Tankless water heater

Short definition

A tankless water heater is a heater that produces hot water on demand without storage. Cold water flows through a heat exchanger and is heated as it passes — no tank, no standby loss. Standard residential gas tankless deliver 7–11 GPM at typical WA winter rise, which usually covers two simultaneous showers but not a shower plus dishwasher.

What it is

A tankless heater is essentially a heat exchanger sized for the entire household. Cold water flows in; sensors detect flow and inlet temperature; the burner (or condensing gas burner, or electric coil) fires at the BTU level needed to deliver the setpoint at the chosen flow rate; hot water flows out.

Two key performance constraints:

1. GPM cap. Tankless heaters are rated in gallons per minute at a given temperature rise. A 199,000 BTU/h gas tankless typically delivers about 9–11 GPM at a 35°F rise (summer in Seattle: 60°F inlet to 95°F shower) and around 5–7 GPM at a 75°F rise (winter: 45°F inlet to 120°F shower). Sizing must match WA winter conditions, not summer.
2. Flow activation threshold. Tankless heaters need a minimum flow rate (typically 0.4–0.6 GPM) to fire. A faintly dripping faucet won’t trigger heating; a low-flow recirculation pump may not either.

Standard residential tankless types:

• Gas, condensing. Most efficient — UEF 0.93+, Energy Star eligible, requires PVC/CPVC venting and a condensate drain.
• Gas, non-condensing. Simpler/cheaper, vents through stainless or AL-29-4C, no condensate drain needed. Lower efficiency.
• Electric, whole-house. Requires 120–200+ amp service to deliver useful flow rates. Rare in residential WA.
• Electric, point-of-use. Small 3–5 kW unit serving one fixture. Eliminates dead legs to far bathrooms.

Why it matters to a homeowner

The tankless decision in WA usually comes down to four questions:

1. What’s your gas-line capacity? A 199,000 BTU/h tankless usually needs a 3/4-inch or 1-inch gas line. Many WA homes have 1/2-inch supply, which means a gas-line upsize ($1,500–$3,000 additional). Sometimes the meter needs upgrading too.
2. Do you have a place to vent? Condensing tankless requires a sidewall PVC vent and a condensate drain. Non-condensing requires a stainless flue.
3. Where will the condensate drain go in WA winter? Frozen condensate lines knock out tankless heaters every January in Seattle, Bellevue, and Kitsap. Route through conditioned space or heat-trace.
4. Is your peak-hour demand really high? A 50-gallon gas tank can handle most 4-person households. Tankless’s “endless hot water” is real, but the GPM cap is also real — running two showers and a dishwasher simultaneously is borderline at WA winter inlet.

Cost in 2026 WA:

• Tankless equipment: $1,200–$2,800 (condensing gas, residential)
• Pro install with permit, gas-line upsize, vent, condensate, expansion tank: $3,500–$6,500
• Operating cost: $150–$350/year on natural gas (vs. $200–$400 for tank gas)
• Replacement frequency: 15–20 years (vs. 10–15 for tank)

When a contractor’s quote on a failed tank pitches “tankless conversion” at the same price as tank replacement, that’s almost certainly cutting corners — gas-line upsize, vent, and condensate routing add real cost. Get a detailed line-item breakdown.

When you’ll encounter this term

• A failed gas tank quote includes “tankless conversion” as an upgrade option.
• A new-construction or major-remodel WA plan submits tankless for WSEC compliance.
• A January cold snap takes out your tankless and the error code points to the condensate drain.
• An annual maintenance reminder requests “vinegar descaling” or “service-valve flush.”

Common variants and what tankless is not

• Gas tankless vs. electric tankless. Gas dominates WA retrofit market because of high BTU output without massive electric service. Whole-house electric tankless is rare.
• Condensing vs. non-condensing. Condensing extracts latent heat (UEF 0.93+, Energy Star), needs PVC venting and condensate drain. Non-condensing simpler/cheaper but less efficient.
• Tankless vs. HPWH. Tankless = continuous hot, no storage, fast hot water always available. HPWH = stored hot, slower recovery, lower operating cost. Different use cases.
• Whole-house tankless vs. point-of-use tankless. POU is a 3–5 kW electric unit serving one fixture (e.g., a remote bathroom). Whole-house serves the entire dwelling.

Common failure modes

• Cold sandwich. Brief burst of cold during a multi-shower sequence as the heater shuts down between draws and re-fires. Newer models with internal buffer mitigate.
• GPM cap reached. Two simultaneous showers + dishwasher exceeds the heater’s rated flow. Sized incorrectly for actual peak demand.
• Scale fouling on heat exchanger. Hard-water Spokane and Eastside requires annual or bi-annual descaling (vinegar circulation through service-valve kit). Soft-water Seattle Cedar/Tolt: less frequent.
• Condensate drain freeze. January 2024 took out thousands of WA tankless heaters this way. Relocate the drain through conditioned space or heat-trace it.
• Combustion-air starvation. Tankless installed in a tight closet with no make-up air; flame fails or backdrafts.
• Ignition failure. Igniter or flame sensor; service call.

Washington note

WA-specific install considerations:

• Permit required. Almost every WA AHJ requires a permit for tankless install. Verifies gas-line sizing, vent termination, condensate drain, expansion tank, seismic mounting.
• WAC 51-56-0500 governs water-heater install. WA Mechanical Code (WAC 51-52) governs venting. WA-amended IFGC governs gas connection (sediment trap and flex connector still required).
• Seismic mounting. Tankless heaters are wall-mounted; the manufacturer’s installation instructions specify minimum stud engagement and fastener size. WAC §507.2 doesn’t apply directly (no tank), but the wall mount must resist horizontal displacement.
• Recirculation loop with tankless. Requires aquastat or demand control. Tankless without a buffer doesn’t fire on tiny recirc flow rates — install a small buffer tank or use a smart recirculation control.
• Cold-snap freeze risk. January 2024 was the canary. Route the condensate drain through conditioned space, sleeve and heat-trace any exposed sections, and consider a small point-of-use buffer heater on the cold inlet for redundancy.
• Rebates. Energy Star-certified condensing tankless qualifies for some WA utility rebates (PSE, SCL, Tacoma Power, Snohomish PUD have varying programs). Verify current 2026 amounts before quoting.

FAQ

Is a tankless water heater worth it in WA?

For most WA homeowners replacing a working gas tank, the savings ($50–$150/year) take 10–20 years to pay back the higher install cost. Tankless makes more sense when: you’re remodeling and the gas-line work is happening anyway, you have a high peak-hour demand, you want to free up the floor space, or your existing tank has failed and the replacement cost difference is small. For households running an electric tank, HPWH usually beats tankless on payback in WA.

Why does my tankless quit during a cold snap?

Almost always a frozen condensate line. Condensing tankless heaters produce a continuous trickle of acidic condensate, and the small drain line freezes when temperatures drop into the teens — common in WA’s January cold events. The fix is preventive: route the line through conditioned space, sleeve and heat-trace any exposed segments, and use a manufacturer-approved freeze-protection accessory. After-the-fact thawing is just emergency triage.

How often do tankless water heaters need maintenance?

Annual or bi-annual descaling is standard. Hard-water Spokane and Eastside homes should descale every 12 months; soft-water Seattle homes can stretch to 24 months. The procedure: connect isolation valves with hoses to a small pump, circulate white vinegar or commercial descaler through the heat exchanger for 30–60 minutes, flush with water, restore service. Total cost: $0 DIY (if you have the pump) or $200–$400 for a service call.