ankless water heaters are quite popular as of late, yet many people don’t really know what the differences are between a standard storage type water heater and a tankless water heater.
Storage or Tank Type Water Heaters
Let’s begin with storage type water heaters. Basically you have a big pot of water (water heater) over a hot fire (burner or electric heating element). A thermostat turns the flames on when the water gets cold and off when it reaches the desired temperature. When you want hot water for something there is a big pot of it just waiting for you to turn on a hot water faucet, and the hot water is on its way.
When the flames are turned off, the tank will begin to slowly cool, even if it is well insulated, just more slowly. The water in a storage water heater is always changing temperature, it’s either being heated by the flames (or by an electric heating element), or it’s cooling off. The outlet water temperature can vary widely. A water heater with the thermostat set to turn on when the temperature of the water drops to below 120 can reach temperatures of over 160 degrees due to stacking.
As the user of the hot water you probably don’t notice the difference in actual water temperature. You are usually using a mixed flow of cold and hot and just adjust the water flows for the temperature you desire.
As you use the hot water cold water is flowing into the heater and after a while the temperature begins to drop until it’s no longer considered “hot” water.
Tankless Water Heaters
Tankless hot water heaters are quite popular these days and with the energy tax credits you can save money over the life of the heater. Tankless heaters don’t have a big tank full of water cooling off all the time, so they don’t have standby losses.
They won’t run out of hot water either. If you need a continuous supply of hot water and a storage heater won’t handle it, then a tankless heater may be the preferred choice. The alternative I suppose would be to use multiple storage heaters.
Heating water on the fly is much more difficult than it first looks. Rather than a big pot of water you now have to picture a coil of copper tubing (heat exchanger) in the flames (burner or heating element) with water flowing through it.
It sounds simple, but consider what happens if you put the copper tubing in the flames without running water through it. It melts. Ok, so we run water through the pipe. If we run the water too slowly it turns to steam in the pipe and possibly blows the pipe up. If we run the water through the pipe too quickly then the water doesn’t get very hot.
Changing the size of the flames has the same affect as changing the water flow, the lower the flame the longer it takes to heat the water.
Tankless water heaters therefore have valves to adjust the size of the flame (or the amount of electricity being directed to the heating elements) and/or the amount of water flowing through the heat exchanger.
They also have a flow switch that turns the heater on only if a minimum flow rate is achieved… typically 1/2″ or 3/4 gallons per minute. That way the heat exchanger won’t melt from overheating. If you try to use too high of a flow rate then your water won’t reach full temperature.
Tankless water heaters have temperature monitors at the inlet and outlet, and for gas models, in the exhaust stream. Tankless gas water heaters also monitor gas flow and/or pressure or both.
As you can imagine, tankless heaters are far more complex than a standard storage type water heater with temperature sensors, flow sensors, multi-speed fans, remote controls, and even microprocessors built into them.
Because there is a minimum and maximum flow rate one has to choose the set point outlet temperature carefully. If the temperature is set too high, then you will have to mix a lot of cold water with it to take a shower. But if you have to use a large percentage of cold water to reduce the temperature then the hot water flow can drop below the minimum flow rate for the heater and the heater shuts off. You find out when the cold water finally reaches you in the shower.
If you set the temperature too low then you will possibly exceed the capabilities of the heater to deliver full temperature water. With these heaters you cannot get a small stream of warm water since at the minimum you will need to run at least 1/2″ gallon per minute to keep the heater on, but that will be full temperature. To reduce the temperature you will have to increase the flow of cold water. Not a good way to practice water conservation.
Water Heater Installation and Maintenance
Proper installation of a gas tankless water heater is extremely important. Since tankless units need much bigger flames than storage heaters they require a lot more ventilation and bigger exhaust pipes. Storage heaters are typically supplied by 1/2″” gas pipes, but tankless units require at least 3/4 and often 1″ pipes.
If the exhaust condensation drain isn’t installed properly the corrosive liquid can destroy the heat exchanger after a couple of years.
Whole house electric tankless hot water heaters require very heavy wiring and usually their own circuit breakers. Make sure your electric service is heavy enough to handle the load. Often a homeowner will need to upgrade the service to handle the amperage.
Interesting article about the perils and pitfalls of tankless water heater installation: Tankless Water Heater Installation
With a storage tank type heater when minerals precipitate out of the hot water they fall to the bottom of the tank and are referred to as sediment.
In tankless heaters they minerals form scale on the inner surface of that copper pipe, restricting the flow of water. Too much scale will cause the heat exchanger to overheat and become damaged. Most tankless heaters need to have the scale cleaned out every year. Some warranties will be void if your water is too hard. Some require you to use a water softener if your water is too hard. You should read the owner’s manual before you buy anything.
Repairing a storage water heater is pretty easy and any plumber can easily handle a repair or replacement. Not so with a tankless heater. They are complex with valves, sensors, and microprocessor chips. They have displays that show error codes, and many even have remote controls for changing the temperature.
When a storage water heater breaks down it’s quite often a leaking tank. You notice it because there is a small leak and arrange for a replacement with minimal inconvenience. With a tankless unit when it quits you simply won’t be able to get hot water. There won’t be any warning.
The installation of a tankless water heater should be handled by a trained professional with the brand of heater being installed. That is probably the most important aspect of a tankless heater… correct installation.
I’ll leave you with a few things to think about…
If there is a natural disaster and the water gets shut off your storage water heater tank is a source of clean drinkable water…
In a power outage your gas storage water heater will still work and an electric will still have hot water for a while… most gas tankless heaters won’t work if the power goes out.
Most tankless water heaters will not work with hot water circulating systems, however they will work with hot water demand systems.