Water heater types field guide: tank, tankless, and heat pump

The water heater choice and what it costs you later

A water heater is the appliance that makes domestic hot water for a building, and it does it one of two ways: it stores a tank full and keeps it hot, or it heats water as it flows. Inside those two it splits again by fuel, gas, electric resistance, or a heat pump, and by how it handles the flue gas if it burns gas. That is the whole map. Everything else is a variation on storage versus on-demand and which fuel makes the heat.

The choice trades five things against each other, and they pull in different directions. First cost: a basic gas storage tank is the cheapest thing you can hang in a mechanical room. Operating cost: a heat pump unit runs on a fraction of the energy, so it costs more to buy and far less to run. Space: a tank eats floor area, a tankless hangs on a wall, a heat pump needs a room full of air. Fuel: you install what the building's gas and electric service can actually feed. Recovery: how fast the unit makes hot water once the stored gallons are spent.

This guide is the type comparison: how each kind works, what it costs to own, what it needs to run, and how to match it to the job. The capacity question, how big, how much recovery, how much storage for the peak hour, is its own problem, worked in the water heater sizing and selection guide. Pick the type here. Size it there. The two decisions feed each other on a real job, so run them together.

What is the difference between a tank and tankless water heater?

A tank water heater stores 40 to 120 gallons of water and keeps it hot around the clock, ready to deliver. A tankless heater stores almost nothing and fires a burner or element only when a tap opens, heating the water as it flows through. That single difference, stored reserve versus heat on demand, drives every other tradeoff between the two.

The tank gives you a buffer. Open three fixtures at once and the stored gallons cover the spike while the burner catches up, so the tank is forgiving of a demand it was not perfectly sized for. The cost of that buffer is standby loss: a mass of hot water sitting in the room bleeds heat through the tank wall every hour, whether anyone draws a drop or not. The tank also has a recovery ceiling. Drain it faster than it can reheat and the shower goes cold until it catches up.

Tankless flips both. No stored mass means no standby loss and, in theory, endless hot water, because it makes hot for as long as water flows. The limit is not gallons, it is flow. A tankless unit can only raise so many gallons per minute across the temperature rise, so it has a hard flow ceiling instead of a hard volume ceiling. Run more fixtures than its rated GPM at the cold-inlet rise and every one of them goes lukewarm. The two heaters fail in opposite ways, and that is the heart of the choice.

Gas, electric, and heat pump: the fuel drives the choice

Fuel is the first fork, and the building's available service often makes the call before anything else does. Gas gives you high heat input cheaply, which means fast recovery for the money, which is why gas dominates storage tanks and high-output commercial work. Electric resistance is simple, compact, and vent-free, but the recovery is capped by how big a circuit you can feed the elements. A heat pump runs on electricity but moves heat instead of making it, so it delivers two to four times the hot water per kilowatt, at the price of needing space and air.

What you can install is bounded by what is already there. A building with a 1/2 in gas branch and a 175,000 BTU meter cannot feed a 199,000 BTU tankless without a utility upgrade. An all-electric building with a full panel cannot absorb a heavy resistance heater without a service look. The cleanest type on paper is the wrong type if the fuel to run it is not at the heater.

Cost of the fuel matters as much as availability. Where gas is cheap, a gas tank is hard to beat on operating cost. Where electricity is expensive and gas is unavailable, a heat pump is the only electric option that is not punishing to run, because resistance heat at high electric rates is the most expensive hot water there is. Price the fuel over the life of the unit, not the day you buy it.

The storage tank water heater

The storage tank is the classic, and for good reason: cheap to buy, simple to install, and forgiving of a demand you did not size perfectly. A steel tank, glass-lined inside, holds the water. A gas burner underneath or electric elements inside the side wall keep it hot. A thermostat cycles the heat. A dip tube carries cold inlet to the bottom so it does not dilute the hot at the top. That is the whole machine, and it has not changed much in decades because it works.

The two costs you sign up for are standby loss and the tank itself. Standby loss is the heat the stored gallons shed all day, which a thicker insulation jacket reduces but never eliminates. The tank is a consumable. It rusts from the inside, and the only thing standing between the water and the steel is the anode rod, a sacrificial magnesium or aluminum rod that corrodes in place of the tank. Once the anode is gone, the tank goes next. On hard or aggressive water the anode can be eaten in a few years, so a tank that never gets its anode checked is a tank on a short clock.

Gas and electric tanks behave differently in the same shell. A gas tank recovers faster because a burner puts out more BTU than an element draws, so it refills the stored volume quicker after a heavy draw. An electric tank recovers slower but converts almost all its input to hot water and needs no flue, which is why it shows up where there is no gas and no room to vent. Recovery is the number that separates them, and it is set by the input, the efficiency, and the cold-inlet rise, worked in the sizing guide.

The tankless or on-demand water heater

A tankless heater hangs on the wall and makes hot water only while a tap is open. Flow through the unit trips a sensor, the burner or element fires, and a heat exchanger raises the water to setpoint on its way to the fixture. Close the tap and it shuts off. Nothing is stored, so nothing is lost to standby, and the unit runs for as long as water flows, which is where the endless hot water claim comes from.

The ceiling is flow rate, not volume. A tankless unit is rated for a certain gallons per minute at a stated temperature rise, and that rating collapses as the rise grows. A unit that delivers 5 GPM raising water 45 degrees might give only 3.5 GPM when the inlet is cold and the rise climbs past 70. Size it for the worst simultaneous draw at the coldest inlet, or it goes lukewarm the moment two showers run on a winter morning. This is the single most common tankless complaint, and it is almost always an undersizing or a fuel-supply problem, not a defective unit.

Tankless carries two items a tank does not. Hard water scales the heat exchanger, so a tankless on hard water needs a descale flush on a schedule or it loses output and eventually fails. And it has the cold-water sandwich, covered next. The higher first cost and the bigger fuel supply it demands are the real price of admission, and they are why a tankless that pencils out on paper still gets value-engineered back to a tank when the gas line and the budget will not stretch.

What is the cold-water sandwich?

The cold-water sandwich is the slug of cool water you hit mid-stream on a tankless heater between two hot draws. Someone runs hot, shuts off, and a moment later turns it back on. The water already in the pipe is still warm, then comes a slug of cooler water that entered while the burner was off and re-firing, then the freshly heated water arrives. Hot, cold, hot. That is the sandwich.

It happens because a tankless unit is not instant. There is a short lag between flow starting and the heat exchanger reaching setpoint, and the water that moves through during that lag is under-heated. On a single long shower you never notice it. On short, stop-and-start draws, washing dishes, a quick rinse, back-to-back showers, the cold slug shows up every time the burner has to re-light.

The fixes are known. A small buffer tank or a built-in mini-tank on the unit smooths it out by holding a little hot water through the lag. A recirculation loop with the right control keeps the line hot so there is no cold slug to push. Some newer units shorten the lag in the firmware. It is rarely a reason to walk away from tankless, but it is worth telling the owner about, because a homeowner who was promised endless hot water and gets a cold burst mid-shower will call it a defect.

What is a heat pump water heater?

A heat pump water heater, also called a hybrid, is an electric tank that makes most of its hot water by moving heat out of the surrounding air into the water, the same refrigeration cycle a refrigerator or air conditioner runs in reverse. Because it moves heat instead of generating it, it delivers far more hot water per unit of electricity than a resistance heater. ENERGY STAR units commonly carry a UEF in the range of 3.3 to 4.1, three to four times the energy into the water for every unit of electricity drawn, with a coefficient of performance near 3 to 4.

That efficiency is the whole reason it exists, and it is the strongest operating-cost case on the electric side by a wide margin. The catch is where the heat comes from. The unit harvests heat from the room air, so it needs air to harvest. Manufacturers commonly call for something on the order of 450 to 700 cubic feet of free air around it, or ducting to a larger space. Put one in a small closet and it runs out of air, falls back on its resistance elements, and you have bought a heat pump that runs as an expensive resistance tank.

It also changes the room. A heat pump water heater cools and dehumidifies the space it sits in, which is a bonus in a hot mechanical room or a southern garage and a problem in a finished basement it makes cold and clammy. It runs slower and louder than a tank, and recovery falls as the source air gets cold, so in an unconditioned space in a cold climate it leans on the resistance backup all winter, exactly when demand peaks. Confirm the room volume, the condensate drain, and the cold-weather recovery before you commit it to a load that cannot run short. The refrigeration-cycle fundamentals behind it are a topic of their own; cross-link the heat pump basics on a real job.

Condensing gas: tank and tankless

A condensing gas heater pulls so much heat out of the flue gas that the water vapor in the exhaust condenses, and capturing that latent heat is what pushes its efficiency past what an ordinary gas unit can reach. A non-condensing gas unit commonly lands around 0.80 to 0.85 UEF; a condensing unit runs roughly 0.90 to 0.96. The gain comes from a second heat exchanger that wrings the last heat out of the exhaust before it leaves the building.

That cool, wet exhaust changes the install. Flue gas leaves a condensing unit below roughly 130 degrees F, far cooler than a conventional flue, so it vents in listed PVC or CPVC instead of metal, which makes the venting cheaper and easier to route. It is a sealed, positive-pressure system, a different vent category from an atmospheric heater, and the two are not interchangeable. The deep treatment of vent categories and combustion venting is its own topic; cross-link it on the job.

The condensate is the part people forget. The water that condenses is acidic, with a pH commonly around 3 to 5, low enough to attack metal drain lines and concrete over time. It runs through a neutralizer, a canister of media that raises the pH before the condensate reaches the sanitary drain, and that media gets replaced on a schedule, commonly every 12 to 24 months. Skip the neutralizer or let it exhaust and you are draining acid into the building's plumbing. A blocked or omitted condensate line backs up into the heater and shuts it down.

Point-of-use, indirect, and the niche types

Not every hot water need is a whole building. A point-of-use heater is a small electric tank or tankless unit installed right at a remote fixture, a hand sink in a far corner, a break-room kitchenette, a single lavatory at the end of a long run. It exists to kill the wait. Rather than run hot water a hundred feet from the central heater and waste the cold slug every time, you put a few gallons of heat at the fixture. On a small remote draw it beats both a long branch and a recirculation loop on cost.

An indirect water heater has no burner or element of its own. It is a storage tank with an internal coil fed by the building's boiler, so the boiler that heats the building also makes the domestic hot water. Where a boiler already runs, especially a high-efficiency one, an indirect tank is efficient and quiet, and it adds no flue or electric load of its own. The tradeoff is the dependency: no boiler, no hot water, and the boiler has to be available and sized to carry the domestic load on top of the heating load. In a cold climate with hydronic heat it fits naturally. In a building with no boiler it makes no sense.

Other niche types are worth knowing by name. Solar thermal systems preheat water with roof collectors and almost always pair with a conventional backup. Combi boilers make space heat and domestic hot water in one appliance for smaller buildings. Each fits a narrow case. Match the type to the actual draw and the fuel already in the building, not to the most efficient unit in the catalog.

Commercial and the scale differences

Commercial hot water is the same physics at a different scale, and the scale changes the answers. A restaurant, a hotel, a gym, a hospital draws far more hot water in its peak hour than a house, and the equipment grows to match: large gas storage tanks, high-input gas instantaneous units, semi-instantaneous heaters with tight temperature control, often banked in multiples. The single-tank logic of a house does not carry up.

Two things separate commercial selection from residential. First, redundancy. On a critical load, healthcare, food service, a hotel, one heater is a single point of failure, so the design runs more than one unit and sizes the bank to still meet the core demand with one offline. That is the N-plus-1 thinking the rest of the building's critical systems already use. Second, the methods. Commercial sizing runs off occupancy demand data and recovery-plus-storage selection, not off a residential first-hour-rating label, and that method work lives in the sizing guide.

Fuel and venting decisions get heavier too. A bank of high-input gas units needs serious combustion air, gas supply, and venting, all sized to the total input, not eyeballed. A commercial electric or heat pump installation can swing the building's electrical service. The type comparison still holds, storage versus on-demand, gas versus electric versus heat pump, but the consequences of getting it wrong scale with the building.

Which water heater is most efficient?

By the DOE efficiency metric, the heat pump water heater is the most efficient type sold, commonly carrying a UEF in the 3.3 to 4.1 range against roughly 0.90 to 0.95 for electric resistance, 0.60 to 0.70 for a standard gas tank, and 0.90 to 0.96 for a condensing gas unit. A UEF above 1 is only possible because a heat pump moves heat rather than making it, so it delivers more energy to the water than it draws from the wall. Treat these as typical ranges from the manufacturer ratings, since the exact UEF is stamped on the unit and varies by model and draw pattern.

UEF, the Uniform Energy Factor, is the rating that replaced the old Energy Factor, and it folds in recovery efficiency, standby loss, and cycling under a standard draw profile. It is rated against a draw bin, from very small to high use, so compare two units in the same bin or the numbers do not mean the same thing. Larger commercial equipment is rated on thermal efficiency and standby loss instead of UEF, so a residential UEF and a commercial thermal efficiency are not the same yardstick.

Most efficient on the meter is not always cheapest to own. A heat pump wins on energy but costs more up front and can lose its edge in a cold space where it runs on resistance backup. A condensing gas unit wins where gas is cheap and electricity is dear. The efficient number on the label only pays off if the install lets the unit actually run at that efficiency, which is the next two sections.

First cost versus life-cycle cost

The cheapest water heater to buy is almost never the cheapest to own, and that gap is the whole argument for tankless and heat pump units. A basic gas storage tank has the lowest first cost by a wide margin. A tankless unit costs more to buy and more to install, between the unit, the upsized gas line, and the venting. A heat pump unit costs the most up front of the common residential types. Stop the comparison at the price tag and the tank always wins, which is exactly the mistake that gets made.

Life-cycle cost is first cost plus the energy and the maintenance over the years the unit runs. A heat pump drawing a third to a quarter of the energy of a resistance tank pays back the price difference over its life, faster where electricity is expensive. A tankless saves the standby loss every hour and lasts longer than a tank, so its longer life spreads the higher first cost over more years. The payback period depends on the fuel rates, the usage, and the install cost, so run it for the actual building rather than quoting a generic number.

The blunt version: pick on life-cycle cost, not first cost, but be honest about the install. A heat pump in a space too small to run efficiently never earns its payback, because it runs on resistance backup and you paid heat-pump money for resistance hot water. A tankless undersized for the gas line never delivers, so the savings are theoretical. The efficient type only pays back if it is installed where it can perform.

Recovery, first-hour, and how each type delivers

Each type delivers hot water on a different curve, and that is what separates them in use. A storage tank delivers its usable stored gallons fast, then drops to its recovery rate once the reserve is spent, so its delivery is a strong start that fades to a steady output. A tankless delivers a flat output indefinitely, capped at its rated flow, so it never fades and never surges past the ceiling. A heat pump delivers like a tank but recovers slowly, so it leans harder on its stored volume and its resistance backup under a heavy draw.

The residential shorthand for a tank is the first-hour rating, the gallons it delivers in the busy first hour from full, which folds usable storage and recovery into one number on the EnergyGuide label. The commercial shorthand is recovery and storage carried as a pair. Both are sizing questions, and both are worked in the water heater sizing and selection guide. The point here is that the type sets the shape of the delivery curve before any sizing happens.

Match the curve to the draw. A spiky, short peak, a house at shower time, a dorm, rewards a tank that rides it out on stored gallons. A long, steady pull, a laundry, a back-to-back commercial peak, rewards recovery that never falls behind, which favors a high-input gas tankless or a banked system. Put a slow-recovery heat pump on a relentless commercial peak and it runs out, no matter how efficient it is.

Space and venting by type

Each type takes up the building differently, and the available space often makes the call when two types both meet the load. A storage tank needs floor area and headroom, more than people remember once you add the clearance for the T&P discharge, the connections, and service access. A tankless hangs on a wall and frees the floor, which is half its appeal in a tight mechanical room or a condo. A heat pump needs not just floor space but a volume of air around it to harvest, commonly hundreds of cubic feet, plus a condensate drain.

Venting splits the gas types hard. An atmospheric gas tank vents on natural draft into a metal flue or chimney and needs combustion air drawn from the room, which makes it sensitive to a tight space. A power-vent unit uses a fan to push the exhaust out, freeing the vent routing. A condensing unit vents in plastic because its exhaust is cool and wet, the easiest gas venting to route but with the condensate to manage. Electric resistance and heat pump units need no flue at all, which is a real install savings where running a vent is hard.

The clearances are not optional. A tankless needs its listed clearances around the vent termination and the combustion air openings. A heat pump needs its minimum room volume or ducting and the clearance for airflow over the evaporator. Crowd any of them into a space that does not meet the listing and you have an install that either fails inspection or never performs. Confirm the clearances and venting against the manufacturer's instructions and the fuel-gas code for the specific unit.

Fuel and utility requirements: the install gotchas

This is where good selections die on install day. A tankless gas unit fires a much higher input than a tank, often 150,000 to 199,000 BTU per hour against a tank's 40,000, so it commonly needs a 3/4 in or 1 in gas line and a meter that can feed it. Many older homes run a 1/2 in branch and a meter sized for the old load. Drop a 199,000 BTU tankless on that and it cannot reach its rated input, the recovery you sold never shows up, and the symptom looks like an undersized heater when the real problem is the gas supply. Verify the meter capacity and the gas line before you quote a tankless, not after.

The electric side has its own gotchas. A heat pump water heater runs on a dedicated circuit, commonly 240 V, and it needs the condensate drain and the room volume on top of the power. An electric resistance tank with strong recovery pulls a heavy circuit, and a commercial electric or instantaneous unit can swing the building's service size, so the load has to land in the electrical load calculation before the panel is done, not as a surprise after.

The rule across all of it: the type is only as good as the utility you can feed it. A condensing unit needs the condensate drain and a neutralizer path. A tankless needs the gas line and the meter. A heat pump needs the air, the circuit, and the drain. Confirm every one of those is present before you commit the type, because the cheapest fix is choosing a type the building can actually run.

Maintenance by type

Every type has a maintenance burden, and it is different for each, so the owner inherits a different chore list depending on what you install. A storage tank needs its anode rod checked and replaced before it is spent, because the anode is the only thing protecting the steel, and it needs a periodic flush to clear the sediment that settles in the bottom and buries the burner or element. A tank that loses capacity is more often full of sediment than undersized, and a flush brings the recovery back.

A tankless needs a descale flush, especially on hard water, because scale builds on the heat exchanger and chokes the output. Run a tankless on hard water with no descaling and you will be replacing a heat exchanger years early. A water softener or a scale filter stretches the interval. A heat pump unit adds an air filter that has to be cleaned so the evaporator keeps breathing, and a condensate drain that has to stay clear, on top of the same anode and sediment care a tank needs, since it is still a tank.

A condensing gas unit adds the condensate neutralizer, whose media gets replaced on a schedule, commonly every 12 to 24 months, or it stops neutralizing and the acidic condensate goes to the drain raw. Two safety items cut across all types: the T&P relief valve gets exercised on a schedule so it does not seize, and any thermostatic mixing valve gets its delivery temperature verified so it has not drifted into a scald or a Legionella risk. Hand the owner the schedule for the type you installed, because the right heater still fails early if nobody maintains it.

Life expectancy by type

How long a water heater lasts depends more on water chemistry and maintenance than on the badge, but the types do carry different typical lives. A storage tank commonly runs about 8 to 12 years, and the anode is what decides where in that range it lands, so a tank with a maintained anode reaches the high end and a neglected one rusts through early. Hard or aggressive water shortens it. These are typical figures, not guarantees, and the manufacturer warranty is the honest floor.

A tankless unit commonly lasts longer, often 15 to 20 years, because there is no large tank to rust through, and many have replaceable parts that extend it further. That longer life is part of the life-cycle-cost case for tankless, since it spreads the higher first cost over more years. The condition is the descaling. A tankless that never gets flushed on hard water does not reach its potential life, because the scale that kills the heat exchanger does not care how long the unit was rated to last.

A heat pump water heater is still a tank, so its tank life sits in the same range as a storage tank, while the compressor and the refrigeration components are the parts that can fail and define the service it needs. An indirect tank can outlast the others where the water is kind, since it has no burner or element of its own to fail. Across all of them, the anode and the water chemistry move the number more than the type does. Track the install date and the maintenance, because the life you get is the life you maintain for.

The efficiency-standard push and electrification

Efficiency standards have been pushing the whole category toward the high-efficiency types for years, and that direction sets what you can even buy. The DOE energy conservation standards set minimum UEF levels for covered residential and light-commercial heaters, and successive updates have effectively pushed larger electric tanks toward heat pump technology and tightened what a gas unit has to achieve, which moves the market toward condensing on the gas side. The exact thresholds and effective dates shift with each rulemaking, so confirm the current standard for the unit class rather than relying on what was true last cycle.

Electrification is the other force, and it shows up in codes, utility rebates, and local rules that favor or require electric equipment in new construction. Where that pressure lands, the heat pump water heater is the electric answer that is not punishing to run, so it is the type the incentives steer toward. Utility rebates can swing the first-cost math enough to change the selection, so check what is available for the jurisdiction before pricing the job.

For the field, the practical effect is narrower than the policy debate. The high-efficiency types are getting cheaper, more common, and in some cases mandatory, while the cheapest atmospheric gas units are getting squeezed out of the catalog. Confirm what the adopted energy code and the local rules require for the building, because the type you would have defaulted to a few years ago may not be a legal install today.

Which water heater is right for the application?

The right type is the one that matches the building's demand curve, fuel, and space, not the most efficient unit in the catalog. The application sets the answer. A single-family house with gas and a busy morning peak is usually a gas storage tank or, where space and fuel allow, a heat pump for the operating-cost savings or a tankless to free the floor and never run out on a long draw. The house with no gas and an expensive electric rate is a heat pump, because resistance hot water at that rate is brutal.

Step up to a restaurant and the picture changes. A dish machine and a back-to-back dinner rush is a long, heavy, steady draw, which rewards high recovery, so a high-input gas tankless or a banked semi-instantaneous system fits where a slow-recovery heat pump would run out. A hotel slams at 7 a.m. when a floor showers at once, a sharp spike that rewards storage to ride it out. Same trade, opposite answers, set by the shape of the draw.

The edge cases are worth naming. A data center or an industrial facility runs its mission load on process cooling, and the domestic hot water is a small comfort load for the people, often a point-of-use unit at a break-room sink rather than a central system, but it still has to be sized and code-compliant. A break area for a handful of staff does not need a commercial plant. Match the type to the actual draw, and do not specify a hotel's hot water plant for a server room's coffee station.

Recirculation and thermal expansion by type

Two system details interact with the type you pick, and both bite if ignored. The first is recirculation. A recirculation loop keeps hot water at the far fixture, but it interacts badly with a plain tankless unit, because the loop's low, steady return flow may not be enough to trip the unit's minimum flow sensor, so the burner never fires and the loop goes cold. The fix is a tankless rated and equipped for recirculation, often with a built-in buffer tank and pump, or a small storage tank on the loop. Pair a recirc loop with the wrong tankless and you get a loop that does not stay hot. The loop sizing, the pump, and the controls are a full topic in the water heater recirculation sizing guide.

The second is thermal expansion. Any water heater on a closed system needs expansion control, and most systems are closed the moment a backflow preventer, a check valve, or a pressure-reducing valve sits on the cold supply. Water expands about 2 percent from cold to hot, and on a closed system that volume has nowhere to go but up in pressure, so an expansion tank takes it. This is true for every type, tank, tankless, or heat pump. Skip it and the pressure spikes every cycle and the T&P relief weeps until it fails.

The T&P relief valve is the last safety device on any heater, and you never compromise its discharge. The full treatment of expansion control, the cold-side PRV, and the T&P discharge details lives in the recirculation and sizing guides. The point for type selection is that none of the types escape these requirements. Pick the type, then build in the expansion control and the recirculation strategy that the type needs.

What to document

The day the hot water runs short or the owner questions the bill, someone will ask why this type went in over the others, and only a written reason answers it. Record what drove the type, not just the model number on the cut sheet. The next person needs to see why a tankless went in over a tank, or why the heat pump is in the garage and not the closet.

Capture the type and fuel, the efficiency rating the unit carries, the reason the type was chosen over the alternatives, and the utility requirements it depends on, the gas line and meter for a tankless, the circuit and room volume for a heat pump, the condensate path for a condensing unit. Tie it to the sizing record from the sizing guide so the type and the capacity are documented together. The table below is the field shorthand for the types, their typical efficiency, where the cost lands, and the job each one fits.

Common mistakes

• Choosing on first cost alone and ignoring life-cycle cost, so the cheap tank costs more to run for fifteen years.
• Dropping a high-input tankless on an undersized gas line or meter, so it never reaches its rated output.
• Putting a heat pump water heater in a closet too small to feed it air, so it runs on resistance backup and never pays back.
• Picking the wrong fuel for the load, like resistance electric on an expensive rate where a heat pump or gas would cost a fraction to run.
• No expansion tank on a closed system, so pressure spikes every cycle and the T&P relief weeps until it fails.
• No descaling plan for a tankless on hard water, so the heat exchanger scales and fails years early.
• Pairing a recirculation loop with a plain tankless that cannot sense the low loop flow, so the loop goes cold.
• Skipping the condensate neutralizer on a condensing unit, so acidic condensate drains raw into the plumbing.
• Matching the type to a demand curve it does not fit, like a slow-recovery heat pump on a relentless commercial peak.

Field checklist

Standards and references

The plumbing and mechanical codes set the framework, and the adopted edition with local amendments controls. The International Plumbing Code and the Uniform Plumbing Code address the safety pieces common to every type: thermal expansion control on a closed system, IPC Section 607.3 and the corresponding UPC provisions; the temperature and pressure relief valve and its discharge; and the delivery temperature limits at scald-sensitive fixtures. Gas units add the fuel-gas code for combustion air and venting, with the vent category and material set by the specific unit's listing. Confirm the section numbers against the edition the jurisdiction has adopted before citing them on a submittal.

Efficiency is the DOE's domain. The Uniform Energy Factor, UEF, is the DOE rating for covered residential and light-commercial heaters that replaced the old Energy Factor, and the DOE energy conservation standards set the minimum UEF by unit class. Larger commercial equipment is held to thermal efficiency and standby loss limits, and ASHRAE Standard 90.1, Section 7, Service Water Heating, sets minimum equipment performance, insulation, and controls on commercial work. ENERGY STAR criteria identify the high-efficiency tier, particularly for heat pump units. The exact thresholds shift with each rulemaking cycle, so confirm the current standard for the unit class.

The selection itself is governed by the design and the manufacturer. The stamped design, the manufacturer's ratings and installation instructions for the specific unit, and the equipment listing control the actual choice, the efficiency, the venting, and the utility requirements. The capacity sizing and the Legionella and recirculation temperature controls are covered in the companion water heater sizing and recirculation guides. Cite the standard that controls the point, and let the project specification and the equipment listing override any rule of thumb here.

Units, terms, and conversions

Water heater work mixes a few units and a few names for the same idea, so the same value reads differently across a schedule, a cut sheet, and a spec.

Efficiency is UEF, the Uniform Energy Factor, on residential and light-commercial labels, and thermal efficiency or standby loss on larger commercial units. A heat pump's instantaneous efficiency is its COP, the coefficient of performance. Heat input is in BTU per hour for gas and in kW for electric, converting at about 3,412 BTU per kW. Storage is in gallons, recovery in gallons per hour, GPH, and tankless output in gallons per minute, GPM, at a stated temperature rise. Temperatures are in Fahrenheit on most US drawings. A heat pump water heater is also called a hybrid; a tankless is also called on-demand or instantaneous.

UEF

Uniform Energy Factor, the DOE efficiency rating that replaced the older Energy Factor

COP

Coefficient of performance, heat delivered per unit of energy drawn, how a heat pump beats UEF 1

Storage tank

A heater that keeps a tank of water hot and ready, sized by volume plus recovery

Tankless / on-demand

A heater that makes hot water as it flows, sized by flow at the rise, with no stored mass

Heat pump (hybrid)

An electric tank that moves heat from the air into the water at two to four times resistance efficiency

Condensing

A gas heater that condenses the flue gas to capture latent heat, vented in plastic with acidic condensate

Indirect

A tank with a coil heated by the building's boiler, with no burner or element of its own

Anode rod

Sacrificial rod that corrodes in place of the steel tank and must be replaced

Cold-water sandwich

The cool slug between two hot draws on a tankless while the burner re-fires

Standby loss

Heat a stored tank sheds to the room around the clock, whether or not anyone draws water

Related tools

Calculators and readiness checks for this work

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