RESPONSE TO: "It took me awhile to fix. I started with cleaning the fire box. Then the symptoms were as if the flame sensor was not working. It would spark and light off but after a few seconds it would it would shut-down amd recycle. So I replaced the flame sensor and ignightor since I noticed the gasket was torn and missing 1/2 inch of material. Same problem but when it did light and I gave the unit a high demand it would blow itself out. So the tech on the phone told me it was probably a gas supply problem. They were right. I replaced the gas supply line since it was undersized and got new high flow ball valves and replaced the pressure regulator. It's working fine now."
I have read through this entire post and everything mentioned previously exactly mirrors my experience with a KD-20. My installer and I have done all of the recommended fixes mentioned here, usually with only temporary success.
We have proper sized gas lines and when we checked pressure we got readings of close to 7" WC and and just over a 1/2" drop when the KD-20 kicked on. Thinking that the regulator at the gas meter was flakey, we had it checked but it received a passing grade by the Gas Co.
Which is the more likely scenario: 1) I have a failing gas control valve in the KD-20, or 2) I have a failed regulator at the gas meter and the Gas Co. guy was ducking a repair?
Overall, I like the way my HW system works (when it works!) but after only 4.5 years of service, I'm considering switching back to a gas tank heater.