I have a TK-2 about 10 years old that was having the same problem. Every evening the heater would not light for about three hours like clockwork and then start working again. Called Takagi and ruled out any fuse issue. They emailed me a procedure for cleaning the burner which I did. However I think the problem was actually a clogged heat exchanger. There was build up of a white to greenish powder in the heat exchanger which I found out the hard way is carbonic acid or some other nasty acid. Part of the procedure for cleaning the exchanger is blowing compressed air through it, however Takagi's instructions actually say to contact them for further instructions if you see this powder. Like an idiot did not. As a result I burnt the crap out of my lungs even though I was wearing a respirator (apparently not fitting as well as it should). TAKE EXTREME CARE IF YOU SEE THIS POWDER AND DO NOT GENERATE OR BREATH ANY DUST. I fixed the problem with the heater but at great cost to myself. Not sure how Takagi recommends handling this scenario, but its worth finding out.
