Fast Plumbing Answers: Tankless Cold Water Sandwich

Your hot shower is interrupted by bursts of icy water. This “cold water sandwich” effect is all too typical for homeowners with tankless water heater systems. But what causes this interruption of hot water delivery, and is there anything you can do to fix it?

Don’t worry- we’ll answer all your questions in 2 minutes or less.

We’re Mother- a refreshingly modern Dallas-Fort Worth plumbing company. Our team of licensed techs is deeply educated and trained to service on-demand hot water heaters. DFW residents frequently complain about cold water sandwich issues in their homes- thankfully, they’re usually avoided with a simple change in how you use the system.

If your home is plagued by inconsistent hot water delivery, there may be an issue with your tankless unit’s recirculation pump. Call us 7 days a week for a free inspection and efficient tankless water heater repair- we’ll have a plumber there within 24 hours.

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Here’s What Causes the Cold Water Sandwich Effect

We accompanied our Journeyman Plumber Dillon Ashton on a service call to inspect a tankless water heater unit in Highland Park, TX. He explained that cold water sandwich issues are usually a normal part of owning a tankless unit.

“When you first turn on the faucet, the heater fires up and gives you hot water,” Dillon explains. “But after you shut the water off for a short period, there’s still a bit of heated water left in the pipe.”

When you turn the hot water tap back on, you’re instantly delivered the remaining hot water in the pipe. Dillon says that gives homeowners a false sense that the tankless system’s ready to go.

“Here’s the catch,” he continues. “While the heater senses flow and reignites the burner, there’s a brief delay before it gets back up to your desired temperature. During that gap, cool water that’s been sitting in the heat exchanger passes through.”

“That’s why you get a slug of cold water wedged between two sections of hot water.”

Most Cold Water Sandwiches Don’t Require Maintenance

Dillon reassures homeowners that a cold water sandwich “isn’t usually a sign of a bad unit- it’s just the nature of how on-demand systems work.”

Instead, these bursts of cold water are due to how we’re used to hot water delivery from a conventional hot water heater tank. The brief delay in hot water generation in on-demand and instant-on water heaters doesn’t exist the same way in tank units.

Homeowners who switch from conventional water heaters to tankless systems aren’t accustomed to waiting those few extra seconds for the ignition and delivery process. 

Dillon says cold water sandwich incidents primarily occur when homeowners quickly shut the hot water off, then turn it back on again shortly after.

“Say you’re soaping up in the shower, or someone quickly rinses dishes,” Dillon says. “These scenarios leave that leftover hot water in the pipe, which is followed by the new burst of cold water as the system fires up.”

How to Avoid Cold Water Sandwich Effects

Eliminate most cold water sandwich events by simply waiting a few more seconds to let your tankless heater deliver more how water.

Remember- the first burst of hot water through your system is left over from the last time you used the fixture. Allow the short cold water flow to pass through the pipes before stepping into the shower or using hot water from the sink- you’ll miss that “cold water sandwich” effect entirely.

2 Tankless Water Heater Components That Reduce Hot Water Interruption

“Some tankless setups use specific on-board components to minimize the delay in hot water delivery,” Dillon explains.

These two components keep a small amount of hot water ready to go, so you don’t feel that cold blast of water:

1. Buffer tank: A small, insulated tank that acts as a hot water reserve. It uses a small thermal component to heat a limited supply of hot water while the tap is off.
2. Recirculation system: A dedicated return line brings unused cold water in your pipes back to the tankless system. A recirculation pump brings unused hot water back to the tank as well. This loop allows a more consistent on-demand hot water experience.

Opt for tankless water heater products that contain these components, and you’ll reduce the cold water sandwich effect in your plumbing lines.

6 Signs Your Cold Water Sandwich is a Bigger Problem

You waited a bit longer to get in the shower, and you’re still experiencing cold water bursts. Your tankless unit has a buffer tank and recirculation pump, but you still suffer from inconsistent hot water delivery.

Here are six symptoms that indicate there’s an underlying tankless water heater repair issue beyond a simple cold water sandwich:

1. Reduced hot water pressure
2. Fluctuating water temperature (especially from hot to lukewarm)
3. Tankless water heater won’t ignite
4. Frequent error codes
5. Gurgling and popping sounds in the system
6. No hot water at all

How poor water quality affects tankless hot water delivery

Our DFW technicians see it all too often- a combination of sediment buildup, hard water mineral deposits and increased city water pressure is detrimental to tankless systems.

A majority of hot water delivery problems in tankless water heater units aren’t about a bad product- they’re about bad incoming water quality.

If you live in an area with exceptionally hard water (high in calcium and magnesium), consider a water softening system to reduce the effects of scaling and damage in your tankless water heater and its associated pipes.

Be sure to descale your tankless water heater at least once per year to maintain optimal performance and hot water delivery. If your home has hard water, descale your tankless unit every 6-8 months.

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Call Mother for DFW Tankless Water Heater Repairs

Most cold water sandwich issues don’t require maintenance- just an adjustment to the way you use your tankless water heater system.

If you tried our suggestion and still get bursts of cold water in the shower or sink, it’s time to call a plumber with deep experience in tankless system diagnostics.

Mother’s entire team of licensed technicians is routinely trained on best practices, modern repair options and tech-driven solutions to tankless hot water issues. 

Bring on-demand hot water back to your daily routine. Call Mother 7 days a week- we’ll provide a free estimate within 24 hours.

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