Vented a rad, now pressure doesn't hold but no evidence of leaks?

[rt18]

My closed central heating system has been fine for years. 6yr old Baxi combi boiler, 11 radiators. One bedroom radiator was discovered to be full of air (more air than water I reckon), so I vented it. In doing so the system pressure dropped, as you'd expect, and the air stopped coming out, so I closed the valve and topped up the boiler pressure back to 1.5 bar, then continued to vent the radiator. The pressure dropped again, so I went to top up the pressure on the boiler only to find when I returned to the radiator that I'd failed to close the vent completely and there was water on the floor. With the air now entirely vented I closed the vent fully and checked the pressure on the boiler and topped it up again to the green, around 1 - 1.5 bar.

Later on that same day I checked the boiler pressure and it was right up in the red, well beyond the green safe zone and close to the upper limit of the dial, nearing 4 bar. I panicked and quickly vented some water from the nearest radiator (kitchen) to reduce the pressure in the system back down to green.

That was all several days ago. Since then I've monitored the pressure and found that first thing in the morning the pressure is at zero and the boiler in error. I have to top it up (takes about 4 seconds) and it comes on fine. During the day the pressure sits between Green 1.5 and Red 3 bar (depending on temperature of water I guess) and runs all day. Heating goes off overnight, then next day it's back at 0 bar and needs topping up again. Note the boiler has an over-pressure release valve set to 3 bar (according to manual).

I've got two questions:
1. Since venting the air I now cannot maintain the pressure in the green zone, it always reaches red. Do I need to get air back into the system somehow? It feels that with no air in the pipework it cannot accommodate the expansion of the water when heated and the system goes over-pressure. Shouldn't there be an expansion vessel in the boiler for this purpose? Can I test if it's failed somehow?
2. I have to keep topping it up, which suggests it's losing water. I checked every accessible pipe joint, rad valve etc., and found no water anywhere. Could the boiler be releasing water into the overflow pipe that goes outside? This might explain why I can't track any actual water loss in the house. What does the safety discharge valve look like and where is it? I don't mind sticking my nose in the machine to find it and check for water.

Details: Baxi DuoTec combi, 28kW, Condensing Combination Boiler. Fitted about 6 years ago. Detached 4 bed house.

 

[rt18]

OK, so I checked my pressure discharge outlet, which is outside and faces the brickwork - it's all wet there. So I can assume that's doing its job correctly and releasing the system pressure when it exceeds 3 bar.
So now I need to understand why the pressure reaches 3 bar - perhaps the expansion vessel has failed. Is there a way to test an expansion vessel without removing it from the boiler? Does it have a gauge on it or a means of checking it is still working?

 

[terryplumb]

Your expansion vessel isn't functioning. It may just need to be recharged.
Your radiator that was full of air has been acting as an expansion vessel.

 

[rt18]

I see online that Baxi's expansion vessels have a bike tyre style valve, which means I can cool down the heating system (ie wait til tomorrow morning), then try briefly pressing the pin in the valve to see if it's under pressure. I'm assuming:
A) If water comes out then the vessel has burst internally.
B) If air comes out then the vessel is functioning ok.
C) If no air comes out (or only a little air) then the vessel might be ok and just needs repressurising. Requries monitoring.

I assume a bike pump will do? The working pressure of the vessel appears to be 0.5 bar according to the "Min Pre-Charge Pressure" value of the boiler technical data.

Might have an update tomorrow on this

 

[terryplumb]

That's not quite the way to do it ,and water present at the EV's valve doesn't necessarily mean it's burst internally.
A bike pump with a pressure gauge would be handy . You can't just put a pump on the EV valve to recharge you need to follow a specific procedure.

 

[rt18]

Sure, I assume I need to cool, isolate and drain the boiler first. The working pressure would only apply with the vessel emptied of water, otherwise I'd get a false measurement.

 

[terryplumb]

Yes, electrically isolate the boiler ,drain the boiler and leave the drain cock open whilst pumping up the EV.
Good idea to attach a hose to the drain cock ,directed to a drain ,as quite a bit of water can be expelled when pumping up the EV.

 

[rt18]

OK, I'm impatient, so I took a look now. The heating was in an idle lull (between runs) so although not cold it wasn't hot either. Note it was still at maybe 2 bar. I turned off the boiler first and then looked for the expansion vessel.
The expansion vessel valve is on the top of the vessel and when the internal pin was depressed and released (really short depression of less than half a second), water slowly rose out and kept rising out, maybe 4 drips in total, then seemed to ease and stop before I replaced the valve cap. The water didn't come out under pressure, and no air came out at all. But it did seem to keep coming out after I released the valve pin.
This leave me wondering what situation could result in a valve releasing water when air is expected, without pressure, even though the vessel is connected to a 2bar central heating system?

 

[terryplumb]

There is 2 bar of pressure present ,acting against the bladder in the EV,releasing the pin in the valve allows water out under that pressure,water doesn't run uphill unless there is pressure behind it.
Half a second depression of the pin in the valve tells you nothing.
If you keep it depressed and water flows non stop out the EV ,the bladder is likely perforated.

 

[rt18]

Thanks. I guess I don't understand the vessel design properly. I thought the bladder contained the water, and the vessel contained air. The bladder is connected to the CH and the vessel to the valve. The vessel is pressurised to 1 bar at manufacture and thus compresses (deflates) the bladder. When the CH pressure rises above 1 bar it begins to expand the bladder, compressing the trapped air in the vessel. In this design there should never be air at the valve unless the bladder has split, and thus the air in the vessel can also find it's way back into the CH. When I released the valve briefly, the presence of water suggests to me the bladder has a problem. There must be zero air left in the vessel otherwise that would have been at the top, above any water, and escaped first. Hence I only pressed the valve nib briefly, to see if there was any air at all.

I'll do a better test tomorrow, pressing the valve for much longer, and note my observations

 

[rt18]

Ok, system cold, pressure at 2 bar, long press on the EV valve = water. No air.
I've ordered an new EV.

 

[rt18]

Final follow up. New expansion vessel arrived. I successfully swapped it out, and now all is very good. Pressure holds and remains steady. Very happy.
In short: If you bleed your radiators and suddenly find your boiler over-pressurised when heating is on, consider that you expansion vessel might have failed.