Baxi Luna 310fi - intermittent burner cycling DHW

Notch48

Hi all,
I'm having an intermittent problem when calling for DHW. Everything will be working as expected and then the flame will go out and then the unit takes a minute to restart. In the video, you can see it's running normally and then for no reason the flame shuts off and the temperature lights flash. Then you can hear a relay (or relays) clicking several times and both the pump and fan indicator lights flash briefly. All the while the light indicating DHW call is lit. Then the unit starts again and it may run fine for the duration of the DHW call or it may cycle again at some random time interval.
See the video here: Video
Has anyone seen this behavior before? Any thoughts on where to look would be greatly appreciated as the Mrs. has threatened extreme measures the next time this happens while she is in the shower
Thank you!

unclejohn

Gas pressure vent piping flame sensor to name a few.

Notch48

Thank you @unclejohn for the reply. Would you suspect one over another based on the sequence of events in the video? If not, I'll start with the easy one (flame sensor) and see where that leads. I'm dumb, but not dumb enough to mess with the gas plumbing or venting. Appreciate any and all thoughts/suggestions. Thank you!

kcopp

This may help...
https://www.baxiboilers.com/sites/default/files/LUNA 310 Fi_Troubleshooting_0.pdf

Alan (California Radiant) Forbes

It could be a diaphragm that's developing a rupture.

When there is a flow of water for DHW, it pushes against the diaphragm. The diaphragm pushes a pin and the pin forces two contacts to close to prove flow and turn on the burner. If there is a hole or split in the diaphragm, it won't push the pin.

You can see this all happening if you remove the cover of the boiler.



https://www.parts4heating.com/Baxi-5405330-DHW-PRESSURE-SWITCH-DIAPHRAGM-p/5405330.htm

superdave

I don't think its the Diaphragm the DHW light does not drop out. Is this doing it on heating also?? I might be the Board or try changing out the NTC sensor on the DHW or better yet as a trick pull the plug off the NTC sensor and put it on new onw with out it installed and see if the boiler fires up and stays going just make sure you have high flow going through the DHW side.

clammy

If the burner is cycling on temp it may be that the plate exchanger is scaled and the boiler is just cycling on it s limit. I have the same model w out the plate and when the indirect gets lined up it cycles on temp ,I de scale the tank and it runs until it’s up to temp . Peace and good luck clammy

Notch48

Finally got to the bottom of this issue. It turned out to be a damaged ground wire. There is a ground wire that exits from the printed circuit board area and goes up into the area under the combustion chamber. That wire is supposed to be routed through a notch so that when the lower panel is closed the wire is not pinched. Well, it wasn't routed properly and opening and closing the lower panel was pinching the wire. Eventually the insulation wore through and most of the copper strands were broken. Which explains the intermittent symptoms. Of course I changed the PC board and a number of other parts without success before stumbling on the frayed wire mere moments before I was going to beat the snot out of the unit with a sledge hammer . And I was going to enjoy that too. Perhaps I will someday, but for now the problem is solved. Thanks for all the suggestions.

psb75

14th century English philosopher Wm of Occam's 'razor of logic' cuts to the quick...once again. Eliminate the simplest possibilities first, before going to the upper levels of complexity when confronting a problem. This 'razor' is the finest tool in my kit. And I'm guilty of not pulling it out...often enough.