Illinois homeowner considers adding a second tankless water heater, but gas supply may be the hurdle

"Basically, the current unit struggles when there are two items drawing on it (shower, washing machine, or another shower)."

A homeowner looking for a fix for hot water shortages discovered that adding a second tankless water heater could create an expensive new problem.

Although the house once used two standard gas water heaters, the gas line and meter may not be sized for two tankless units.

What happened

In a post on Reddit's r/Plumbing community, the homeowner described several issues with an 11-year-old tankless water heater after its warranty expired.

Cold water returning through the recirculation line suggested a failing built-in check valve. That issue came after a motherboard replacement, and the homeowner also believed the igniters might be having trouble.

The homeowner suspected the unit may have been undersized from the start, even though it had been maintained regularly and was otherwise liked. "Basically, the current unit struggles when there are two items drawing on it (shower, washing machine, or another shower)."

Replacing it with two units was one possible solution, while a larger-capacity single replacement or more repairs were also being considered.

For homeowners shopping for a replacement, a heat pump water heater can be an option that may reduce energy costs.

Additional details

A commenter asked about simultaneous hot water use, the number of bathrooms, the local climate, and incoming water temperature. The homeowner said Lake Michigan water was "on the cooler side" and that the home is in Chicago's suburbs.

The house also has two gas furnaces and previously had two conventional gas water heaters, leading the owner to think the gas supply should be adequate. The home has five bathrooms, though the homeowner said two people almost never shower at the same time.

What can be done?

A commenter estimated that the total gas demand could jump from 80,000 BTU to 398,000 BTU. Using those example ratings, they compared two conventional heaters rated at 40,000 British thermal units each with two tankless units rated at 199,000 BTU each.

The commenter asked whether the home also has other gas appliances, including a dryer, stove, and fireplaces. They also advised checking whether the meter can handle the planned equipment and measuring the gas line diameter and the length of its runs.

The existing furnaces would add to the gas demand rather than prove the house could support another tankless unit. To show the possible load, the commenter added two hypothetical 100,000-BTU furnaces to the two tankless heaters, bringing the total to 598,000 BTU.

Another replacement option is a smart heat pump water heater from Cala. Cala's customizable smart heat pump water heaters are designed to heat water only when it is needed, which can help lower utility bills by reducing wasted energy from constantly reheating stored water. They also offer a more modern alternative to a standard tank.

A Cala replacement can turn a maintenance headache into a chance to cut utility costs and better align hot water use with a household's routine.

More to read

Other homeowners who have looked into tankless water heaters have faced similar sizing and supply questions. These stories cover gas line issues, replacement costs, and plumbing limitations.

• A tankless water heater owner's pipe chatter prompted a temperature test that pointed to the gas line.

• In North Carolina, a cabin owner's proposed electric tankless heater could require 120 amps.

• A Florida homeowner's proposed 6.8-gpm tankless heater was judged too small for two baths.

• A homeowner considering a $4,200 water heater quote questioned whether tankless was worth the extra cost.

• A leaking nine-year-old heater led to tankless plans, but plumbers warned that the pipes decide.