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Why Does My Tankless Water Heater Go Cold in the Shower?

You’re standing in the shower, finally relaxing after a long day. The water feels just right—until suddenly, it turns ice cold. You jump back, frustrated, wondering why your tankless water heater keeps letting you down. At 2 Sons Plumbing Seattle, we hear this story all the time. The idea of endless hot water sounds great, but when the system isn’t set up or maintained correctly, cold surprises can sneak in. The good news? Most fixes are easier than you think.

Why Tankless Water Heaters Sometimes Run Cold

If you’re getting hit with cold water mid-shower, these common culprits are usually to blame:

Low water flow – Tankless heaters need strong flow to stay activated. If the water becomes too slow, the burner shuts off and you get a cold blast.
Mixing valve issues – A faulty or worn valve may be blending in too much cold water.
Mineral scale buildup – Seattle’s hard water can leave deposits inside the heater, slowing heating and causing inconsistent temperatures.

What You Can Try on Your Own

Before calling a plumber, try these easy fixes:

Turn the handle up – Ensure enough flow for the heater to stay active.
Set the temperature to around 120°F – This helps stabilize output and avoid fluctuations.
Flush the unit yearly – Descaling prevents mineral buildup that cuts off heat.

If you’ve tried these steps and still get cold bursts, the issue likely requires a professional.

When to Bring in a Pro

Call a plumber right away if you notice:

A gas smell near the heater
Error codes flashing on the display
Frequent cold swings even with normal flow and settings

These symptoms often indicate deeper issues that require safe handling and proper tools.

What It Usually Costs

Costs vary depending on the root cause, but most Seattle homeowners see:

Routine descaling and flush: $250–$500
Mixing valve service or replacement: Varies by brand and needed parts

Regular maintenance is the best way to avoid costly breakdowns—keeping scale under control helps your heater run smoothly and last longer.

Stop Putting Up with Cold Showers

If your tankless heater keeps cutting out, call 2 Sons Plumbing Seattle at (206) 487-1757. We’ll send a plumber who works with tankless systems every day and knows exactly what to look for. Sometimes it’s a quick flush, sometimes a valve adjustment—but whatever the cause, we’ll track it down and get your hot water flowing again.

One call is all it takes. No more guessing. No more shivering halfway through your shower. Just steady, reliable hot water—exactly the way it should be.

2 Sons Plumbing, Electrical, Heating & Air

4.9

Based on 3490 reviews

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Deann Minamino

22:05 02 Jan 26

We have used 2 Sons Plumbing quite a few times now and have never been disappointed! Courteous, professional, knowledgeable and efficient describes everyone from the office to the field teams! One of their team members, while here installing a sink fixture, overheard an estimate we were getting from a visiting sales rep for a project. Their team member (Carey) offered that the estimate was way over priced (by over $20k). We had 2 Sons price it out for us and they came in $23k less. We have a couple planned projects upcoming and plan to use them for that as well as any unexpected issues that arise. Just this week, they dispatched a team on NYE as our water heater broke. Once again, good price, courteous, professional and efficient! I/we are so pleased to have found this company! We highly recommend them! (NOTE/FYI: I, the posting party, copied this from my Yelp Review)

Deann Minamino

22:05 02 Jan 26

Ashley Dunn

21:37 01 Jan 26

Justin and Bryce were swift and thorough when we experienced an emergency leak in our daylight basement. They quickly identified the problem as a leaking water heater, provided a quote, and completed the work within just a few hours. The job included draining the old water heater, removing and properly disposing of it, and supplying and installing a brand-new unit. We are incredibly grateful that they came out on New Year’s Eve and ensured we didn’t start 2026 without running water. They also offered a great recommendation for addressing separate water damage caused by another issue. Highly appreciated and highly recommended

Ashley Dunn

21:37 01 Jan 26

Benjamin Grossman

05:40 30 Dec 25

Huge props to Adam. We had been trying to replace our kitchen sink ourselves but ran into some issues, so we called for help. They were able to come out the very same day, which was a huge relief. We needed a new faucet installation, a separate hot water faucet, and water tank set up.

Adam was fantastic. He fixed our kitchen sink and even helped with a stubborn screw we could not get undone (I spent 4 hours myself before trying to get it out) , all with incredible patience and friendliness. He explained everything he was doing clearly, which made the whole process easy to understand and feel confident about. The new installation looks fantastic. Will definitely use them again.

Benjamin Grossman

05:40 30 Dec 25

Heejae Yang

02:27 30 Dec 25

Great service for people who hate money

This is a wonderful company to repair your plumbing issues, including storm drains and sewer lines—ONLY if you have the disposable income of a tech bro or sis. I don't.
I called because the storm drain receiving my downspout was clogged. It was adjacent to work they have done before. Without even coming to the site, a technician diagnosed a total failure over the phone. Their solution? Demolish the concrete, access and remove the P-trap, and redo the entire install for a "discounted" manager promo of $4,000.
I ended up fixing the issue in 5 minutes with a Shop-Vac. Total cost: $0.
I’ve used them for a sewer line repair before and now realize I likely funded their entire holiday party. Why fix a drain for free when you can pay $4,000 to have your walkway ripped up? This is the team for you if your budget is infinite and your patience for common sense is zero.

UPDATE: A real pro checks for a simple clog before recommending a sledgehammer. We all know they were looking for a payday.

Heejae Yang

02:27 30 Dec 25

Maggie McNish

00:43 30 Dec 25

Adam came to my house and gave excellent service and worked very efficiently. He was incredibly professional and personable and kind. He educated me on my plumbing situation so that I’m better equipped to prevent issues in the future and to preserve integrity of items and appliances I use and in a way easy for me to understand. I hope every experience for every customer with 2 Sons goes this way. Everyone I’ve had come to my house with 2 Sons has been great — competent, kind, professional, and transparent. Adam was terrific.

Maggie McNish

00:43 30 Dec 25

Melissah Watts

02:23 12 Dec 25

I had a wicked combo of 50 yr old bathtub faucets ( the hot water completely frozen in the “on” position) and a 13 year old hot water heater …. Yep, on Saturday … 2 Sons sent Carey out on Sunday and he advised me to wait until Monday when he could really solve my problem with high quality hardware and a new hot water heater! Carey came out Monday with Micah , beautiful copper pipes, a new hot water heater and a modern bathtub faucet. In 5 hours I had a new bathtub faucet and a new hot water heater. On top of really good craftsmanship , they respected and protected my existing tub/ toilet / sink. Thanks for a job well done!! M & M Watts

Melissah Watts

02:23 12 Dec 25

Anon

20:53 08 Dec 25

Have used 2 Sons multiple times to help with the plumbing in our house and every time has always been great. This time was no exception. Dave was amazing - very knowledgeable and reasonable in his suggestions and treatment of our plumbing issues. He was also very kind and was respectful of our house and space. Would definitely hire him again.

Anon

20:53 08 Dec 25

Marshall Harnish

21:22 05 Dec 25

2 Sons Plumbing did a fantastic job! I had a difficult water line replacement job, and they were friendly, quick and professional. The work turned out great. I had Dave as the project manager and he was honest and great to work with.

Would use them again for sure.

Marshall Harnish

21:22 05 Dec 25

Mark Ondrake

23:58 04 Dec 25

The 2 Sons company was prompt, thorough, clean, and performed precise, high-quality work. The office staff and plumbing technician showed professionalism and a focus on details. The finished repair exceeded our expectations, the price was very reasonable (high value for the quoted price), and the technician left the Job site spotless. See attached photo, they used the Big Bertha Motorized Snake to clear a clogged drain, the obstruction was 25 feet away!!

Mark Ondrake

23:58 04 Dec 25

Laura Mueser

23:11 03 Dec 25

Great service over a holiday weekend. Total failure of our sewer line was fixed with a ten year warranty. Clean, great communication, full fix. It cost a lot -- I mean, it was a major repair in a hundred year old house-- but was reasonable for what we received, and the pricing was very transparent. Seth, Jorge, Shane, Dan, and others were all perfectionists and appreciated.

Laura Mueser

23:11 03 Dec 25