Boiler constantly losing pressure.

If the isolation valves were fully closed I'd be looking at the heat exchanger.
If the water was coming from the prv pipe you would have noticed that and you've previously stated the boiler insides are dry.

Get the gas engineer to carry out the same isolation test if he doesn't believe your conclusions.

 

The white pipe at the far right. Pull this down about an inch. This will then let us see if water is coming from the heat exchanger.
Obviously top up the pressure and don't use the heating or hot water.

 

Sounds like the first line of a classic R&B riff. You appear to have covered all the bases, I'm stumped.

 

Is their an auto air bleed on the boiler at the top. It may be that the pressure you are losing is trapped air not water.

 

Hmm. Interesting reading.

I don't know, I was just thinking... if heat exchanger is leaking, shouldn't be there some water inside and under the boiler?

That water must end somewhere, right?

So I was thinking further... IMO, the only place where you don't realize any leak is your tap. Isn't it possible that diverter valve failed ( in the very strange way ) and it diverts small amount of CH water to DHW system...?

 

Tom ,if heat ex is leaking the water is evacuated with the condensate from the boiler ,so you wouldn't notice as the condensate pipework is always dripping , whilst boiler is on. Regards terry.

 

Did you over pressurise the system yesterday. Or maybe not fully closed the filling loop ?

 

Check all valves that you have been working on are fully open ,except filling loop which should be fully closed. Otherwise it points to a problem with EV .

 

Yes ,or not done properly. Valve leaking air out maybe.

 

Or the secondary heat exchanger leaking?

 

I would think so as they should have tested for gas security during the commissioning stage. if it was the expansion vessel at fault, you should have seen water being discharged from the vent pipe, you did test for this. For when they return, get a hitching rail outside for them to tie up their horses.

 

I'm glad it's all sorted, but it's still a mystery to me how those symptoms were caused by a faulty expansion vessel. I'm guessing it's an internal EV (as in built in to the boiler), so probably around 8 litres. In a correctly balanced system, that's 4 litres of air and 4 litres of water each at somewhere around 1 bar. If the EV was faulty, the diaphragm was pierced and the water ended up flooding the entire EV, so 4 litres of water from the CH system displacing 4 litres of air. This seems reasonable when the CH system was still connected, but it doesn't seem reasonable when it was isolated, but not when it was just the water in the boiler. Where was all this water going on each subsequent repressurising every morning? I guess we'll all remain in the dark on this one, unless someone can provide an explanation that puts us out of our misery.

Did they also happen to replace the filling loop with its double check valve? I've had both ends of a filling loop fail before, which contaminated my drinking water with CH water when the pressure in the CH system rose to be higher than my mains (which are regulated to 3 bars).