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[uk_ducati]

i have another hot water problem on a baxi combi 80e
the problem is that when a mixer shower is used that is on the combi boiler the boiler keeps turning on and off. i have noticed that the diverter valve keeps going in and out, thus causing the boiler to turn on and off. This does not happen if a tap is turned on as the boiler will stay fired up. i have noticed that if a tap is turned only slightly on the boiler will not fire up untill there is quite a lot of flow rate. i think it might need a new diaphragm but was just wondering why would the diverter valve keep moving in and out while the shower is on. could it be from pressure backing up from the shower head or do i need to look at another thing

cheers

paul

[htg engineer]

I would change the diaphragm (it's a doddle) if it's a thermostatic mixer shower, then it'll mix the water to a set temperature, if the set temperature is met the hot water flow will be restricted this may be why the boiler is cutting off - as you say if you reduce the DHW flow rate at a tap - the pressure differential valve pin does not move.

Is the shower going hot and cold ?

There should be a NRV on the hot water. To prevent back flow.


htg

[uk_ducati]

hi

yes the shower is going hot and cold. this goes cold when the diaphragm closes. also the presure at the shower head increases when the hot stops. i assume this is because it has now only got mains cold water going to it.

when changing it do i need to drain the primary circuit as in the manual it says to drain it but for some reason they make you take all the diaphragm/diverter head off. i would have just thought it would be easier to remove front cover, replace diaphragm then refit cover.

also could you tell me what the micro switch does on the diverter valve as i am getting confused with what that does and what a differential pressure switch does

by the way htg engineer ive passed the acs. cheers for you help

paul

[htg engineer]

You're right, no need to drain. Just isolate the cold water to the boiler, and open a hot water tap. Remove microswitch, remove front 10mm tube, then undo the 6 screws - replace diaphragm and refit in reverse order.


htg

[htg engineer]

The top microswitch is basically a pump proving switch, when the pump runs it pushes the diaphragm and pin up to make contact with the microswitch - if this connection isn't made the sequence will not go any further - basically if the pump's not going to boiler cannot light.

The pressure differential valve is just for the hot water, when the hot tap is opened the pressure behind the diaphragm is greater than the pressure in front so the diaphragm moves forward, pin moves out and makes contact with the microswitch - which starts the pump, pump proven ?

The same as an Air pressure Switch is to prove the fan is running.

Alot of fault finding is knowing the boiler sequence, easy on the Baxi's,
Pump runs ... pump proved ?
fan runs ...... fan proved (APS) ?
Ignition ......... ignition/flame proved (flame sensing electrode) ?
Main flame.

Watch the sequence and see where it fails.

The Baxi 105he's have the condensate sensor connected with the flame failure, so if you get one flashing flame failure also check the condensate trap isn't full or blocked.


htg

[uk_ducati]

hi

thanks for that htg. make sense now

will let you know how i get on

paul

[uk_ducati]

hi

thanks for the help

that must be the easiest diaphragm change ever

paul

[htg engineer]

They are very easy, very easy boiler to work on. You may have seen I have recommended Baxi's on the grounds they are cheap, chaep for parts and very easy to work on - which keeps labour costs down.

You never spend more than 20 mins changing a part on the baxi combi range, unless the boiler has to come off the wall for a replacement expansion vessel.


htg