I went back on this after downloading a manual for what was comparatively the Takagi TK-3, which does not appear to have its own manual, and i'm wondering if the model is discontinued. Anyway...I treasure feedback given after the problem has been solved so I will include it here.
The front cover to the heater was removed, exposing the tiny window where the pilot and ignition could be observed. On this heater the ignitor is nearly silent and cannot be heard unless your ear is really close to the heat exchanger. On some tankless you can hear the rapid spark of the ignitor from several feet away. You have to be eye level and look at a slight downward angle to see the spark.
A spark was observed but no pilot. Turning the hot water on and off several times the pilot finally lit once, and the burner ignited.
The gas supply was turned off and the three screws securing the gas inlet were removed, thereby loosening the inlet assembly. Next, all the perimeter screws on the gas inlet manifold were removed, the circuit board and some wires were moved out of the way, and the gas manifold was able to be pulled away from the bottom of the burner area. About 60% of the burner inlets were completely blocked by an accumulation of lint and maybe some aromatic fabric softener. There was some small pieces of "debris" atop the combustion fins, directly below the spark electrode area, and i figured that moving the heater up one inch to accommodate a new bypass water valve assembly had knocked loose some stuff in the vent above.
Using compressed air and a small brush the burner inlets were cleaned; the amount of material that blew out was substantial. The heater had been installed in '08 and was in a semi-enclosed back porch with open windows and a defective clothes dryer exhaust. I replaced the exhaust and re ducted about two years ago when in this room repairing the washing machine, after telling the customer that lint causes all kinds of problems if not properly exhausted, never thinking about the water heater. There are two dog beds in this back porch/laundry too, and the dogs are highly active and a bit shaggy. Enough said.
The only real problem with this was finding the tiny flame window (appears to be made of Mica and is about a dime in size only thinner) that fell to the ground when removing the ignitor and sensor assembly. I dropped it four times until figuring a tiny piece of electrical tape would hold it until I could get the assembly back on, and then carefully pull the "tail" of the tape out, leaving the window in place. These are sealed assemblies and the pilot will not operate safely or correctly without the window. I suggest placing a large and clean towel below the heater to catch screws, etc., especially when working in an attic ( I did another one of these on Sunday.) I also suggest exercising diligent care when removing the ignitor/sensor assembly as the gasket is friable and delicate and may come apart in your hands - be careful!
Other than that my experience with repairing one of these was a bit terrifying at first, but after downloading the factory repair and service guide, watching a few utube videos, and a nearby refrigerator full of imported beer, I was able to use my education in the heating industry and my experience in working on stoves and ovens to comfortably approach this and find the simple solution.
Photo from T-K3 TAKAGI Water Heater Campbell.jpg 79.68KB
Photo from T-K3 TAKAGI Water Heater Campbell.jpg 60.44KB