New Valves, inhibitor & balance yet 1 radiator needs constant bleeding..why?

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We have a megaflow cylinder and boiler on the ground floor and have recently had new thermostatic valves fitted on all radiators (apart from the towel rail), radiators balanced, system drained and inhibitor put through.

For the first couple of days all was well but we've noticed a bubbling sound from the top floor radiators (loft conversion) and when checked they were full of air and needed bleeding until water emerged.
One radiator on the middle floor doesn't have an air bleed plug (just 2 caps) so not sure if this has any impact as it's never been bled, but we now find we have to bleed the top floor radiators daily; sometimes 3 times a day just to get them to stay hot!

Any ideas as to what the issues could be would be much appreciated as we've already spent quite a bit and don't want to invest loads more chasing possible solutions for which there may be many options thanks
 
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Show us pics of the radiator that you think has no" air bleed plug".
Are the rads that collect air vertical rads ?
Is the system sealed/ pressurised ?
 
Please post photos of the pump, and the pipes around it, especially the joints on the suction side. Look for any green or limescale marks
 
Picture of the plug that is on both sides of the middle floor radiator is attached along with a picture of the rad that constantly needs bleeding
The system is sealed with a pressure relief valve (which incidentally often leaks via the pipe outside)
 

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Picture of the plug that is on both sides of the middle floor radiator is attached along with a picture of the rad that constantly needs bleeding
The system is sealed with a pressure relief valve (which incidentally often leaks via the pipe outside)

Ah. And you keep topping it up with fresh water?

What does the pressure gauge read (1) fully cold and (2) fully hot?
 
We topped up the water earlier as cold it was a quarter away from where it is now but surely should not have to keep doing that?
Pictures of pump and tundish also attached to see if anything sheds light
 

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Please take the actual readings, hot and cold.

For 24 hours, don't bleed it, and don't top it up.

How old is your red expansion vessel?
 
ok will do so over the weekend and report back thanks - do you have a suspicion as to what it is?
Expansion vessel is around 20 years old
 
@terryplumb may have a better idea, but I think it possible the expansion tank needs pumping up, or replacing, or is too small, and the cold-to-hot pressure swings are causing the venting, followed by low pressure and topping up. Or your fill valve might be passing.
 
I think it possible the expansion tank needs pumping up, or replacing, or is too small,
That is also what I would be looking at, often a car tyre type valve so you can top it up. Inside there is a diaphragm to keep gas and water independent, if this has ruptured it could cause the problem, or if the gas has come out.
 
As already asked ,tell us the system pressure cold ,and what it rises to when central heating has been on for an hour.
Have a bleed valve fitted to the rad that doesn't have one.
Tell us the boiler make and model.
 
Hot and cold readings attached ..interestingly the middle floor rads this morning are now just warm and the top floor ones are piping ..it's almost like the rads take it in turns to share the heat

Boiler also attached and It's been pointed out there should be a filter on it but I don't think that would affect what's going on
 

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Those pressure readings are fine. When heating is turned off and system cools down ,does pressure return to its original position or is it lower ?
At what point after heating was turned on did you notice the different radiators temperatures , shortly after turning on ,or after an hour or more ? Did they all eventually heat up fully ?
 
It was after an hour that we noticed the middle floor rads just warm but top floor ones were still hot.
As it was bled and topped up a lot yesterday we will leave it for a whilst and see what the readings are after a full days cycle of on and off times including putting it on just for an hour when needed ..will report back!
 
Assuming a 12L E.vessel and pre/fill pressures of 1.0/1.2bar then the final pressure will be 1.5bar with a hot 75L system, you can see the E.vessel volume and you know the number of rads so can make a good estimate of final pressure once known.
As suggested above, install a vent on that middle floor rad, the readings will then be more realistic.
 

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