Could be as much as 2.5 wc. What puzzles me is it worked like this the last few years and I only took over this pool last pool season so nothing would have changed other then a component within the heater, that's what my brain tells me. Yes, the meter on the house is tool small for all but when the pool season is on then the heat in the home is off and as I said before, it has been working flawlessly up to now.
Is the heater gas supply hard-plumbed or flex? If flex, that can cause problems, usually immediately, but sometimes it shows up later. I can't tell you how many times I have heard, "It always worked that way before." Then I hard-plumb the heater and it works again.
A small gas meter can still be an issue, even if house heating is off.
Most residential gas meters are able to pass 250 cu. ft./hr. unless they are upgraded or the house is very large. That's already too small for a 335K heater. With gas at a property the water heater is most likely a gas appliance as well as the range in the kitchen and a gas clothes dryer.
The average rate of gas usage of a water heater is 1cu. ft./hr. per gallon of capacity (more efficient ones do use a bit less). So a 40 gallon water heater will be requiring 40K BTU/40 cu ft. worth of gas per hour, and since the supply to that appliance is likely closer to the meter, it will get it before the pool heater. A large home would likely have a larger water heater, maybe a 50 or 60 gallon model.
If the 40-gallon water heater is running and your meter is already small, that leaves you short 125K BTU of gas. Then, they may also have a gas clothes dryer running. They usually require about 22K BTU/22cu. ft. when the burner is on (which is not continuously).
Add and older heater and the problem can show up eventually. Having enough pressure, measured in w.c., does not always mean there is enough supply.
