Why Is My Tankless Water Heater Running Hot and Cold?

If you have a tankless water heater, then you know that it may run hot and cold on occasion. This is because tankless heating units are designed to run without any delays as they heat water. It’s not uncommon for tankless heaters to be running on high one second and low the next.

The problem is that this may potentially make your tankless water heater less efficient, which may cause your energy bill to go up! Let’s find out why your tankless water heater runs hot and cold, what causes these fluctuations, and how they can be fixed.

Cold Water Sandwich

Although the situation is rare, you might come across a tankless water heater problem known as a cold water sandwich. Your water heater will provide hot water for a few seconds, followed by a blast of cold water (cooler), then ample warm water. It almost feels like your tankless water heater shuts off for a second before heating water, but that’s not the case.

This can occur when one fixture is turned on immediately after another fixture has been turned off. You’ll get some hot water stored by the faucet, followed by cold water from the tankless water heater before the gas burner can fully heat the heat exchanger.

If you’re washing your hands, a cold water sandwich probably isn’t a huge deal. But it’s annoying if you’re getting a cold shower when you want hot water. Here’s how you can fix this annoying water temperature fluctuation.

How to Fix It

You have two options for fixing a cold water sandwich: installing a recirculating system or putting in a small tank water heater. In fact, some manufacturers today already offer this buffer tank.

If you don’t need a lot of hot water at once, adding a recirculating system–or return loop–to your tankless unit will resolve the heat exchanger delay. However, it’s important to note additional energy is consumed in the process.

Have more significant hot water needs? You can bolster your tankless water heater’s capacity and eliminate that burst of cold water by adding a small tank water heater. This water heater connects via the return loop. It only has a five-gallon storage tank, making it very energy efficient.

But don’t forget that when properly sized, a tankless water heater’s capacity is practically endless.

Wrong Gas Line Size

Most tankless water heaters require you to (size) extend your gas line for ample gas supply. If you’ve already completed this, but it’s still running hot and cold, then your line is the wrong size. Upgrading your gas meter might be necessary to ensure your water heater is receiving enough fuel.

How to Fix It

The right line size for a tankless water heater should be between 1/2″-3/4″ in diameter. If yours is too small, call a Service Experts Heating, Air Conditioning & Plumbing Expert for gas line replacement.

Improper Plumbing System Crossover

Tankless water heaters require you to have a direct connection with the supply and return lines. If plumbing was done incorrectly or a pressure balancing shower valve needs repair, cold water can seep into the hot water line, making the hot water lukewarm. You’ll get bursts of hot and cold water until that line heats up after constant use. Diagnosing this problem can be difficult on your own, without help from a professional plumber.

How to Fix It

You have two options when it comes to fixing an improper plumbing system crossover. You can either raise the thermostat on your tankless water heater or install a pressure balancing valve.

If you choose to raise the thermostat, then you’ll need to constantly use hot water at every tap. This will steadily raise the temperature of your hot water supply, allowing it to heat up in your return line.

If you choose to install a pressure-balancing valve in your gas line, it monitors the temperature of your home’s incoming gas line. It will also balance out the excess pressure when cold water enters the system, so you won’t get bursts of hot and cold water.

Broken Flow Sensor

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. This ensures you get enough hot water throughout your home promptly.

How to Fix It

You need to diagnose the source of the problem. Take a look at your system’s gas pressure, thermostat or water heater assembly for issues. If you have a malfunctioning flow sensor, it’s important to have Service Experts Heating, Air Conditioning & Plumbing replace it so your tankless water heater operates efficiently.

Dirty Water Filter

The water filter used on the cold water inlet of instantaneous (tankless) water heaters prolongs its life span and maintains energy efficiency by filtering (sediments), calcium, lime and iron. If it gets too dirty, you’ll not only be getting uncomfortable bursts of cold and warm water, your natural gas bill can skyrocket. You might need to descale your tankless heater if there’s excessive mineral buildup.

How to Fix It

Regularly change your system’s filter so it can function at its best. We suggest doing this about every six months or so.

Corroded Lines

Corroded lines (in your home) are a common reason why you’ll experience bursts of hot and cold water. A few signs of this problem include discolored water or rusty-brown particles in the water. As water flows through your home’s plumbing system, internal corrosion can break free and become trapped in your tankless heater’s filter.

How to Fix It

You need to call a Service Experts Heating, Air Conditioning & Plumbing plumber for an inspection. Corroded lines can cause more serious problems beyond tankless systems and are best replaced.

Mineral Buildup

Similar to a dirty water filter, mineral buildup from hard water can cause issues with how tankless water heaters provide hot water. In fact, this kind of buildup is the number one cause of temperature problems.

How to Fix It

It’s best to call Service Experts Heating, Air Conditioning & Plumbing for a plumbing inspection. Depending on the severity of the issue, we might need to descale your tankless systm or install a water softener.

Hard water is a common problem for North American homeowners. Nearly 90% of households in the U.S. and 85% of households in Canada have hard water.

Not having soft water can cause a lot of issues, such as:

    • Scaly pipes and water heater
    • Soap scum and detergent buildup
    • Dry skin
    • Streaky dishes and faded clothing
    • Lime deposits in other hot water appliances like your dishwasher and clothes washer

Let the Experts Handle Your Tankless Water Heater Needs

One of the most frustrating things about a tankless system that runs hot and cold is how hard it can be to determine why your water heater is doing this. Your Expert plumber in the U.S. will offer professional advice as we explain what needs to happen next.

This includes diagnosing tankless water heater problems to fixing any issues we might uncover during our visit. Don’t let frustration get the best of you! Contact us at today to schedule your appointment.

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