Why Is My Tankless Water Heater Not Getting Hot?

A tankless water heater that will not get hot is the most common complaint on this site — and the most misunderstood. The water is cold, or lukewarm, or hot for a minute and then cold again, and the instinct is to blame the unit. But a tankless unit that will not heat is usually telling you something specific, and the cause is almost always one of six things.

The first thing to understand is how a tankless unit decides to heat. It does not heat water because it is switched on. It heats water because water is flowing through it, and the flow is strong enough to trip the flow sensor. No flow, no fire. Low flow, weak fire.

The five-minute checks

Do these before anything else — they fix a surprising number of cold-water mornings.

• Gas supply on. The gas valve to the unit fully open, other gas appliances working. On an electric unit, this check is the breaker.
• Water supply on. The cold-water valve to the unit fully open. A valve that is only half open starves the unit of flow.
• Power on. The unit plugged in, the display lit, the breaker not tripped.
• Inlet filter clean. A clogged filter is the most common simple cause of no hot water. It is a small screen where the cold water enters the unit.
• Vent clear. Nothing blocking the intake or the exhaust. A blocked vent will shut a gas unit down rather than let it run unsafely.
• The temperature setting. Check what the unit is set to. A unit set low will give you lukewarm water no matter how well it works.

If the unit shows an error code, write it down exactly, with the brand, and look it up on our error codes page. The code will tell you which of the causes below to look at first.

Not enough flow

Most tankless units need a minimum flow of water before they fire at all. Open a tap just a trickle, and the unit may not fire — the flow sensor never trips, the burner never lights, and the water stays cold. Open the tap fully, and the same unit fires at once. That is not a fault.

The problem comes when the flow is low for a reason. A low-flow shower head, a tap opened only partway, a partly closed valve, or a clogged inlet filter can all hold the flow below the level the unit needs. If one tap gets hot water and another does not, flow is the first thing to check.

Flow also falls as scale builds. A heat exchanger narrowed by scale passes less water than a clean one. Lukewarm water with a dropping flow rate is the signature of a unit that needs a flush — see our flush frequency page.

The cold-water sandwich

Hot, then cold, then hot again. That pattern has a name — the cold-water sandwich — and it is normal for a tankless unit, even a healthy one. You turn a tap on, the unit fires, and hot water flows. You turn the tap off to soap up or to shave. The unit stops firing. A little cold water sits in the pipe between the unit and the tap. You turn the tap on again, and that slug of cold water reaches you before the unit has fired up again.

It is worst when taps are opened and closed quickly, and worst of all in a shower. Some units have a small buffer tank or a recirculation setting that softens it. Most do not. It is not a fault, and it is not fixed by turning the temperature up — the cold slug is cold because the unit was off, not because the setting is low.

Scale in the heat exchanger

Scale is the slow cause of lukewarm water. Every gallon of hard water leaves a little mineral behind, and inside a tankless heat exchanger that mineral becomes a layer of scale that insulates the water from the heat. The water runs lukewarm, the flow drops, and eventually the unit overheats and shuts down.

How fast it happens depends on your water. In Casper, Wyoming, where the water runs about 19 grains per gallon, a unit that has never been flushed will show it within a couple of years. In Medford, Oregon, where the water runs 20 to 40 parts per million, scale is barely a factor. Your city page states the local hardness — and our flush and descale page covers what a flush does.

Gas or electric supply

On a gas unit, no hot water can mean no gas. The valve may be closed, the supply may be interrupted, or the gas pressure may be too low for the unit to fire. Other gas appliances in the house are the quick check: if the stove works and the water heater does not, the problem is at the unit, not the supply.

On an electric unit, no hot water can mean no power, a tripped breaker, a failed heating element, or a failed thermostat. Check the breaker first — and if it has tripped, do not just reset it and walk away. A breaker that trips again is pointing at a fault inside the unit. A burning smell is the other warning: turn the unit off at the breaker and call.

Too much demand at once

A tankless unit can only heat so much water at once. Two showers and a dishwasher running together need more hot water than one unit can heat, and the temperature at every tap drops. That is not a fault either. It is capacity — endless hot water at one tap, and a limit when several taps run together.

If the water is hot when one tap runs and lukewarm when two run, the unit is fine and the household is asking more of it than it can give at once. The fix is to stagger the use, or to talk to a provider about whether the unit is sized right for the household.

When to call

Call when the five-minute checks do not fix it, when an error code comes back after a single reset, or when the water is lukewarm and the unit has never been flushed. Call at once when you smell gas — leave the area first, and call your gas utility's emergency line from outside. Call at once when water is leaking from the unit, and shut the water off to it first.

When you call (877) 418-1589, say what the water is doing — cold, lukewarm, hot then cold — whether the unit shows a code, and the brand if you know it. That is enough for the provider to know what to bring, and whether the visit is a flush, a part, or a look.

Questions people ask

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