Point-of-use and mini-tank water heaters

A point-of-use water heater is a small heater placed at or near one fixture, usually under a sink, instead of in the basement or garage with the main heater. ENERGY STAR describes the idea simply: the water is heated very near the sink, shower or bath where it is used, rather than at a central heater, and the units are often used to boost the temperature at a fixture that sits a long way from the main tank.

Two different machines share the name. One is a mini-tank, a small electric storage heater with its own thermostat, element and relief valve. The other is a small electric tankless unit that heats water only while it flows. They solve the same complaint in different ways, and they fail in different ways, so it helps to know which one a store listing or a plumber’s quote is describing before you compare them.

What problem a point-of-use heater solves

The usual complaint is the wait. You open the hot tap in a far bathroom, and cold water runs for a long time before anything warm arrives. That cold water is not coming from the heater.

It is the hot water from the last draw that sat in the branch pipe and cooled to room temperature. It has to be pushed out of the pipe before new hot water from the heater can reach you.

The ENERGY STAR technical bulletin on demand water heaters makes the same point about long runs. Depending on the diameter and length of the line to the farthest fixtures, a large amount of hot water can sit in the pipe, cool down, and be wasted down the drain the next time someone waits for a shower to warm up. The longer the distance, the longer the wait. For far-flung fixtures, the bulletin names a small individual tankless unit installed near the fixture as one answer.

The arithmetic is easy to check at home. Half-inch copper has an inside diameter of a little over half an inch, which works out to roughly one gallon for every 80 feet of pipe. If the far sink is 50 feet of pipe from the heater, there is a bit more than half a gallon of cooled water standing between you and the hot water.

EPA’s WaterSense program labels bathroom sink faucets that use a maximum of 1.5 gallons per minute. At that flow, the roughly 0.6 gallon in a 50-foot run takes about 25 seconds to clear, and that is before the pipe itself has warmed up. A low-flow aerator saves water on every use, but it stretches the wait at a distant tap, because the same half gallon has to come out more slowly.

A point-of-use heater shortens the wait by shortening the pipe.

The heater sits a few feet from the faucet, so there is very little standing water to purge. That is the whole mechanism. It does not make the main heater bigger, hotter or faster.

Mini-tank or small tankless

A mini-tank is a scaled-down storage heater. It holds a few gallons at a set temperature and reheats them with one electric element after you draw from it. Because it stores water, it can deliver a short burst of fully hot water at whatever flow the faucet allows, and the small element only has to rebuild that little reservoir between uses.

The limits come from the same reservoir. Once the stored gallons are used, the outlet temperature drops to whatever the element can keep up with, which is much less. A mini-tank also keeps its water hot all day, so it has its own small standby loss. Anything that stores hot water under pressure needs a temperature and pressure relief valve with a discharge line, the same as a full-size tank.

The International Code Council’s CodeNotes on water heater safety walk through where that discharge line may terminate: the floor, a pan, a waste receptor or outdoors, in a way that does not cause injury or damage. Under a vanity, that usually means planning the discharge line before the heater goes in.

A small tankless unit has no reservoir. A flow sensor switches the elements on when water moves and off when it stops. The ENERGY STAR bulletin describes demand heaters in general this way: when water starts moving, an element or igniter is activated, and cold water flows through a heat exchanger that raises its temperature on the way to the fixture.

There is no standby loss, and there is no stored burst either. What you get at the tap depends entirely on how much heat the unit can add per minute, which is limited by the electrical circuit feeding it.

So the choice tends to follow the use. A mini-tank suits a sink used in short bursts where the user wants fully hot water at once, such as a sink for washing hands or rinsing dishes. A small tankless unit suits a fixture with low, steady flow where you do not want a tank holding heat around the clock. Neither is a substitute for a shower, which is a much larger flow, unless the unit was specifically sized for that.

The flow and power math

The same rule that sizes any tankless heater applies here. Gallons per minute equals the heat input in Btu per hour, divided by about 500 times the temperature rise in degrees Fahrenheit. One kilowatt is about 3,412 Btu per hour, and nearly all of an electric element’s output ends up in the water. The temperature rise is the difference between the incoming water and the temperature you want at the tap, so winter groundwater asks for a bigger rise than summer groundwater does.

The table runs four illustrative element ratings through that formula. They are round numbers for the arithmetic, not the ratings of particular products, and the current column is simply watts divided by volts.

Read it against the faucet. A 1.5 gallon-per-minute bathroom faucet with a 60°F rise needs about 45,000 Btu per hour, which is roughly 13 kilowatts. A small 120-volt unit cannot supply that, so the water comes out warm rather than hot unless the user runs the tap more slowly or mixes in no cold at all.

That is not a defect. It is the arithmetic. It is also why small units are usually paired with a single sink, where each draw is short.

The Department of Energy’s older water heating guide gave a rough benchmark for full-size demand heaters: a 70°F rise at about five gallons per minute through gas units and about two gallons per minute through electric ones, and it described demand systems as best suited to low-demand or remote applications. A point-of-use unit sits at the small end of that range by design.

Where point-of-use makes sense

ENERGY STAR lists the situations where a point-of-use unit, or a decentralized approach in general, is more likely to save energy and money:

• Some fixtures sit far from the other fixtures in the house.
• Daily hot water use is low, for example less than 20 gallons per day.
• The fixture is a remote bathroom or a hot tub.

It also names specific cases to consider: new construction where the home cannot be laid out to keep pipe runs short with an efficient central heater, an addition where you could run only cold water to the new space and heat it there, and a hot tub if electricity is the heating fuel. In every case it says to weigh the costs and benefits before going decentralized.

ENERGY STAR adds a comparison that matters in older houses. Adding a point-of-use heater at a distant fixture may be a better choice for system-wide efficiency than turning up the main tank so the far tap feels hotter. Raising the whole tank also raises the scald risk at every other tap. The Consumer Product Safety Commission urges households to set water heaters to 120°F and gives burn times that fall sharply above it, which is covered in temperature, scalds, and Legionella.

Where it is the wrong fix

A point-of-use heater fixes distance at one fixture. It does not fix a main heater that runs out of hot water, a failed dip tube, a dead lower element, or a mixing valve stuck partway. If every bathroom fades together, the problem is upstream, and the hot water runs out too fast is the place to start.

It is also not a backup for a solar or geothermal system. ENERGY STAR says plainly that point-of-use heaters are not suitable backups there, because they cannot support a household’s full hot water demand when the main system is not running.

Finally, it is not the only answer to a long wait. A recirculation pump moves the cooled water out of the line before you need it. The DOE guide describes on-demand recirculation, where a user trips a switch at the faucet and the pump stops once hot water reaches it.

The ENERGY STAR bulletin warns against continuous circulation, which keeps the heater firing, and points to a push-button demand pump with a temperature-sensing shutoff instead. If several fixtures share one long branch, a single pump can serve all of them, where point-of-use would need a heater at each one.

Installation and safety points

Most point-of-use heaters are electric, so the circuit is the first constraint. The table above shows why a small tankless unit can need a dedicated 240-volt circuit and a breaker in the tens of amps, and why a larger one can be beyond what a crowded panel can take. Use the current and breaker size printed on the nameplate and in the installation manual, not an estimate. The page on electrical safety explains why water heater circuits are treated as continuous loads.

Check these before you buy:

• The nameplate voltage, wattage and required breaker, and whether the panel has room for it.
• For a mini-tank, where the relief valve discharge line will run, and whether the cabinet floor needs a pan.
• For a tankless unit, the minimum flow it needs to switch on, which is listed in the manual, because a faucet with a very low-flow aerator may never wake it.
• Whether the unit is meant to preheat with water from the main heater, or to start from cold supply only, since that changes the rise it has to make.
• Whether local code requires a permit for the circuit and the plumbing connection.

Treat a mini-tank like any other storage heater for maintenance. Its relief valve should be tested as described in testing the relief valve. A small tankless unit in hard water can scale like a large one, and its screen or inlet filter can clog and starve it of flow.

The make and model manual sets the service steps for either type. Manufacturers such as Rheem publish installation and use manuals for their point-of-use models, and the one for your model is the one to follow.

The short version

A point-of-use heater is the right tool when one fixture is far from the main heater, uses little hot water, and the complaint is the wait, not the supply. Choose a mini-tank for short bursts of fully hot water, and plan its relief valve line. Choose a small tankless unit for low, steady draws, and check the flow-and-power arithmetic against the faucet before you trust the label. If the whole house is short on hot water, fix the main heater first.

Sources

ENERGY STAR, “Point of Use (POU) Water Heaters” (energystar.gov), for the definition, the remote-fixture boost use, the comparison with raising the storage tank setting, the list of situations to consider, the 20-gallon-per-day example, and the note that point-of-use units are not suitable backups for solar or geothermal systems.

ENERGY STAR, “Technical Bulletin: Demand Water Heaters” (energystar.gov PDF), for how demand heaters switch on, wasted water in long lines, the point-of-use option for far-flung fixtures, and the warning against continuous recirculation. U.S. Department of Energy, “Water Heating: Energy-Efficient Strategies for Supplying Hot Water in the Home,” DOE/GO-102001-0785 (energy.gov PDF), for the five and two gallon-per-minute benchmark and on-demand recirculation.

U.S. EPA WaterSense, “Bathroom Sink Faucets” (epa.gov), for the 1.5 and 2.2 gallon-per-minute faucet flows. U.S. Consumer Product Safety Commission, “Avoiding Tap Water Scalds” (cpsc.gov PDF), for the 120°F recommendation. International Code Council, “CodeNotes: Water Heater Safety in the I-Codes” (iccsafe.org), for relief valve discharge termination. Manufacturer installation manuals for nameplate ratings, minimum flow, and service steps.