Greenstar 24Ri boiler revving

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Hi all. My one year old installation has started to show some signs of issues with the central heating. Before describing the problem, my boiler (see title) was installed from new last year by a qualified plumber who provided a level of certification required to satisfy building regs signoff.

For one year my central heating and hot water has been fine. Recently due to colder weather I have noticed short periods of loud revving coming out from the boiler, not the pipes, pump, cylinder etc. The pipes are very hot to tough on both the flow and return. This is also the case for the pipe after the automatic bypass.

To give a little context, the layout of the central heating is a traditional S Plan Layout and is a fully pumped system. Control is performed by a wireless stat that (I assume) is wired correctly at the receiver since it calls for hot water (HW) or Central heating (CH) as I would expect. The two one-way valves are controlled via the boiler and seem to be doing their job ok. In addition a Honeywell Bypass is installed just after the pump as I have many rads with TRVs and as additional protection in-case any of the gate valves fail (saves the pump over time). There is a header tank in the loft that has a working water supply and a cylinder in the cupboard for hot water (also new).

So, the revving noise appears to happen continuously when the CH and HW has reached temperature and one call for heat is removed, a stat stops calling for heat on HW for example. If I manually turn any call for heat off then the noise no longer occurs. The pump does continue running for a few minutes after removing calls to heat which I assume is to dissipate the heat stored in the water. One thing that bothered me was the automatic bypass valve since the return piping was getting very hot, almost as hot as the flow. I get the function of the valve but it the return pipe after the valve heats up just like the flow just after start-up. My perception was that this valve would not open until a certain pressure was reached (e.g. when rads shutdown) and forced the valve to open and allow some flow through the bypass. I was convinced that maybe the bypass valve was the problem so I replaced the valve with a like for like replacement. This i hoped would also help some of the newer rads get hotter. However the new bypass valve gives the same results as before and continues to rev. PS - During the drain down I took the opportunity to install X100 inhibitor.

Moving on, I then assumed that either the water is getting to hot in the pipes (not dissipating enough heat) and causing some build-up of heat energy in the exchanger causing revving (I hate guessing) or my stat and radiators need balancing. My approach was to set the stats to 21 degrees and balance radiators to evenly dissipate heat. In addition, Ive opened the more of the rad lock(? dunno their real name) valves to allow more better flow when TRV is still open. I've also aligned times better now to increase overlap between HW and CH demand times. This seems to have improved things but very early days.

Does anyone have any thoughts on this situation of a revving boiler? Shouldn't a boiler ignore call for heat when return pipe water temp is similar to flow? What sensor(s) could be faulty? I'm not a plumber but have enough tools and experience to change water pipe configurations outside of the boiler so any suggestions would be appreciated.
 
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SammyInnit

Is the pressure gauge fluctuating in sequence with the revving sounds you're getting?
 
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Where would I find the pressure gauge on the boiler? Looking at the boiler manual it reference a pressure gauge on sealed systems - don't know if that's what you mean? My system is vented if that is relevant.
 
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sounds like a combustion problem, you need to get it serviced for the warranty after 12 months anyway so mention it to your service engineer
 
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Could also be a sticking microswitch on one of the two port valves.
 

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