Yes. Tankless water heaters cost more to install when the gas line or venting must be upsized, take longer to deliver hot water, won't fire below a minimum flow, stop during power outages, and need regular descaling in hard water like Phoenix's. For many busy households the benefits still outweigh those trade-offs.
Why can installation be the biggest hurdle?
Because a tankless burner is several times larger than a tank burner, and the gas pipe has to keep up. A standard 50-gallon gas tank on Bradford White's spec sheet has a 40,000 BTU per hour input. Rinnai's sizing table lists whole-house tankless models rated from 120,000 up to 199,000 BTU per hour.
That much gas needs a bigger pipe. Rinnai's FAQ walks through sizing the gas line by gas type, inlet pressure, allowed pressure drop, the other appliances on the line, and the total load under the National Fuel Gas Code. In its natural gas table, the 199,000 BTU models top out at 40 to 50 feet of equivalent half-inch pipe, and Rinnai notes that an undersized gas line can stress a tankless system. A house plumbed years ago for one 40,000 BTU tank may need a bigger line.
Venting is the other piece. A DOE Building America guideline says retrofit costs can vary significantly based on venting requirements, electrical availability and gas line upsizing needs. A DOE-hosted field study presentation lists high installation costs in some situations, naming increased gas line size, increased gas meter size and difficult venting. In new construction those costs shrink, since the pipes go in sized right from the start.
### Is electric tankless any easier to install?
Usually not for a whole house. Rinnai, which makes no electric tankless model, explains why: an electric unit delivering 3 gallons per minute with an 80°F temperature rise would need about a 150 amp, 230 volt circuit. Electric tankless shines at a single sink or a casita, as our page on electric tankless water heaters in Arizona explains.
How does daily use feel different?
Three things change at the tap: wait time, low-flow behavior and outages.
### Hot water takes longer to arrive
The unit has to sense flow, fire and heat up before it sends hot water. The DOE guideline says tankless units experience a time delay due to start-up sequences, especially when the heat exchanger has fully cooled off from the prior draw. It adds that delays of 20 seconds or so are not uncommon.
That delay stacks on top of the time hot water already takes to travel the pipes. A recirculation system can close the gap, at the cost of some energy. Rinnai notes that without a recirculation system a slight 1 to 2 second temperature drop is normal as water in the pipes clears the line.
A related complaint is the cold water sandwich, a burst of cold water between two hot draws. Our page on a tankless that turns cold mid-shower explains what causes it.
### A trickle won't fire it
Every tankless needs a minimum flow to switch on. Rinnai says its units need at least 0.4 to 0.6 gallons per minute through the hot side to ignite, and 0.26 gpm to stay running. The DOE guideline puts typical gas tankless minimums at 0.4 to 0.75 gpm and notes that shaving or rinsing dishes at a low flow may not trigger the burner.
In practice, a barely-open faucet stays cold. Most people adjust quickly. Some never stop noticing.
### It stops in a power outage
A gas tankless uses electricity for ignition, controls and the fan. A. O. Smith says gas tankless water heaters stop working during power outages, and Rinnai says its units will not operate in an outage. A standard gas tank with a standing pilot keeps heating. If outages worry you, battery backups exist, and our page on whether a gas water heater works during a power outage compares the types.
How much maintenance does hard water add?
More than a tank, and Phoenix water makes it a regular job. Scale forms inside the narrow passages of the heat exchanger. Rinnai's manual says hard water must be treated to prevent scale build up or damage to the heat exchanger, and its diagnostic guide points to scale as a cause of heat exchanger overheat codes.
The City of Phoenix reports total hardness of 10 to 17.6 grains per gallon, so descaling on schedule isn't optional here. See how often to descale a tankless water heater in Phoenix for intervals and steps.
So who should skip tankless?
Skip it, or think twice, if:
- Your gas line or meter is already near capacity and upsizing would be a big job.
- You lose power often and don't want a battery backup.
- Your household mostly uses hot water in small, low-flow bursts.
- You won't keep up with descaling.
It's a strong fit if you have gas, a big household with long or back-to-back showers, and a place to vent it. The DOE field study found tankless units had acceptable performance for all ten homeowners studied and did not increase daily hot water use in any of those homes. If you're weighing tankless against a heat pump tank, read tankless vs hybrid water heaters. HQ installs and services tankless units across the Valley; see our water heater service.