Alpha CB24 pressure dropping regularly

Hi everyone,

I have an Alpha CB24 boiler that was installed in 2001. I bought the place two years ago and had it checked, the gas engineer said it was fine but the seals may need replacing in the next couple of years. I've no idea what the seals are but he didn't seem concerned in the slightest.

It's been working fine since then, but during the summer, the pressure was dropping every couple of months or so. I'd only know when the hot water stopped working, and it only started happening after the system was drained to replace some TVR's. It's been drained again since to remove a couple of radiators.

It's been chilly here so I checked the pressure two days ago before I put the radiators back on, and it was at the very top of the green "normal" range. Today it had dropped completely and the heating wasn't coming on. I've not had the radiators on much as I'm a "jumper and blanket first" person, so this is a bit worrisome.

Does this sound like it would be an expensive thing to fix given the age of the boiler?

The Alpha CB24 is a bit of a dog. A poor design that few like working on. It depends where the leak is...have you checked the obvious such as radiator valves/joints, the boiler pressure relief valve.

Gasguru said:

The Alpha CB24 is a bit of a dog. A poor design that few like working on. It depends where the leak is...have you checked the obvious such as radiator valves/joints, the boiler pressure relief valve.

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Thanks. That might explain why the engineer didn't want to come back out!

I had no idea a leak could be the cause, so I hadn't checked for that. I know zip about boilers and my plumbing knowledge extends to swapping taps and changing washers. It's already been repaired at least once before I bought the place as there's a red ball thing on the floor that's connected to it, the engineer told me that was due to a repair. There was water damage on the carpet near the boiler too, so I assume that was a leak.

I'll check the radiator valves and joints, and the boiler pressure relief valve, is there anywhere else obvious I should check? If it's a leak, would a bit of plumbers tape or something hold it until I can get someone out?

Find the leak ,if you have one ,then you can explore the options of how to proceed. Have you " bled " air from any of your radiators recently , that would also reduce the system pressure.

Gasguru said:

The Alpha CB24 is a bit of a dog. A poor design that few like working on. It depends where the leak is...have you checked the obvious such as radiator valves/joints, the boiler pressure relief valve.

Click to expand...

Absolute rubbish, the CB and the Baxi 80 and 105 are the exact same Hydraulic set up as they are in fact the same boiler, built in the same factory so to come out with that is total tosh, the boiler has its problems like most but that statement is completely false

terryplumb said:

Find the leak ,if you have one ,then you can explore the options of how to proceed. Have you " bled " air from any of your radiators recently , that would also reduce the system pressure.

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I've inspected it as best I can, the only thing I can find that could be a leak is a stain on the carpet right underneath where the pipework was for a radiator we had removed. I don't know is this is a leak or from where it was removed. I've put some tissue paper under it to see what happens. Everything else seems to be in order apart from a corroded pipe in the downstairs bathroom, but it's not leaking.

I haven't bled any radiators, I did a check to see if the bottom was cold and they were all OK, so I left them alone.

Have you checked ,outside ,the pressure relief pipework for signs of water having been expelled from it ?

terryplumb said:

Have you checked ,outside ,the pressure relief pipework for signs of water having been expelled from it ?

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I had a quick glance earlier but nothing you could describe as an inspection. The external pipework is below the bedroom window and my eyesight is rubbish, so I'll have a look in the daylight tomorrow.

The pressure on the boiler is topping out at maximum when it's on, but I don't know if that's normal or not.

Looking at some other threads on here, they've talked about repressuring the expansion vessel when the pressure drops on a CB24. Which is turns out is that "red ball thing" I described underneath the boiler. So maybe that's the issue, I've never done this since I lived here.

The first thing is check the air pressure in the expansion unit.
It probably wasn't checked at the last yearly boiler service.

That needs fixing to a wall the correct way up pipe connection should be at the bottom of the unit.

ianmcd said:

Absolute rubbish, the CB and the Baxi 80 and 105 are the exact same Hydraulic set up as they are in fact the same boiler, built in the same factory so to come out with that is total tosh, the boiler has its problems like most but that statement is completely false

Click to expand...

You're getting your boiler models mixed up. Immergas in Italy made the Alpha 240 series of boilers. AFAIR Baxi then bought Immergas and produced several boilers under the Baxi and Potterton brands names...the Baxi 80eco, Potterton Performa etc and the multitude of variants. The hydraulics are very similar and some parts the same as the Alpha 240. The Alpha 240 was a very robust and well designed boiler. The CB24 is definitely not. The licensing agreement allows Alpha to continue to import Immergas manufactured boilers despite Baxi ownership.

The CB series have several design faults...

The horseshoe DHW flow switch must be the most unreliable out there.
The diverter is terrible to work on (although the diaphragm can be removed if the pump is taken out).
The grub screws on the manifold below the diverter corrode and shear off making diverter removal a pain.
Removing the plate is likewise aggro.
It uses the cheap CEME water pressure switch...the switch point gradually rises resulting in owners over pressurising the boiler.
The thermistors are poor quality.
The o rings on the primary heat exchanger go brittle and often leak.
The wrap around combustion chamber can be very difficult to remove and the casing seals nearly always require replacement.
Parts can be very expensive...in the early days I had a string of failing pcbs and at £200 alone they were expensive repairs 15 years ago.

They are a dog...

Baxi never bought Immergas, Baxi bought the old factory and production line, Immergas are still the same company based in Nepicar and make many boilers that are re branded , one of them being Alpha