On Sat, 16 May 2015 09:29:54 -0500, Ignoramus6769
wrote:

I bought a 200,000 BTU pool heater, to be installed outdoors.

The model is Hayward H200FDN.

The size of the piping that leads to it is somewhat iffy and appears
just a bit undersized according to various tables. Depending on the
actual pressure in the system, may be inadequate and not able to
supply enough gas.

The manual says:

``Based upon an inlet gas pressure of 0.5 psig or less at a pressure
drop of 0.5 in-wc''

Later it says:

``Hayward will not be responsible for heaters that soot up due to
improper .. natural gas line sizing''.

If I cannot provide this heater with adequate gas flow, I can upgrade
most of the piping from 3/4" NPT to 1" NPT, it is roughly a day or
work and some money.

To question is, do I need to do it?

So, I thought, I could turn to measurement of the gas pressure right
at the inlet of the pool heater. If the gas pressure at the inlet,
when the heater is running, is above the recommended value, then
I am fine.

I do have a pressure gauge that I could use.

My question is, and here's where I am not sure, what is that
"recommended inlet pressure". Is that 0.5 PSIG when the heater is
running? Am I reading that right?

thanks
You are correct. You need the pressure at the appliance when it is
running. With the appliance NOT running you would have the correct
pressure on even a 1/4" line.