The Cold Water Sandwich Effect

Sep 18, 2022

At first glance, the “Cold Water Sandwich Effect” sounds like it could be the title of a fanciful novel or play – kind of like The Effect of Gamma Rays on Man-in-the-Moon Marigolds from the 1970s. Instead, it refers to an annoying water heater issue that occurs when you have fluctuating water temperatures. 

Picture this – You’re taking a nice, relaxing hot shower when, suddenly, the water turns ice cold! But then the water quickly returns to being warm again. You have just experienced a cold water sandwich. This is a common phenomenon with tankless water heaters. It’s an unwelcome surprise and not something you want to experience – even on very hot, humid days like we are having in San Diego this summer!


Causes of the Cold Water Sandwich Effect

Sometimes there is some warm water that remains stored at the mouth of the faucet or other outlets. So, when you run the tap or shower you’ll get hot water instantly. This happens, for instance, when someone has taken a shower just before you. However, because tankless water heaters heat water on an as-needed basis (which saves you money) it takes a short time to heat up the exchanger for your shower. During that time, some cold water may get through the exchanger and reach the user before more heated water comes in. Thus, the warm – cold – warm effect.

This sandwich effect is more noticeable in the summer because the water entering the house is much warmer than in the winter which confuses the heater. Should I stay on or turn off?

More likely than not, the cause is the flow restrictor inside the shower head, which limits flow. Tankless water heaters need to sustain flow in order to keep the hot water flowing. Again, this is more noticeable in the summer as you are using less hot water and the groundwater can be over 80 degrees.

Not sure if it’s a flow issue? Try taking your normal shower but also run the hot water in the sink at the same time. If the problem goes away it’s a flow issue (and not the water heater). 



So, what can you do about this? First, remove the showerhead. Then, either clean or, preferably, remove the flow restrictors which are plastic washers behind the screen. This will resolve the problem 99 percent of the time. **Simply buying a new showerhead will not solve the problem and may even make the problem worse. The reason for this is that all new showerheads come with flow restrictors no matter the size.** In short, the more hot water flow, the better (particularly in the summer months). 


Another Summer Tip!

Lower the temperature on your tankless water heater to 110 degrees during the summer. It will lower your energy costs and, because you are saving energy it helps the environment.


Contact Arnett’s Water Systems Today

Arnett’s Water Systems is the #1 provider of Noritz tankless water heaters in San Diego County. Contact us today to discuss personalized, energy-efficient water system options. We’ll help you find the perfect solution for your budget.

Experience the difference with better water!