Why Does My Tankless Water Heater Go Cold?

You’ve most likely heard of the many benefits of tankless water heaters; the reason why you got one in the first place. Things like never missing out on hot water whenever you need it and saving on your energy bill.

But what about when the heater you so much like fails to work according to plan? When instead of heating water, all you get is cold water? Or when the temperature of the water fluctuates from hot to cold?

Although tankless water heaters have a multitude of benefits, they aren’t immune to problems. This guide will take you through the reasons why your tankless water heater goes cold from time to time. You’ll also get some tips to help you prevent this from happening again.

Reasons Why a Tankless Water Heater Goes Cold

1. The Cold Sandwich Phenomenon

The cold water sandwich is the situation where the water delivered from a tankless water heater is hot for a few minutes and then changes back to cold again. A very unpleasant situation, especially when taking a shower.

If your tankless water heater doesn’t boast features that can help minimize this fluctuation, it’ll be subjected to this power variation, especially if it’s electric-powered.

When the water heater is on, it provides hot water, and when off, the unit turns off. Therefore, the water in the pipes gets cold. When you turn the unit on again, the cold water is pushed out before the hot water reaches the fixture hence the burst of cold water.

The Solution

Although the tankless water heaters heat water fast than tank models, they are actually not instantaneous. The few seconds of cold water isn’t a flaw, and thus no need to be worried. It’s just how the system works, and you can’t fix it in any way.

The solution, therefore, is to wait for the gush of cold water to pass before stepping into the shower.

2. Clogged Heat Exchanger

If you live in a place with hard water or use well water, mineral buildup can be a major problem. Hard water is rich in minerals like magnesium and calcium.

Over time, hard water deposits these minerals onto your heater’s components. This buildup corrodes the pipes and damages the heat exchanger.

Do you have an idea of how your water gets cold now? The heat exchanger is the most important component of your tankless water heater. It consists of a group of pipes that heat the water it carries inside.

Now that the heat exchanger is clogged with limescale, can it heat your water? Of course not. You also don’t want to have a clogged heat exchanger since it can be hazardous.


The heat exchanger will continue to heat, and remember, the water required to cool it down doesn’t pass through.

The Solution

Ensure that you flush your water heater regularly to prevent limescale and mineral buildup. If buildup has already occurred, you’ll need to get a professional to help clean the lines.

3. Clogged Water Filter in the Cold Water Inlet

If not getting enough hot water, you need to check the cold water inlet. It is a small plastic attachment that lets cold water enter the tankless water system.

The cold water inlet filter stops any debris from entering the water heater. However, over time, the filter can get clogged with buildup. Should this happen, no water can enter the water heater, hence the non-heated water.

The Solution

To reverse this situation, you only need to remove and clean the dirty filter. You can also replace it if necessary. Doing so will enable water to enter the heater for the healing process to take place.

4. Low Water Pressure

Another reason why your water heater might get cold is if it stops working due to low water pressure. If the flow within your tankless heater is low, below the activation flow rate, it shuts down the gas burner. This results in a sudden drop in the outlet’s hot water temperature.

What causes this low water pressure?

If your tankless water heater capacity is smaller than the demand, and all additional fixtures in your home are opened or closed, there will be a change in water flow. As a result, you’ll see a fluctuation or temperature drop, and the result will be cold water coming out of your showerhead and an uncomfortable shower.

The solution

Depending on the severity of the problem, you can fix this issue by either:

  • Upgrading your water heater’s capacity
  • Limiting simultaneous use of taps/showers/water outlets
  • Invest in a second heating unit

If noticing the problem on rare occasions, limit the simultaneous use of the fixtures in your home. In simple terms, let your spouse know when you’re using hot water in the kitchen when they’re taking a shower.

For severe cases, upgrade your existing unit. And if the current unit doesn’t meet your household’s hot water needs, get a second unit. Sounds extreme? It’s actually worth the effort.

Instead of continuing with a heater that doesn’t meet your entire household’s hot water needs, getting a second unit is a great option. It’ll save you from wasting water and gas as you wait for the unit to produce hot water when needed.

5. Plumbing Crossover

Problems in the plumbing can also be the reason for the temperature fluctuations, and plumbing crossover is one of them. The crossover is caused by a faulty mixing valve, faulty plumbing, or a single lever faucet.

So, how do you tell whether you have a plumbing crossover issue?

Check all the faucets. Close the main valve and open the hot water tap, then wait for 5 minutes. If the water is still running, you have issues with the plumbing crossover.

The Solution

If you are a DIY-er, plumbing crossover shouldn’t be hard for you to repair. Depending on the location, you just need to check the valve or mixing valve or replace the cartridge.

You should, however, keep in mind that replacing the cartridge may not solve the problem. However, buying a high-quality faucet or cartridge can prevent the crossover problem.


All the above-listed problems could be the reason why your tankless water heater goes cold. However, should you follow all the guidelines discussed, you should be able to solve this problem. In addition, if you find it a little difficult to solve the problem on your own, get a professional plumber to help you with the task.

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