Collects sun into a storage tank and leaves a backup heater to finish the job.
also: solar thermal water heater, solar hot water system
The basics
What to know about water heaters
What it has to do
A water heater takes water arriving from the main at 40 to 70 F and hands it to your taps at about 120 F, or 49 C. Two ways of doing that split the market. A storage heater keeps 30 to 80 gallons hot around the clock in an insulated steel tank and reheats it as you draw it down. A tankless heater keeps nothing: it senses flow the moment a valve opens and heats the stream on its way past a heat exchanger. The rest of the names here describe which energy source does the work.
How a tank works
Inside a tank, heat goes in at the bottom and hot water leaves from the top, because hot water floats. Gas models burn fuel under the tank floor and carry exhaust up a flue through the middle. Electric models use two immersion elements, upper and lower, wired so only one draws power at a time. Recovery falls straight out of that: a 4,500 watt element adds about 20 gallons an hour at a 90 degree rise, and a 40,000 BTU burner adds about 40, BTU per hour being the rate at which a burner releases heat. Efficiency is printed as UEF, uniform energy factor, meaning heat delivered divided by energy bought. Electric resistance sits near 0.92, atmospheric gas near 0.60, condensing gas near 0.80, and a heat pump near 3.5, because it moves heat rather than making it.
Size by output, not gallons
Size by output, not by the number on the box. Storage heaters carry a first hour rating: gallons delivered in one busy hour starting from a full hot tank. This is volume plus roughly an hour of recovery. Build your own worst hour: count about 10 gallons of hot water per shower, 6 for a dishwasher cycle and 7 for a warm wash, then buy within a couple of gallons of that total. Tankless units are rated in gallons per minute at a stated temperature rise, and the rise catches people out. A 199,000 BTU condensing model gives close to 7.5 gallons per minute where the main arrives at 70 F, and under 5 where it arrives at 40 F in February. Two showers and a kitchen tap already want 5.5, so a unit that never falters in a warm state stalls in a cold one.
Match it to the building
Match the format to the building and the household; the guides below carry the detail. Gas Storage suits a home with a compliant gas line and a working flue, and a standing pilot version keeps making hot water through a power cut. Electric Storage is the cheapest box to buy and the most expensive to run. A Heat Pump Water Heater repays fastest in a warm garage or an unheated basement, wants about 750 cubic feet of air around it, and chills that space a few degrees while it runs, the same trade a Heat Pump Clothes Dryer makes with laundry. Gas Tankless earns its place in a house of four taking back to back showers. Electric Tankless and Point-of-Use models belong at one fixture far more often than at a whole house. Solar covers 50 to 80 percent of a year's hot water where the sun cooperates.
Running cost repeats forever
Every choice sends a bill somewhere. Running cost repeats forever. Heating water for four people takes roughly 4,000 to 4,800 kWh a year on resistance elements, 1,200 to 1,500 kWh on a heat pump, and 200 to 250 therms on gas. Redo that with your own rates, since cheap gas and costly power flip the answer. Installation pulls the other way. Swapping a tank for the same kind of tank is a half day, while moving to gas tankless drags in a larger gas line, its own sealed vent and a condensate drain, which is why the quote can triple. Gas connections, new circuits and venting are licensed work nearly everywhere, and permit rules differ by city, so confirm yours first.
Read past the brochure
Read past the brochure. UEF only compares units tested on the same draw pattern, labeled very small, low, medium or high, so a small heater with a flattering number is not beating a large one. Check the anode rod count and length: that sacrificial magnesium or aluminum rod corrodes so the steel tank does not, and a 12 year warranty usually buys a second rod rather than a better tank. Demand isolation valves with service ports on any tankless job, or descaling later starts with a saw. Set the thermostat to 120 F, since a serious burn takes about 5 seconds at 140 F and more than 5 minutes at 120 F. Confirm the temperature and pressure relief valve drains through a full size pipe reaching within 6 inches of the floor, with no shutoff and no threads on the end.
How long each type lasts
Plan on 8 to 12 years from a tank and 15 to 20 from a tankless unit, since water chemistry decides where you land. Sediment settles on a gas tank floor and insulates the steel from the flame. This is what the rumbling means, and on an electric tank it buries the lower element until it burns out. Scale narrows a tankless heat exchanger until flow drops and error codes start. The anode is your cheapest lever: pull it at year three, replace it once more than 6 inches of bare core wire shows, and a 30 dollar part adds years to a 1,500 dollar installation.
Buyer checklist
Before you buy water heaters
A major appliance must fit the space, the utilities and the service network. Running cost and repair access belong in the first comparison.
01
Measure the full installation
Include doors, handles, hoses, vents, clearance, service access and the delivery path, not only the cabinet opening.
02
Confirm every utility
Check voltage, circuit, plug, water, drain, gas and ventilation requirements against the exact model and local rules.
03
Compare ongoing use
Use the local energy or water label and include filters, detergent, bags and other consumables.
04
Check parts and service
Look for local technicians, realistic warranty coverage and the availability and price of common replacement parts.
Measure the route before the appliance. A 50 gallon tank stands about 60 inches tall and 22 inches across, weighs 140 pounds empty and close to 550 pounds full. An 80 gallon heat pump model reaches 76 inches. It travels upright, so check door height, stair headroom and the turn at the bottom of the stairs.
Leave the tank's own height in clear space above it, or the anode rod cannot be drawn straight out. Under a low ceiling, plan on a segmented flexible rod and 12 inches of clearance instead, and settle that at installation rather than discovering it at year three.
Total your peak hour rather than your household size. Four showers is 40 gallons of hot water, a dishwasher cycle 6, a warm laundry load 7, washing dishes by hand 4. A family of four running morning showers and a dishwasher needs a first hour rating near 46, which a 40 gallon gas or 50 gallon electric tank clears.
For tankless, add the fixtures that will run at once at 2 gallons per minute per shower head, 1.5 for a kitchen faucet and 4 for a tub filler. Subtract your coldest month's incoming water temperature from 120 to get the rise, then read the flow quoted at that rise. Below 50 F incoming, whole house gas units start around 180,000 BTU.
Check the electrical supply against the appliance. An electric tank with 4,500 watt elements needs a dedicated 240 volt, 30 amp circuit in 10 gauge wire. A 27 kW whole house electric tankless needs 112 amps across three 40 amp double pole breakers, which usually forces a 200 amp service. Any new circuit is licensed electrician work, and voltage and wiring rules vary by country.
Confirm gas and venting before ordering a tankless unit. A 199,000 BTU burner is roughly five times a tank burner. That means the half inch line feeding your old heater often has to grow to three quarter inch or larger depending on run length and meter capacity. The unit needs its own sealed vent rather than the furnace flue. Gas work is licensed in most places.
Care
Caring for water heaters
Drain 2 to 3 gallons from the bottom valve into a bucket every six months, until it runs clear. Sediment builds a layer on a gas tank floor that insulates the steel from the flame. This is the rumbling you hear, and on an electric tank it buries the lower element until that element fails.
Pull the anode rod at year three, then every two years, and fit a new one once more than 6 inches of bare steel core wire shows. That rod corrodes in place of the tank, costs about 30 dollars, and softened water consumes it roughly twice as fast.
Lift the lever on the temperature and pressure relief valve once a year and let it snap shut, standing clear of the discharge pipe because the water leaving it is 120 F or hotter. If it keeps dripping afterward, replace the valve. Never cap that outlet or add a shutoff to the line.
Circulate 4 gallons of white vinegar through a tankless heat exchanger for 45 minutes every year on hard water, or every two to three years on soft. Scale narrows the passages until flow drops and error codes start, and the job is only possible if isolation valves were fitted at installation.
Rinse the air filter on a heat pump model every three months and check that the condensate line runs free. A blocked filter starves the evaporator, the unit quietly falls back to its resistance elements. The only symptom you get is a bigger bill.
Look at the draft hood of an atmospheric gas heater once a heating season. Rust flakes or soot there mean exhaust is spilling into the room instead of rising up the flue. Keep a carbon monoxide alarm on the same floor and press its test button, because backdrafting gives no other warning.
Questions
Common questions
What size water heater does a family of four need?
A first hour rating near 50 gallons, which usually means a 40 gallon gas tank or a 50 gallon electric one. Build the number yourself: four showers at 10 gallons of hot water each, plus 6 for the dishwasher, is 46 gallons in the busy hour. Buy within a couple of gallons of your total rather than adding a safety margin, since a bigger tank costs more to keep hot.
Is a tankless heater actually cheaper to run?
Yes, by roughly 8 to 14 percent in a home using 85 gallons a day and 24 to 34 percent in one using 40 gallons or less, because the saving comes from ending standby loss rather than from better burners. Installed cost runs two to three times higher, so payback often lands between 10 and 20 years. A longer service life closes part of that gap.
What temperature should I set it to?
120 F, or 49 C, on the thermostat. A serious burn takes more than 5 minutes at that temperature, about 30 seconds at 130 F and about 5 seconds at 140 F. If anyone in the house has a weakened immune system, store at 140 F to suppress Legionella bacteria and fit a thermostatic mixing valve at the tank so taps still deliver 120 F.
Why does the shower run cold after ten minutes?
A tank gives up only about 70 percent of its volume before the outlet cools. So 50 gallons is really 35 gallons of usable hot water, or a 17 minute shower at 2 gallons per minute once mixed. If it used to last longer, suspect a broken dip tube dropping cold water at the top of the tank, or a failed lower heating element.
Can a heat pump water heater go in a closet?
Only with help. Most need 700 to 1,000 cubic feet of surrounding air, about a 10 by 10 room with an 8 foot ceiling. A louvered door of the size the maker states, or a ducting kit that draws air from one space and exhausts to another, solves a tight closet. Sealed in without either, it starves and falls back to its backup elements.
How do I know it is time to replace rather than repair?
Water on the floor coming from the tank body ends it, since a rusted through tank cannot be repaired. That is exactly why a drain pan piped to a legal drain matters on any floor above a finished room. Elements, thermostats, gas valves and relief valves are all worth fixing. Past 12 years, put the repair money toward a replacement instead.
Do I need an expansion tank?
You do if a pressure reducing valve or a backflow preventer sits on the incoming line, which makes your plumbing a closed system. Heating 50 gallons expands it by roughly a gallon with nowhere to go, pressure climbs past 150 psi and the relief valve starts weeping. Many codes now require the expansion tank, so ask what applies where you live.
Why does my tankless heater go cold when I turn the tap down?
Most need 0.4 to 0.75 gallons per minute of flow before the burner lights. A modern 0.5 gallon per minute bathroom faucet sits right on that line. Opening the tap wider fixes it. A separate effect, the cold water sandwich, pushes a cool slug through when you shut a tap and reopen it inside a minute.
