Try clocking your meter to ensure you’re getting the proper input. To do this, simply turn all other gas appliances to pilot or off mode. Flip on your pool heater and wait for it to light. Using a stopwatch, time how long it takes for the 2’ dial to go around 5x. My 400Mbtuh Pentair runs about 1 min 40 seconds to run 5x.
Divide the answer by the number of revolutions for the average. 100sec/ 5 rev = ~20sec / rev.
in other words you’re using 2cuft of gas every 20sec.
since there’s 60 sec in a min, and 60 min in an hour the multiplier is 3600
There‘s ~1100 btus per cuft of natural gas.
From there the math is simple.
divide the volume of the dial read 2cuft / secs per rev x 3600
example (2 / 20) x 3600
360cuft per hour x 1100btus= ~396,000 btuh input capacity.
If it’s low you could have something as simple as a clogged gas orifice from spider webs, or it could be more complicated. Newer heaters have a hidden orifice inside a union fitting.
My heater is rated at ~82% efficiency, so my output capacity is 324,720btuh.
My water flow rate will determine my temperature Δ.
I have a 14° temperature Δ across my heat exchanger.
gpm = (btu input x efficiency) / (Δ temp x 500)
My example:
(396,000 x 0.82) / (14 x 500) = ~46 gpm
BTW, the 500 is derived from (60 min taking gpm to gph) x 8.34 lbs per gallon. As it takes 1btu to raise 1 lb of water 1°.
If you’re measuring at the skimmers and jets you‘ll need more accuracy. My salt chlorinator is immediately downstream of the heater, and it provides an outlet temperature. The heater input screen reads 1° higher than my chlorinator, so I adjust for the differ. For increased accuracy you could install
Pete’s plugs and use the same thermometer.
If your gas and water flow rates are correct and you’re still only getting a 2° Δ, then your efficiency is way off. This is indicated by a very high flue gas temperature or high CO readings. A combustion analysis would be needed.
for every 50° increase in flue temp there is a 1% efficiency loss. However, too low of flue temperatures can result in corrosive flue gas condensate. Too low of flame temperature and CO is increased.
So, yes I should increase my water flow, and I’ll likely get a bit of a net efficiency gain.
Again, if you’re measuring at the pool jets, you’re way off and you’re losses are likely due to underground piping.
