Please help! Boiler loosing pressure, but only when the heat

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Hi all.
We have an ARISTON Combi A Series boiler situated in our loft. The last number of years we have had a number of instances where it has either cut out for no apparent reason or lost pressure.

The cut outs are very infrequent and so are not at this time a concern. The pressure loss however is.

The first time it lost pressure, a number of years ago, during the winter months of 2012-2013, We were advised to swap out the Pressure Relief Valve and got someone in to do so. For a while it appeared to have worked.
The following winter of 2013-2014, it started to loose pressure again. It lost pressure slowly at first, but as time progressed, it needed topping more frequently, until it got to the point where it would only last a day before needing filling up.

When the boiler first started leaking I placed a tray under the boiler. Thinking back, I don’t think there was any water on the tray. Sometimes when we needed to re-pressurise the boiler it did leak water from the compression fitting on the inlet of the filling loop. I was always careful to wipe up the water leaked from the compression joint so that I could properly monitor the situation. However, there were occasions during this time when it appeared that there were damp patches on the ground outside, below the pressure relief valve discharge pipe. We were told that the replacement PRV had failed and decided to replace this one year old PRV with yet another one.

It seemed to work at first, but sometime during the end of last winter (2013-2014) it once again started losing pressure.

Ever since the first occurrence of the pressure loss, I had noticed the ‘tell tail’ sign of a leak the appeared to originate from a sensor above the pressure relief valve, according to the boiler manual, this is the pressure sensor. There was a line of white residue running down from the base of this sensor continuing down the PRV housing. But, because there was never any trace of water on the tray below, I assumed that this was a previous leak that had now been rectified.

We once again asked someone to look and following a little investigation it was decided that this sensor could be the cause of the pressure loss. He had to remove the metal panel casing from the boiler to access this sensor and when he did so, it looked much more likely that this had been leaking and decided to remove it. When the sensor was removed it was discovered that the ‘o ring’ seal was, well I can’t actually remember what he said was wrong with it, but I do remember it was worse for wear. We didn’t have a replacement sensor or ‘o’ ring and so some kind of temporary measure was agreed.

I should probably add at this point that during every partial drain down to replace each of the PRVs the expansion vessel was checked. Now, the valve for the vessel is in a very awkward location and so getting the connector of the air pump on it is difficult. But, the fitter was fairly confident (Not 100% though) that the expansion vessel is ok. There has never been any water on the top of the valve and no water has discharged when deflated, releasing the pressure.

Anyway, after doing the temp repair to the sensors ‘o ring’, we fired up the heating and all appeared to be well. The boiler has held pressure ever since, all the way through the summer, never loosing any pressure it would seem. Until now! Today, I went into the loft to see how the pressure was, predominately because there have been a few occasions during September of this year (2014) that we have switched on the heating to get some washing dry. And low and behold it has lost pressure again! No sign of leaks on the tray. I have not noticed any signs of wet on the floor beneath the PRV discharge pipe.

So, other than the sensor ‘o ring’ temp repair I am non the wiser. Obviously I am not a Gas Safe fitter, or plumber.

WHY HAS THE BOILER NOT LOSSED PRESSURE THROUGH THE ENTIRE SUMMER?
Even when there has been countless occurrences where the boiler has been called on to generate hot water. i.e baths, showers, dishes.
I’ve been in the loft many times during the summer and it has held constant at 1.5 bar.

IS THE WATER FEEDING THROUGH THE PRESSURE SENSOR HOT AND THEREFORE EVAPORATING INSTEAD OF DRIPPING ONTO THE TRAY BELOW?

IF THE SENSORS DODGY ‘O RING’ NEEDS REPLACING, WHICH I ADMIT IT PROBABLY DOES, WHY DID IT NOT LEAK DURING THE SUMER MONTHS

COULD IT BE THAT THE RADIATORS OR PIPEWORK BE THE LEAKING AND CAUSING THE LOOS OF PRESSURE?
BUT IF SO, WHY DID THEY NOT SLOWLY LOOSE PRESSURE OVER THE SUMMER?

I have looked around each of the radiators during the summer and can see no damp patches on the carpets or drips on the compression fittings of valves, or bleed valves.
There is one radiator that has rust around the fitting incorporating the bleed valve. Beneath this bleed valve on the radiator tail, there is some rusty staining, which I assumed was from above. However, both of these rusty components are not wet in any way, either when the system is hot or cold.

Can someone please take a little time to expain the answers to these questions and give some sensible advise?

It gets very expensive having people come out to look at it and you just never know if we are being lead up the garden path.

Many Thanks for taking the time to read this very lengthy message.
 
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