Main Combi 30 HE, hot water from tap cold unless on full

[phoenixw]

Hi,

The classic "water leaking from the pushrod on the front of the baxi DHW diverter valve" problem afflicted our boiler, so I bought the 5111137 kit, replaced the diaphragm and everything seemed OK. It almost seemed *too* easy.

Except... Now the hot water only comes on if the tap is turned on quite a lot. It used to be the case that you only needed to crack the tap open slightly to get very hot water. If you turn the tap on a little, you can see the pushrod comes out a little bit from the diverter valve and fails to switch on the microswitch.

I've opened up the front of the DHW diverter valve two more times and put it back together how I originally found it to no avail. The diaphragm is in perfect condition, but even the original one in place doesn't seem to work right.

The other thing to maybe mention is that since I did this job, if I crack open the hot water tap just a little it sounds like air is coming through the pipe along with the water. The central heating is absolutely fine.

So, any ideas? I'm tearing what little is left of my hair out here!

 

[Agile]

It should turn on at about 2 li/min.

I dont know what you have done wrong!

Tony

 

[Tom1981]

The two differential pipes to and from the diverter valve connect to the hydro block on the right side of the boiler. Within that block there will be a venturi, it's more than likely thats shattered, without it you won't get a pressure difference in the pipes so no movement in the diaphragm.

Fairly easy to change, the filter is access by the nut on the left hand side of the block (One with the pump on it)

 

[Agile]

I discounted that because he seemed to be saying it was working ok until he worked on the boiler.

Are you sure that the venturi unit can be removed without removing the gss valve?

 

[phoenixw]

Would a shattered venturi cause a restriction in the movement of the diaphragm as in my case or a complete failure in movement? What would have caused the venturi to shatter during the replacement?

Are there more detailed diagrams of the venturi assembly anywhere? The Partsarena drawings for the boiler hydraulics seem to omit it.

 

[Tom1981]

Yes the venturi can be removed without having to touch any gas carrying parts. It's spring loaded, the ones I've seen have split causing the spring to fall out. If it's broken the tap has to be on full rate to even get the diaphragm to lift.

Can always check the pipes before cracking the nut.

I can have a look for it's part number.

Nothing really causes it to shatter, but if theres a sudden refill of water that could cause damage I suppose, it's fairly fragile.

 

[phoenixw]

I'm assuming that the part is the 248051 "Hydraulic Venturi"? The drawings on parts arena (rather the lack of them) also indicates an o-ring/assembly (248053) is it worth replacing that as well or just leaving as-is?

Is it straight forward to replace? I'm hoping it's just a case of draining the hot water, cracking the nut on the left of the block (it looks easy enough to get to), replacing the venturi and putting everything back together.

Thanks for all your help.