Is a Tankless Water Heater Worth It in Hard Water?

By the Plumbing Experts Queen Creek team · Updated September 2026

Tankless water heaters are good equipment and the marketing around them is mostly accurate. What the marketing does not emphasise is the interaction with hard water, and in this town that interaction is the whole decision.

This page is the version with the trade offs left in.

How it differs from a tank

There is no reservoir in a tankless heater. Water runs through an exchanger and comes out the far side hot, which gets you hot water that does not run out, a box on the wall instead of a cylinder on the floor, nothing being kept warm around the clock for no reason, and no large vessel of water sitting in the garage waiting for its turn to split.

That last point is worth dwelling on locally. A storage tank in a garage against a wall shared with living space is one of the more common origins of a serious water escape in a Queen Creek house. A tankless unit removes that failure mode entirely, and for some households that alone settles the question.

What happens inside the heat exchanger

Every gallon the household uses passes through narrow passages inside the heat exchanger and is heated directly. At this hardness, mineral deposits inside those passages.

Deposit inside an exchanger causes trouble in a defined order. First it sits between the flame and the water like a blanket, so the burner has to run longer for the same result. Then it shrinks the bore of the passages, so the flow the unit can sustain drops away and it starts cycling on and off. In the end the control board notices, throws a code and stops the unit to protect it.

A tank hides this for years by simply collecting sediment in its base. A tankless unit cannot, because there is nowhere for it to go. That is not a design flaw; it is the direct consequence of heating water in a narrow passage.

Descaling, and the valves that decide how easy it is

The remedy is a descale: a weak acid is pumped around the exchanger for about an hour, the deposit dissolves, and the whole lot is flushed clear. Nothing is dismantled and nothing is repaired. It is simply the housekeeping this equipment requires, and at our hardness it comes round sooner than a manual written for average water will tell you.

How painful the visit is depends on one detail decided years earlier. If a pair of isolation service valves was fitted when the unit went in, a technician connects hoses to them, runs the pump and disconnects. If they were not, the pipework has to be cut into and made good again, every single time, and an hour becomes an afternoon.

So the instruction at installation is short: those valves are not optional. They cost very little on day one and they discount every service visit for as long as you own the unit. On an existing installation without them, adding them during the next descale usually pays for itself within two visits.

Softening changes the arithmetic completely

This is the point that determines whether tankless is a good idea in a given Queen Creek house.

On untreated supply, a tankless unit is committing you to frequent descaling, and a household that skips it will get error codes, falling output and eventually a damaged exchanger. On softened supply, the descaling interval extends substantially and the unit behaves the way the brochure describes.

So the honest advice is that a tankless installation and a softener should be thought about as one project rather than two. Fitting the heater without treating the water is buying the expensive option and then declining to protect it.

Why replacing a tank with tankless is not a swap

Homeowners frequently expect a like for like exchange and are surprised by the quote. The reasons are physical rather than commercial.

The burner in a gas tankless heater is dramatically more powerful than the one in the tank you are removing, because it has to heat water instantly rather than slowly. Existing gas pipework frequently cannot feed it, which means gas line work as part of the project. Venting requirements differ and frequently mean a new vent run rather than reusing the old flue. Electric models have their own substantial demands on the electrical supply. Condensing models produce condensate that needs draining. And the unit mounts to a wall rather than standing on the floor, which sometimes means relocating it.

None of that makes it a bad idea. It does mean the comparison should be made against the full installed cost rather than against the price of the unit, and a quote that treats it as a straight swap has probably not surveyed the gas supply.

What not to do with a demand heater, and who it suits

It suits households that run out of hot water regularly, that want the garage or closet space back, that intend to stay in the house long enough for the efficiency to matter, and that are willing to treat the water and keep to a descaling schedule.

It suits less well a household that will not maintain it, a property where the gas and venting work would be substantial, or a situation where a failing tank needs replacing this week and there is no time to do the project properly. In that last case, replacing like for like now and planning the tankless conversion for later is the more sensible sequence, not the lesser one.

The flow rate conversation nobody has before buying

A tankless unit does not run out of hot water, which is the headline. What it can do is fail to supply enough water at once, which is a different limitation and one people meet unexpectedly.

Output is rated in flow at a given temperature rise, and temperature rise is the gap between incoming water temperature and the temperature you want. In winter, incoming groundwater is colder, the required rise is larger, and the available flow at that rise is lower. A unit that comfortably runs two showers in August may not in January.

Sizing therefore has to be done against winter conditions and against peak simultaneous demand rather than against a summer afternoon. Ask what temperature rise the sizing assumed. A quote that has not considered it is a quote that has sized the unit for the easy half of the year.

A final note on recirculation. Households that want hot water quickly at a distant bathroom sometimes add a recirculation system, and it interacts with a tankless unit in ways worth understanding. Some models support it natively with a built in pump and buffer; others need a specific external arrangement and will short cycle unpleasantly without one. Raise it during specification rather than adding it afterwards, because retrofitting recirculation onto the wrong model is the sort of expensive correction that a five minute conversation at quote stage avoids entirely.