All I did was connect the two wires going to the switch together to complete the circuit. Thought the switch was bad. Then I removed the switch with the vacuum tube connected and sucked on the tube. Heard a clicking sound indicating the switch was working. Reinstalled and heater igniter glowed and burners lit. Great to have heat again.

You should be able to read the flow on the digital controller on your water heater. Check in operating instructions and find the sequence. I do not have a Nortiz, but can read the actual flow through the unit to within .1 gpm. That info can determine what is going on with your whole system. You could also have too small a gas line. Tankless water heaters are designed to guarantee temperature at the expense of pressure, meaning if you turn every faucet in the house on and you exceed the capacity of the unit it will slow down the flow to ensure the temperature set point is met. Check out the ground water temp with a thermometer. Check the unit specs for output at a specific temp rise. I suspect your ground water temp is close to 60F, so set the unit at 120* see what Noritz says the output will be at 60* temp rise. If the unit reaches that flow rate then your gas line is likely ok. FYI, the number one mistake in tankless installs is improper gas line sizing.

We have had a Noritz (N-069M) for 2 years now w/o problems. Next-door neighbor has a Rinnai. I'd look at the costs and see if the payback is worth it to you. Potential extra costs include running a new/bigger gas line (sounds like you may be doing that anyway) and venting. You could potentially locate your unit on an outside wall to eliminate the venting cost. For as infrequently as it gets below freezing there, I doubt you'd have problems. Personally, I think for most people it won't make economic sense over a better gas-fired tank heater. But there are other drivers- saving space in a small home, locating the heater nearer to point of use, etc. The one drawback that may affect you in FL like it does us in GA is the low-flow cutoff. Our thermostatic shower valve this time of year (when incoming water temp is at peak) can cut back flow enough that the burner on the Noritz turns off. We set our heater at 105F to mitigate this (showers are still plenty hot). Hasn't happened this summer except when I tried to take a lukewarm shower to cool off after exercising. We'll see how we do in August. Oh, and make sure you get the flushing valve kit so that you can periodically clean the heater by running vinegar through it. Soft water here, but lots of FL water is from wells and has higher hardness.

Maxi-Therm heater is located 4 stories below deck and is short cycling. Replaced the header because of leaking and installed a new by pass, but it still is short cycling.. Any suggestions anyone?