Air trapped in unvented cylinder coil with X-Plan

[HouseholderQuery]

Hi there. I've just had a brand new heating system installed. Worcester Bosch system boiler with optional internal diverter kit, HW output connected to Gledhill unvented cylinder and CH connected to the rads. I believe this is referred to as PDHW or X-Plan - the boiler is either heating the cylinder or the CH system, never both. I asked for this set-up in order to ensure fast recharge of the HW cylinder (6 people in the house and don't want to run out of HW). However, what's happened is that whenever the boiler is heating the cylinder, there is a repeated gurgling sound (it sounds like a loud stomach noise followed by a rush of falling water). This video might help understand, or I can wait until it starts again and make another recording:

According to the installer, that is air trapped in the coil. You can see he installed a bleed valve on a pipe coming out of the middle of the cylinder, which he has replaced with an AAC since I took this video (see photos below), but that has not (yet) solved the problem.

He is telling me that ordinarily air in the cylinder coil would travel up towards the boiler and escape into the CH circuit where it could be bled from the rads, but because of the diverter valve the air is trapped in the cylinder circuit and there is no way to release it. He claims he's spoken to Worcester Bosch who've confirmed there is nothing else he can do, and that it's my fault for asking for this set-up. He said to leave it for a month and see if the air works itself out through the AAC, but if it doesn't then there is no solution and the diverter will have to be replaced with a regular zone valve(?) - and I'll be charged for the work.

Does this sound right?

 

[Johntheo5]

Certanly doesn't sound right to me, PDHW is quite common.

Can you confirm that the circulation through the HW coil is from top to bottom, top pipe will obviously be hotter than the bottom. Also a photo of the AAV, position on piping etc.

What is the boiler pressure?.

Does the Gledhill Lite have a external expansion vessel or does it have its own, internal air bubble?.
I read now it hasn't but has a corrugated heating coil, don't know if this encourages noise or not due to air getting trapped in the corrugations, its also a suspended coil "which can lead to noisy operation if the flow through it is too high".
I would give Gledhill a tinkle and get them to listen to your video.

 

[HouseholderQuery]

It's a 300L Gledhill StainlessLite Plus HP model # HP300IND. It's attached to a gas boiler for rapid heat-up. The HW coil is top to bottom. It has an external expansion vessel, mounted above the cylinder. The AAV is in the photo above (on top of the vertical pipe to the left of the cylinder - if it's not clear I can post another pic - you can also see it in the video, when it was still a manual valve). Plumber says the boiler itself also has an internal AAV - boiler is located on the floor above, directly above the cylinder.

 

[Mister Banks]

According to online sources the diverter valve can be placed in "manual" mode which would allow the trapped air to be released.

Edited…….

 

[HouseholderQuery]

Online sources for the boiler? I don't think I said which boiler I have - it's the Greenstar 30i System Boiler. I believe it has the optional integral diverter kit installed. If there is a mode that can be activated to release air through boiler settings it would be great to know how that is done - but my plumber says there is an AAV in the boiler that he's stripped and checked to make sure it's working, so why would you need to activate a manual mode?

 

[Johntheo5]

I suppose having the boiler higher than the cylinder doesn't help but one would think it should be possible to get rid of that air, there are probably a good few systems about like that.

In PDHW mode, the boiler flow temperature is normally boosted up to maybe max, say 80C, but don't know if this is settable from the boiler menu, the heating system should certainly be accessible so maybe boost this up to 80C ish for a few hours when the CH is on, it might help to vent more air. Some installers like to shut off rhe boiler's AAV after commissioning so maybe ask if this was done here, even if unlikely since it has been cleaned out and checked.
Does the coil AAV emit anything, even occasionally/.

 

[HouseholderQuery]

I'm sure the boiler's AAV is open - that's why he checked to make sure it was working.

The coil AAV was put in yesterday to replace a manual bleed valve and yes, that valve was emitting small amounts of air when bled. Now that it's automatic I can't tell of course, but I assume it is still emitting some air - perhaps over time that will solve the problem.

How would it help to raise the CH flow temp, since that water goes through the other side of the diverter valve and doesn't touch the coil? I think he said that the flow temp for the coil is set by the cylinder, but I can see whether it's possible to boost that up.

 

[Johntheo5]

It may not help at all to raise the CH temperature but it's the same body of water that's circulated and it may contain some entrained air that can be more easily removed via rad vents or whatever when it's hotter??.
This looks like your control panel?.

 

[Mister Banks]

Online sources for the boiler? I don't think I said which boiler I have - it's the Greenstar 30i System Boiler. I believe it has the optional integral diverter kit installed. If there is a mode that can be activated to release air through boiler settings it would be great to know how that is done - but my plumber says there is an AAV in the boiler that he's stripped and checked to make sure it's working, so why would you need to activate a manual mode?

My mistake, further investigations reveal that on the Greenstar there is no manual procedure on the diverted valve and the boiler relies on an automatic air purge mode.

 

[Johntheo5]

My mistake, further investigations reveal that on the Greenstar there is no manual procedure on the diverted valve and the boiler relies on an automatic air purge mode.

For interest, what does this manual procedure do?, does it, like some A rated circ pumps, speed up/slow down/almost stop the pump, for about 10 minutes?.

 

[Razor900]

Gledhill cylinders are notorious for coil resonance. Can't really make it out on the video but I would strongly suspect that's the real cause and it hasnt got air in it.

PDHW can make it worse because usually the boiler pump will ramp up to maximum when its recharging the cylinder. It does warn you in Gledhills instructions

 

[Razor900]

 

[HouseholderQuery]

That's interesting, but I don't think that the boiler is capable of delivering anywhere near 25l/min through the coil - I don't have the spec to hand, but at high temps I think it's around 10-15l/min. Also it's not a constant sound, it's a periodic gurgling which does very much sound like air.

 

[Johntheo5]

I would think that the coil would emit at least 20kW especially if/with a flowtemp of 80C. I wouldn't expect a coil dT of more than 10C, if so then the flowrate required would be 28LPM, IF the actual flowrate is 15LPM, then at 20kW, the coil dT would have to be 19.1C, again unlikely.
If the coil has a gate valve on the return, then shut it and reopen it about 3/4 of a turn and see if the noise dies down.