Pool Heater BTU Input vs Output: Key Difference

What Input and Output BTU Mean on a Listing

British thermal units, written BTU, are a unit of heat energy. A pool heater listing uses a BTU number to state a heat amount per hour, often written BTU, BTU/h, or BTU per hour. Some listings show 1 BTU number, and some show 2. The 2 numbers are the input and the output.

The input figure states the energy the heater takes in. For a natural gas or propane heater, the input is the heat content of the fuel the heater burns. For a heat pump, the input is the electricity the unit draws, often shown in kilowatts instead of BTU. The output figure states the heat delivered to the pool water. On a listing that shows both, the output figure is the one that describes heat reaching the pool. The BTU versus kilowatt article explains how the 2 units relate.

• BTU input or input BTU for the fuel or electricity the heater takes in
• BTU output or output BTU for the heat the heater delivers to the water
• BTU/h, BTU per hour, or BTU for a heat amount per hour
• Not stated by the seller when a title gives 1 figure without a label

Why the Two Figures Differ: Efficiency

The input figure and the output figure differ because no heater sends every unit of its input into the pool water. A gas or propane heater burns fuel, and part of the heat leaves with the exhaust or is lost around the burner. The output is therefore lower than the input. The ratio of output to input is the heater's thermal efficiency figure, usually shown as a percentage. A higher efficiency value means a smaller gap between the input and output figures on a listing. The thermal efficiency article explains how that value appears on a spec sheet.

A heat pump works differently. It moves heat from the surrounding air into the water instead of generating heat by burning fuel. Because of that, a heat pump's output BTU can be larger than its electrical input when both are expressed in heat units. A heat pump listing may show an output BTU figure and an electrical input figure in kilowatts; those are 2 different kinds of measures and are not compared the way a gas heater's input and output are compared. The coefficient of performance article covers how heat pump output is expressed.

Reading a Heater Title That Shows Both Figures

A listing may state an input figure and an output figure, such as a high input value and a lower output value. The gap between the 2 is the efficiency loss stated by the maker. When a title gives 1 figure only, treat it as the stated figure on the listing and read the maker's manual for the other figure. If the manual does not show it, the phrase 'not stated by the seller' applies to the missing figure.

The 2 heater families read differently. A natural gas pool heater title commonly labels an input figure, and a propane pool heater title does the same. A pool heat pump title more often labels an output figure and states the electrical input in kilowatts. Read the label before comparing any 2 numbers.

BTU Figures on Gas Heaters Versus Heat Pumps

On gas and propane heaters, the efficiency relationship is direct: the output is part of the input, so the output figure is always lower than the input figure. The 2 figures appear in a title or spec sheet as input BTU and output BTU. On heat pumps, the relationship is different because the unit gathers heat from the air. A heat pump's output BTU and its electrical input in kilowatts describe 2 separate quantities.

The heater type comparison article puts gas, propane, heat pump, and solar heaters side by side in listing terms. This article does not claim that one type heats faster or costs less; the site compares stated details on listings, not measured performance.

Questions to Ask Before You Buy

Before a purchase, ask a few questions from the listing: which figure the title labels, whether the maker's manual lists the other figure, what efficiency value the maker states for a gas heater, and what output BTU and electrical input in kilowatts a heat pump listing shows.

• Which figure does the title label: input, output, or neither?
• Does the maker's manual list both the input and the output figure?
• For a gas heater, what thermal efficiency value does the maker state?
• For a heat pump, what output BTU figure and what electrical input in kilowatts does the listing show?
• Who confirms the gas and electrical supply? A licensed gas installer handles the gas supply, a licensed electrician handles the electrical supply, and the maker's manual and the local building office decide each connection.

What to pick for your situation

Questions

What is the difference between input BTU and output BTU on a pool heater?

Input BTU states the heat content of the fuel a gas or propane heater burns, or the electrical input a heat pump draws. Output BTU states the heat delivered to the pool water. For a gas heater, output is lower than input because some heat is lost; the ratio is the thermal efficiency value.

Why does a heater listing show 2 BTU figures?

A maker may state both the input and output figures so a shopper can see the efficiency relationship. A title may label 1 figure only; the manual or spec sheet usually lists the other. When a listing gives 1 figure, the missing figure is not stated by the seller.

How does efficiency connect input and output BTU?

Efficiency is the ratio of output to input. For a gas heater the result is lower than the input because some heat leaves with the exhaust. For a heat pump the relationship is different: the output includes heat moved from the air, so the output figure is not compared to the electrical input the way a gas heater's figures are.

Do heat pumps use the same input and output BTU comparison?

A heat pump moves heat from the air into the water instead of generating heat by burning fuel. Its listing commonly states an output BTU figure and an electrical input in kilowatts. The output can be larger than the electrical input expressed in heat units because the heat is moved, not created.

What should a shopper do when a title states only 1 BTU figure?

Treat the figure as the stated number on the listing, then read the maker's manual for the other figure. If the listing says nothing about the missing figure, the phrase 'not stated by the seller' applies. The maker's sizing page and an installer handle any pool size questions.