Water heater sizing and selection field guide for plumbers

What sizing a water heater actually solves

Sizing a water heater is the decision that sets the storage volume, the heat input, and the recovery rate so the building has enough hot water at the hour it draws the most, and not much more. The target is the middle of a window. Too small and the far shower goes cold during the morning rush. Too big and you pay for the tank, the floor space, and the standby heat loss every day for the life of the unit, to serve a peak that lasts twenty minutes.

Two numbers do the work together: recovery and storage. Recovery is the gallons per hour the heater can make, set by the heat input and how cold the incoming water is. Storage is the gallons sitting hot and ready before recovery has to keep up. A unit can carry a small tank with fast recovery, or a big tank with slow recovery, and both can serve the same peak. The combination is what you size, never one alone.

This guide is the heater itself: the demand, the recovery, the storage, the type, and the fuel. The recirculation loop that carries that hot water to the far fixtures and keeps it warm is a separate sizing problem, covered in the water heater recirculation sizing guide. The cold-water distribution that feeds the heater is sized by fixture units in the water supply pipe sizing guide. Cross-link them on a real job. The loop and the distribution both change the load the heater sees.

How do you size a water heater?

You size a water heater to the peak-hour hot water demand, which is the most hot water the building will draw in its busiest single hour, measured in gallons per hour at the use temperature. That number, not the fixture count and not the floor area, is what the heater has to cover with recovery plus usable storage. Everything else in the selection serves it.

The peak is almost never the sum of every fixture running at once. Diversity is the reason. Not every shower, sink, and dishwasher draws at the same moment, so the design applies a usage factor by occupancy. An office runs a flat, light demand with a small midday bump. A restaurant spikes hard at the dinner rush behind the dish machine. A hotel slams at 7 a.m. when a floor of guests showers inside the same twenty minutes. A gym, a dorm, and a multifamily block each draw on their own clock. Same fixture count, very different heaters.

The application sets the shape of the curve, and the shape sets whether you lean on storage or on recovery. A short, sharp spike rewards storage that rides it out. A long, steady pull rewards recovery that never falls behind. Get the occupancy and its peak hour right first. A recovery and storage pair sized to the wrong demand curve is wrong no matter how good the equipment is.

What is recovery rate?

Recovery rate is the gallons per hour a heater can raise from the cold inlet temperature to the setpoint, and it is the number that decides whether the unit keeps up once the stored hot water is spent. It is a function of three things: the heat input, the efficiency, and the temperature rise the heater has to make. Change the rise and the recovery moves with it.

The arithmetic is fixed by physics. It takes about 8.33 BTU to raise one gallon of water 1 degree F, because a gallon weighs 8.33 pounds and water's specific heat is 1.0. So recovery in gallons per hour is the input in BTU per hour times the efficiency, divided by 8.33 and by the temperature rise. Run the numbers on a 40,000 BTU per hour gas heater at 80 percent efficiency lifting water from a 55 degree F inlet to a 120 degree F setpoint, a 65 degree rise: 40,000 times 0.80 is 32,000, divided by 8.33 times 65 is about 59 gallons per hour.

Here is the trap that sinks undersized jobs. Recovery falls as the rise grows, and the rise grows in winter when the building uses the most hot water. The same burner that recovers fine on a 50 degree summer rise can drop 30 to 40 percent in a northern climate when the inlet hits 40 degrees F and the rise stretches past 80 degrees. Size on the coldest-month inlet, not the average and never the summer one. The heater has to keep up on the day it is asked to work the hardest.

BTU_in

Heat input in BTU per hour, from the burner rating or the electric element wattage converted to BTU/hr

Eff

Thermal efficiency or recovery efficiency as a decimal, from the manufacturer's rating

8.33

BTU to raise one gallon of water 1 degree F, since a gallon weighs 8.33 lb at a specific heat of 1.0

ΔT

Temperature rise, the setpoint minus the cold inlet, in degrees F, taken on the coldest-month inlet

Storage versus recovery, and the usable fraction

Storage and recovery trade against each other, and the design picks where on that line to sit. A tank full of hot water buffers the peak, so a building with deep storage can ride out a sharp spike on stored gallons and refill in the quiet hours behind it, which means it needs less recovery. Cut the storage and you have to make the gallons in real time, which means more recovery. There is an infinity of combinations that satisfy a given load, and the engineering judgment is choosing the one that fits the demand curve, the fuel service, and the mechanical room.

Lean on storage when the demand is spiky and short, like a dorm or a gym where a crowd hits the showers and then it is quiet. Lean on recovery when the demand is long and steady, like a laundry or a back-to-back peak that never lets the tank refill. The reference data make the swap concrete: for a given occupancy, doubling the recovery rate per person can cut the required storage by more than half. They are dials on the same dashboard.

The catch that gets missed: you do not get the whole tank. As you draw hot off the top, cold inlet enters the bottom and mixes upward, so the delivery temperature sags before the tank is empty. Usable storage is commonly figured at about 70 to 80 percent of the tank volume before the outlet falls too far. Size on the usable fraction, not the nameplate gallons. A 100 gallon tank gives you roughly 70 to 80 gallons of useful hot water, and the rest is recovery's job.

What is first-hour rating?

First-hour rating, FHR, is the gallons of hot water a storage heater can deliver in one hour starting from a full tank, and it is the number stamped on the residential EnergyGuide label that the DOE requires. It folds storage and recovery into a single figure: the usable hot water already in the tank, plus the gallons the burner or element can recover during that hour. It is the residential answer to the same question commercial sizing asks with recovery and storage separately.

The formula is the storage times the usable fraction, plus the recovery in gallons per hour. A 50 gallon heater with a 40 GPH recovery figures at 50 times 0.70 plus 40, about 75 gallons in the first hour. That is why two heaters with the same tank size can carry very different first-hour ratings: the one with the bigger burner recovers more during the hour and delivers more, even though the tanks match.

Match the FHR to the peak-hour demand, not the tank size to the household. A family that draws 60 gallons in the busy morning hour needs an FHR near 60 with a little margin, and a 40 gallon tank with strong recovery can beat a 50 gallon tank with a weak burner. On residential work the FHR is the honest comparison. Reading tank gallons alone is how people buy a heater that runs cold.

Tank, tankless, or semi-instantaneous?

Three approaches cover most hot water, and the choice is the storage-versus-recovery trade made physical. A storage heater carries a hot reserve and modest recovery, sized by storage volume plus recovery in gallons per hour. It is forgiving on a spiky peak because the tank absorbs the spike, and it carries a standby loss because a big mass of hot water bleeds heat to the room all day.

A tankless or instantaneous heater holds almost no water and makes hot on demand, so it is not sized by gallons at all. It is sized by flow rate in gallons per minute at the required temperature rise. The same physics that drives recovery drives the flow it can deliver: gallons per minute equals the input times efficiency, divided by about 500 and the rise, where 500 is the 8.33 BTU per gallon times 60 minutes. The hard part is the rise. A unit rated for 5 GPM at a 35 degree southern rise may give only 3.5 GPM at a 50 degree northern rise, because the cold inlet eats its capacity. Undersize the input and the far shower goes lukewarm the moment two fixtures run together on a cold morning. Tankless has no standby loss, but it has a flow ceiling, and you size to the worst simultaneous draw at the coldest inlet.

A semi-instantaneous heater sits between them: a small built-in volume with fast, tightly controlled recovery, often paired with a stratified storage or accumulator tank to cover the peak that pure on-demand output cannot. On commercial work the bank is common: several units staged together so capacity scales and one unit out of service does not take the building down. The table is the shorthand. The real selection runs off the demand curve, the available gas or electric service, and the room, not off a preference for one type.

What temperature should a water heater be set to?

Store at 140 degrees F and deliver tempered at or below 120 degrees F. Those two numbers do not contradict each other; they are the whole reason a mixing valve exists. Water has to be stored and circulated hot enough to suppress Legionella, and it cannot reach the fixture that hot or it scalds. The setpoint serves safety on both ends at once.

The Legionella side first. The bacterium grows in the warm band, roughly 77 to 108 degrees F, and only starts dying above about 131 degrees F. Because a tank stratifies and the bottom runs cooler than the setting, 140 degrees F is the common minimum storage temperature so the whole tank stays out of the growth band. Store at 120 to chase comfort or efficiency and you have built an incubator, especially in the cool lower volume and the dead legs.

The scald side is blunt. At 140 degrees F a serious burn happens in about five seconds, faster for children and older adults with thinner skin. At 120 degrees it takes far longer, which is why the delivery limit sits there. A thermostatic mixing valve bridges the two: a master valve listed to ASSE 1017 tempers the bulk hot water leaving storage, and a point-of-use valve listed to ASSE 1070 limits the temperature at the scald-sensitive fixture, set never to exceed 120 degrees F. Storing hot also keeps the tank turning over out of the stagnant band. The deep treatment of the loop temperatures, the master and point-of-use valves, and the Legionella water management program lives in the water heater recirculation sizing guide. Set the storage and deliver tempered. Confirm the delivery limit for the occupancy against the adopted code, since some uses call for tighter.

Fuel and heater type

The fuel is half the selection, because it sets the recovery you can buy, the venting you have to build, and the efficiency the energy code will hold you to. Gas, electric, and heat pump each behave differently, and the building's available service often makes the call before the demand curve does.

Gas heaters split by how they handle the flue gas. An atmospheric heater draws combustion air from the room and vents on natural draft, simple and cheap but the least efficient and the most sensitive to a tight mechanical room. A power-vent or power-direct-vent unit uses a fan to push the flue gas out, which frees the venting routing and raises efficiency. A condensing heater pulls so much heat from the flue gas that the water vapor condenses, reaching the highest efficiency and producing an acidic condensate that has to be drained and usually neutralized. Gas gives high recovery for the money, which is why it dominates commercial storage and instantaneous work.

Electric covers resistance and heat pump, two very different animals. A resistance heater is cheap, compact, vent-free, and nearly all of its input ends up in the water, but its recovery is modest because the elements are limited by the circuit you can feed them. A heat pump water heater moves heat instead of making it and runs two to three times more efficient, but it pulls that heat from the surrounding air and works best in a warm, open space. An indirect tank is the fourth path: no burner or element of its own, just a coil fed by the building's boiler, which is efficient where a boiler already runs. The efficiency metric you will be measured against is the Uniform Energy Factor, UEF, the DOE rating that replaced the old Energy Factor.

Gas input, combustion air, and venting

A gas heater is only as good as the air it gets and the flue it vents into, and both are sized to the input, not guessed. Starve the burner for combustion air in a tight mechanical room and it burns dirty, soots the heat exchanger, loses the recovery you sized for, and can spill carbon monoxide into the space. The combustion air openings, whether to outdoors or borrowed from an adjacent volume, are figured from the total input of the appliances in the room under the fuel-gas code.

The venting has to match the heater's vent category and the input. An atmospheric, natural-draft heater vents into a Category I system on the buoyancy of hot flue gas. A condensing heater is typically Category IV, sealed and positive-pressure, vented in a listed plastic because the condensing flue gas is wet and acidic and would destroy metal vent. The two are not interchangeable, and mixing them up is a dangerous mistake. Vent material, diameter, length, and termination are all listing-specific.

The gas piping that feeds the heater is its own sizing problem, set by the input in BTU per hour, the length of the run, and the allowable pressure drop, and it is sized by the fuel-gas code, not by matching the connector at the heater. Cross-link the gas pipe sizing by topic on a real job. A heater fed by an undersized gas line cannot make its rated input, so the recovery you calculated never shows up, and the symptom looks like an undersized heater when the real problem is the pipe. Confirm the combustion air, the vent category, and the gas supply against the fuel-gas code and the manufacturer's instructions for the specific unit.

Electric input and the demand it puts on the panel

An electric resistance heater trades the flue for the panel, and the kilowatts it draws become a real load on the electrical service. Recovery on an electric tank is limited by how much element wattage the circuit can feed: convert the kW to BTU per hour at about 3,412 BTU per kW, run it through the recovery formula, and you see why electric recovery is modest unless you pull a large circuit. A heavy commercial electric heater can carry several elements and a serious feeder.

That demand has to land in the building's electrical load calculation, not show up as a surprise after the panel schedule is done. A bank of large electric heaters or a commercial electric instantaneous unit can swing the service size, and the conductor, the overcurrent device, and the feeder all have to suit the nameplate. Cross-link the electrical load calculation by topic so the hot water load is in the total before the service is sized, not bolted on after.

The instantaneous electric path is real but power-hungry. A whole-building electric tankless unit pulls a very large current to make the rise on demand, which is why the gas equivalent is more common on big commercial loads where the gas service exists. Where electric is the only fuel, the recovery and the flow you can buy are capped by the service you can build to the heater. Size the electrical to the listing, and put the load on the panel schedule.

The heat pump water heater

A heat pump water heater moves heat out of the surrounding air into the tank instead of making heat with an element, which is why it runs two to three times more efficient than resistance, often with a coefficient of performance near 3. The efficiency is real and the operating cost is low, and the energy codes increasingly push toward it. The catch is that the efficiency comes from the air it pulls heat from, and that changes where it can live.

It needs air volume. A heat pump water heater in a small closet runs out of air to harvest, drops onto its backup resistance elements, and loses the efficiency that justified it. Manufacturers call out a minimum room volume or ducting to a larger space for a reason. It also cools and dehumidifies the room it sits in, which is a feature in a hot mechanical room and a problem in a conditioned space or a finished basement where it makes the room cold and clammy.

Cold climate is the other limit. Recovery falls as the source air gets cold, so a heat pump water heater in an unconditioned space in a cold region leans harder on its resistance backup through the winter, exactly when the inlet water is coldest and the demand is highest. It still works, but you size it knowing the heat pump capacity is not the whole rating in January. Most units carry a hybrid mode that brings the elements in to keep up. Confirm the room volume, the drainage for the condensate, and the cold-weather recovery before you commit a heat pump unit to a critical load.

Commercial sizing methods

Commercial sizing starts from the occupancy, not the fixture list, because the recognized methods are built around how a building type actually uses hot water. The ASHRAE Handbook, HVAC Applications, gives demand data and a sizing approach by occupancy, and it defines a storage heater by exactly the two numbers this guide hangs on: the storage volume in gallons and the recovery rate in gallons per hour at a stated rise. You pick the storage-and-recovery pair that covers the occupancy's peak.

The fixture-unit path is the other recognized method. A hot water fixture unit value is combined with a Hunter-style demand curve to estimate the probable peak flow with diversity baked in, then a one-hour or two-hour peak duration is applied to size the equipment. It is the same logic as the cold-water fixture-unit method in the water supply pipe sizing guide, turned to the hot side and the heater. The two methods do not always agree, and both have a documented habit of overshooting, which is how oversized, wasteful heaters get specified.

Whatever method you start from, the governing source is the stamped design and the manufacturer's sizing tables, which carry the equipment's real recovery and storage for the project's rise. Treat the rules of thumb here as the framework for understanding the numbers. The engineer's calculation and the equipment listing control the actual selection on any submittal, and the local code and the AHJ control what is acceptable.

How the recirculation loop adds load

A recirculation loop is not free hot water. It keeps the distribution hot so the far fixture gets hot water fast, and it does that by circulating water that constantly sheds heat into the building, which the heater has to make back. That standby heat loss is a load on the heater every hour of every day, separate from any fixture draw, and it has to be added to the heater's duty.

On a building with a long, poorly insulated loop, the recirculation load is not trivial, and a heater sized for the fixture demand alone can run behind because the loop is quietly eating its recovery. The fix starts upstream: insulate the hot and recirc piping, which cuts the loop heat loss by roughly 70 to 80 percent and shrinks the load the heater has to carry. Then add the residual loop loss to the peak-hour demand when you size the recovery.

Sizing the loop itself, the flow, the pump head, and the balancing across risers, is a full topic in the water heater recirculation sizing guide. The point for heater selection is narrower: the loop adds a continuous load, count it, and insulate the piping so it stays small. Skip that and the heater that looked right on the fixture math runs short on a cold morning.

Thermal expansion and the T&P relief valve

Every water heater on a closed system needs thermal expansion control, and most commercial systems are closed. Water expands about 2 percent by volume from cold to hot, and on a system closed by a backflow preventer, a pressure-reducing valve, or a check valve, that extra volume has nowhere to go but up in pressure. An expansion tank, a small sealed tank with an air cushion behind a diaphragm, takes the volume as the water heats and gives it back as it cools. Precharge it to the static system pressure before it goes in, or it does almost nothing.

The temperature and pressure relief valve, the T&P, is the last safety device on the heater, and it is the one you never compromise. It opens if the water exceeds a temperature limit, commonly 210 degrees F, or a pressure limit, commonly 150 psi, dumping water to keep the tank from becoming a pressure-vessel failure. The discharge piping is where inspectors catch the most violations: it runs full size from the valve, never reduced, downward, terminating to an air gap near the floor or over a waste receptor, with no valve, plug, or cap that could disarm it.

Skip the expansion tank on a closed system and the pressure spikes on every heating cycle, so the T&P becomes the only relief and weeps, and a relief valve that weeps repeatedly fouls its seat and eventually fails to reseal. Plug or valve the discharge and you have disarmed the last protection on the heater. The full treatment of the expansion tank, the cold-side PRV, and the T&P discharge details is in the water heater recirculation sizing guide. The requirement lives in the plumbing code, IPC Section 607.3 and the corresponding UPC provisions; confirm the section against the adopted edition.

Codes, efficiency, and the altitude de-rate

The energy code controls the minimum efficiency you are allowed to install, and the metric is the Uniform Energy Factor, UEF, the DOE rating for residential and light-commercial units that replaced the old Energy Factor. Larger commercial equipment is held to thermal efficiency and standby loss limits instead. On commercial work the reference is ASHRAE Standard 90.1, Section 7, Service Water Heating, which sets the minimum equipment performance, pipe insulation, and control requirements the design has to meet. The adopted energy code and local amendments control which edition applies.

Efficiency is not a free upgrade in the sizing math. A condensing or heat pump unit costs more up front and earns it back on the fuel, but it only pays if the standby losses around it are controlled too, which means insulating the tank and the piping and not oversizing the storage. An oversized high-efficiency heater can lose at the meter what its rating promised, because a bigger tank than the load needs bleeds more standby heat every hour.

Altitude is the de-rate people forget on gas. A gas burner makes less heat in thin air, so above a threshold elevation, commonly around 2,000 ft, the input has to be de-rated, often on the order of a few percent per 1,000 ft, per the manufacturer's instructions and the fuel-gas code. The recovery you calculated at sea level is not the recovery you get at altitude. Confirm the de-rate factor and the high-altitude listing for the specific unit before you trust the rating in the mountains.

Redundancy and critical-facility hot water

On a large or critical load, one heater is a single point of failure, and the design answer is more than one unit. A bank of two or more heaters staged together does two things: it scales capacity to a big peak, and it keeps hot water flowing when one unit is down for service or has failed. Size the bank so the building still meets its core demand with one unit out, which is the N-plus-1 logic the rest of the building's critical systems already run on.

Healthcare, lab, and food-service facilities are where redundancy stops being optional, because losing hot water shuts down sterilization, sanitation, and patient care, not just comfort. Those facilities also usually carry a documented water management program for Legionella, so the redundancy and the temperature control have to hold even with a unit offline. The standby unit has to keep its own storage hot and stay out of the growth band while it waits, not sit cold until it is needed.

Data centers and industrial sites are a different hot water profile worth naming. The mission load is process cooling, and domestic hot water is a small comfort load for the people, but the building still needs it sized and code-compliant, and the same redundancy thinking applies where the facility runs continuously. Treat the hot water as a real system on those jobs even when it is the smallest mechanical load in the building, and size the critical pieces against the project's availability requirement, not the average occupancy.

Install and commissioning

Commissioning is where the sizing on paper becomes a heater that performs, and most callbacks on a new water heater are commissioning nobody finished, not equipment that failed. Set the storage temperature and verify it at the tank, not at the thermostat dial, which can read off by several degrees. Set the master mixing valve and confirm the tempered delivery at the fixtures with a thermometer, because a valve set on the bench is not a valve verified at the tap.

Confirm the safety pieces before the unit is buried behind finish. Check the T&P discharge is full size, downward, to an air gap, with nothing that can cap it. Set the expansion tank precharge to the measured static system pressure with the system depressurized. On a gas unit, verify the draft or the power-vent operation and that the combustion air openings are clear. On a heat pump unit, confirm the room has the air volume and the condensate drains.

Then prove the recovery, not just the setpoint. Draw the tank down under a known load and watch how fast it recovers against the rate you sized, on the actual inlet temperature of the day. If the heater falls behind, you find it now, during commissioning, when the fix is a setting or a missed gas-supply problem, not from a tenant complaint in January. Write down every reading. The commissioning record is what the next person checks against when something drifts.

What the owner has to maintain

A water heater is sold once and maintained for fifteen years, and the maintenance is what decides whether it reaches that age. The anode rod is first. It is a sacrificial magnesium or aluminum rod that corrodes in place of the steel tank, and once it is spent the tank itself starts to go. On hard or aggressive water the anode can be eaten in a few years, so it gets checked and replaced on a schedule. Pull the anode at the right interval and the tank outlives it. Ignore it and the tank rusts through from the inside and floods a mechanical room.

Sediment is the second job. Minerals drop out of the water and settle in the bottom of the tank, where they insulate the burner from the water on a gas unit, bury the lower element on an electric one, and cut the recovery the building was sized for. A periodic flush clears it. A heater that seems to have lost capacity is often a tank full of sediment, not an undersized unit, and the flush brings the recovery back.

Two safety items round out the schedule the owner inherits. The thermostatic mixing valve needs its delivery temperature verified periodically, because a valve that drifts high scalds and a valve that drifts low lets the loop fall into the Legionella band. And the T&P relief valve gets tested on a schedule by lifting the lever to confirm it opens and reseats, because a relief valve that has never been exercised can seize shut. Hand the owner the schedule with the equipment, because the heater you sized right still fails early if nobody maintains it.

What to document

Lose the inputs behind a heater selection and you lose any way to show the unit was sized to the demand rather than outgrown by it. When the building runs short of hot water two winters from now, the record is what answers whether the unit was ever sized right or whether the load grew past it. Capture the design inputs and the as-set values, not just the equipment cut sheet.

Record the unit and type, the storage volume, the heat input in BTU per hour or kW, the recovery in gallons per hour, the temperature rise it was sized on with the cold-inlet assumption, the peak-hour demand it was matched to, and the mixing valve setpoint with the verified delivery temperature. Note the fuel, the venting category, and the expansion control. The next person reading the record needs to see the demand the heater was sized to meet and the inlet temperature that drove the recovery, because those are the two assumptions that quietly go wrong.

Common mistakes

• Sizing on tank size instead of the peak-hour demand and the recovery, so the gallons look right and the unit runs cold.
• Calculating recovery on the summer inlet, so the heater falls behind on the cold-inlet winter peak.
• Counting the full tank as usable instead of the 70 to 80 percent that delivers before the outlet sags.
• No thermostatic mixing valve, so the fixture delivers stored water hot enough to scald in seconds.
• Storing below 140 degrees F to chase comfort or efficiency, leaving the tank in the Legionella growth band.
• No expansion tank on a closed system, so pressure spikes every cycle and the T&P weeps until it fails.
• Tankless undersized for the worst simultaneous flow at the cold-inlet rise, so it goes lukewarm at peak.
• Oversizing a high-efficiency unit, so standby loss eats the efficiency the rating promised.
• Ignoring the recirculation loop load and the gas-line or panel limit, so the rated recovery never shows up.

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 heater's safety and temperature pieces: 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; the delivery temperature limits at scald-sensitive fixtures; and the combustion air and venting through the fuel-gas code for gas units. Confirm the section numbers against the edition the jurisdiction has adopted before citing them on a submittal.

Sizing and efficiency draw on ASHRAE. The ASHRAE Handbook, HVAC Applications, gives the service-water-heating demand data and the storage-plus-recovery sizing approach by occupancy, and ASHRAE Standard 90.1, Section 7, Service Water Heating, sets the minimum equipment efficiency, insulation, and controls on commercial work. The DOE energy conservation standards set the Uniform Energy Factor minimums for covered residential and light-commercial heaters. Temperature control devices carry their own listings: a master mixing valve to ASSE 1017, a point-of-use temperature-limiting valve to ASSE 1070. For Legionella, ANSI/ASHRAE Standard 188 sets the water-management framework and ASHRAE Guideline 12 the supporting practice.

The sizing itself is governed by the design engineer and the manufacturer. The stamped design, the manufacturer's recovery and storage tables for the project's rise, and the equipment listing control the actual selection, and they do not always agree with the rules of thumb here. Cite the standard that controls the point, and let the project specification and the equipment listing override any rule of thumb in this guide.

Units, terms, and conversions

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

Recovery and peak demand are in gallons per hour, GPH, while tankless output is in gallons per minute, GPM. Heat input is in BTU per hour for gas and in kW for electric, and they convert at about 3,412 BTU per kW. One gallon of water takes about 8.33 BTU per degree F, which is the 500 constant once you account for 60 minutes per hour. Temperatures are in Fahrenheit on most US drawings and in Celsius on Legionella references, so storage at 140 degrees F is 60 degrees C and a 120 degree F delivery is about 49 degrees C. Efficiency shows up as UEF on the label and as thermal efficiency on larger commercial units.

Recovery rate (GPH)

Gallons per hour the heater raises across the cold-inlet temperature rise

First-hour rating (FHR)

Gallons a storage heater delivers in one hour from full: usable storage plus recovery

Usable storage

The fraction of tank volume delivered before the outlet sags, commonly 70 to 80 percent

Temperature rise (ΔT)

Setpoint minus cold inlet, in degrees F, taken on the coldest-month inlet

UEF

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

Semi-instantaneous

A small built-in volume with fast recovery, often paired with a buffer tank

Diversity

The usage factor recognizing not all fixtures draw hot water at once

Anode rod

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

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