Combi shower going Hot-Cold-Hot problem

[gas112]

turn shower on and have someone feel for it going hot and cold as you look at the boiler the HW demand light should stay on all the time . but i bet its going on and off.
a very common fault on 105 that shows up on showers is the venturi in boiler falls apart and when the shower gets hot and the flow of hot water through it gets slowed down by thermostatic cartridge in shower the diaphragm drops back taking pin away from microswitch so water goes off once it cools down water flow through shower increases making diaphragm move back out making microswitch and firing boiler back up . simple 20 minute repair and 15 quid part

 

[gas112]

plastic bit falls apart

 

[ianmcd]

The boiler has to be removed from the mounting to do it I think, and its about 50kg. As mentioned in post 4, if its got to come off the wall I may as well change exchanger for a new one.

The Plate heat exchanger can be removed without taking the boiler off the wall

 

[buildersteve]

Well that's another thing I didn't know about. Thanks gas112 for that. I've got one on a ticket at my local boiler parts shop who have one with the part number 248051.
Looking at it on the boiler, its been leaking for some time and there's a lot of white crud all over it. Got to be worth a look by the man, when I find one!

I have two part numbers for this venturi
HYDRAULIC VENTURI 248051 248050
Which one do I buy? Thanks

 

[gas112]

248051 is just the end bit which just un screws so that's all you need but if 050 is whole bit as in picture makes it bit easier. It is in the left hand side of the brass housing on the righthand side of boiler just were the two we pipes go in. You will need a 24 mm spanner to get it out once out put finger in hole and get the broken bits out before fitting new one back in.


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No need to drain heating just turn cold off and open hot tap

 

[ianmcd]

When doing anything on this boiler always cover the electronics panel as any leaking water goes straight in there

 

[buildersteve]

Thanks for all that gas112. You are correct. When I got to the shop it was only the end bit so I've gone on ebay and bought the whole fitting (not the block). Something has been leaking around the fitting as its got a lot of white powdered crud on it. I'm going to put some lemon juice on it to dissolve the crud and wait for the part to get here. Thanks also for the fitting instruction. I think I can manage it without the aid of calling someone.

Hi Ian. Yes, I'll protect the electronics when I do it as I don't want any more problems!
When I was at the shop there was a Baxi rep there so I quizzed him on the problem. He was very helpful giving me the part number (which I already had) but he said that as the boiler was nearly 20yrs old he wouldn't have the knowledge for that boiler. That's as bad as Jaguar! They drop parts and info on models that are over five years old and they cant even read the fault codes on my 97 XK8!
Anyway, I'll let you all know how I get on. Looking at the heat exchanger, I think the whole pump block would need to come off before it would come out without removing the boiler from the wall. Doesn't look that difficult though.

 

[buildersteve]

I took this pic to show the parts guy. You can see what mess has crusted itself round the fitting. It may have leaked from the small union above it.

 

[buildersteve]

Just waiting for the venturi to arrive but this morning it went right back to the way it was when all this started. The shower is cold all the time and the basin hot tap in the same room is running cold. Neither will switch the burner on so the moods in the house are not good!
I've now got a reliable pressure tester and the reading on the outside tap is just over 3 bar or 45psi.
Its difficult to see if the pressure has dropped as the pressure isn't that great anyway. As soon as I have the venturi I'll fit it. Meanwhile we'll use the electric shower in the downstairs shower room.
Any further comments are welcome!

 

[buildersteve]

You couldn't make it up! Went onto the local water supplier website and they have a power problem about four miles away from us which has dropped our water pressure. The result was the same as before. No hot at the shower or the basin tap in that room.
Got me wondering if they dropped it a week ago for some reason best known to them and just waited to see if anyone noticed.

 

[buildersteve]

Success! It was indeed the veturi. The plastic support had dried out and had fallen off, letting the spring and valve seat fall out. I retrieved all the bits except one small leg on the valve. Just couldn't find it. All working as it should now.
Many thanks to all who contributed to this fix, especially gas112 for his valuable fix info. If I had got on here before I phoned Brit Gas I would have saved a hundred quid.

 

[gas112]

Glad to have been of assistance dont worry about the wee missing bit it will prob be in the plate and wont cause any harm

 

[buildersteve]

Bit of an update on this Baxi.
Working well. Lots of constant hot water and the shower no longer going cold.
However, the boiler does "sing" when any hot water is drawn. I seem to recall that there was a mod for this boiler regarding this fault. Can anyone remember exactly what it was? Thanks. Steve

 

[buildersteve]

Could really do with a bit of help on this as the whine is getting on my wick a bit! Especially when taking a shower.
It started a couple of days after the venturi valve was fitted so I'm guessing it may be something to do with that but the guy from Brit Gas did fit a kit to the DHW valve. I remember he had to delve into a load of opened kits to retrieve a very small washer if that's got anything to do with it.
Also, the whining only comes on when a hot tap is opened and not when its turned off. Once I know what it is I'll have some instructions for the next boiler engineer!
Thanks. Steve

 

[gas112]

It's o rings in divert or valve was quite a common fault on instants when they first came outdo a search on here and something should come up remember and add instant onto 105e search