baxi 105e too hot

[rghid]

I have got a baxi 105e that is going hot and cold..... from stone cold on HW it fires up and the water gets up to temp coming out of the tap, it then goes cold after a few minutes but comes back again hot and cycles like this ... Have put new senors on it so know they are ok, and have took out plate heat exchanger and checked this it seems perfectly clear.

I am get 10.5 v DC to the modulating coil during HW demand, as the red lights get upto 70 degrees the flame stays at high with no drop in voltage till the flame goes out completley flame does not drop down at all.. Would i be right in assuming it is a faulty board

 

[corgiman]

could be the primary HE is partially blocked mate

that would give that symptom

what temp diff are you getting across the DHW HE and the P HE?

 

[rghid]

I do not have anything to check temp . but as the heat is getting from the secondary heat exchanger (plate) into the hot water outlet( they are getting up to normal temp) i would imagine heat is being adequately transferred from the primary heat exchanger into the secondary one.

The hot water is leaving the boiler past the hot water sensor hot enough. so i would imagine resistance in this should alter , altering voltage through the board to the modulating part of the gas valve and thus cutting down flame to keep an even temp to the tap outlets. It does not modulate at all and voltage stays at between 10 and 10.5 volts DC till burner just goes out. I would expect voltage to drop on temp rise to drop flame down and keep boiler outlet at right temperature

 

[corgiman]

what about the CH thermister have you checked that??

maybe a fault gas valve

 

[rghid]

Thanks for replying CM . have changed heating sensor at same time as HW one as they are only a couple of quid..
Fault finding chart says either gas valve or board. I am suspecting board purely because there is no drop in voltage to the modulating coil, if the voltage dropped and flame stayed high i would have thought it was the gas valve. As both parts are around a ton i don`t want to get it wrong.
Thanks again for your input

 

[baxpoti]

Sounds more likely to be the plate heat exchanger

Get new h/ex seals as well x4.

 

[rghid]

Spot on baxipoti. Trouble is i tried it with stat set at max and water seemed just as hot as normal just not modulating down, bit dopey never tried it with stat at half which when i did it modulated and worked ok. So thought i would check the plate heat ex again . There was a good flow though it and no sign of scale looking in and giving it few wallops.

So left it soaking with a descaler in it today, flushed it through and its seems all ok now. System is 4 years old and was all flushed through properly on installation , inhibitor added x100 and has an inline scale reducer on cold inlet. system has original water in it , only had to top it up slightly once in the 4 year it` been in.
So what could cause the partial blockage.

Corgiman sorry for red herring of plate heat exchanger being ok.

 

[namsag]

Baxipot a mate said today that local baxi engineer told him that duo techs should be run at 1.5 bar to stop scale forming have you heard anything about this also why would this stop scale forming.

 

[baxpoti]

Baxipot a mate said today that local baxi engineer told him that duo techs should be run at 1.5 bar to stop scale forming have you heard anything about this also why would this stop scale forming.

Not heard of this and can't see any reason for it!