Why Does My Tankless Hot Water Heater Go Cold?

When you purposefully purchase a tankless water heater, the last thing you expect is for the machine to stop providing hot water when you need it.

To understand why your tankless water heater doesn’t always perform as you expect it to, it’s important to know how they work and what you can do to make yours more efficient.

Beyond knowing why your tankless water heater isn’t doing its job and heating your cold water, it’s important to know how they work and when to call a plumber.

5 Ways To Fix the Problems

You won’t be able to fix your tankless water heater until you know what’s causing it not to head up properly. Each method is accompanied by the issue causing a water heater to give out cold water.

1. Buy a New Heater – Your Heater Is Too Small

In some cases, the issue is that the heater is too small for the property that it’s in. The downside of this being the issue is that you’re never going to see any positive changes in the time it takes for hot water to appear in your home.

Suppose it’s simply too inconvenient to wait for the water to heat up (which is understandable because you probably brought a tankless water heater in the first place). In that case, you might consider returning/exchanging/buying a new heater.

Alternatively, you can buy a second heater or limit the use of some of the fixtures in your home.

2. Your Gas Line Isn’t Supporting Your Heater – Extend the Line

This is a slightly more complicated issue. First, you must extend your gas line to include your tankless water heater during installation.

An appropriate size for your gas line is 1″ if you’re using a heater like this. If your gas line isn’t big enough, your heater may fail to function.

3. Sudden Cold Water Interruption – Your Heater Is Delayed When Heating

It’s unpleasant to turn on your hot water taps and get a rush of warm water followed by cold water before it starts warming up again.

This is usually leftover warm water from the previous use, which is why you get that blast of icy water in between.

What’s happening here is your heater is struggling to warm up the water fast enough, so you’re getting leftover water and cold water before the heater kicks in.

Some tankless water heaters have a feature to reduce this event, but it is very common and, unfortunately, not a fixable issue.

4. The Water Filter Is Clogged – Check the Inlet

For households experiencing tepid water, it’s worth having a quick look at the cold water inlet. This little, plastic connective part allows cold water into the heater.

When blocked, which happens naturally over time, water can no longer freely enter your heater. When that happens, you get cold water.

To fix this issue, take the connection out and clean or replace it. Only take out the cold water inlet once you’ve identified and ensured that you have the correct part of the heater.

5. Your Heater’s Heat Exchanger Is Blocked – Contact a Plumber

If you live in an area with hard water, minerals can build up on the parts of your tankless system. This can then rust the heater’s pipes and damage the part that heats your water.

You should be flushing your heater regularly to avoid this if you know that your residence is in an area where mineral build-up is common.

If you suspect mineral deposits are to blame for your lack of hot water, you’ll need to call a plumber. But, unfortunately, this is not something you can fix yourself.

How Does a Tankless Hot Water Heater Work?

A tankless water heater does not store readily heated hot water as tanked devices do. However, the average tankless heater can store up to 50 gallons of water, which heats quickly upon activation.

This saves energy because the water in the tank doesn’t need to be kept hot all the time. Tankless heaters will only heat your water when you turn on a hot tap or your shower in the warm settings that it has.

Despite being called “instant” heaters, they take a few seconds to a couple of minutes to heat the water from your taps. The time it takes to heat the water is based on a few things, including:

1. Any plumbing issues.
2. The size of your heater.
3. The age of your heater.
4. The condition of your heater.
5. Rust, mineral deposits, and damage on your heater or plumbing.
6. Plumbing crossovers.

Cold water enters your heater from the bottom of the system through the cold water inlet. This water moves through the heater, heating up as it goes, and then comes out through the pipes it’s being directed to.

When you turn off your hot water taps, the heater will stop heating the water and naturally cool down.

Takeaway

There are many reasons that the cold water going through your heater is still coming out cold, despite the burner coming on when you turn on a hot tap.

Before you panic, give the cold water some time to heat up. If it’s still coming out cold after a few minutes, it’s time to troubleshoot your tankless water heater and plumbing to find the issue.

When in doubt, always contact a plumber. The last thing you want is to mess around with your home’s plumbing and accidentally make the situation worse or damage something.