How a tankless water heater works
A tank water heater keeps a full tank of water hot around the clock. A tankless unit, also called an on-demand water heater, stores none. When you open a hot tap, a flow sensor feels the water moving, the burner fires, and the water heats as it passes through a heat exchanger. Close the tap and the burner shuts off. That’s why ENERGY STAR rates tankless units by how many gallons of hot water they produce per minute (GPM), not by how many gallons they hold.
Sizing: flow rate and temperature rise
Two numbers decide the size. The first is flow rate: the gallons per minute of hot water your home uses at its busiest moment, such as two showers and the kitchen sink running together. The second is temperature rise: how many degrees the unit has to add, from the temperature the water comes in at to the temperature you set.
The bigger the rise, the fewer gallons per minute a unit can deliver. That’s why the same unit can keep up easily in August and fall behind in a cold snap, when the water coming in from the street is colder. For scale, ENERGY STAR requires a certified gas tankless unit to deliver at least 2.8 gallons per minute over a 67°F rise. Size for your coldest incoming water and the most fixtures you run at once, not for a summer afternoon. Your technician will work through your household’s numbers with you.
Gas supply: the pipe and the meter
Many whole-house gas tankless units are rated up to about 199,000 BTU per hour, several times what a typical gas tank burns. The unit only fires that hard while hot water is running, but when it does, the gas pipe and the gas meter have to keep up while the furnace, range or dryer may be running too. A pipe that’s too small or too long for the load starves the burner, and that shows up as weak heat, error codes, or a unit that shuts off mid-shower.
Each manufacturer’s installation manual sizes the gas pipe by the unit’s rating, the length of the run and the type of gas. The meter belongs to CenterPoint Energy, and adding a large gas appliance can mean CenterPoint has to upsize it.
Venting: condensing or non-condensing
A gas tankless unit makes exhaust that has to leave the house safely, and ENERGY STAR advises working out how the combustion gases will be vented before a unit is chosen. There are two main types.
Condensing units pull extra heat out of the exhaust, which leaves it cool enough that many models can vent through plastic pipe, in the materials their manual allows. Cooling the exhaust also makes acidic water (condensate), so these units need a condensate drain, often with a neutralizer.
Non-condensing units send hotter exhaust out and usually need stainless-steel vent pipe.
The old metal flue from a gas tank generally can’t be reused for either type. Some models are built for outdoor mounting and vent straight out of the cabinet. Most gas tankless units also need a standard electrical outlet nearby for the controls and fan. If there isn’t one where the unit will hang, you’ll need an electrician; we don’t do electrical work.
Houston’s hard water, and why descaling matters
The City of Houston’s 2025 water quality report puts hardness on the main system at an average of 135.5 mg/L, about 7.9 grains per gallon, with readings from 43.5 to 229 mg/L. The U.S. Geological Survey classes 121 to 180 mg/L as “hard.”
USGS explains why that matters: when hard water is heated, calcium carbonate can form solid deposits, and that scale can shorten equipment life and raise heating costs. If scale shows up all over the house, a water softener ahead of the heater cuts it at the source. A tankless heat exchanger is a set of narrow passages heated hard and fast, so scale there does more harm than sediment in the bottom of a tank. As it builds, it slows heat transfer and narrows the passages. Hot-water flow drops, the unit runs hotter, and some models post a scale or overheating code.
Descaling, often called flushing, clears it out. The water is shut off at the unit’s service (isolation) valves, a pump circulates a descaling solution through the heat exchanger, and then the unit is rinsed and its inlet water filter cleaned. How often depends on the manufacturer, your water and how much hot water you use, so go by the schedule in your unit’s manual. If your unit was installed without service valves, they can be added so every future flush is simpler.
Error codes, weak flow, or a unit that won’t fire
Tankless units report problems as codes on the unit or the wall controller, and each brand uses its own. Most fall into a few groups: ignition failure (the burner didn’t light), flame loss (it lit, then went out), overheating, a venting or fan fault, and on some models a scale warning. Write down the code and the model number before you call. The code tells us where to start looking, though not always what failed.
When a unit won’t ignite at all, common causes are the gas supply (a closed valve or an undersized line), a blocked air intake or vent, a dirty flame sensor, a worn igniter, or no power to the unit. Resetting it once is fine. If the code keeps coming back, stop resetting it and have it checked. If you smell gas, don’t reset the unit: leave the house and call 911 and CenterPoint Energy from outside.
Weak hot-water pressure while the cold side is fine points to the unit itself, often a clogged inlet filter screen or scale in the heat exchanger. If hot and cold are both weak, the cause is likely elsewhere in the plumbing.
Sources: ENERGY STAR, gas tankless water heaters · ENERGY STAR, tankless benefits and savings · ENERGY STAR, water heater criteria · City of Houston 2025 water quality report · USGS, hardness of water
