Enter the number of bathrooms, peak fixtures, and your groundwater temperature — get the required GPM, BTU output, and a realistic installed cost to build your quote.
Pre-built quote forms for tankless and tank water heater installs, drain jobs, and service calls — professional, ready to print.
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A Rinnai RL94 rated at 9.4 GPM will deliver 9.4 GPM in Florida. In Minnesota in January with 37°F groundwater, it may deliver 5–6 GPM at 120°F output. The same unit, half the capacity. This is the most common sizing mistake in tankless installations — speccing to the manufacturer's rated GPM without adjusting for actual groundwater conditions.
Required BTU/hr = GPM × 8.33 × 60 × temperature rise. A 2-bathroom home needing 3 GPM at 65°F rise requires: 3 × 8.33 × 60 × 65 = 97,000 BTU/hr. That maps to a 100,000–120,000 BTU unit. Most residential gas tankless units run 140,000–199,000 BTU — choose based on your peak GPM demand after temperature rise adjustment.
A whole-home electric tankless water heater (Stiebel Eltron, EcoSmart) typically requires 150–200 amps of dedicated 240V capacity. Most existing panels don't have this available. Before quoting an electric tankless, evaluate the panel — what's the total service amperage, what's currently loaded, is there room? Panel upgrade adds $1,500–$4,000 to the install and completely changes the quote.
Tank-to-tankless conversions often require: new gas line sizing (larger diameter for high-BTU units), new venting (direct vent vs. existing B-vent), water line modification, and a dedicated 120V outlet for the unit's electronics. The unit itself is $700–$1,400 in material. The retrofit work can easily add $500–$1,500 in labor. Set expectations on both cost and the scope of work before you show up.
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Tankless water heater installation is significantly more complex than a tank swap — it may require gas line upsizing, new venting, electrical upgrades, and water treatment if hardness is an issue. This calculator prices the full installation including unit markup, all associated work, permits, and labor time. Use it to give customers an accurate installed cost and avoid surprises when the job scope expands on site.