Worcester Bosch 24i Junior: Air in CH after DHW demand

[kayble]

Dear all,

My first post so apologies if I've missed a post that answers this question previously. I have a Worcester Bosch 24i Junior combi boiler in my first floor flat. There are five radiators all with TRVs and a bath with shower fed by the boiler.

I have started having a problem recently whereby air appears to be drawn in to the CH system after a longish demand for DHW. For example, the timer will set the heating to come on at 6.45am and go off at 8.00am. If during that time someone takes a shower, when the shower finishes and the boiler switches back from DHW to CH, there is a brief high pitch noise of air (sounds like it's escaping from a valve in the boiler of some description) followed by a woosh of air bubbles within the CH circuit for 5-10 seconds as the CH comes back on, after which everything goes quiet again.

I've not noticed this with shorter DHW demands like from a tap in the kitchen for example. The boiler is otherwise working well. CH pressure needs a small topup every 8-9 months. The last work it needed was to replace the gas valve - back in mid 2010.

I have a video of the sound, taken this morning - will figure a way to upload this and post it - but if anyone has any ideas in the meantime?

Best,

John

 

[plumbplumb]

All with TRV's. Should have one without to act as a by-pass
If all rads are at temperature (I.e by the room) and the boiler finishes a HW demand and spins the pump to shift any excess heat there is nowhere for this to circulate

 

[kayble]

Thanks for the response.

Another issue has come to my attention. There is no visible thermostat on this system. I originally operated the CH system by setting all TRVs to desired temperature and configuring the mechanical timer on the boiler to operate CH at times when the flat was inhabited. In this configuration, the CH pump would operate at all times as defined by timer and with flat up to temperature, the boiler would only indicate that gas was being burned during brief intervals via indicator on boiler fascia.

From reviewing other posts, I take it that this configuration is not efficient/correct and I should instead have a room thermostat installed (for example in the hallway). I should then remove the TRV on the rad in the hallway (so that it doesn't conflict with thermostat) and replace TRV in bathroom which I removed in response to the above post from yesterday. Having then configured the thermostat I can expect the boiler to only burn gas and operate the pump according to the thermostat, with additional room specific temp selection being available with the TRVs. I would be very grateful if someone could confirm my understanding?

FYI - system was installed in early 2005.

 

[kayble]

Quick update.

Today, I have installed and configured a Hortsmann HRFS1 wireless thermostat. The install was easily done from the boilers external controls wiring block and i also removed hallway trv and replaced bathroom trv.

The cumulative change of removing hallway TRV, adding wireless thermostat and bleeding rads has resulted in a much quieter system when CH pump is running and also when DHW demand is closed off as mentioned before.