For maintaining heat, the heater rating is not very important.
What matters is the loss rate.
The heater will reach an equilibrium where the gain rate and loss rate are fairly equal.
The loss rate is what determines your heating demand.
So, you need a way to monitor exactly how much the heater runs and how much gas is consumed over a period of time.
Does the heater have its own gas meter or is the main meter only feeding the heater?
If yes, you just have to monitor the meter to see how much gas you are using and multiply by the cost of the gas per unit.
Also, you can monitor the runtime from the panel.
So, check the gas meter and the runtime and note the amount of gas used and the total runtime, then recheck the gas meter and the runtime to see how much was used.
If you can do this on a regular basis, you can figure out the amount of gas that is being used under various conditions like weather, set temperature, covered or uncovered etc.
The heater will use 266 cubic feet of natural gas per hour.
For example, if the runtime increases by 35 hours in a week, that's 5 hours per day, which is 1,330 cubic feet per day or 9,310 cubic feet per week.
The meter in cubic feet should match the runtime hours x 266 cubic feet per hour pretty closely.
If your cost is $12.00 per 1,000 cubic feet, then the cost per day is $15.96 (For 5 hours runtime per day) and the cost per week is $111.72 or about $480.00 per month.
The price of gas just went up by about 1.5 times.
So, what would have cost 480.00 per month would now cost $720.00 per month.
What types of costs are you seeing?
What is the cost of gas per 1,000 cubic feet?
The cost has gone up substantially recently as shown in the below chart.
Is the pool indoors or outdoors?
If indoors, what is the air quality management in terms of humidity and air temperature?
What is the surface area of the pool?
