Expansion vessel pressure keeps dropping

dharm999

nofoollikeold said:

1. Are you sure that expansion vessel is for the boiler? 
2. Boiler EV's are usually red, whereas those for unvented cylinders are normally white, sometimes blue.
3. The unvented hot water cylinder should have what is called a tun dish in the pipework from the temperature and pressure relief valve (side of cylinder) which may be shared with that from the combination valve (cold water into cylinder).
4. I'm aware the Megaflow has an internal expansion space and does not normally need an external EV.  However, external EVs are somtimes fitted if the internal space is compromised.  

Yes, definitely for the boiler.  

The Megoflow is placed about 8 feet away from the boiler and has an overflow (if that’s the right phrase) that goes in to the wall behind it,  No idea where it exits the house, need to have a look outside, to see if I can find it.

dharm999

A quick update.  Plumber has been and installed some isolation valves for the boiler, so I can now isolate it.  Will do the isolation test in the next couple of days 

Pump under the boiler, Grundfos Alpha, has been playing up.  First thing in the morning it hasn’t been working, but once disconnected and reconnected, it works and runs ok all day.  It comes on at night for the heating and the heating goes off at 9pm, before coming back in at 6.45am the following morning.  Last two mornings it hasn’t been on, display has two dashes on it.  Could a faulty pump be causing the pressure drop?  The pump isn’t leaking, and does get very hot when running.

Grumpy_chap

dharm999 said:

The pump is a Grundfos one, and I set it to 2, max is 3, and it has an Auto adapt function.

We have a system with many similarities to the OP and also experience a similar issue with the expansion vessel.
Our system also has the Grundfos pump with the auto-adapt cleverness.  I particularly specified that as I thought it would save energy despite the plumber saying it was a waste of money and never achieved the outcome anticipated.  How right he was!  Disabled the auto-adapt and wacked the pump to the max setting and it made the system work better, though did not entirely eliminate the pressure dropping.

WIAWSNB

dharm999 said:

A quick update.  Plumber has been and installed some isolation valves for the boiler, so I can now isolate it.  Will do the isolation test in the next couple of days 

Pump under the boiler, Grundfos Alpha, has been playing up.  First thing in the morning it hasn’t been working, but once disconnected and reconnected, it works and runs ok all day.  It comes on at night for the heating and the heating goes off at 9pm, before coming back in at 6.45am the following morning.  Last two mornings it hasn’t been on, display has two dashes on it.  Could a faulty pump be causing the pressure drop?  The pump isn’t leaking, and does get very hot when running.

Blimey, your boiler didn't have isolating valves? Looking forward to the results.
No, I can't see any way the pump can affect the sealed system pressure, other than if it fails to work, your boiler will 'boil' until the overheat stat kicks in. This temperature increase could well cause an associated pressure increase which could pop the safety discharge valve. Once you've lost system water, the pressure will be lower once it's cooled down.

dharm999

Had read that if the pump was faulty that it could cause airlocks which could then manifest as a pressure drop, is that likely do you think?

WIAWSNB

dharm999 said:

Had read that if the pump was faulty that it could cause airlocks which could then manifest as a pressure drop, is that likely do you think?

You have a sealed, pressurised, system, which does not usually suffer from air locks. Since the system at a notably higher pressure than the 'outside', air just cannot get in.
If there is air in the system, it must either have always been there, such as dissolved in the initial fill of water, or perhaps has escaped from inside a faulty EV.
You can also have gas (H2) forming from internal corrosion, but regardless of source, all of this will end up inside a radiator, from where it can be bled. Or, it's vented pug automatically via an auto air vent in the boiler.
I'm not aware of 'airlocks' as such forming in a pressurised system; it's technically possible, but would be at some poorly-designed pipe high spot. 

dharm999

Thanks.  @WIAWSNB, any ideas what could be causing the pump to work intermittently?  It was ok this morning, as I reduced the time between when it went off last night to when it came on this morning.  Maybe that was just a coincidence, but if not, why would it not start having been off for around 9 hours.

Thanks

WIAWSNB

dharm999 said:

Thanks.  @WIAWSNB, any ideas what could be causing the pump to work intermittently?  It was ok this morning, as I reduced the time between when it went off last night to when it came on this morning.  Maybe that was just a coincidence, but if not, why would it not start having been off for around 9 hours.

Thanks

No idea. But if it's starting to wear out (how old is it?) or has some sludge build up (no idea), then the 'start-up' inertia is usually the hardest thing to overcome. Sometimes it will, sometimes it'll fail - and I suspect 'Smart' pumps will have a safeguard which will kick in so it doesn't overheat trying.

My WM's drain pump would operate intermittently, and I knew it had some hair wrapped around the spindle. Until I removed it for a proper cleaning, it was seemingly pure 'chance' whether it would fire up or not. 

Niv

Interesting read. I cant help at this point but poked my head in to say, I have a very similar sounding issue, Significant bleed on the radiator a couple of times a year, never found a leak, plumber replaced the expansion vessel last year, it made no difference, so I continue to bleed and refill a couple of times a year. I do not have a magnetic filter - I asked the plumber to install one and he said there is no good place to put it, I said put it in a not good place then as something better than nothing but he didnt..

I am replacing a radiator in the spring so will use this as an opportunity to put in some more inhibitor, maybe that will help.

dharm999

The boiler is around 10 years old, and the house was built around 23 years ago, so the boiler has been replaced, at least once.  No idea if the pump was changed at the same time as the boiler, or even in the time since the boiler was changed, is there any way to work out its age, if I can find a serial number for the pump?