Tankless Water Heater Cost Calculator

Add the fixtures that could run at once: a shower draws 2–2.5 GPM, a faucet 1–2, a dishwasher 1.5–2, a washing machine 2–2.5. HomeGuide's guidance is a 3.5 GPM minimum for 1–2 simultaneous fixtures, 5 GPM for 2–3, and 7+ GPM for 3–4. Then derate for cold groundwater: Fixr's example shows a 4.5 GPM-rated unit delivering only 3.5 GPM at a 35°F groundwater intake but 7.1 GPM at 77°F — northern homes should round up a size.

A like-for-like gas swap — new unit, mounting, gas and water connections, no new venting — runs about $2,770 in this model. A conversion from an old tank, with new condensing venting, isolation valves, permit and haul-away, lands near $4,420. That brackets the published anchors: Fixr averages $2,811 ($2,500–$4,500) for an installed 150,000 BTU unit, Angi averages $2,640 ($1,406–$3,902), and HomeGuide's full gas range is $2,100–$5,600.

A tankless burner exhausts at far higher temperature and pressure than a storage tank's naturally drafting flue, so old B-vent is never rated for it — HomeGuide flags vent replacement ($100–$600) as a standard line in every conversion. The calculator prices the venting trade-off directly: on the 6 GPM gas default, a condensing unit with PVC/concentric venting lands about $400 above the same job on a stainless-vented non-condensing unit ($4,420 vs $4,020), because the $300 equipment premium plus the $350 concentric kit outweigh the cheaper pipe. Most homeowners take it anyway — condensing units recover exhaust heat for higher efficiency, and PVC penetrates one wall instead of chasing a chimney.

Usually, yes on older homes. A tank draws roughly 40,000 BTU/hr; a whole-house tankless draws 150,000–199,000 BTU/hr, so a half-inch branch that fed the tank often can't feed the new unit. HomeGuide prices upsizing the supply at $350–$750 and a brand-new line at up to $2,000; this calculator models $550 for an upsize and $1,000 for a new run. Have the installer verify meter capacity, too.

Annually — every 6 months on hard water (Fixr's maintenance guidance). Professional flushes run $150–$300 per visit in Fixr's $45–$400 per-visit range, with around $225 typical. The isolation/service valves that make a flush possible add about $325 installed here (retail kits run $150–$180) — insist on them at installation, because retrofitting valves onto a unit that lacks them means cutting the unit out of the wall.

Frequently. A whole-house electric unit runs 18–36 kW through 3–4 double-pole breakers — 100+ amps of spare capacity many older 100A panels don't have. HomeGuide prices a dedicated circuit at $250–$900 and panel replacement at $850–$1,700; this model adds $575 for a circuit and $1,275 for a new panel. A full electric conversion lands near $3,625 installed — cheaper than gas up front, pricier to run.

It's the fix for the two tankless complaints: the wait for hot water at far fixtures and the lukewarm "cold water sandwich" between draws. A demand-controlled pump and install run about $600 here (published pump prices $200–$750 plus $300+ of labor), and because it only runs on trigger, it doesn't erase the efficiency gain. It's cheapest to plumb a dedicated return line during new construction — retrofit loops cost more.

HomeGuide puts tankless life at 15–20 years against 8–12 for a tank — and tankless units have serviceable parts (heat exchangers carry 12+ year warranties), so you repair rather than replace. The lifespan is maintenance-bought, though: an annual professional flush runs $150–$300 in Fixr's $45–$400 per-visit band, so plan on roughly $2,300–$4,500 of flushing across 15 years — cheap next to a mid-life replacement, ruinous to skip, because scale on the heat exchanger is the #1 killer. The $325 isolation-valve kit at installation is what makes those flushes possible. Run the tankless-vs-tank calculator for the full 12-year total-cost comparison.