Water Heater · Service

Tankless installation, including the case against it

What actually makes a tankless retrofit expensive, when it genuinely pays, and the four situations where it is the wrong buy. Free matching with installers who have done more than a handful.

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In short

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.

Triggers

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 →

The job

How tankless water heater installation actually works

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 for: A 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.
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 for: This 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.
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 for: Any 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.
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 for: On 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.
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 for: Isolation 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.
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 for: Condensate 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.
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 for: Gas 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.
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 for: Assuming "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.

Money

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 →

Straight answer

When tankless water heater installation is the wrong call

Everyone else ranking for this is paid when you say yes. Here's when you shouldn't.

You have one bathroom and a modest, non-overlapping hot water draw. A tankless unit solves running out of hot water. If you have never run out, you are paying a large retrofit premium to fix a problem you do not have, and the running-cost saving on a small draw takes a very long time to return it.
Your gas line is undersized and cannot be practically upsized. A ¾-inch branch feeding a long run is the classic blocker. Where the new pipe would have to cross a finished ceiling, a slab or a shared wall, the gas work alone can exceed the cost of a good storage tank — and no unit choice makes that go away.
You have very hard water and no intention of treating it or descaling it. Scale on the heat exchanger is the number one cause of tankless units disappointing people early. If a softener is not in the plan and an annual descale is not something you will actually do, a storage tank is the more forgiving appliance and will fail more slowly and more predictably.
You are moving in the next few years. The payback is in running costs accumulated over a long stay, and buyers rarely pay a premium for the unit on the wall. A short remaining tenure turns a tankless retrofit into a gift to the next owner.
You want electric tankless in a cold-groundwater climate on a small panel. The colder the incoming water, the more electrical demand it takes to deliver usable flow, and older services often cannot support it. This combination produces more regret than any other tankless configuration.
Your real complaint is waiting for hot water at a distant tap. That is a distribution problem — pipe length, no recirculation — and swapping the heater will not shorten the wait by a second. Fix the loop, not the appliance.

Your options

Types of tankless water heater installation

Gas tankless

The mainstream route, and the one where gas line capacity and venting decide the price.

Electric tankless

No venting, but a serious electrical demand. Panel capacity is the gate.

Stay with a storage tank

More forgiving of hard water, cheaper to install, and often the honest answer.

Heat pump water heater

The other efficiency route — storage-based, with its own space requirements.

Compare water heater options side by side →

Before you sign

What to check when hiring for tankless water heater installation

Have you traced my gas line back to the meter and confirmed its capacity for this unit?

The single largest hidden cost on a gas tankless job. A quote written without this answer is provisional, whatever it says on the paper.
What flow will this unit deliver at my winter incoming water temperature?

Rated flow with no temperature rise attached is a meaningless number. This is the answer that tells you whether two showers at once will actually work in January.
Where does the new vent run, and how is it terminated?

The route is often the constraint, not the unit. It decides whether this is a half-day job or one that involves opening up a wall.
Are isolation valves for descaling included?

Leaving them out saves a small amount today and turns every future descale into a callout. It is the most consequential corner an installer can quietly cut.
What is my water hardness, and what do you recommend about it?

An installer who has not asked about hardness is not planning for the failure mode that will actually end this unit's life early.
How many of these have you installed, and will you service it afterwards?

Tankless installs punish inexperience in ways that only show up months later. Somebody who installs them regularly and returns to descale them has skin in doing it right.
Will I still be waiting the same time for hot water at the far bathroom?

The honest answer is yes, unless recirculation is in scope. Getting that on the record now prevents the most common post-install argument.

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Related work

Often done at the same time

Water Heater Replacement

Compare all four routes first

Gas Water Heater Installation

The storage alternative on the same fuel

Water Heater Maintenance

Descaling and what else it needs

Heat Pump Water Heater

The other efficiency route

All water heater services →

FAQ

Tankless Water Heater Installation — questions people ask

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Is a tankless water heater actually worth it?

It is worth it when you run out of hot water, when the space matters, or when you are staying long enough for the running-cost difference to accumulate — and when your gas line and venting do not need heavy work. It is a poor buy in a low-draw home with hard water, an undersized gas line and a short remaining stay. The appliance is good; the question is entirely whether your house is a cheap or an expensive host for it.

Why does my tankless unit give me cold water in the middle of a shower?

That is usually the "cold water sandwich" — a slug of cooled water sitting in the pipe from a previous short draw arrives before the unit's newly heated water does. It is a characteristic of on-demand heating rather than a fault. Recirculation, a small buffer tank on some models, and avoiding very short on-off draws all reduce it, and it is worth asking any installer how they plan to handle it.

How often does a tankless water heater need descaling?

It depends on your water hardness far more than on a calendar. Soft water may go a long time between services; genuinely hard water can need attention every year or sooner. Have your hardness measured rather than guessing, and make sure isolation valves are fitted so the service is quick. Skipping this is the most reliable way to make a tankless unit underperform.

Can I put a tankless unit exactly where my old tank was?

Often yes physically, since it is much smaller and wall-mounted — but the location has to work for the new venting, for gas or electrical supply, and for condensate drainage on a condensing unit. Interior closets with no nearby exterior wall are where the plan tends to change. The tank's old footprint is a starting suggestion, not a guarantee.

Do electric tankless water heaters avoid all this hassle?

They avoid venting and gas work entirely, which is genuinely simpler. They replace it with an electrical demand that is substantial, especially where incoming water is cold, and older electrical services frequently cannot support one without a service upgrade. Have the panel and load assessed before you treat electric tankless as the easy path.

Will a tankless water heater give me hot water instantly?

No — those are two different claims. "Endless" refers to never depleting a tank. The delay you experience at a tap is the time it takes for hot water to travel the pipe, and that is unchanged by removing the storage tank. If the wait is your actual complaint, the fix is a recirculation loop, which should be designed in rather than bolted on later.