How machines work
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Tankless water heater
The big idea: it only heats what flows. No water moving, no fire.
Run the steps up to Water heats first, then slide.
Water moves first. Opening the tap pushes cold water through the heater. A little wheel in the pipe spins and tells the brain.
Then the fire. The fan clears the box, a spark lights the gas, and a sensor checks the flame is really there.
Fire matches flow. More water, bigger flame. Less water, smaller flame. Stop the water and the fire stops.
Where the numbers come from
One small model drives every number and every moving part. Heat needed is 500.4 × GPM × °F of rise BTU per hour (8.34 lb per gallon × 60 minutes), which is exact physics. At 1.5 GPM and a 50 °F to 120 °F rise that is 52,540 BTU/h of heat into the water, so at 84% efficiency the burner settles at 62,550 BTU/h, about a third of its 199,000 BTU/h maximum.
Typical values, which vary by model: it fires above 0.5 GPM and stops below 0.4 GPM; 199,000 BTU/h maximum input, 15,000 minimum; 84% thermal efficiency for a non-condensing unit. Estimates: a 2 s fan pre-purge, about 1 s to prove the flame, a 3 s post-purge, a 2.5 s warm-up time constant at the outlet, and 0.15 gallons of pipe between heater and tap (about 16 ft of ½ inch PEX), which is why the tap runs cold for about 6 s after the flame lights.
At a 70 °F rise this heater can keep up with 4.77 GPM. Above that it runs flat out and the water comes out cooler: at 6 GPM, about 106 °F.