Why does my tankless water heater go cold then hot again?

Author: Norris Hickle  |  Last update: Monday, October 6, 2025

A damaged flow sensor can cause your tankless water heater to run hot and cold. The flow sensor measures the water entering your system and sends that information to your unit's control board. If there's a burst of cold coming into the system, then the controller adjusts the flame.

Why is my tankless water heater going hot and cold?

Fluctuating hot and cold water could be a couple of things. Firstly, it could just need flushed with a descaling liquid such as vinegar. Secondly, it could be a dirty flame rod not sensing the flame well enough. Both of those things are usually addressed when you do a tankless service. Have you had it serviced?

Why does my water run hot then cold then hot again?

It's possible your water heater tank is allowing retrograde (backwards) flow from the tank through its cold water supply pipe into other cold water pipes, causing variable amounts of warm water in place of cold at some of the taps, particularly th...

Why does my tankless water heater keep going on and off?

The issue may be the flame rod, clogged exhaust/intake screen, clogged condensate drain, plugged up heat exchanger, bad mixing valve after the unit, etc. You'll need to get a tankless specialist out to find out what's going on with it.

Why does my hot water start hot then go cold?

Blocked heat exchanger

Sludge and sediment buildup is fairly common in older boilers, especially when the filters are old. This blockage will then cause water to run hot and cold intermittently.

Why does my tankless hot water heater go cold?

Why does my tankless water heater go cold in the middle of a shower?

Overloaded Demand: Tankless water heaters provide hot water on demand, but they have limitations on the amount of hot water they can produce simultaneously. If multiple fixtures or appliances are drawing hot water simultaneously, the heater may struggle to keep up with demand, resulting in cold water.

What are the symptoms of a faulty diverter valve?

Symptoms of a faulty diverter valve often include inconsistent hot water flow and radiators not heating properly. Homeowners may notice that radiators remain cold even when the boiler is on. Inconsistent water temperatures are common, with water switching from hot to lukewarm or cold unexpectedly.

Why won't my tankless water heater stay hot?

Tankless water heaters rely on ignition to heat water on demand. If the ignition fails, you'll be left with cold water. This could be due to a variety of reasons such as a faulty igniter, gas supply issues, or a dirty burner. In such cases, it's best to consult a professional technician to diagnose and fix the problem.

How to tell if a tankless water heater is going bad?

To help you identify when you have a tankless water heater in trouble, we've listed several warning signs to watch for:
  1. A drop in hot water pressure. ...
  2. Lukewarm water. ...
  3. The water heater keeps shutting off. ...
  4. Unusual behavior from other appliances. ...
  5. Leaks.

Why does my tankless water heater stop heating after a few minutes?

One common cause of a drop in water pressure and a loss of heating in a tankless water heater is a clogged water filter. The water filter is responsible for keeping sediments and dirt out of the unit, but over time, it can become clogged and restrict the water flow.

How do you fix fluctuating water temperature?

3 Ways to Fix Fluctuating Water Temperatures
  1. Check the Size – Is the current size of your water heater right for the size of your home? ...
  2. Check the Age – How old is your water heater? ...
  3. Check the Maintenance Records – When was the last time your water heater was flushed, let alone inspected by a plumber?

Why does my hot water keep coming and going?

Water heater leaks are frequently behind water heaters that run constantly. When a water heater leaks, the leak essentially drains the hot water tank, and the system loses hot water. If you find signs of a leak, call your plumber right away to schedule water heater repairs.

How do I keep my tankless water heater warm?

Anti-Freeze Measures for Tankless Water Heaters

Insulating exposed pipes, keeping the area around the heater warm, and regularly using your hot water are effective anti-freeze measures. These steps help maintain a consistent temperature around the unit, reducing the risk of freezing.

Why does my instantaneous hot water go cold?

Having intermittent hot and cold water is quite a common problem with instantaneous systems. This can be caused by a range of things including: The diaphragm on the gas meter could be faulty. Not enough gas may be getting to your hot water system (which can be caused by an under-sized gas bottle).

What maintenance does a tankless water heater need?

The most complicated and time-consuming tankless water heater maintenance is typically descaling and flushing out the unit. Even with the inline water filter, lime and calcium deposits can build up inside the tankless system. If you have hard water, it's a good idea to descale and flush the unit once a year.

Why does my tankless hot water heater go cold?

A tankless water heater requires a certain amount of water pressure to function properly. If the incoming water pressure is too low, the unit will not be able to generate enough hot water. This is often caused by a plumbing system crossover, where the cold and hot water lines are connected.

How often does a tankless water heater need to be flushed?

Just like a tank unit, tankless water heaters need to be flushed once per year. Homeowners with hard water should consider flushing and cleaning the unit even more frequently, perhaps every six to nine months because of the excess amounts of magnesium and calcium found in hard water supplies.

What is the life expectancy of a tankless water heater?

One of the most common questions asked by consumers about tankless water heaters is how long they will last. A tankless water heater can last anywhere from 15 to 20 years with proper maintenance and operation. In comparison, a standard tank water heater normally has a working life of 10 years or less.

Why does my tankless hot water heater fluctuate?

These water temperature fluctuations can be caused by:

A plumbing system crossover where there shouldn't be one. Improperly lengthened venting pipes. Damaged flow sensor — something that should be checked during your annual plumbing maintenance appointment. A dirty water filter on the cold water inlet.

Why does my hot water go cold after a few minutes?

Are you scratching your head and asking, “Why does my hot water heater run cold so quickly?” If your hot water keeps going cold, the culprit could be sediment buildup, a broken heating element, a faulty dip tube, or simply high water usage in the household.

How do you know if your tankless water heater is broken?

No hot water coming from taps, or water is lukewarm. What's the issue when you have no hot water in house but have cold water? This is the most common problem you can expect with a tankless water heater.

How do I know if my diverter valve is stuck?

Signs a diverter valve is stuck

If your diverter valve is sticking, you'll soon know about it. Whether that's standing under a cold shower because you've got no hot water or facing a chilly evening because your radiators won't switch on. Other things to look out for include: No hot water from the taps.

How much does it cost for a plumber to replace a diverter valve?

$100 – $350 shower cartridge replacement cost

Plumbers charge $100 to $300 in labor alone to replace a shower mixing valve or diverter valve.

How do I know if I need a new diverter valve?

The most common symptoms for a faulty diverter valve are:
  1. I have hot showers but my radiators stay cold. A diverter valve may be faulty if the hot water taps still work while the radiators do not turn on.
  2. I can only get lukewarm water from my taps. ...
  3. My hot water only turns on when central heating is on.

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