Comparing Tankless Hot Water Heaters to Storage Water Heaters
Tank type Water Heaters
Tank type Water Heaters
Tank type water heaters are simple to install, operate, and repair. They are reliable and often the first thing to fail is the tank which begins to leak. In my experience most storage water heaters never need flushing or any other maintenance. Some manufacturers recommend flushing periodically, but I know very few people who have ever performed any maintenance on their water heater. I know people who aren't even sure where it is.
Tank type, or storage, water heaters simply turn on when the temperature drops below a set point and turn off when the upper set point is reached. It's the same for gas water heaters as electric. Gas water heaters have a thermostat near the bottom of the tank built into the gas valve, and electric water heaters have two thermostats, a lower and an upper thermostat, which are easy to replace.
With tank type water heaters you set the temperature, or more likely just leave it at the factory setting, and you forget about it.
Whatever the temperature is of the incoming cold water, the water heater still ends up at the cut-off temperature of the water heater. Colder water just takes longer to heat up.
Tankless Water Heaters
Tankless water heaters are sophisticated complex pieces of equipment bearing little resemblance to a traditional storage type water heater.
There are several problems that must be overcome when designing a water heater. In order to heat water rapidly a very large amount of heat must be transferred to the water quickly. A heat exchanger is used for that.
One problem is that any fluctuation in the flow rate will cause a corresponding change in outlet temperature unless the heat source is modulated. Without having a pre-set regulated outlet temperature you would never know how hot the water coming of the outlet would be, it would depend on the water flow rate.
The same holds true for inlet temperature. The outlet temperature would depend on the inlet temperature, so if the inlet temperature dropped 5 degrees so would the outlet temperature. In most areas it would be substantially different in summer than winter.
Another problem is protecting the heat exchanger. If the flow of water is too small, it won't cool the heat exchanger enough and the heat exchanger could become damaged. The heater could become a steam generator.
In order to obtain good temperature regulation and protect the heat exchanger tankless water heaters use sensors to detect things like the inlet water temperature, outlet temperature, water flow rate, gas flow rate, gas pressure, and flue temperature. It's very complex. Some tankless heaters regulate not only the energy input, gas or electricity, but also regulate the water flow.
Gas tankless water heaters also need a large gas line to supply all those Btu's and a large venting system for all that exhaust.
One of the problems with high efficiency is that the highly corrosive exhaust gasses condense on the walls of the venting and can damage the venting, the heat exchanger, and other parts if it is
If you have a dishwasher make sure that it draws water at a high enough flow rate to turn on the tankless heater. Many do not. Washing dishes in cold water is not very effective.
Tankless Water Heater Maintenance
Scale must not be allowed to build up in tankless water heaters as it can restrict the flow of water through the heat exchanger. Generally the warranty of a tankless water heater will contain a figure for water hardness that should not be exceeded. If your water is too hard it will void the warranty. They recommend a water softener ahead of the heater in those cases.
Remote Controls
Many tankless water heaters have remote controllers. Tankless heaters only work within a certain range of output temperatures. If your temperature is set too high or too low for a specific task or flow rate, then you need to adjust the output temperature for that use. There is usually a control panel on the water heater, but it can be inconvenient to run out to the garage when you are about to shower and notice the temperature is set wrong.
Tankless manufacturers therefore offer remote control units so you can adjust the outlet temperature to meet your specific requirements for whatever you are going to use it for.
Electric Tankless Water Heaters
Electric tankless hot water heaters are not very popular do to the amount of electricity they require. For a whole house type application you typically need to run extra heavy electrical wiring and have a separate set of breakers for the water heater. Often you will need an upgrade from the utility company to get that much electricity to your house from the grid.
Water Conservation with a Tankless Water Heater
Cold water must pass through the heat exchanger to reach full temperature. The water in the heat exchanger when the heater comes on is cold. It takes time for the water to flow all the way through the exchanger to the outlet to become fully heated.
It therefore takes longer for the hot water to reach the fixtures. Longer waits for hot water means more water running down the drain, which leads to more wastage of water, not water conservation.
The Australian government did a study of a number of brands and models of tankless water heaters and found that on average a tankless water heater to from 10 to 20 seconds longer to supply water to the fixtures.
Tankless Water Heater Installation
Installing a tankless water heater is not something for the amateur. You should have a factory certified technician install your tankless heater. Poor installation is the number one cause of problems associated with tankless water heaters. A poorly installed tankless water heater can wait years before acting up.
Venting is critical in gas heaters, as is gas line sizing, water flow, combustion air, and condensate drainage if needed is also very important. Freeze protection also becomes an issue with tankless heaters. The warranty won't cover a heat exchanger damaged by freezing.
Sizing is another critical issue. Too small or too large a heater for your application will cause you problems.
Comparing Tankless Hot Water Heaters to Storage Water Heaters
By: William Lund
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