Tankless water heater installation, with the caveats stated up front

Why people switch, and which reasons hold up

Three reasons come up in nearly every conversation, and they are not equally solid.

Endless hot water. This one is true and it is what most people are actually buying. A tankless unit heats on demand, so it does not run out. For a household where the third shower of the morning is a gamble, that is a real change in daily life.

Space. Also true. Replacing a tank in a garage corner or an interior closet with a wall-mounted unit gives back usable floor area, and in a tight utility closet it can be the difference between storage and none.

Efficiency and lifespan. Partly true, with an asterisk. There is no standby loss from keeping a tank hot, which matters more than usual in a garage that passes a hundred and ten degrees in July. Service life is typically longer than a tank. But both advantages assume the unit is descaled on schedule, and in Arizona that assumption does a great deal of work. A neglected tankless unit on hard water does not quietly underperform, it fails.

A fourth reason rarely gets mentioned and deserves to: failure mode. A tank fails by rusting through and putting forty or fifty gallons on the floor. A tankless unit stores nothing, so when it fails you lose hot water rather than your flooring. On an interior closet or attic installation, that is worth something.

How it actually works

Cold water enters, a flow sensor detects movement, and the burner or element fires to raise the water to the set temperature as it passes through a heat exchanger. Nothing is stored. Close the tap and the unit shuts down.

Two consequences follow, and between them they explain most tankless complaints.

First, there is a minimum flow rate below which the unit will not fire at all. A thin trickle from a bathroom tap may not trigger it, which is why a very small draw can come out cold. Second, output is limited by how much heat the unit can add per minute rather than by a reservoir. It cannot borrow from a tank of already-hot water to cover a spike. Ask it for more than its rated capacity and the temperature falls rather than the supply stopping.

Sizing is flow rate plus temperature rise

This is the number that decides whether you end up satisfied, and it is a completely different calculation from tank sizing. Two figures matter.

Simultaneous flow. Add up what you might genuinely run at once, in gallons per minute. A shower is roughly one and a half to two and a half depending on the head. A kitchen tap is around one and a half. A tub fills faster than either. Two showers plus a kitchen tap is a materially different requirement from one shower.

Temperature rise. The gap between incoming water temperature and your target. Here Arizona is genuinely unusual, and for once it works in your favor. Incoming municipal water is warm for much of the year, in high summer, distinctly warm, so the rise required is small and a unit delivers close to its maximum rated flow. In winter, and especially in the high country where incoming water is far colder, the required rise increases and usable capacity drops with it.

The mistake is sizing on a summer figure. A unit that comfortably runs two showers in July can struggle with those same two showers in January in Prescott, because it is being asked to add far more heat to every gallon. An honest sizing conversation uses your coldest-month incoming temperature, not the flattering one.

The gas line question, which is usually the real cost

Here is the part that surprises people. A tank heater might draw thirty to forty thousand BTU. A whole-house gas tankless unit commonly needs somewhere between one hundred and two hundred thousand. That is not a marginal increase, and the existing gas line to that location was sized for the tank.

So a conversion frequently requires a larger line run from the meter, and occasionally a meter capacity review as well. The cost depends almost entirely on distance and routing, a unit going on a garage wall a short run from the meter is a modest job; one replacing a heater in an interior closet at the far side of the house is not.

This is the most common reason a tankless quote lands higher than expected, and it is why the conversation belongs before you have chosen a unit rather than after. See gas line services for how that work gets assessed.

Ask for the gas work as a separate line item. It makes competing quotes comparable and it shows you what you are actually paying for.

Venting and condensate

A tankless unit cannot reuse a tank heater’s vent. The combustion characteristics differ and the requirements are specific to the appliance.

Venting is typically a dedicated stainless or approved plastic run, either through a wall or up through the roof depending on the unit and its position. A wall termination is usually simpler and cheaper; an interior location may force a longer roof run. Clearances from windows, doors and air intakes are set by code and are not a matter of preference.

Condensate. Condensing units, the more efficient type, produce mildly acidic condensate that must drain to an approved point, sometimes through a neutraliser. That means a drain line at the unit. In a garage this is usually easy. In an interior closet it is often the detail that decides where the unit can go at all.

Electric tankless, briefly and honestly

Electric units avoid venting, combustion air and gas sizing entirely, which sounds like it sidesteps the whole problem. For whole-house duty in most homes, it does not.

Whole-house electric tankless demands substantial electrical capacity, frequently several dedicated high-amperage circuits and, in older housing, a service panel upgrade. That can cost as much as the gas work it was meant to avoid.

Where electric tankless genuinely earns its place is point of use: one distant bathroom, a casita, a workshop sink, a guest suite at the end of a long run. One fixture, modest demand, and no ninety-second wait for hot water. In that role we would recommend it without hesitation.

Recirculation and the cold-water sandwich

Two things people are told tankless will solve, and what is actually true.

The wait for hot water. Tankless does not shorten it. If your far bathroom takes ninety seconds today, it will take roughly ninety seconds afterwards, because the delay is the volume of cooled water sitting in the pipe between heater and fixture. Some units make it slightly longer while the burner fires. If a long wait is one of your reasons for switching, what you want is a recirculation arrangement, either a unit with built-in recirculation capability or a dedicated return line. Decide it before choosing the unit, because retrofitting is harder than specifying.

The cold-water sandwich. A short slug of cool water arriving mid-draw, most often when a tap is closed and reopened quickly. It is the water that sat in the exchanger and outlet piping while the unit was off. Modern units with a small buffer tank largely handle it. It is real, it is minor, and it is better known in advance than discovered.

The hard-water commitment, stated plainly

This is the part of the decision that matters most in Arizona and gets the least airtime.

A tankless heat exchanger has narrow passages and heats water rapidly, precisely the conditions that drive dissolved calcium and magnesium out of solution as scale. In water as mineral-heavy as Arizona’s, scale begins accumulating inside that exchanger from the first day of use. It restricts flow, insulates the exchanger so the burner works harder, and eventually produces error codes or outright failure.

Descaling is therefore not optional here. It is a condition of ownership, typically annual on untreated water, sometimes more often. Skipped for several years, a tankless unit in this state can fail well inside its expected life, and manufacturers can decline warranty claims where scale damage is evident and no maintenance was carried out.

Two decisions at installation make it manageable. Isolation service valves, a pair of valves with hose ports that let the unit be descaled in about an hour. Without them, descaling means partially disconnecting the unit and the labor roughly doubles, every single time. Insist on them: they cost very little at install and are never cheap to add later. And treatment upstream, which changes the accumulation rate substantially, see water softener installation.

The full maintenance picture is on the tankless descaling and maintenance page.

Where tankless fits, and where a tank is the better answer

Good fit: a household that genuinely runs out of hot water with a correctly sized tank; a property where floor space is at a premium; a home already being repiped or renovated so the gas and venting work rides along with existing disruption; an owner who will keep up with descaling, or who has softened water; and point-of-use duty for a distant fixture or casita.

A good tank heater is the better answer when: it is a straightforward like-for-like replacement and the existing tank was adequate; the gas run is long enough to make conversion cost disproportionate; the household will not maintain it, since a tank is far more forgiving of neglect; the property is a rental or is being sold soon, so the payback will not land with you; or the water is very hard with no plan to treat it.

Neither answer is a sales position. It is a genuine fork, and the honest recommendation depends on your gas run, your water and your appetite for maintenance. If a tank is right for you, the water heater installation page covers it properly.

What should be in the quote

Ask for these itemised, because tankless quotes vary enormously in what they include, and a low number usually means several are missing.

The unit and its rated flow at a stated temperature rise. Gas line work priced separately, with size and route noted. Venting, with termination type and location. Condensate drain and neutraliser where the unit needs them. Isolation service valves. Electrical work for controls, or the full electrical picture on an electric unit. Removal and disposal of the old heater. Permit and inspection. Any water treatment being recommended alongside, priced on its own.

Then two questions worth asking before you sign anything: what descaling interval does the manufacturer require for warranty purposes, and what does that service cost. Knowing the annual figure in advance is what separates a considered decision from a surprise.