Grant Vortex Pro Combi 26 - Gurgling Air noise on Direct Hot Water side

4 Jan 2010
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United Kingdom
Hi All,

I posted on this subject back in 2018 and never really got to the bottom of it!

But I have an issue where air seems to be trapped in the heat store/exchanger on the hot water side of things. It fizzes and gurgles, knocks and bangs in various degrees of loudness and frequency.

At the end of August it was banging away so I took the cover off to have a look. I hadn't looked inside for many months previous. I noticed that the pressure was down to about about 0.5 bar. I assumed the worst and thought I had leak somewhere as some pipework for a new radiator (now behind plasterboard) was put in a few months ago (radiator is not fitted yet). I then noticed that the AAV on the water store looked like it had been leaking as I could see a limescale/rusty mark running down it.

So I slowly topped up the pressure and waited. The pressure was holding steady at 1.0 bar (when hot water is up to temp) and the knocking and gurgling completely stopped on the hot water store for the first time in years! It worked perfectly throughout September and into October.

So my assumption was that there had been air stuck in there for years, the AAV had finally blown it out and the pressure had dropped.

Yesterday, my central heating flicked on for the first time this year. Now the knocking is back on the hot water side!!

The pressure is OK, still at 1.0 bar.

Could this just be a coincidence?! Why would this reappear after the central heating comes on?

I have heard the AAV working a couple of times today but the knocking persists. Could this be an intermittent fault on the AAV?

Is there anything else it could be?

I had learned to live with the noise but since its has been working fine, I've also noticed how little the boiler actually fired when running water. A lot of the time the pump would just circulate and only really fire up if drawing a lot of water (bath/shower). Now it seems to fire most times you run a tap! So concerned about burning oil now.

I am really at a loss here.....any help and advice greatly appreciated.
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The auto air valves on the earlier Grant combi's were/are dreadful things always leaking and not the easiest to replace.
Current model has manual air valves much better!.
We prefer to replace original ones with manual problem gone for good.
The store temperature is controlled by a thermister there is flow thermistor as well plus flow switch that triggers it into action.
Maybe time for a competent oil engineer?
Thanks Exedon. That is interesting about the newer models. I've just looked at the manual on the Grant website and can see the new version has several manual vent points now!

There still seems to be an AAV on primary heat store on the new model. Is it possible/advisable to change that to a manual air valve?
We leave it as it is but close black plastic screw on its outlet.
Would also add we get rid of pressure switch as well (has a habit of snaping off! ) that's not on new model either.
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On my regular services on these and other oil combi boilers, I used to close off the AAVs after repressurising.
Thanks for the responses guys. Just as an update: I have had the central heating off for the past few days and the air in the store has been expelled and the boiler is behaving itself.

I think that some air in the central heating made its way to the store after it came on for the first time in 6 months and my slightly dodgy AAV too its time getting rid.

I'm going to check the radiators for any air later today.

I've got a radiator going on at some point in the next few weeks. I'll replace the AAV at the same time.

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