Themaclassic F24E pressure problem

3 Dec 2012
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United Kingdom
Hi there!

I have a problem with a boiler Themaclassic F24E, it looses pressure just after the heating is on when it cools down. So basically, i have to raise pressure to 1bar for it to work again. When domestic hot water is used there is no problem.
The pressure release valve was changed, the expansion vessel is at the right pressure (0.5 bar), the radiators were bled and there is no sign of leak..
Does anyone have any idea of what the problem might be?

Thank you in advance
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The expansion vessel might be at the right pressure (although your not stating what it is)
But there is another condition which is just as important.
When checking or re charging, the vessel must be empty of water.
The water side must be open to atmosphere so any water in the vessel can be discharged out.
The correct operation of the vessel depends on having the maximum volume of air to compress. With a vessel full of air and no water the air pressure set say at 1bar would rise to say 2bar or maybe slightly over.
This is OK because the pressure relief valve is set at 3bar.
Due to the vessel losing some of it air, the volume is reduced and when the same conditions apply the pressure goes well past 3bar. This opens the PRV and pressure drops to zero or maybe 0.5bar and you have to top with water again on a regular basis.
Also to consider is the rubber diaphragm in the vessel could be 'holed' so water gets to the air side.
There is also flexible pipe connecting the vessel to the system and this could be blocked so the vessel can't take in the extra water.
So a fault by the vessel gives a quick discharge, but there also times when the pressure drops over a longer period, say overnight when heating is not on.
This would most likely be due to the PRV not sealing properly due to dirt on the valve seat. placing plastic bag on discharge pipe is good for checking what comes out overnight.
You need to take note of the set pressure and the max it goes to with heating on.
There is no problem with the HW because part of the system water is diverted and it goes around a much smaller circuit and the volume of system water that gets heated is reduced, so not as much expansion, not as much pressure
Hi Mandate!

Thank you very much for your help!
The pressure recommended by the manufacturer is 0.5bar. The boiler pressure gauge was going over the limit so I checked the pressure,
discharged the boiler with the pressure release valve (which i thought was enough) the pressure was 1.1 bar so I released it till the 0.5bar but according to you, and it makes sense, if there is water in the vessel it will not get out unless i really open it to air! This will obviously not let the vessel work properly.
Next weekend will try to get the vessel open that way and will let you know the result! Its in the back of the boiler so quite a bit of work..

Thanks again!
You may be able to drain off water from a radiator, depending on what type valves you have, so you don't need to touch the PRV. But you still have to have access to the valve on the vessel, so stand by with foot pump and pressure gauge
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Last time I gave it a bit of pressure after releasing a bit too much and the vessel is fine cause i checked next day and the pressure didn't change.
To release the water on the vessel I opened the PRV and what I should have done was drain the system on a radiator or in the boiler drain valve. No need to disconnect the vessel from the boiler right?
Agree no need to disconnect vessel, unless you suspect the connecting pipe to be blocked
If vessel is half full with water you will only need to discharge half the volume of vessel. I would feed it into a bucket so you'll know how much you have discharged.
Then charge vessel with air to instructions, normally about 0.8bar (check with tyre pressure gauge) making sure water pressure gauge reads zero.
Then top up the water side to instructions, normally about 0.8 bar or just slightly over.
Under these conditions you can see the diaphragm will still be biased over to the water side.
If when running CH if the pressure still rises to 3bar it would suggest the connecting pipe is blocked. Or the diaphragm holed, in which case water would exit from the air valve when you depress the valve pin.
Hope it all works out OK
I am quite sure the vessel pipe is not blocked. At the moment the problem is about loosing pressure and not about getting over pressure. I don't know if the vessel has any water cause its hidden at the back of the boiler and i hardly manage to access the air valve on the top.
My plan is drain the water from system (from a radiator) so the vessel drain any water too, give the right pressure on the vessel with air pump (0.5bar) then fill the system again with water to the right pressure (1bar) and bleed air on the radiators. Am I missing anything?
If I drain the system from the radiators how will I know how much water was on the vessel? If I drain the central heating from the boiler will that be any better?
Thank you
It does matter where it is drained from, although better not to use the PRV if it might not seal correctly.
You don't need to drain the whole system, if you drain off the same capacity as the vessel that would be adequate. Pushing air in the vessel will force the excess water out. I suggest catching it so you will know the volume compared to that of the vessel. Vessel sizes vary but most don't exceed about 10.litres.
As you know bleeding the radiators will allow the pressure to drop so you will need to top a time or two.
Last time i did it by the PRV cause i installed it new so its safe and I thought it would work. Didnt..
The Vessel capacity is 5L. The radiators / system is not completely drained for at least 4 years so will cease to do it now.
Yeah, top up is easy, at the moment need to do it on a daily basis..
Thank you for your help, will let you know when done!
Hi Mandate,

Drained the boiler and pressurized the expansion vessel. After filling up the central heating system with water again the pressure on the expansion vessel raised for almost double the pressure so I had to release some air. Bled air on radiator and all ok so put the boiler working again.
After working twice on sunday it didnt fail but the pressure dropped and was stable at minimum (0.7bar) normally should be 1-1.5bar. Will work tonight, will let you know what happens tomorrow.

On this boiler the small braided hose at the bottom of the vessel can very easily become blocked. If its blocked the vessel cannot take the expansion when the water is at a higher temperature. It may need cleaning/replacing. I believe the current part has a slightly larger internal diameter to prevent this issue. Also make sure that you dont put any major bends in it when you put a new one in. it can kink the internal hose causing the same problem.

Hope this helps.

Hi Mandate,

Unfortunately the boiler lost again pressure to zero.

Hi Naanbread,

Thank you for your advice. I tested the pressure on the expansion vessel at different points and it varies, if it was blocked I think it wouldn't, what do you think?
I recon the vessel is faulty..
This is my girlfriends boiler and she is contacting British gas to repair it anyway as I will not be around this week.

Thank you both for the support.
She has evidently become fed up with your inadequate efforts!

Top up boiler pressure to 1.5 Bar cold and let ALL of the air out of the EV. Plenty should come out but no water!

The pressure on the boiler should then have fallen to less than half!

Reduce pressure in boiler to zero and pressurise EV to 0.9 Bar but ensure the boiler repains at zero until you have finished pumping air.

Then pressurise boiler to 1.5 Bar.

If all is well she can cancel BG and give you something to thank you.

Hi Tony!

You seem to know this boiler and that procedure makes sense to me!
Will try to convince my girl to cancel British Gas.
Will let you know the result probably this weekend!

Thank you very much
You should have done it today!

In the heating buisness time is of the essence! next weekend is too late!


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