Tankless installation, including the case against it

What is tankless water heater installation?

A tankless water heater heats water only while you are drawing it, so it is rated by flow rate at a given temperature rise rather than by stored gallons. That single difference drives everything about the install: to deliver heat that fast, a gas unit typically needs far more fuel capacity than the tank it replaces, and an electric one needs serious dedicated circuits. The unit is rarely the expensive part of a tankless retrofit - the gas line, the venting and the electrical work to support it usually are. It is an excellent appliance in the right house and a poor investment in the wrong one.

Signs you need tankless water heater installation

• You genuinely run out of hot water - back-to-back showers, a large soaking tub that a tank cannot fill hot
• Your water heater lives somewhere you would rather use for something else, and a wall-hung unit would free the floor
• You are replacing anyway and gas already runs to the space with capacity to spare
• The heater sits over finished space and the standing risk of a tank rupture is what keeps you up
• You are on a vacation or rental property where the heater spends most of the year keeping water hot for nobody
• You are already opening up walls or the mechanical room for other work, so the retrofit labour is partly sunk

Seeing something not on this list? Browse water heater problems by symptom →

How tankless water heater installation actually works

1. 1

   Establish the temperature rise your climate demands

   A tankless unit is not rated in gallons; it is rated to raise incoming water by a certain amount at a certain flow. The colder your groundwater, the harder that work is, so the same unit delivers meaningfully less usable flow in a northern winter than in a warm climate. This is the number that decides which unit fits your house, and it is the number product listings are vaguest about.

   Watch forA flow figure quoted with no temperature rise attached to it. It is marketing, not a specification. Ask what flow the unit delivers at your local winter incoming temperature, and ask to see it on the manufacturer's own chart rather than in a brochure.

2. 2

   Gas capacity survey - the most common blocker

   A gas tankless unit fires at a far higher rate than the storage tank it replaces, because it has to make hot water on demand instead of slowly. The existing branch line, and sometimes the meter itself, is frequently not sized for that. Upsizing means running new pipe from the meter, through whatever is in the way, and occasionally asking the utility about the meter.

   Watch forThis is where honest quotes and cheap quotes separate. If nobody has traced your gas line back to the meter and checked its size and length, the quote does not yet include the biggest single line item on a gas tankless retrofit.

3. 3

   Venting - assume replacement, not reuse

   Tankless units do not vent into the old flue. They use their own sealed venting, run in the manufacturer's specified material, along a path with its own length and termination rules. The route matters as much as the material: an interior mechanical room with no easy exterior wall can turn a straightforward install into a framing job.

   Watch forAny suggestion that the existing chimney or B-vent will be reused. It will not, and a shared flue that suddenly loses its main appliance can create a separate problem for whatever else was venting into it.

4. 4

   Electrical - yes, even on gas units

   Gas tankless units still need power for their controls, ignition and fan, so a receptacle or dedicated circuit has to reach the unit. Electric tankless is a different order of magnitude: it needs substantial dedicated circuits and appropriate service capacity, and in a cold-groundwater climate the demand is high enough that many older panels simply cannot host one.

   Watch forOn an electric tankless proposal, insist that someone has actually looked in your panel and calculated the load. Discovering on install day that you need a service upgrade turns a same-day job into a multi-week project with an unplanned electrician's bill attached.

5. 5

   Water quality assessment and the descaling plan

   Hard water is the leading cause of early tankless disappointment. Scale forms on the heat exchanger, where it insulates the surface and chokes flow, and the failure is gradual enough that people blame the unit rather than the water. The fix is boring: know your hardness, treat the water if it is high, and descale on a schedule.

   Watch forIsolation valves - sometimes called service or flush valves - being left off the quote to shave the price. Without them, every future descale becomes a plumbing job instead of a maintenance task, and most people simply stop doing it.

6. 6

   Mounting, plumbing and condensate

   The unit is hung on a suitable structural surface with the manufacturer's clearances, tied into the water lines, fitted with isolation valves, and - on a condensing unit - given a route for the acidic condensate it produces, which usually means neutralisation before it goes to a drain.

   Watch forCondensate handled as an afterthought. It is corrosive, it is not welcome in every drain, and a condensate line routed downhill into nothing is a slow, quiet source of damage.

7. 7

   Commissioning, temperature setting and the walkthrough

   The unit is fired, combustion and gas pressure verified under load, flow and temperature checked at a fixture, and the setpoint chosen. You should leave the walkthrough knowing how to read an error code and where the isolation valves are.

   Watch forGas pressure not verified while the unit is actually firing at full rate. A line that reads fine at rest can sag badly under a tankless unit's demand, and that is exactly the fault that shows up as intermittent cold water three months later.

8. 8

   Recirculation, if you want hot water quickly

   Tankless removes the wait caused by storage, not the wait caused by pipe length. If the far bathroom took ninety seconds before, it will still take about ninety seconds unless a recirculation loop or a dedicated pump is part of the plan. Some units support this natively; retrofitting it later is more disruptive.

   Watch forAssuming "endless hot water" means "instant hot water". They are unrelated claims, and the disappointment of finding that out afterwards is the single most common tankless complaint that has nothing to do with the unit working correctly.

What does tankless water heater installation cost?

A tankless retrofit commonly runs roughly $3,000 to $8,000 installed, and the spread is mostly infrastructure - gas line upsizing, new venting, electrical - rather than the appliance. A like-for-like tankless swap where that work already exists sits far lower. See the cost guide for the breakdown.

See the full tankless water heater installation cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →