Point-of-Use Water Heaters
A point of use water heater is a small heater mounted right at the fixture, usually under a sink, so hot water arrives in seconds instead of after a long wait. It comes in two forms: a mini-tank that stores a few gallons, or a small electric tankless unit that heats water as it flows.
Both suit a sink. Neither replaces your main water heater for showers and laundry.
Who a point-of-use heater is for
The honest test is distance. If the sink is a long pipe run from the main heater and you stand there running water for ages, a point-of-use unit fixes that. A.O. Smith's pitch is the same: hot water arrives fast because it doesn't travel far, and you waste less water and energy pushing cold water out of a long line.
The common spots, going by A.O. Smith and Rheem:
- a kitchen or bathroom sink at the far end of the house
- a room addition or garage
- a guest bath, pool house, outdoor kitchen, detached workshop or basement wet bar
- a small apartment with light hot water needs
It's the wrong tool when you need a lot of hot water at once. A.O. Smith says plainly that its point-of-use units are designed for sinks, not showers. Showers need a whole-house unit with enough capacity.
Mini-tank vs tankless vs the alternatives
| Option | What it is | Best for | The catch |
|---|---|---|---|
| Mini-tank (under-sink tank) | Stores roughly 2.5 to 7 gallons | One or two sinks, or a single low-flow shower | Runs out faster than a full-size tank |
| Point-of-use tankless (electric) | Heats water as it flows, no tank, no venting | Hand washing and light use at one fixture | Flow limits and higher up-front cost |
| Whole-house tankless | One large unit for the house | Several fixtures at once | Still has the long-pipe wait at far taps |
| Recirculation pump | Keeps hot water near the fixtures | Long waits everywhere in the house | Adds pump energy use, no extra capacity |
For most people fixing a single slow sink, we'd pick the mini-tank. It plugs the gap with the least electrical work, and it handles the bursty, short draws a sink sees. Go tankless when you have the circuit for it and the fixture sees steady, light flow.
If the wait happens at every tap, look at a hot water recirculation pump first. One pump can fix the whole house, where you'd need a point-of-use heater at each sink.
Mini tank water heaters
A mini-tank is a small version of your main heater: an insulated tank with an electric element and a thermostat. A.O. Smith's mini-tank range runs 2.5 to 7 gallons. Its EPU models come in 2.5, 4 and 6 gallon sizes, each with a 1440-watt element. The 4-gallon model runs on 120 volts, which is why mini-tanks are often the easier install.
It connects directly to the cold water supply near the fixture. Some installers plumb it in series after the main heater instead, so the tank covers the wait and the main heater's water takes over once it arrives. Ask your plumber which setup suits your sink.
The downside is capacity. A few gallons goes quickly if someone fills a pot or a basin. And it's another tank to keep an eye on.
Tankless and instant under-sink water heaters
An instant under-sink water heater heats water with electric coils as it passes through, so there's no tank and no standby heat loss. It mounts on the wall near the fixture and needs no vent. Rheem describes the same compact, wall-mounted, vent-free design for point-of-use electric tankless units.
Two numbers decide whether one will keep up: how much water flows through it and how much it has to heat that water. The U.S. Department of Energy's rule of thumb is to assume 50°F incoming water unless you know otherwise, and to target 120°F, which means a 70°F temperature rise. Electric demand heaters typically manage that rise at about 2 gallons per minute, against about 5 for gas units. Colder incoming water in winter cuts what any unit can deliver.
Compare that with what the fixture uses. Rheem's rough figures are about 0.5 gallons per minute for a bathroom faucet and 1 to 2 for a kitchen sink. A bathroom sink is an easy match. A busy kitchen sink is borderline for small units.
Power is the other limit. A.O. Smith's own comparison shows a 240-volt, 3,500-watt tankless point-of-use unit next to a 120-volt, 1,440-watt mini-tank. The bigger the heating element, the bigger the circuit it needs. Check the spec sheet for the breaker size before you buy, and check that your panel has room.
Tankless point-of-use water heater for a shower
People ask about this a lot. It can be done, but not with the small under-sink units. A.O. Smith says its point-of-use tankless units aren't designed for showers, and a shower needs a unit with real capacity. Rheem's figure for a shower is about 2 gallons per minute, right at the limit DOE gives for a typical electric demand heater. In a cold-water region you'd need a much larger electric tankless unit and the electrical service to match. See electric tankless water heaters and how to size a tankless water heater.
"Point of use" in water treatment
The same phrase shows up on water filters and softeners, and it means something different. A point-of-use reverse osmosis filter or treatment system treats water at one tap, usually the kitchen sink, rather than the whole house. A point-of-use softener is rare. Softening normally happens where the water enters the house. If you're looking at treatment rather than heating, start with well water filters or our water softeners guide.
What to skip
Skip any unit that doesn't list its flow rate at a stated temperature rise. Skip a tankless model your panel can't power without an upgrade you haven't priced. And skip the idea that a point-of-use unit will rescue a main heater that's running out of hot water. That's a sizing or repair problem. Start with water heater runs out of hot water.
Installation notes
- Mount it where you can reach it. Under-sink cabinets are tight, and you'll want access for service.
- Put a shutoff valve on the cold supply to the unit.
- Tank models need a temperature and pressure relief valve like any water heater. Follow the manual on where it discharges.
- Tankless models need the correct breaker and wire size from the manufacturer's spec sheet. This is electrician work.
- Most towns want a permit for new circuits. Check with your local building department.
A.O. Smith calls its mini-tanks low maintenance. The plumbing is simple; the wiring is where DIY jobs go wrong.
FAQ
Can I put a tankless water heater in my bathroom?
Yes, if it's an electric point-of-use model sized for the fixture. Rheem lists master and guest bathrooms among the typical spots for point-of-use electric tankless units, which need no venting. A gas tankless unit is a different job because it needs combustion air and a vent.
Are there any disadvantages to a tankless point-of-use water heater?
A.O. Smith lists several: it serves one fixture, flow limits suit light use, it isn't designed for showers, and it costs more up front and may need electrical upgrades. For a single sink those are usually acceptable trade-offs.
Do tankless water heaters need a drain line?
Electric point-of-use units don't burn fuel, so they don't make condensate. Condensing gas tankless units do, and their condensate line has to drain freely. Tank-style mini heaters need their relief valve piped to a safe spot.
What is the best-rated under-sink water heater?
We haven't tested these units, so we won't name a winner. Pick on the numbers: tank size or flow rate at your temperature rise, the voltage and breaker it needs, and the warranty. A.O. Smith's mini-tanks, for example, carry a 6-year warranty.
Sources
- A. O. SmithMini-Tank Point-of-Use Water Heaters (opens in a new tab)
- A. O. SmithTankless Point-of-Use Water Heaters (opens in a new tab)
- RheemIs a Tankless Electric Water Heater Right for Your Home? (opens in a new tab)
- U.S. Department of EnergySizing a New Water Heater (opens in a new tab)