Rinnai tankless water heater goes cold after a few minutes

Have a tankless water heater that’s giving you problems?

If so, we’re here to help. In this article, we’ll share:

• 3 common tankless water heater complaints
• The problem behind these complaints
• How to solve those problems

Need professional tankless water heater help? Just contact us.

1. “My tankless water heater runs hot, then cold, then hot again.”

The problem behind the complaint?

First off, this is a common tankless problem called a “cold water sandwich”. This problem happens because the heat exchanger (the part that actually heats the water) takes a while to warm up. Before it fully warms up and starts heating the water, you get a short run of cold water. Then, once the heat exchanger heats up, you get a steady flow of warm water.

So where does the initial blast of hot water come from? Well, usually there’s still hot/warm water left in the faucet from the appliance’s last hot water run.

How to solve the problem:

If you get that “hot water, cold water, hot water” surprise, consult a professional to see if you should combine your tankless unit with a “mini” tank water heater.

The small tank water heater provides you with hot water while your tankless unit’s heat exchanger warms up. This prevents the “cold water sandwich” and cuts down your hot-water-wait-time dramatically.

2. “I’m not getting any hot water from my tankless water heater.”

The problem behind the complaint?

A lot of different tankless water heater problems could cause you to lose hot water completely.

For starters, regardless of whether you have a gas or electric tankless unit, it could very well just be that you’re pushing the unit to its limits. Because tankless units only heat water as it’s needed, running too many hot water appliances at the same time could mean the unit just can’t keep up.

For example, if you’re showering, running the dishwasher and trying to wash your hands at the same time, you might get hot/lukewarm water in the shower and dishwasher but no hot water at all when you’re washing your hands.

On the other hand, your “no hot water” problem could be cause by a bigger issue such as:

• Plugged heat exchanger (due to hard water)
• A blocked vent/air intake
• Gas supply issues
• Bad ignitor/flame rod
• Dirty burners

How to solve the problem:

First, try running only one hot water appliance at a time. If you get hot water when only one hot water appliance is running, ask a plumber if you need to upgrade to a tankless unit with a higher flow rate.

If you get no hot water when only one hot water appliance is running and you have a gas tankless unit, make sure that:

1. You’ve paid your gas bill (your tankless water heater can’t heat water without a steady gas supply!)
2. Your water heater vent is unobstructed (be careful—these are usually located on the roof)
3. The gas valve feeding your tankless unit is turned ON, not OFF.
4. You’re scheduling regular maintenance. Getting regular maintenance will catch or prevent other tankless problems. Your plumber may also recommend you need a water softener to combat hard water.

Still don’t have hot water? Have a professional inspect the unit to diagnose and fix the problem.

3. “My tankless water heater shuts off during showers.”

The problem behind the complaint?

If you have an older tankless water heater (10+ years old), the problem is most likely that the minimum flow rate is too high.

You see, every tankless water heater has a minimum “flow rate”—which is the minimum amount of water, measured in gallons, that the tankless unit needs flowing through it every minute (gpm) to produce hot water.

If the amount of hot water you’re asking for is below the minimum flow rate for your unit, your unit is probably shutting itself off as a safety precaution. You see, if not enough cold water is flowing over the heat exchanger, it runs the risk of overheating. So to avoid damage to the heat exchanger (and to avoid scalding you with super hot water), your tankless unit will shut itself off, leaving you with cold water in the middle of your shower.

How to solve the problem:

If this is a persistent problem ask a plumber if you should upgrade your unit. Most older water heaters have a minimum flow rate of 1/2 to 3/4 gpm. However, newer units offer very low minimum flow rates (as low as 1⁄4 gpm), meaning your unit will run even when there’s only a small demand of hot water.

Need professional tankless water heater repair in Phoenix?

Just contact us.

We offer 24/7 plumbing repairs and licensed, green plumbers you can trust.

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