looked identical to the original but was 3mm deeper and wouldn't fit around the casing. So check the size if you have a alpha CD28C. The body needs to be 70mm not 73mm. I took the original apart and found that the brass push rod had been fitted with a steel E clip which had rusted and come off. Got a box of E clips for £2.70 from toolstation and fixed in minutes. All working fine now. Many thanks to all those who gave me advise and steered me in the right direction as i probably would had bought an unnecessary new pump.

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