Baxi Combi 105 HE Pressure system questions

[Darth_Pistachio]

Hi - i've searched the forums and found a few answers to my questions but not all so forgive me my inexperience in advance.

I've recently had a Baxi Combi 105 HE fitted - about 10 days or so now and the other night noticed excessive clanking and banging from the pipes.

I wandered about the house today and noticed that a few of the radiators directly related to the banging part of the circuit were stone cold at the top and nice and hot at the bottom - so I proceeded to bleed the air from them.

The first rad I bleeded had a lot of air in it and finally when a spurt of water came through I shut the bleed valve and proceeded to the next Rad. This one started venting air nicely and then stopped but with no water this time. Now A lot of you are going to guess immediately what happened but I've been a slave to an old servowarm gravity fed system for the last 15 years so forgive my innocence!

After a few minutes I popped to the boiler and saw that the pressure gauge was now at zero so I knocked the boiler to the off position when I realised the system had lost pressure and then got out the installation guide. I soon figured out how to repressurise the system with the permanent refill loop the installer had fitted so I did so and took it back up to about 1 Bar. I do have a few questions though that are still bugging me.

The installation manual explains how to initially pressurise the system but nothing on subsequent top ups due to drain downs and leaks etc.. it informs you to have all Hot taps running to purge the DHW circuit - Am I correct in assuming this step doesn't have to be followed after the initial prime up and you can leave it out for future re-pressurisations?

Is the correct way to bleed air from rads to knock the boiler off - bleed a rad - re-pressurise via the filling loop and repeat until all rads are bled and then set final pressure to 1 Bar again?

Is there any way I could have cocked up the pressurisation of the expansion chamber with what I initially did? (i.e. boiler was on with first pressure drop to CH circuit)


I've also noticed that the bleed valve on the radiator that is in the same room as the boiler has been replaced by the installer with some small circular brass protrusion screwed in with PTFE tape. Is there a specific reason for this? As that radiator is full off air ( top half is stone cold) and I now have no way of bleeding it off.

Thanks in advance for any answers to my paranoia driven questions!

 

[EliteHeat]

You're right, the HW circut does not need to be re-primed when topping up the pressure.

The procedure you used to bleed the radiators is correct. The only way you can really damage the boiler is to over-pressurise, so be very careful not to allow the pressure guage to go much above 1.5 bar ever.

You can still bleed the radiator with the 1/2" brass plug in it. Just loosen it off a bit using a spanner, until water starts coming out. The installer probably lost the bleed screw. You can buy complete replacement 1/2" bleed inserts for about 60p.

 

[chrishutt]

The only way you can really damage the boiler is to over-pressurise

Not really true. Even if you pressurised as high as 3 bar during filling the PRV would open and release the excess pressure. The only harm that might be done is that the PRV may not seal shut properly afterwards, possibly requiring replacement. Temporary pressures during filling of up to 2.5 bar are fine (unless you have a week joint somewhere about to fail). The higher the pressure the faster the air venting and the less frequent the repressurisations.

Otherwise notbovvered is spot on.

 

[ChrisR]

You shouldn't have that much air in the system 10 days after fitting

 

[croydoncorgi]

And are you sure it's 'air'?
Might be hydrogen.
To test: get the curtains out of the way, open a radiator bleed screw SLIGHTLY and attempt to light what comes out. If there's a mini-blowtorch effect (!), blow it out. You've found hydrogen.

(It's probably best NOT to do this test without first bleeding the whole system and then waiting a couple of days for the gas to re-accumulate. Hydrogen / oxygen mixture (produced by electrolysis - which is usually what's going on) is explosive, but unless there's a lot of air in the system anyway, the oxygen reacts quickly with the steel radiators, leaving just the hydrogen.)

Stopping electrolysis (and the other cause of hydrogen - bacterial action) can be tricky.

 

[Darth_Pistachio]

thanks for the replies guys - very helpful.

As there was never a pressure drop during running of the boiler before I let out all the pressure I can safely assume there isn't an actual leak on the CH circuit - so I'm wondering where the air could come from.

So the suggestion of Hydrogen build up does seem to make a lot of sense - especially seeing as the radiators are old and have had a treatment or two shoved down them.

I have to assume the installers flushed the whole system before they primed and started the boiler - I have no accurate way of knowing that they did - and possibly air could have got into the system then?

If it is Hydrogen build up what is the suggested method/s of getting rid of that problem - It can be caused by electrolysis or bacteria I understand?

cheers

Fab'