Baxi 105HE, Repaired it, but no hot water unless two taps on

[DCGas]

Hi,
I am looking at a Baxi 105HE. Customer had no hot water, as the main burner was cycling on/off due to heat build up. After some diagnostics I replaced the plate heat exchanger and the DHW diaphram, which cured the problem and the burner now stays on when hot water demand is called for and all burner pressures etc are spot on. HOWEVER??? when you turn a hot tap on now the water is barely luke warm (even though the burner is going at full rate, unless you turn on another hot tap in the house, whereupon the hot tap you originally turned on immediately goes hot and stys hot until you turn the other tap off, when it then reverts back to chucking out luke warm water again. Never seen anything like this before and would appreciate some ideas as I don't like a boiler beating me. (which I suppose it already has)!

 

[Steve69the69Gas]

Have you bled the pump?

 

[DCGas]

Thanks for the reply. Yes the pump is working fine (heating working fine to)

 

[combispecialist]

sounds to me as if the diverter is passing,with heating off turn hot tap on and check to see if heating flow pipe gets hot,if it does it needs a new diverter valve

 

[45yearsagasman]

Not an easy fault to find but you may have a defective mixer tap/valve allowing cold water to mix after going through combi.Check the hot water temp directly below the boiler to prove the boiler is ok and that the diverter valve is not passing to heating circuit.

 

[MUFC1999]

If you isolate the water under the boiler CW inlet open hot mixers and they should run dry, if cold side is passing in should carrry on running and running and running at atleast 1 of the taps

 

[DCGas]

Already checked flow and return pipes with boiler in DHW mode only and they are not getting hot. Also there are no fault diagnostic lights showing and the diverter valve is switching over OK from DHW to CH. This one is a real weird one. If all DHW taps are connected to each other as they should be, because they have all had hot water coming out of them in the past, then why not now. I can't understand how one tap can flow luke warm then suddenly go red hot when another hot tap is turned on, but the second hot tap that was turned on only flows luke warm????? As the combi stands now it appears to be working perfectly, apart from the fact that you can only get hot water out of only one tap at a time and only if you have turned another hot tap on elsewhere in the house? I'm stumped.

 

[DCGas]

I think the only mixer valve tap is in the kitchen, all the others are singles. I'll let you know the outcome (if I can get the straight jacket off to type). Cheers

 

[mickyg]

have you taken a gas rate while its doing this? if the gas rate is correct then the heat must be going somewhere. either down the heating or into the hot water. You can't make heat energy dissapear.

 

[DCGas]

[/quote]burner now stays on when hot water demand is called for and all burner pressures etc are spot on.

Yep burner pressure is 12mbar, I totaly agree with you, the heat has to go somewhere, thats why I am so stumped, it's weird

 

[DCGas]

Had another look today, turned out to be the thermostatic shower passing cold water into the hot water pipes as 45yearsagasman & MUFC1999 suspected. Thanks to everyone that offered help. Just goes to show you never stop learning in this game.