How manufacturers size gas pool heaters, why your existing gas line limits the replacement, and why bigger isn't automatically better.
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Difficulty: measuring and math only; all gas work is for a licensed pro
Sources: Pentair MasterTemp Installation and User's Guide; Hayward heaters FAQ
"What size heater do I buy to replace mine, and why can't I just go bigger?"
Hayward's H-Series brochure gives these area formulas:
For indoor pools, Hayward says to divide the area by 3.
Source: Hayward Universal H-Series brochure (2023)
Hayward's knowledge base recommends these H-Series models for swimming pools by surface area. Hayward says to pick the model whose area is "equal to, or just greater than" yours.
| Hayward model | Pool surface area (sq ft) |
|---|---|
| H150 | 450 |
| H200 | 600 |
| H250 | 750 |
| H300 | 900 |
| H350 | 1,050 |
| H400 | 1,200 |
Hayward's recommendation assumes a 30 °F temperature rise, 3½ mph average wind, and elevation up to 2,000 ft. Hayward says "a pool heater should heat the pool, on average, one degree an hour," and that going up one or two sizes shortens heat-up time.
Source: Hayward knowledge base: What power supply do I need?; Hayward H-Series brochure (2023)
Pentair's commercial MasterTemp / Max-E-Therm (ASME) brochure sizes by temperature rise per 24 hours, assuming 5.5 ft average depth:
| Rise in 24 h | Model 200 | Model 250 | Model 400 |
|---|---|---|---|
| 30 °F | 16,230 gal (394 sq ft) | 20,288 gal (493 sq ft) | 32,461 gal (788 sq ft) |
| 20 °F | 24,346 gal (591 sq ft) | 30,433 gal (739 sq ft) | 48,691 gal (1,182 sq ft) |
Source: Pentair MasterTemp / Max-E-Therm commercial heater brochure
The two brands' charts use different assumptions, so don't mix them. Use the chart for the brand you're buying.
This is why you can't just go bigger. Hayward says that when you replace a heater, the new one is "limited by the diameter of the existing gas supply pipe," and that a heater larger than the pipe can supply "will result in a sporadic or non-operating heater." Have a licensed gas professional check your gas line before you choose a larger model.
Source: Hayward knowledge base
Using one model family as an example, Pentair's MasterTemp guide lists sizes of 175K, 200K, 250K, 300K and 400K BTU/hr; factory settings of 78 °F pool and 100 °F spa, adjustable from 65 to 104 °F; an external 120/240 VAC supply; and direct connection to Schedule 40 PVC. Check the same items in your replacement's manual. Hayward's 2023 brochure lists thermal efficiency of 82.7% to 84% depending on model; get the figure for a specific model from its own spec sheet.
Source: Pentair MasterTemp Installation and User's Guide; Hayward H-Series brochure (2023)
Hayward says most of a pool's heat is lost in the evening through evaporation, and that a solar cover or other thermal barrier means less gas is needed.
Source: Hayward heaters FAQ
Fine to DIY: measuring your pool, doing the area math, reading the old heater's rating plate, comparing manufacturer charts, using a pool cover.
Call a licensed gas / pool-heater professional: gas piping and gas line sizing, venting and clearances, fuel conversion, gas valves, burners, igniters and heat exchangers, and installing the new heater. Call a licensed electrician: the heater's electrical supply.
See our full When to Call a Pro page. Follow your model's manual; this site isn't a substitute for it, and it isn't legal advice.
The numbers on this page come from specific manufacturer documents and models. If your equipment isn't one of them, don't assume the same numbers apply: find the model number on the rating plate, open that model's manual on the manufacturer's site, or contact the manufacturer. You can also email us with the brand and model number.
Related guides: Gas pool heater won't heat or keeps cycling
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