Bosch Worcester 24 Cdi boiler problem

[RonA]

Hoping there may be a few central heating experts out there.
During last summer I replaced the two radiators in my lounge area, the room had always been cold & the old rads didn't kick out much heat.
The replacement rads are triple finned units where the old units had just been doubles without any fins. In addition I changed the rad valves to Drayton thermostatic ones.
All was well until the weather got really cold & the CH was brought into play. Now the pressure in the CH system hovers just below the point at which the PRV lifts & I find I am now having to top up the system press daily, which drops from 0.8 bar down to 0.4 bar overnight. Any thoughts please.
RonA

 

[the snooper]

sounds like a leak, have you checked the safety valve isn't leaking

 

[Kent]

If its not a leak its the pressure reducing valve blowing off and reducing it that way. PRVs do become faulty and if your pressure is likely to change and come in to high consider fitting a pressure limiting valve on the flow in rather than rely on the PRV to drop it.
due to the work you did I think leak in the heating circuit though

BTW I am not a qualified heating engineer, just learnt a bit along the way

 

[Rig Pig]

Is this an oil boiler? My old Bosch has developed the odd leak around the storage tank fittings, it's all nice and hot so the water evaporates so look for crusty deposits not wet patches. Also check the accumulator is charged and the bladder hasn't ruptured.

 

[RonA]

Is this an oil boiler? My old Bosch has developed the odd leak around the storage tank fittings, it's all nice and hot so the water evaporates so look for crusty deposits not wet patches. Also check the accumulator is charged and the bladder hasn't ruptured.

Hi I have checked the accumulator pressure & it's set upto 0.8 barg, presumably if the bladder had rupture you'd get water out of the air valve?
RonA

 

[daleyd]

Is it a combi?

Ours (28 CDi junior) lost a bit of pressure, and when the heating came on the pressure would rise, and blow off the PRV, which would then drop the pressure when cold. It turned out there was an accumulator (Apologies if its the incorrect term, but I'm more used to hydraulics than boilers!!) pressure storage vessel that has an inflatable bladder inside. This had lost air pressure and just needed inflating again.

Edit, sorry I didn't realise the bladder had already been mentioned.

 

[the snooper]

Is it a combi?

Ours (28 CDi junior) lost a bit of pressure, and when the heating came on the pressure would rise, and blow off the PRV, which would then drop the pressure when cold. It turned out there was an accumulator (Apologies if its the incorrect term, but I'm more used to hydraulics than boilers!!) pressure storage vessel that has an inflatable bladder inside. This had lost air pressure and just needed inflating again.

Edit, sorry I didn't realise the bladder had already been mentioned.

expansion vessel

its one of those things I already mentioned the safety valve PRV but no doubt it will pop up many times

 

[Kayos]

How old is it? Early early (non condensing ones) have a dual pass heat exchanger (so no seperate dhw calorifier) and can give faults that look like failed prv or vessel. Most likely is a leak or prv failed, then vessel then heat exchanger failed

Check for discharge outside, then a leak

 

[Highway_Star]

So, I have the 28 CDi version. Same boiler just smaller.

A couple of years ago we had similar issues.

When the heating was running hot, the pressure would massively rise, and the PRV would blow off. The system would then need topped up. It got worse and also the hot water became rather hit or miss.

So I called in a trusted local firm who found some faults.

PRV weeping, so pressure was always leaking away. - replaced.

Expansion vessel knackered. Quickest most cost effective fix was to install an external one.

DHW pressure switch - Microswitch knackered. Replaced the entire assembly as the diaphragms are prone to failure after a few years.

It's been spot on ever since.

I've go the service manual if it's any help.

 

[Kayos]

So, I have the 28 CDi version. Same boiler just smaller.

A couple of years ago we had similar issues.

When the heating was running hot, the pressure would massively rise, and the PRV would blow off. The system would then need topped up. It got worse and also the hot water became rather hit or miss.

So I called in a trusted local firm who found some faults.

PRV weeping, so pressure was always leaking away. - replaced.

Expansion vessel knackered. Quickest most cost effective fix was to install an external one.

DHW pressure switch - Microswitch knackered. Replaced the entire assembly as the diaphragms are prone to failure after a few years.

It's been spot on ever since.

I've go the service manual if it's any help.

I used to hate that, it may be quick and easy but it's not exactly hard to change a vessel on any Worcester of that age

 

[Fergus]

Any hints and tips for changing the vessel? I need to do the one on my Worcester boiler and I've never really done much with boilers.

 

[the snooper]

de pressurise the system then swap it for the new one, make sure new one has correct air pressure in it they are simple

 

[waddycall]

When you check the expansion vessel make sure the heating system is depressurised first. Expansion vessels should be pre-charged so you should get a pressure reading at the Schraeder valve around 0.8 - 1.0 bar when the gauge on your boiler reads zero.

 

[RonA]

Is this an oil boiler? My old Bosch has developed the odd leak around the storage tank fittings, it's all nice and hot so the water evaporates so look for crusty deposits not wet patches. Also check the accumulator is charged and the bladder hasn't ruptured.

The boiler is about 8 years old & has had the DHW changeover diaphragm changed twice. This note got me thinking, that's why I asked if the press vessel diaphragm had ruptured would you get water out of the Schrader valve?
RonA

 

[Fergus]

When you check the expansion vessel make sure the heating system is depressurised first. Expansion vessels should be pre-charged so you should get a pressure reading at the Schraeder valve around 0.8 - 1.0 bar when the gauge on your boiler reads zero.

Thank you both, I think the biggest problem with mine is the rather tight installation, I'm going to have to shift the lathe to get at it too.

 

[waddycall]

Thank you both, I think the biggest problem with mine is the rather tight installation, I'm going to have to shift the lathe to get at it too.

You can get water out of the schaeder valve but not always as it depends if the schraeder valve is at the top of the expansion vessel. You can get an air/nitrogen pocket at the top even if the diaphragm is ruptured so you won't get water out of the valve until you bleed the air out.

 

[marfyboy]

Any hints and tips for changing the vessel? I need to do the one on my Worcester boiler and I've never really done much with boilers.

I can't remember were they are if its on the back of the boiler you might have to drop the boiler

 

[marfyboy]

The boiler is about 8 years old & has had the DHW changeover diaphragm changed twice. This note got me thinking, that's why I asked if the press vessel diaphragm had ruptured would you get water out of the Schrader valve?
RonA

tap the vessel if it really dull sounding it could need air or bladder is split as above start by pumping up with an empty boiler (heating water)

 

[Fergus]

I can't remember were they are if its on the back of the boiler you might have to drop the boiler

If you drop the panels off the boiler unit the vessel is inside the unit between the level of the control panel and the boiler itself.

 

[marfyboy]

If you drop the panels off the boiler unit the vessel is inside the unit between the level of the control panel and the boiler itself

then if memory serves right it should be a sliding clip at the back held in place with a Philips screw. remove screw push clip up and undo screw on front bracket then its a case of wiggling the vessel out trying not to get the contents all over the electrics. replacing is a reverse but change the o ring and stick some silicon grease on it as they have a tendency to leak. but make sure its drained down first. if you use the safety to drain it they can sometimes pass after as well. I might be totally wrong! because I am thinking of the vessel that's shaped like a small air compressor receiver not the round pancake type? hope it helps!