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Andrew Smith
Andrew Smith, Plumber
Category: Plumbing
Satisfied Customers: 3591
Experience:  City and Guilds qualified plumber
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I have a Baxi duotec combi. CH works fine, when hot water demanded,

Customer Question

I have a Baxi duotec combi. CH works fine, when hot water demanded, the Hall effect sensor works fine, the water thermostat is working fine, the diverter is fine, the pump runs ok but the burner does not ignite. Any ideas?
Submitted: 1 year ago.
Category: Plumbing
Expert:  Andrew Smith replied 1 year ago.
***** ***** : Can you confirm the hot water flow switch is clicking?
***** ***** : If you try with the power to the boiler off and your ear against the case
***** ***** : You should get a light click when you run the tap
JACUSTOMER-n2kspyjf- : The flow switch is fine. The hot water is now coming on and getting to temperature but then the burner disengages and it runs cold before the burner kicks in again. It continues to cycle like this.
JACUSTOMER-n2kspyjf- : i am beginning to suspect this may be the diverter valve after all.
***** ***** : What the divertor while it is doing this - see if moves up and down
***** ***** : It could also be a scaled up plate heat exchanger - this would mean that the return from the plate is hot so the boiler turns off
***** ***** : Whereas the heat cannot be transferred into the hot water fast enough due to it being scales up and the boiler turns off, this will only fire again once the return water from the plate has dropped in temperature again.
JACUSTOMER-n2kspyjf- : The diverter motor does not seem to move at all when hot water is called and the CH is off. The PHE is brand new I installed less than 6 months ago along with a magnaclean in the CH return pipe so will be very surprised if that has clogged already. When CH only is called the diverter motors fully down.
***** ***** : OK, next check - can you run the hot water can you see if the heating pipes warm up under the boiler?
JACUSTOMER-n2kspyjf- : No, the heating pipes remain cool. The 3 way valve has a bypass pipe for the PHE, and there are two separate valve bodies not a single cartridge. The baxi part number is(###) ###-####if you want to see more clearly.
***** ***** : Are you able to follow the heat when the burner is firing but you have no hot water?
JACUSTOMER-n2kspyjf- : When I run the hot water the temperature window shows a steady climb to exactly 60 degrees then the burner cuts out, the temp drops to around 40 and the burner re-fires. When I had a similar problem with the blocked PHE the temp rise was significantly more rapid and hit the 90s before the boiler cut out. This time it seems more controlled, it could just be coincidence but the maximum selectable water temp is also 60 degrees.
***** ***** : You have full control but have poor het
***** ***** : heat transfer into the hot water
***** ***** : This again points to the plate HEX?\
JACUSTOMER-n2kspyjf- : The heat transfer seems fine as the water runs hot to seemingly the correct temperature but the boiler isn't holding it at that temp
***** ***** : ok, perhaps the main boiler thermistor is having problems maintaining the heat on the primary side?
***** ***** : How is the heating performing?
JACUSTOMER-n2kspyjf- : CH is working fine though?
***** ***** : Perhaps it would not be noticed if it was on and off alot as it would maintain the temperature quite well
***** ***** : It would show immediately when running hot water
JACUSTOMER-n2kspyjf- : Quite possibly but initial observation doesn't seem so, I have set the CH to various temps on the boiler and it controls to the requested temp just fine
***** ***** : Do you want to try and replace this first and see how you go?
JACUSTOMER-n2kspyjf- : Just had a closer look, CH is controlling and maintains set temperatures exactly as expected. I won't be able to get parts until Tuesday now anyway so I'll come back to you after the weekend. My gut feeling is the 3 way valve and if not the PCB.
***** ***** : If there was a problem with the 3 way valve not opening fully then you would get heat creepage into the heating, this is why i asked you to hold the heating pipes under the boiler when on hw only

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