The lines start out filled with air.
You can have problems getting a stable flame (or any flame) until the air is pushed out and gas alone is flowing in the line.

You need to actually measure the gas pressure to find if it is low.
The air in the line goes to the gas pressure as soon as the valve is opened connecting it to the gas supply.
The fact that you can get the burner to light means that there is probably not any air remaining.

The line may not be sized correctly (longer runs at low pressure get pretty large quickly).

The unit could also have a problem.
If there is ANY debris in the line it may have been pushed into the gas valve if a 'drip leg' was not installed correctly.

I haven't tried running the unit any more but I did do some double checking on the gas lines and think I have found a problem that would affect the sytem when it is heavily loaded.

My gas meter is a "pound meter" running 1.2-2.2 psi. The gas meter feeds an Earthquake shutoff valve with a 1" inlet and outlet, which then feeds the low pressure regulator with a spring set at 6-8" WC. The problem I see is the outlet of that regulator is only 1". The 1" section is only 2 inches long after which it transitions to 1-1/4" pipe... but if I look at the gas tables a 1" pipe can only flow 286 CFH (~ 286K Btu/hr) at 50 ft and 259 CFH at 60 ft, the approximate distance to the furthest appliance.

With all appliances running full tilt my load is about 485,000 Btu/hr. Thus, I do not believe the 1" section is large enough.

The installation does not have a gas line drip leg before the Noritz unit. If that was an oversight by the gas piping contractor, the inspector also missed it, or we don't have the requirement in our local area.

I'll be talking to the contractor about the 1" pipe section and lack of a drip leg this week.

One thing I wonder about is the Noritz startup gas load... when it fires up the exhaust blower runs at high speed, so I'm guessing the unit requires a lot of gas to get going, after which it drops down once the cold water is up to temperature. If the unit is attempting to start while the furnace and old gas WH are running, I wonder if the 1" restriction in the piping is causing the low gas pressure?

It's all very frustrating.

I have heard other people having problems with the corrugated flexable gas connectors. Once removed and hard piped or CSST (3/4" up to 25') was used everything was fine.

It's kinda like kids blowing through those carnival snake noise makers.

Also I see you used lever handle ball valves, are they full port?

The gas flex line I have is rated 255,000 Btu/hr. I purchased it from Cinnabar Equipment, the supplier I used for the Z-Vent stainless exhaust ducting. That doesn't mean it's not part of the problem, just that is should be OK.

I need to purchase a digital manometer to check the gas pressure.

All the ball valves are full port.

Yes a pressure reading close to the inlet of the heater under all firing conditions will tell all.

On the hose behind the filter, is that a rating tag tightly wrapped around it or one of those in-line safety shuttoffs?

The heater does start at full fire to heat water in exchanger and then modulates down after reading temp on outlet thermister and flow at water valve, then the circuit board adjusts firing rate.

Good luck, I'm betting on the flex connector

No need to buy a manometer, you can make a simple one at home. I made one of these a few years ago...and it worked great.

Here is a link that might be useful: manometer

Yes, that's a rating tag wrapped around the flex line. I tie-wrapped it in place. I guess I need to read the fine print :-) It is supposed to have an internal diameter of 3/4" but I can tell it is not that large at the fittings.

If the heater starts at full power, which it would when the water is full cold, that's a theoretical load of 236,000 Btu/hr. If the furnace happened to be running (yes, it is still darn cold here in Seattle), that's up to 75,000 Btu/hr, and if the old gas water heater that's still in the system were also running, that's 36,000 additional. That's a potential 347K Btu/hr, which I think could be a problem for the 1" pipe at the main regulator.

The odd thing is the very 1st time I tried to run the Noritz tankless is started right up without any fluttering or combustion problems. But, the furnace and other water heater may have been off at the time.

The home-made manometer looks interesting, but I think I will spring for a digital manometer, which is less than $100, and will let me monitor the system much more easily during start-up.

Thanks.

As we now know from another note I posted the problem with the installation was an INOP flue damper.

The gas flex connector works just fine. As measured the pressure drop from static to operating is less than 1/2" WC.

Tom

You can make a manometer with a bucket and a pieces of tubing.
Fill the bucket with water, than connect the tubing to the gas line.
Immerse the open end of the tubing in the bucket until gas just stops flowing.
Measure the depth to the open end of the tube.
You have just measured 'inches of water'.

This method breaks down at higher pressures when finding a deep container becomes a problem, but for normal HVAC and many gas burner measurements works very well.

I had a tech working for me that complained about dragging out his manometer to check HVAC duct pressures.
Tubing and a glass of water are all you really need to measure less than 6 inches of water column.

So you had a blocked vent?