🔥 FL Tankless Water Heater Sizing
Calculate peak GPM demand, BTU input, and temperature rise using Florida's 60–76°F groundwater. Size gas or electric units correctly for FL.
Fixture GPM defaults: Standard shower 2.0; rainfall head 3.0; garden/soaking tub fill 4.0; kitchen faucet 1.8; bathroom faucet 1.0; dishwasher 0.7; clothes washer 2.0; utility/mop sink 1.5.
Sizing Philosophy: Simultaneous Use
- Low (1–3 GPM): 1 shower + kitchen faucet. Studio, condo, 1BR.
- Moderate (3–5 GPM): 2 showers + dishwasher. Standard FL 3BR/2BA — most common range.
- High (5–8 GPM): 2+ showers + tub fill + dishwasher. Larger home, master bath with rainfall head.
- Very High (>8 GPM): Multiple showers + heavy use. Two units or commercial-grade.
FL tip: Clothes washers rarely run hot in FL's warm climate — most residents use cold wash year-round. Don't oversize for the washer unless used for hot cycles.
FL Groundwater Temperature Advantage
Florida's warm groundwater (60–76°F year-round) means far less temperature rise than cold-climate states (40–50°F). A unit rated for 5 GPM in Minnesota may handle 8+ GPM in South FL at the same BTU input. No seasonal penalty — FL groundwater stays nearly constant; the unit you size for summer is the unit you need in January.
| FL Region | Groundwater | ΔT to 110°F | vs. Northern ΔT |
|---|---|---|---|
| South FL | 76°F | 34°F | 48% less BTU |
| SW FL | 75°F | 35°F | 46% less BTU |
| Central FL | 72°F | 38°F | 40% less BTU |
| Space Coast | 70°F | 40°F | 36% less BTU |
| NE Florida | 67°F | 43°F | 31% less BTU |
| N Central FL | 63°F | 47°F | 24% less BTU |
| Panhandle | 60°F | 50°F | 20% less BTU |
| Northern US | 40°F | 70°F | Baseline |
#1 FL Issue: Hard Water Scale Buildup
Florida's hard water (6–22 GPG) is the leading cause of premature tankless failure. Without treatment, scale can reduce heat exchanger efficiency 20–40% per year and void warranties in 2–3 years. The heat exchanger has narrow channels (some under 1/16") that concentrate deposits directly on the highest-heat surface. At 15+ GPG, visible scale can build within 6–12 months.
Scale protection options: - Annual descaling flush (minimum): $125–$250/year. White vinegar or citric acid. Required in most FL counties to maintain warranty. - Polyphosphate scale inhibitor filter: $75–$200 installed, $40–$80 cartridge every 6 months. Reduces (doesn't eliminate) scale. Good for <14 GPG. - Electronic/magnetic scale inhibitor: $200–$500 installed. No salt, no waste water. Effective under 16 GPG. - Water softener (best protection): $800–$2,500 installed. Eliminates scale via ion exchange. Required by Navien, Noritz, and some Rinnai models to maintain warranty in FL.
| FL County | Hardness | Scale Risk | Recommendation |
|---|---|---|---|
| Collier | 22 GPG | Extreme | Softener required |
| Lee | 20 GPG | Extreme | Softener required |
| Miami-Dade | 19 GPG | Very High | Softener required |
| Broward | 17 GPG | Very High | Softener required |
| Palm Beach | 15 GPG | High | Softener recommended |
| Sarasota | 14 GPG | High | Softener recommended |
| Hillsborough | 13 GPG | High | Scale inhibitor minimum |
| Orange / Orlando | 8 GPG | Moderate | Annual descale |
| Brevard | 10 GPG | Moderate | Scale inhibitor or descale |
| Duval | 6 GPG | Low | Annual descale only |
FL Code & Permit Requirements
- Gas tankless must use sealed combustion (direct vent) or power vent. Open-combustion gas appliances require combustion air provisions rarely available in FL airtight homes. (2023 FBC §501.7 / NFPA 54 Ch. 10)
- Permit required for all water heater replacements regardless of fuel type. Homeowner permit available in most counties for single-family residential. (FL Statute 489.103(7)(a))
- Gas water heaters must meet minimum 0.82 Energy Factor. Condensing tankless (0.94–0.98 EF) qualify for FPL / Duke Energy rebates. (FL Energy Code §R404.1)
- CSST flexible gas pipe serving tankless requires arc-fault bonding; bonding wire to ground panel within the same room. (2023 FBC / NFPA 54 §7.13)
- T&P relief valve discharge pipe must terminate within 6" of floor or outside. Condensing units produce acidic condensate — drain must comply with FPC neutralization. (FPC §504.6)
- Electric tankless (24–36 kW) require dedicated 240V circuits, 100A–150A. Confirm main panel capacity (most FL homes have 150A or 200A) before purchasing. (NEC 422.11 / FBC Electrical)
Gas vs. Electric in Florida
| Factor | Gas Tankless | Electric Tankless |
|---|---|---|
| Unit cost | $600–$1,800 | $250–$700 |
| FL install cost | $800–$2,200 | $400–$1,200 |
| FL annual energy cost | ~$220–$350/yr | ~$400–$650/yr |
| Max flow rate | 4–11 GPM | 2–4 GPM |
| Hurricane outage | Works (battery pilot) | No hot water |
| Scale sensitivity | Moderate | Higher (element coils) |
| Lifespan | 20–25 years | 10–15 years |
| Best for | Whole-house | Point-of-use / supplement |
Gas tankless pays back the higher install cost in 3–5 years vs electric in FL, assuming FPL average rates of $0.14–$0.16/kWh and natural gas at ~$1.20/therm.
FL Installation Cost Reference (2024–2025)
| Item | Low | High |
|---|---|---|
| Gas tankless unit (condensing) | $900 | $1,800 |
| Gas installation labor (FL) | $800 | $2,200 |
| Gas supply line upgrade | $400 | $1,500 |
| Electric tankless unit | $250 | $700 |
| Electric installation (FL) | $400 | $1,200 |
| Panel upgrade (if needed) | $1,200 | $3,500 |
| Water softener (recommended FL) | $800 | $2,500 |
| Scale inhibitor filter | $150 | $400 |
| FL permit (typical) | $75 | $350 |
| Annual descaling service | $125 | $250 |