Water heater types

The heater in the closet is not “a water heater” in the way a troubleshooting list assumes. A standing-pilot gas tank, a power-vented condensing tank, an electric tank with two elements, a heat pump parked in the same footprint, and a tankless unit on the garage wall fail differently and are sized differently. Using the wrong page wastes a Saturday and, around gas and electricity, can make a small fault worse. This section is the map. The symptom pages assume you already know which machine you are looking at.

Five machines, not one

A storage tank holds a reservoir and reheats it as you draw. Cold water enters through a dip tube that should discharge near the bottom. Hot water leaves from the top. The energy comes from a gas burner under or beside the tank, from electric resistance elements, or, in a heat pump model, from a refrigeration circuit that pulls heat out of the room and dumps it into the water. Standby loss is the tax you pay for that reservoir: heat leaking through the insulation all night. The anatomy, including what the dip tube, anode, and thermostat each do, is how a storage tank works.

A tankless heater, also called a demand heater, has no reservoir that can go cold halfway through a second shower in the way a tank can. It has a heat exchanger, a flow sensor, and a burner or elements that modulate to hold a set temperature. It can still run out of temperature. If the flow is too high for the temperature rise the unit can make, the water gets lukewarm. If the flow is below the minimum activation rate, it does not fire at all. Both limits are in how a tankless heater works.

Fuel is a separate choice from shape. A tank can be gas or electric. A tankless unit can be gas or electric, and a whole-house electric tankless unit is a large electrical load that many panels cannot accept. Gas brings a vent and combustion air. Electric brings a 240-volt circuit and no flue. Outages, operating cost, and what “efficiency” even means are compared in gas or electric, without pretending one fuel wins in every zip code.

A heat pump water heater is an electric storage tank with a compressor on top or, less often, in a split system outdoors. It can be two to three times as efficient as a resistance element when the room is suitable, and it falls back toward ordinary electric use when the air is cold or when you lock it in resistance mode. Space, noise, condensate, and the conflicting published air-volume figures are in heat pump water heaters.

A condensing gas heater, tank or tankless, cools the flue gas far enough to squeeze water out of it and capture heat that an atmospheric unit sends outdoors. The vent is plastic listed for the job, not the old metal B-vent, and the condensate is acidic. Treating a condensing unit like an atmospheric tank is how new heaters get connected to chimneys they cannot use. That distinction is condensing gas heaters.

What this section leaves in one paragraph

A few heaters are real and do not have their own guide, because a full page would either be thin or would wander into boiler service.

A point-of-use heater is a small electric tank, or a small tankless unit, under a distant sink. It solves a long wait and a large slug of cooled water in a branch line. It does not fix a whole-house heater that is undersized, and it needs its own circuit sized from its nameplate. If the far bathroom is the only complaint, a point-of-use unit can be the right tool. If every bathroom fades together, the central heater or a crossed fixture is the problem, which is the runs-out guide.

A tankless coil or an indirect tank hangs off a boiler. The Department of Energy’s comparison article treats tankless coils as a poor fit outside cold climates, because they ask the boiler to fire for domestic draws in mild weather, and treats a well-insulated indirect tank on an efficient boiler as a different and often better arrangement. Sizing and servicing those systems is boiler work. This site does not tune boilers.

Solar thermal systems with a backup tank are sized from collector area and storage volume, not from a first-hour rating alone. The older Energy Saver sizing guide describes rules of thumb a contractor uses, including collector area per person and a storage ratio on the order of 1.5 gallons per square foot of collector. Any solar page that skips freeze protection, stagnation in summer, and the backup heat source is incomplete. If you have a solar preheat tank, the backup heater is still one of the machines above, and its troubleshooting still applies to that half of the system.

If you already know you are choosing between a tank and a tankless unit, the decision page is tank or tankless, not a second anatomy lesson. Brand comes after that choice. How the major brands differ is about parts and sales channels, not a ranking.

What to read next

If the heater is already misbehaving, leave this section for troubleshooting once you know the type. If you are replacing it, the type choice is mostly a fuel, a vent, a room, and a first-hour or flow requirement. Those are in sizing a tank and sizing a tankless heater. Do not buy a type because a relative likes it. Buy the one the room, the fuel, and the peak hour can support.

Sources

Department of Energy, Energy Saver, “Which Water Heater Is Right for You?” (energy.gov). Department of Energy, Energy Saver, “Sizing a New Water Heater,” including the solar collector notes reproduced from that guide. ENERGY STAR, residential water heaters (energystar.gov/products/water_heaters).