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Tips to Fix Tankless Water Heater Problems

Updated on September 1, 2009

Tankless Water Heaters

This article can be viewed in full in the January 2008, edition 192 of Fine Homebuilding. "Why Add a Tank to a Tankless Water Heater?" was originally published by Michael Chandler.

Also, known as demand water heaters, these compact units have long been used in Europe and in other parts of the world where energy costs are high. Here in the United States, the early adopters remember that in the 1980s, the installed units roared like jet engines. The slower the hot ware ran, the hotter it got, so adjusting the water temperature in the shower was often a challenge. Many people who installed tankless water heaters, ended up going back to replace them with tank-type heaters.

Today's tankless water heaters have better control circuitry and perform more efficiently than louder models.

Instant water heaters save energy and offer endless hot water, but not without some problems. A small tank and a big pump can fix the flaws.

Efficiency of Tankless Water Heaters

Rather than storing hot water in a tank and heating it all day, tankless water heaters heat water as it runs through the pipe. This approach is more efficient but it can lead to a few problems. Some of these problems can be corrected by adding a small tank to your tankless hot water heater. This approach still guarantees your energy efficiency, but it ensures your hot water usage is not hindered.

Common problems include:

  1. Because the pipes typically are insulated by the heat exchanger isn't, the water inside the heater cools more quickly than the water in the pipes beyond the heater. This cold water making its way through the pipes is an unpleasant surprise to an unsuspecting shower-taker.
  2. Heaters can heat only so many gallons of water per minute. If a dishwasher is turned on during a shower, the hot water is split between the two uses, reducing hot-water pressure to each.

Correcting the problems:

  1. A small tank-type water heater stops the cold-water surprise. Adding a 10-gallon electric water heater to a tankless model creates a buffer to eliminate the section of cold water left in the tankless heater's heat exchanger. If located near the bathroom, the tank also can reduce the wait for hot water.
  2. A big pump provides all the hot-water pressure you need. When the dishwasher is turned on during a shower, the hot water is split between two uses, which reduces the pressure. This usually translates into a lukewarm shower. Adding a large pump to the system to satisfy enough water through the tankless heater and into the tank to satisfy the needs of multiple hot-water users simultaneously.

Tips for Tankless Hot Water Heaters

Fine-tuning the system with a tempering valve

Use a big pump if you want the water to flow through the tankless water heater quickly to reheat the small tank in as short a time as possible so tha the pump will shut down and the gas flame will go out quickly. To achieve this brief burn time, you can turn the tankless water heater's thermostat up to about 140F while leaving the tank thermostat set at 125F. When the water in the tank drops below 125F, the pump turns on and starts filling the tank with hot water. The quicker the tank gets back up to temperature, the shorter the flame burns, and the moreefficiently the tankless water heater converts the energy in the propane to the hot water. Now the pump and the tankless water heater will be activated infrequently, but because that tank temperature will fluctuate from 125F to 140F, you can install a tempering valve on the output to the house in order to keep the domestic hot water at a safe 125F.

With a tank of reliably hot water, you can easily add a recirculating hot-water loop to keep members of the house from waiting for hot water.

Water & Energy Conservation

Recirculating Loops

A lot of cold water can do down the drain while you wait for endless hot water. You can save gallons and gallons of water by keeping that hot water handy with a recirculating loop. These loops are either automatic (timer- or temperature- activated) or switch-activated (motion sensor or manual).

Timer-Controlled Systems

The Grundfos and Laing systems keep water at a steady 90F to 100F between the water heater and the farthese hot-water outless in the house. Both systems are designed to run for long periods, so they work well on timers targeting hig-use periods. While they save a great deal of water, they use energy while you're away and double a family's water-heating cost if the hot-water pipes aren't well insulated. For this reason, neither LEED for Homes nor NAHB's green-building guidelines make these systems eligible for green points.

Demand-Activated Systems

The more energy-efficient solution is a deman-activated system, which uses a motion sensor or push button to triggor a high-velocity pump that automatically moves hot water to the faucet. Metlund's D'mand system earns both LEED-H and NAHB green points for water as well as energy conservation. It uses a high-velocity pump to push hot water through dedicated 3/4 inch supply lines to the farthest tap. As soon as the water in the tap is warm, the pump shuts off.

Tank Vs. Tankless


  • Tankless: $300 to $1500
  • Tank: $200 to $900


  • Tankless water heaters are much smaller and can fit almost anywhere. They're designed to be mounted on a wall. Gas-fired units require exhaust venting.
  • Tank heaters require a closet or basement-floor area.
  • If you replace a tank with a tankless heater, both the gas line and the venting usually need to be increased because the heating elements are much larger in tankless heaters than in tank-style heaters.


  • Upkeep on tank-type heaters is simpler, and it's easier to find people who can work on them.
  • Parts for tankless heaters are more difficult to get.
  • Tank heaters are more tolerant of hard water. Lime buildup in tanless heaters cuts efficiency; adding flush ports allow the heater to be cleaned with whit vinegar.


  • A tankless heater generally outlasts an unmaintained tank type.

Other Factors

  • Tank heaters are less likely to be damaged by freezing weather than tankless heaters are.
  • Tankless heaters pose less risk in an earthquake (less weight to restrain).

Will a Tankless Heater Pay for Itself

Whether all this extra work pays for itself depends on your behavior. If you have a tank-type water heater, you're probalby used to limiting the length of your showers so that you don't run out of hot water. With a tankless system, you'll never run out of hot water - even with several teenagers in the house. Of course, this might prompt family members to take more and longer showers, negating the energy saving.

The best thing about a tankless water heater is probalby the pease of mind it affords. Your supply of hot water is endless, but you're paying little or no money to heat water when it's not being used. Of course, a $300 tax incentive is another bonus.

*Note: You can add a solar water heater and radiant floor to tankless water heater systems to boos the value.

Fine Homebuilding

Fine Homebuilding Magazine

The Fine Homebuilding magazine shows in-depth techniques for various home projects. You'll find hands-on construction tips that are more than fully explained for easy comprehension.

The projects range from building foundations to from carpentry and interior finishing to system installations (whether plumbing, heating, or air).

Each issue contains in-depth reviews about tools, techniques, and materials that you may need for fine homebuilding projects.

You'll find information from various departments covering builder's tips, remodeling ideas, and answers to reader's questions.

This is a magazine that I love to peruse on my days off to get ideas for my next project. I have learned wood staining tips that I used to build my corner desk, and I've found tool reviews that I've used when looking for better tools to effectively build various projects around my home.


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    • MrWaterheater profile image


      6 years ago

      I really like the comparisons between tankless and tanked water heaters! Nice hub.

    • whitton profile image


      7 years ago

      Great Hub. This is some very informative information and great tips to fix tankless water heaters.


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