Alpha cd28c pressure not increasing when heating is on

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storedpepper

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Pump runs, hot water going into central heating pipe, first 3 rads getting hot water but last 3 rads warm at the inlet but rads cold. Hot water is working ok. when the heating is on pump is running but only about an eighth of a bar increase in pressure. I'm thinking that there is not enough pressure to push the water around all the rads.If there was a blockage wouldn't the pressure rise? It seems to me that the pump impeller is damaged in some way. Do these wear out? or could there be air in the pump reducing efficiency? Can i just change the pump head or impeller without taking any pipes off as it looks quite difficult to get to the pipes? Its an Alpha pump. Its already set at 3 so i can't increase it anymore to check it. I don't know if this is a common problem with these or not. Any help would be appreciated.
 
What is the boiler pressure when cold ?
Are the rads that dont heat up on ground floor and the ones that heat up ,are they upstairs ? Try closing the trv valves ( assuming you have them) on all rads that heat up ok ,see if others start to heat. And have you bled air from rads that are not heating up ?
 
Diverter valve.

Check that the actuator head is fully functional. It could well be sending hot water through the transfer plate so your hot water will still be getting hot but it's not diverting 100% of the hot water through CH when the thermostat is calling.

This would result in similar symptoms as the flow is going to take the path of least resistance, ie. The plate exchanger.
 
terryplumb said:
Unlikely if three rads are heating up.
Click to expand...

Recently fixed one with the exact symptoms. I've since edited my post with a more detailed explanation.

I'd presume the boiler will have an auto bleed so I'd doubt its an air lock.

There would be some heat even with seized TRVs.

If the pump is running it'll either feed nothing or all.

That points me to a reduced flow rate, which is probably being lost at the diverter.
 
First thought was air so i bled them all next thought trv's faulty so i took the heads off and made sure they were free. When bleeding the cold rads warm water,not hot, did come out. It's a bungalow so no upstairs. I'm pretty sure that when the heating came on before the pressure rose about half a bar. It's set at 1bar now and only just moves when pump is running.
 
I have thought diverted may be sticking half way, Is that common. Hot water is working ok.
 
storedpepper said:
I have thought diverted may be sticking half way, Is that common. Hot water is working ok.
Click to expand...

It's an issue on most boilers.

I'd remove the actuator head and you'll find a push rod, in or out will either be CH and HW.

It'll probably be spring loaded. Give it a push and pull with some pliers just to ensure its free to move.

Let the boiler run, you'll either have HW of CH but not both. If the rod moves freely I'd consider the head isn't working properly. If you can push the rod (or pull) and get the other service working, that's your answer.
 
The pressure guage will raise very slightly when the heating is on but should be very slight about 0.2 Bar, that has nothing to do with your problem, as advised shut off the ones that are heating and see if it makes any difference to the cold ones
 
SammyInnit said:
Diverter valve.

Check that the actuator head is fully functional. It could well be sending hot water through the transfer plate so your hot water will still be getting hot but it's not diverting 100% of the hot water through CH when the thermostat is calling.

This would result in similar symptoms as the flow is going to take the path of least resistance, ie. The plate exchanger.
Click to expand...
The Alpha CD28C doesnt have an actuator, it is Diaphragm controlled
 
Turn the heating off and let the pipes cool, run the HW on and feel the flow and return pipes below the boiler for the heating and see if they get hot when on HW only
 
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