Leak on Unvented Cylinder / Central Heating - A Puzzling Situation

I have a puzzling situation I'm hoping someone can help make sense of... It's also stumped our usual plumber.

I recently moved into a house which has an Unvented Indirect Cylinder. The model is a RM Cylinders Steflow 210l. It is approx. 10 years old, installed when the house was new. It is heated by an Ideal Logic+ Heat 24 boiler. There are two expansion vessels - one red, one white - above the cylinder. The cylinder is housed in an airing cupboard on the first floor.

Shortly after moving in, I noticed the pressure gauge for the red expansion vessel was a little low. I topped it up via the filling loop to around 1 BAR and bled the radiators. Most were fine and had no air, except three heated towel rails which had a fair bit of air in them.

I monitored the pressure over the next few days and saw it was slowly but constantly dropping. About 0.1 BAR (or one notch) a day.

Later that week, I noticed a damp stain on the kitchen ceiling, directly below the cylinder. I tested this with a moisture meter and it was definitely wet. Upon investigation, it seemed as though there was water coming from underneath the cylinder and running along the grouting between the tiles in the airing cupboard.

Over next days, I left the central heating off and there was no further obvious water under or next to the cylinder. The ceiling also began to dry out. However, the pressure gauge continued to drop as before. Hot water from the cylinder is unaffected and is delivered as normal.

I have since performed several tests with the central heating on and it is probably 50/50 whether water appears from underneath the cylinder or not. I've checked all around the cylinder whilst this is happening - there are definitely no leaks from any pipework or fittings, and the tundish is bone dry (I left some tissue poking through it). I cannot see anywhere where the water is coming from except under the cylinder.

When the heating is on, all the radiators are working well. I have noticed that the towel rails on the first floor have cold spots at the top and need bleeding again. However, I believe these are on our hot water system and not the central heating, as they heat up when hot water is on and central heating is off. I must admit I don't really understand how this would work, as the pressure in the central heating drops when these are bled.

I'm aware the issue is likely a leak on the system somewhere. I've checked around the pipework to every radiator and couldn't identify a leak so I'm fully expecting the worst. I just can't get my head around why water only appears under the cylinder when the heating is on...

Any ideas / help gratefully received!

I wonder if a tiny split in the cylinder opens up enough for water to seep through when its contents are hot/warm?

Sounds like it could be a leaking cylinder connection wheere the heating pipes enter it, hence loss of pressure in heating system.
Could be tricky to find if the connection is behind deep insulation.

john12676 said:

I must admit I don't really understand how this would work, as the pressure in the central heating drops when these are bled.

Click to expand...

The CH and the HW are all on the same pipework albeit they are controlled separately by their own valve so bleeding the towel rad will always drop the system pressure.

if there is a leak on the primary pipes/connection to the cylinder then when it leaks then the system pressure will also drop, this may be indicated by the fact that the towel rads are getting cold spots as they are typically where air in the CH system will collect. Air constantly being found in the system (rads) after being bled off can point to a leak.

First point for the plumber would be to check the cylinder connections and if dry then the cylinder tappings (may need some of the metal casing to be cut out. Has the cylinder been faithfully serviced and the paperwork complete?

If it was the cylinder itself, I would expect to see a constant leak given the static pressure it would be sitting at.

1. Worth checking if that drain off cock (....430.jpg) is fully sealing. Try nipping up the square bolt head. Leave a cup or something underneath for a couple of days.
2. As Madrab says, the hot water from the boiler (boiler primary circuit) goes to both the coil inside the hot water cylinder and to the radiators. To which it goes is determined by the motorised valves. So a leak from radiators or hot water coil (which would include the towel rails) would cause a loss of pressure in the boiler circuit (red expansion vessel).