Plumbing service
Tankless water heaters
Short answer
A tankless water heater heats water only as it flows, so it doesn't run out, but it can only supply a limited flow at once. Sizing depends on how many fixtures run together and how cold your incoming water is. Gas models often need a larger gas line and new venting, and all models need periodic descaling, especially with hard water.
Tankless Water Heaters at a glance
- Sized by
- Flow (gallons per minute) at your required temperature rise.
- Gas installs often need
- A larger gas line, new venting (often PVC for condensing units), a condensate drain.
- Electric installs often need
- Substantial electrical capacity; check the panel before choosing electric tankless.
- Maintenance
- Descaling and inlet filter cleaning, commonly yearly in hard water.
Sizing for your home and climate
Add up simultaneous flow
Count the fixtures you run at the same time, such as two showers and a kitchen faucet, and add their flow rates from the fixture labels or specs.
Find your temperature rise
Subtract incoming winter water temperature from your target (about 120°F). Incoming water is much colder in northern states than in the South, so the rise is larger.
Match the unit's rating
Check the manufacturer's flow-at-rise chart, not the maximum flow number on the box.
An undersized unit is the most common tankless complaint. Size for your coldest month.
What the installation involves
| Item | Why it matters |
|---|---|
| Gas line sizing | Tankless burners need a large gas input; an undersized line starves the unit |
| Venting | Condensing units usually vent with PVC; non-condensing need stainless vent; both follow the manufacturer's clearances |
| Condensate drain | Condensing units produce acidic condensate that needs a drain, sometimes with a neutralizer |
| Isolation valves | Service valves let the unit be descaled without draining the house |
| Electrical | Gas units need a power outlet; electric units can need large dedicated circuits |
| Permit | Most jurisdictions require one for water heater and gas work |
Common tankless problems
- Cold-water sandwich: a burst of cold between uses while the unit fires up again. A small buffer tank or recirculation can help.
- Won't fire at low flow: each unit has a minimum activation flow, and a low-flow faucet may not reach it.
- Error codes: often point to scale, ignition, venting or gas supply problems. The manual lists them.
- Fluctuating temperature: scale in the heat exchanger, a clogged inlet filter, or demand over the rating.
Is tankless right for you?
It's a good fit when you run out of hot water with a tank, when space is tight, or when you'll stay long enough for lower standby losses to add up. It's a harder sell when gas line and venting upgrades are expensive or when big simultaneous demand would need a very large unit. See the full tankless vs. tank comparison.
Problems that lead to tankless water heaters
- Water heater leakingWhere a water heater leak is coming from tells you whether it's a cheap fix or a new tank. How to find it safely, and what to shut off first.
- No hot waterWhy a house suddenly has no hot water, or runs out fast, with separate checks for gas, electric and tankless heaters and when to call.
Common questions
How long does a tankless water heater last?
Often longer than a tank model, provided it's descaled on schedule. Neglected units in hard water fail sooner.
Do I need to descale a tankless water heater?
Yes. Most manufacturers call for periodic descaling, commonly once a year and more often with hard water, to keep scale from damaging the heat exchanger.
Can a tankless heater run the whole house?
Yes, if it's sized for your peak simultaneous flow at your climate's temperature rise. Large homes sometimes use two units.