A Tankless Water Heater's Real Downsides — the Full Ledger, Before Anyone Sells You One

We install these units. That's exactly why we'd rather you read the honest list first, instead of hearing it from the person whose commission depends on the sale.

The tankless water heater service page already covers the three downsides most people hear about — the steep upfront cost, the short delivery delay some people call a “cold water sandwich,” and the annual chemical flush — so we won't repeat those here. This is the layer underneath: the mechanical reasons those costs exist, plus the parts of the ledger a salesman tends to leave off.

A high-efficiency condensing tankless is a sealed-combustion, Category IV appliance. It pulls its combustion air straight from outdoors and pushes exhaust out under positive pressure through new plastic vent pipe. The manufacturer is blunt about the limit: you cannot tie that vent into an existing vent, and you cannot run it into a masonry chimney. Whatever flue served your old atmospheric tank heater is dead weight now.

A condensing unit wrings heat out of its own exhaust, and that leaves behind condensate. That water is acidic — a pH of 3 to 5. You can't just run it into any drain and forget it; the manufacturer says disposal has to follow local code, because acid at that strength corrodes drains and the public sewer over time.

The standard answer is an inline neutralizer: a cartridge of media the condensate flows through, which raises the pH before it ever reaches your drain. It works, but it's a wear part. The manufacturer recommends testing the pH once a year and replacing the media when it drops below 6.0. That's a small chore most tank owners never had.

A standard tank heater sips around 40,000 BTUs and runs fine on a 1/2-inch line. A tankless flashes water on demand and can pull up to 199,000 BTUs, with a 3/4-inch gas connection at the unit. Try to feed that much gas through the old 1/2-inch pipe and the heater starves — it can't hold temperature, and it shuts down. Most conversions need a dedicated 3/4-inch trunk line run back toward the meter, which is a real part of the project cost and the reason a tankless heater needs its own gas line.

A tankless heater only lights its burner once it senses enough water moving through it. A Rinnai condensing unit needs about 0.4 gallons a minute of flow just to activate, and roughly 0.26 gallons a minute to keep running. Crack a faucet to a thin stream — the kind you'd use to shave or rinse a single dish — and the flow can fall under that threshold. The burner never fires, and you get cold water at that tap.

Scale is a tankless heater's enemy, and hard water is scale's supplier. The manufacturer states plainly that in a hard-water area the water must be treated and the unit flushed more often than the normal schedule. Ignore it and the risk isn't only performance — scale damage to the heat exchanger is specifically excluded from the warranty, so a missed maintenance habit can leave you paying for a failure the warranty won't touch. Whether that's a heavy tax or a light one depends entirely on your water, which is why we go through what annual descaling actually involves separately.

A gas tankless is an electric appliance. It runs on 120-volt, 60-hertz power and lights with direct electronic ignition — there is no standing pilot to fall back on. When the power goes out, the control board, the blower, and the igniter all go dark, and the unit makes no hot water at all. The manufacturer's own fix is to add a battery backup to keep hot water flowing during an outage.

A tank heater is a simple appliance. A tankless is an electromechanical one — a control board, a magnetic flow sensor, an ignition board, a flame rod, a mixing valve — and most of those parts belong to the brand. That's fine when your plumber stocks them and knows the unit; a fouled sensor becomes a same-day fix. It's a problem when nobody nearby carries the part, and a repair that should take an hour turns into days of cold showers waiting on a shipment.

Two intervals are worth putting on the calendar. The heat exchanger needs an annual descaling flush, and more often than that on hard water. If your unit runs a condensate neutralizer, test its pH once a year and replace the media when it drops below 6.0. The rest is a look, not a stopwatch — the inlet filter screens, the air intake, and the condensate trap all get checked at the same visit. There's no invented “monthly additive” and no secret schedule; the intervals are what the manufacturer publishes.

Some outfits bill up to $800 for a “proprietary chemical flush” — the exact annual descaling described above, dressed up as a patented secret. There's nothing proprietary about it. It's white vinegar or an OEM descaling solution circulated through the heat exchanger, a filter screen rinse, and a trap cleaning. When we do it, it's routine maintenance, not a racket. If you want the mechanics of that job, we lay it out in the descaling walkthrough.

For the right house, none of this is a dealbreaker — it's a list of obligations you take on with your eyes open. Plenty of homes are a clean fit, and the install is a few hours of work. One customer, Andrew F., described a cold-weather swap this way:

Thinking about a tankless heater, or trying to figure out whether the one a salesman quoted actually fits your house? We'll run the gas load, the vent path, and the condensate before we quote anything, and we'll tell you honestly if a tank is the smarter call for your home. Call or text us — 405-519-1868.