Tankless Hot Water Systems: Pros and Cons

A tankless hot water system heats water as it is drawn, rather than storing it hot in a cylinder and keeping it there. The claim that follows from that, endless hot water, is true only within the unit’s capacity, and capacity is where the decision usually turns. This article sets the benefits against the drawbacks in the order a buyer meets them, then works through what the household needs to measure before choosing one.

How a tankless system works

Water enters the unit, passes a heat exchanger, and leaves at temperature. Gas models fire a burner on demand; electric models draw power through elements sized to the flow they must raise. Nothing is stored, so nothing stands and cools between uses. The output is limited by how much water the unit can raise to the set temperature at the incoming water temperature, which in a cold climate in winter is a real constraint rather than a technicality. A storage cylinder with the same nominal output behaves differently, because it can draw on a tank of already-hot water for a short burst of high demand. Neither design is simply better. They fail in different ways.

The advantages

Lower standing losses

A cylinder keeps its contents hot around the clock, and heat escapes through the tank and the pipework regardless of whether anyone uses the water. A tankless unit spends nothing keeping a tank warm, and the fuel it uses tracks the hot water the household draws.

A supply that does not run out

Shower by shower, the water stays hot for as long as the unit can keep up with the draw. The qualification matters: run more outlets at once than the unit can serve, and the temperature at every outlet drops together. A household that never asks for two hot showers at once gets a genuinely endless supply; one that does will meet the limit on the first cold morning it tests it.

A longer service life

A tankless unit has no steel cylinder sitting full of water and corroding from the inside, which is the failure that ends a storage system’s working life. What the unit has instead is a heat exchanger and, in gas models, moving parts and a burner, so its lifespan depends on water quality, servicing and the conditions it operates in. The warranty period is the fair comparison between two specific models, and it is the number worth reading side by side.

Space recovered

The unit is a wall-mounted box rather than a cylinder with a footprint, and in a small house the freed corner is worth more than the specification suggests. The unit does not have to sit where the old cylinder stood, which matters when the cylinder occupied a hallway cupboard or the only dry corner of a laundry.

Less damage when something fails

A tank fails by releasing the water it holds, which is why a burst cylinder soaks floors and walls. A tankless unit holds a small volume inside its pipework, so the failure mode is a leak or a breakdown rather than a flood. It is not damage-proof, but the worst case is smaller.

The drawbacks

Higher upfront cost

The unit and the installation both cost more than a like-for-like storage replacement, and the gap is widest where the installation is complicated. Whether the running savings close that gap depends on household use, local energy prices and how long the household stays in the home, and no general article can settle it for a particular house. The comparison worth making is the ten-year cost, not the invoice.

Flow-rate limits

The unit serves only as much hot water as it can raise at once. Two showers, a dishwasher and a washing machine running together can exceed what a modestly sized unit will deliver, and the symptom is not a warning light but lukewarm water at every tap. Incoming water temperature matters here as well: the same unit that copes in summer may struggle in a cold winter, because it must raise colder water through the same range. The unit is also only as good as the pipework it feeds, and older pipe that has scaled or partially blocked reduces the flow before the unit’s capacity is tested at all. The common plumbing problems that affect pressure are worth ruling out for that reason. What any household can check before buying is a count of the hot outlets it genuinely runs at the same time, in the busiest half hour of the day.

Installation requirements

The site has to suit the appliance, and this is where a retrofit surprises people. A gas unit needs a supply line of adequate capacity, and an existing line sized for a cooktop or a small cylinder may need upgrading. It needs a flue or vent path that the installation rules allow, with the clearances those rules set from windows, doors and boundaries. An electric unit needs a dedicated circuit and the capacity to feed it, which in an older home can mean a switchboard upgrade. The wall itself must carry the unit, which is straightforward on masonry and a matter for the installer on a lightweight wall. None of this rules a system out, but all of it belongs in the quote rather than in the week after the unit arrives.

Delayed delivery at the tap

With no stored hot water, the hot tap runs cold until the unit fires and the heated water reaches it. The wait is the distance between unit and outlet, and the water that runs while waiting is the trade for having no tank. Locating the unit centrally, or fitting a small dedicated unit for a distant bathroom, reduces the problem. In a compact home it is a few seconds; in a long house it is longer, and a reader standing at the far kitchen tap will notice it as much as any specification line.

What sizing involves

The system must match the household’s simultaneous demand, which is a different quantity from the daily total. Two households can use the same volume of hot water in a day and need different units, because one draws it in a single evening rush and the other spreads it across hours.

The check a buyer can run is straightforward. Count the bathrooms. Count the hot outlets that run together most often, and how often that happens. Note the season: the coldest mornings set the requirement, because the unit must raise colder water through the same range. Then read the manufacturer’s flow figures with those numbers in hand, rather than reading the marketing summary above them. A household that cannot state its simultaneous draw does not know which unit size it needs, and the installer can only work with the number it is given. It also helps to know the state of the existing pipework before committing to a system that depends on flow, and the problems that hide in older plumbing are the first thing worth checking.

What the installer needs to check

The constraints are checkable in advance, and a quote that does not mention them is incomplete. The gas line capacity and meter pressure, for gas models. The flue route and its clearances, including the positions the rules prohibit. The electrical circuit and board capacity, for electric models. The wall construction and the length of the pipe run to the outlets that matter. The location rules that apply where the unit will sit, which vary with the installation rather than with the suburb. A licensed plumber confirms what applies at the address, and the point of the pre-purchase check is that a unit which cannot legally or physically be installed at the chosen spot fails before the money is spent, not after.

Weighing it up

A tankless system suits a household whose hot water demand is spread out or modest at any one moment, whose site can take the unit and the pipework, and whose motivation is running efficiency, space or a supply that does not run out mid-shower. A storage cylinder holds its ground where several outlets run at once, where the installation is awkward, or where the lowest purchase price matters most. The deciding facts are the household’s simultaneous flow, the site’s constraints and the ten-year cost, and each of those is knowable before a unit is chosen.