Gas central heating boiler woes

Fresh from the heights of my success in replacing of one of the motorised zone valves on the central heating system, the gas boiler decided nuh-uh 😒 and promptly ceased functioning within a day. Buggrit! 😖

The boiler in question is a Vaillant ecoTEC plus 624 and was giving a fault code of F.29: Failure during operation: Re-ignition unsuccessful. According to the Installation & Maintenance Manual this can be due to:-

1. Gas supply temporarily stopped
2. Flue gas recirculation
3. Condensate duct blocked
4. Defective earthing of the boiler
5. Ignition transformer has spark failure

All very complex. However, as a first diagnostic action I reset the system by pressing the flame button three times (Manual section 13.2) to see if it was just a one off error. Unfortunately it was not. As with the zone valve replacement, I will provide no pictures to protect the incautious.

Next for some basic observations; the system water pressure was OK; I could hear both the water pump and the blower for the burner running; and I could hear the gas valve opening and closing; but could not hear any click of the igniter firing.

Looking at the fault list, there shouldn't be a reason for the appliance to suddenly have an earth fail (#4 - unless it is an internal fault). I had wired the new zone valve into the earth but that shouldn't have affecting anything; I checked anyway and it was fine. Four of the possible causes (#s 1, 2, 4 & 5) involve looking at the actual gas flow system and that is strictly beyond my pay grade (and illegal to mess around with to boot), so I could not rule them out. But I could not see any blockage in the flue gas pipe externally (#2) and I could confirm that there should be no reason for a sudden interruption in the gas supply (#1) as the gas hob in the house was working normally. The one possible cause that I could investigate myself was a blocked condensate duct (#3).

Modern condensing boilers recover heat from the exhaust of the burner by condensing water vapour out of the flue gas (CH4 + 2O2 → 2H2O + CO2 - North Sea gas plus oxygen from the air burns to give water and carbon dioxide and heat) thus giving up the latent heat of condensation. The water produced by the boiler is channelled from the burner into a condensate trap where it collects until there is enough to discharge via a plastic pipe to an external drain (in our case). Thereby avoiding the production of a slow but steady stream of water that can freeze solid in winter and block the pipe. The drain can also be blocked externally or internally by detritus from the boiler as it slowly oxidises the burner (common with aluminium burners, less so with stainless steel) and the resulting particulates are carried away in the condensate stream.

The condensate trap was accessible from underneath the boiler and was attached bayonet-style. When it was removed, there was a bit of detritus in the trap but not very much at all. There could still have been a blockage further up the system, so I took the case off the boiler and unhooked the condensate drain and confirmed that it was clear. The plastic drain pipe was not holding up any water but to check that the drain was fully open I poured some tap water down the drain to confirm that it disappeared smartly. So that ruled out a blocked condensate duct.

Just in case it had been the boiler condensate drain or the earthing of the zone valve. I restarted the boiler and it worked, once, then started failing again. So it appears to have an intermittent but persistent fault. At this point I was running out of options, so I checked some of the diagnostic codes for the igniter system:

Dx code	Description	Value
D.061	Number of ignition device cut offs	4
D.064	Average ignition time	1.7s
D.065	Maximum ignition time	4.1s
D.068	Unsuccessful ignitions at 1st attempt	7
D.069	Unsuccessful ignitions at 2nd attempt	5

According to Gemini,

An average ignition time of 1.7 seconds is normal, but a max ignition time of 4.1 seconds is unusually long (typically, a healthy ecoTEC ignites within 1–2 seconds). Coupled with high retry counts before locking out, this points directly to an internal combustion, sensing, or gas-regulation issue.

It pointed to one of the following, igniter, burner cleanliness, gas valve or spark delivery via the main PCB as the most likely causes. So it was definitely time to call in a more competent man.

The Gas Safe Engineer arrived the next day, confirmed the problem and started with a service replacing seals, cleaning everything as necessary and replacing the pressure sensor. That first step did not solve the problem. A further investigation discovered that the other zone valve on the system was faulty (stuck microswitch) so that was replaced, still no fix. The next step was to replace the gas valve that may have been overworked by the always-on zone valve.

So a couple of days later the engineer was back to replace the gas valve to the boiler and after fitting and a thorough test the boiler was up and running again.

For almost a week…

Time taken: Five hours (at £75/hr ex VAT)
Cost: Service kit and pressure sensor, £46.74; zone valve head , £54.99; gas valve, £257.80. Parts & labour all in, £881.43.