Pool Heater Size Calculator

The energy to warm your pool is set by how much water there is — its volume. But the heat your pool loses, hour after hour, is set by how much water is touching the air — its surface area. Those are two different geometry numbers. A deep, small-footprint pool and a shallow, sprawling one can hold the same water but lose heat at completely different rates, because the shallow one has far more surface exposed.

The proof. Two 20,000-gallon pools, same ΔT, same wind, both uncovered:

Your pool loses heat four ways, and they're not equal. Evaporation is the giant — typically half to two-thirds of the total — because every pound of water that leaves as vapor carries off about 1050 BTU of latent heat with it. Convection (wind cooling the surface) and radiation (heat beaming to a cold night sky) split most of the rest. Conduction into the ground is almost nothing.

That latent-heat mechanism is why evaporation is so costly per pound. The physics doesn't care about your air temperature directly — it cares about the vapor-pressure difference between your warm water and the air just above it. That gap is what humidity and wind set, which is why both matter so much, and why “heat loss = U × area × ΔT” is a useful engineering simplification rather than an exact law. The pool evaporation calculator models that vapor-pressure mechanism specifically — and reconciles to ~60 % of this page's combined surface loss at standard conditions, so the two pages describe the same physics by construction.

We lump the surface losses into a typical-conditions combined coefficient because that's the form a homeowner calculator can actually use. The real number swings 2–3× with your weather — which is exactly why we give the heater size as a range, not a single number. The honesty boundary the §12 negative-space list spells out is built around this fact: the engine is precise; the world isn't.

A cover doesn't just trap a little warmth — it shuts down the dominant loss mechanism entirely. In our standard E1 example, a cover cuts the standing loss from 84,000 to 29,400 BTU per hour — it saves 54,600 BTU every hour the pool sits. Day and night. Continuously.

The sizing payoff: with a cover, the same pool needs a markedly smaller heater (or the same heater heats far faster), because the heater spends its output warming water instead of chasing evaporation. The calculator above always shows you the covered alternative regardless of which toggle you pick — surfacing the lever by construction rather than burying it behind a checkbox.

The cost side of this story — what the saved BTU translate to in dollars per season — lives on the future cost-to-heat calculator (shipping in a later phase). We won't fuel-wash the comparison here; the energy savings are real either way.

A gas heater is a fire hose — 200,000 to 400,000 BTU an hour — so it heats fast and shrugs off cold snaps, but it burns expensive fuel. A heat pump is a slow, efficient pump that moves heat from the air instead of making it: it puts out far less per hour (100,000–140,000) so it heats a cold pool over days, not hours, but it can deliver four to six units of heat per unit of electricity, so it's cheap to run.

The sizing consequence (E5). A 110,000 BTU/hr heat pump on our example pool takes ≈96 hours — ~4 days — to heat from cold. That's fine for a pool you keep warm continuously (its real job is maintenance), wrong for one you want hot by Saturday. Gas wins for fast / intermittent; heat pump wins for steady / economical.

The cost math — what each fuel costs per BTU at your local rates, the COP curve as air temperature drops, the season-long dollars — is the cost-to-heat calculator's job (shipping in a later phase). We won't fuel-wash the comparison here. Both are legitimate choices for the right pool and the right user.

Heat-up energy follows the first-principles definition E = mass × specific heat × ΔT. Water's specific heat is exactly 1.0 BTU/(lb·°F) by the definition of the British thermal unit; the 8.345 lb/gal is the same CRC water-density constant the salt and chemistry pages import from lib/dosing/core.ts. One source for water's weight; the two engines can't disagree about it.

Heat-loss model. The four-mechanism decomposition (evaporation ~50–70 %, convection, radiation, conduction) traces to standard ASHRAE pool-engineering references. We lump the surface losses into a combined U-coefficient that is linear in wind and dropped to ~35 % of uncovered when a cover is in place. This is a typical-conditions engineering estimate, not a constant of nature— evaporation tracks the water-to-air vapor-pressure difference, which swings 2–3× with humidity and wind. Every place we expose U or its components, the label says “estimate.”

Latent heat of vaporization (~1,050 BTU/lb) is the physics behind why evaporation is so costly per pound of water lost, and why a cover (which shuts down evaporation) is the biggest lever. The cover-fraction constant in the engine is the simplification of that physics — calibrated to ASHRAE-typical conditions, not derived from your individual weather.

Heater output classesfollow residential manufacturer product lines (150K / 200K / 250K / 333K / 400K for gas; 100K–140K for heat pumps). The sizing recommendation rounds UP to the next real class and surfaces the next class above as the upper bound of an honest range. We don't print a single fake-precise BTU number; the §12 negative-space list is built around refusing it.

About two coincidental-digit constants. The thermal engine's WATER_SPECIFIC_HEAT = 1.0 and WIND_COEFF = 0.4share digit strings with two chemistry constants on this site (dihydrate calcium chloride purity = 1.0 in the calcium engine, and the Langelier C-term offset = 0.4 in the LSI engine). They are genuinely different physical concepts — water's specific heat is defined by the BTU, and the wind coefficient is a heat-transfer correlation. The grep gate that enforces single-source discipline counts assignments per concept, not digit appearances. Documenting it here so any future “consolidate” instinct is checked.

The heating cluster — now complete. The cost-to-heat, evaporation, and heat-up-time calculators all consume this same lib/thermal/heatloss.ts engine — designed shared, not page-local — the way the chemistry cluster shares acidbase.ts across the pH, alkalinity, and LSI pages. Explore the volume, LSI, and full calculator hub for everything live.