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Full services directory How it worksFor prosAbout Get matched Call (866) 582-8523What your plumber must verify before a tankless retrofit: gas capacity, venting, condensate, electrical and hard-water reality — not another pros-and-cons list.
This page covers tankless water heater as part of a plumbing project. For the full guide — every option, the complete process and detailed pricing — see our main tankless water heater guide.
From the plumbing side, a tankless water heater is less about the box and more about whether your home can feed it. Gas units often need more fuel capacity than the tank they replace, new sealed venting, condensate handling on condensing models, and power for controls. Electric tankless units need serious circuit capacity. Hard water without a treatment plan turns a retrofit into a descaling subscription. Equipment choice detail lives on the water heater guides; this page is the site readiness check.
Gas capacity survey on gas models
Trace pipe size and length back toward the meter and compare to the unit’s BTU demand. Upsizing gas pipe — and sometimes coordinating the meter — is the common cost bomb on retrofits.
Watch for: A tankless price that assumes the existing half-inch branch is fine without measuring. Ask what pipe size they verified.
Venting and combustion air path
Tankless units use manufacturer-specified sealed vents with length and termination rules. Old B-vent flues are not a free reuse. Shared chimneys may need rework when the tank leaves.
Watch for: “We’ll use the old chimney” without a written vent design. That is how carbon monoxide and failed inspections happen.
Water, condensate, isolation valves and service clearances
Isolation valves for descaling, a drain path, condensate neutralisation where required, and clearance for service are plumbing details that decide whether the unit is maintainable.
Watch for: A unit jammed into a closet with no service valves. The first descaling will be a demolition mini-project.
Electrical — even on gas
Gas tankless still needs power for ignition and electronics. Electric tankless is a panel conversation. Coordinate a licensed electrician when load is non-trivial.
Watch for: Discovery of a service upgrade after the unit is on site. Check the panel at survey.
Plumber-quoted tankless work is unit plus gas, vent, water and electrical coordination. The appliance is often not the largest line. Hard-water treatment and gas upsizing move totals more than finish colour. See the water heater cost guides for equipment bands.
See the full tankless water heater cost guide — by material, size and region →
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