Tankless Water Heater Installation Checklist — What to Know Before You Install

Tankless heaters are rated by flow rate (GPM), not tank size. Add up the flow rates of all fixtures you'd run simultaneously at peak usage: shower (2.0–2.5 GPM), kitchen faucet (1.5 GPM), dishwasher (1.5 GPM), washing machine (2.0 GPM). A family of four running two showers and a dishwasher simultaneously needs 6–7 GPM. Undersizing is the most common tankless installation mistake — the unit can't keep up and delivers lukewarm water during peak demand.

Gas tankless units deliver higher flow rates (8–11 GPM) and handle whole-house demand in most climates. Electric units max out at 3–5 GPM in cold climates (groundwater temperature matters enormously). If your incoming water is below 50°F and you need more than 4 GPM, you need gas. Electric units require 150–200 amp service panels with available breaker space for 2–3 dedicated 40-amp circuits — many older homes can't support this without a panel upgrade ($1,500–$3,000).

Gas tankless units consume 150,000–199,000 BTU — roughly 2–3x more than a tank water heater. Most existing 1/2-inch gas lines to the water heater location are inadequate. You'll likely need a 3/4-inch gas line run from the meter, and possibly a gas meter upgrade from your utility. Have a plumber assess your gas line size and pressure (minimum 3.5 inches WC at the unit). Gas line upgrades cost $300–$1,500 depending on distance.

Gas tankless units require Category III stainless steel venting (not standard B-vent from your old tank heater) and can vent horizontally through a wall or vertically through the roof. Wall-mount installation saves floor space. The unit needs clearances from windows (typically 12 inches), air intakes, and combustible materials per manufacturer specifications. Condensing units produce acidic condensate that needs a drain line — plan for condensate drainage to a floor drain or condensate pump.

Calculate your required temperature rise: desired output temperature (typically 120°F) minus your incoming groundwater temperature. In cold climates (40°F groundwater), you need an 80°F rise. Check the manufacturer's performance chart at YOUR temperature rise — a unit rated at 9 GPM delivers that at a 35°F rise, but only 5 GPM at an 80°F rise. This is where most undersizing happens. Your plumber should verify incoming water temperature with a thermometer, not a guess.

Condensing units extract more heat from exhaust gases, achieving 95–98% efficiency vs. 80–85% for non-condensing. They cost $200–$500 more upfront but use PVC venting ($50–$100) instead of expensive stainless steel venting ($300–$800). Condensing units produce acidic condensate that must drain — but the venting cost savings usually offset the condensate drain cost. For most installations, condensing is the better value.

If your bathrooms are far from the water heater, you'll wait 30–90 seconds for hot water to arrive — the same as with a tank heater. A built-in or add-on recirculation pump ($100–$300) keeps the hot water line primed so hot water arrives in 5–10 seconds. Some units have a built-in pump; others require a separate kit. Plan for this at purchase — retrofitting later means a second plumber visit.

Tankless installation requires different skills than tank replacement — gas line sizing, sealed combustion venting, control board configuration, and flow sensor calibration. Ask specifically about tankless installation experience, not just general plumbing. Improper installation voids the manufacturer's warranty (typically 10–15 years on the heat exchanger). Verify the installer will pull required permits — unpermitted work can void your homeowner's insurance coverage.

Most jurisdictions require a plumbing permit and sometimes a gas or electrical permit for tankless water heater installation. Permit costs: $75–$300. The inspection that follows ensures gas connections are leak-tested, venting meets code, electrical is safe, and condensate drains are properly routed. Skipping permits saves a few hundred dollars but risks code violations, insurance claim denials, and problems at home sale.

The installer needs clear access to the mounting wall, gas supply line, water supply and distribution lines, electrical outlet (for the control board and ignition), and venting path. Clear a 4×4 foot area around the installation location. If the unit is replacing an old tank heater, the tank needs to be drained and removed first — some installers handle this, others expect it done. Confirm in advance who handles old tank removal and disposal ($50–$100 if the installer does it).

Hard water (above 7 grains per gallon) causes scale buildup in the heat exchanger — the most expensive component. Scale reduces efficiency, triggers error codes, and can destroy a $1,500 heat exchanger in 3–5 years. Most manufacturers require a water treatment system for warranty coverage in hard water areas. A whole-house water softener ($1,000–$3,000 installed) or a scale inhibitor system ($300–$600) protects the investment. Test your water hardness before installation — test kits cost $10–$20 at hardware stores.

Turn on two showers and the kitchen faucet at the same time — your peak expected demand scenario. Water should reach 120°F within 15–30 seconds and hold steady without fluctuation. If the temperature drops or the unit error-codes under load, the unit may be undersized, the gas line may be undersized, or the unit needs reconfiguration. This is the test that matters most — don't let the installer leave until peak demand works.

Every new gas connection must be pressure-tested and checked with a combustible gas detector or soap solution at every joint. Watch the installer do this — or ask to see the test results. A gas leak is not just an efficiency problem, it's a safety hazard. The permit inspection will catch this, but don't run the unit on gas until connections are verified.

Condensing units produce acidic condensate (pH 3–4) that must drain through a condensate neutralizer (calcite cartridge) and to an approved drain point. Verify the condensate line drips when the unit is firing and that the neutralizer is installed. Without a neutralizer, acidic condensate corrodes metal drain pipes. Neutralizer cartridges need replacement annually ($15–$25).

Set the output temperature to 120°F (the Department of Energy recommendation — hot enough for comfort, low enough to prevent scalding). Ask the installer to walk you through the unit's digital display, common error codes, and how to reset the unit if it locks out. The three most common error codes are ignition failure (gas supply issue), flow sensor error (minimum flow not met), and exhaust temperature error (venting issue). Know what each means so you can troubleshoot before calling for service.

Scale builds up inside the heat exchanger even with treated water — faster with hard water. Annual flushing with white vinegar dissolves scale and restores efficiency. You'll need a flushing kit ($100–$150 for a submersible pump and hoses) or pay a plumber ($150–$300 per flush). The process takes 45–60 minutes: close isolation valves, connect pump, circulate vinegar through the heat exchanger, flush with clean water, reopen valves. Many manufacturers require annual flushing for warranty coverage.

Most tankless units have a small mesh filter at the cold water inlet that catches sediment. A clogged filter restricts flow, causing the unit to not fire (minimum flow not met) or to produce reduced hot water. Removing and rinsing the filter takes 5 minutes: close the cold water isolation valve, unscrew the filter housing, rinse the screen under running water, reinstall. This is the most common cause of 'the unit stopped working' service calls — and it's a free fix.

Check the vent pipe from the unit to the exterior termination point. Look for joints that have separated, corrosion on stainless steel venting (indicates condensate is pooling rather than draining back), bird nests or debris at the exterior termination, and proper slope for condensate drainage. A blocked or disconnected vent allows carbon monoxide to enter your home — this is a safety-critical check.