Comparing Tankless Water Heaters With Storage Water Heaters - What Are The Differences?

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If you are considering the purchase of a tankless water heater then you should read this document.  Tankless hot water heaters are a whole different species from the traditional storage or tank type water heater.


Tankless water heaters are often touted for being a green plumbing product.  They are considered green because they don't have the standby heat losses associated with tank type water heaters.  More about energy and water conservation as related to tankless water heaters later.

For me the main attraction of a tankless water heater would be the ability to have unlimited hot water.  If I was running out of hot water often I would consider one.  The monetary savings from the higher efficiency of a tankless unit would only be around $5.00 a month and that isn't enough incentive for me to put up with some of the characteristics of a tankless heater.
 

Storage (Tank type) Water Heaters

Storage water heaters are pretty simple to install, operate, and repair.  They are very reliable and typically the first thing to fail is the tank which springs a leak.  In my experience most storage water heaters never need flushing or any other maintenance. 

Most manufactures recommend testing the pressure temperature safety valve yearly, and some recommend flushing periodically, but I know very few people who have ever performed any maintenance on their water heater.

Storage water heaters monitor the temperature of the water in the heater, and when the water temperature drops below a set point the heater turns on, and when it reaches the uppers set point it shuts off.   Gas water heaters and electric water heaters both operate the same.

There are a limited number of things that can go wrong and most are easily fixed.  The anode rod inside the heater can need replacing to continue to protect the tank from corrosion. It's easy to replace.

The dip tube which leads the incoming cold water down to the bottom of the tank can break. It is easy to replace.

On electric models the upper or lower thermostat and or the high limit switch can malfunction and need to be replaced. They are easy to replace.  The heating elements can burn out, but are easy to replace. 

On gas models the gas valve can go bad, but rarely do, and are easy to replace.  The thermocouple can go bad and is easy to replace.  The burners can get plugged but can be cleaned fairly easily... again, a rare breakdown.

Water heaters don't have controls for you to play with... you set the temperature, or more likely just leave it at the factory setting and you forget about it.  Basically you install your water heater and then forget about it until or if there is a malfunction.

If you have a gas water heater it probably still work fine if there is a power outage since most tank type water heater don't require any electrical supply.

The incoming cold water temperature won't have much affect on your hot water usage.  Whatever the cold water temperature is it still ends up at the cut-off temperature of the water heater.  Colder water will take a little longer to heat, but unless you run out of hot water it won't have any affect on your usage.

Gas storage water heaters come in a range of efficiencies, and so if you want a high efficiency water heater you just have to pay more.  They will still have the standby losses though.


Tankless Water Heaters (Instant Water Heaters)

Tankless water heaters are far more sophisticated than storage heaters.  Their job is a difficult one... to heat water quickly gets complicated in a hurry. 

To begin with tankless water heaters need a brain. Why you ask?  Well, to begin with there is the problem of temperature rise.  With a tank type water heater you just add heat until the water temperature reaches the shut off temperature.  How much of a temperature rise needs to occur isn't very important.

However a tankless water heater has to carefully watch and regulate the temperature of the water coming out of the heat exchanger.  Too much heat and the heat exchanger could easily spit out scalding hot water and could even turn into a steam generator.

To further complicate matters the flow rate of the water through the heat exchanger would also dramatically affect the temperature.  If the energy input is held steady, then an increase in flow will cause a direct and corresponding drop in temperature a decrease in flow would cause a rise in the outlet temperature. 

If the energy input is held steady and the temperature of the cold water drops, the outlet temperature will drop the same amount and vice versa.

A water heater needs to have a fairly steady temperature at the outlet even with variable flow rates and inlet temperatures.  Not only is the constant outlet temperature needed for safety and comfort, but to protect the heat exchanger as well.  A melted heat exchanger can be quite expensive.

So tankless water heaters need a brain to monitor the outlet temperature and control the size of the flame for gas tankless water heaters or the amount of electrical current passing through the heating elements in the case of electric tankless water heaters.

Some tankless water heaters can modulate both the gas and the water flow through the heaters while some just modulate the gas flow to the burner (or the heating elements).  However, just monitoring the outlet temperature is not enough.  Tankless water heaters have a number of critical sensors.

In order to function properly most tankless water heaters monitor the gas flow and or pressure, the exhaust temperature, the inlet temperature, the outlet temperature and the water flow.

If the "brain" detects anything wrong at any sensor it shuts the heater off.  With so many parts there is a larger potential for something to go wrong than with the simple old tank type unit. 

Gas tankless water heaters burn a lot of gas in a hurry. As a result you will probably need a large gas line than used with tank type units, typically 3/4" but you may even need to go to 1" pipe in some cases.

More fire means more exhaust so tankless hot water heaters need larger venting than storage types.  Often it must be stainless steel due to the corrosive condensate that can accumulate in the venting.  The more efficient a water heater is the cooler will be the exhaust.  If a water heater was 100 percent efficient then all the combustion heat would end up in the water and the exhaust would be cold.

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 properly drained. 

If the corrosive condensate isn't properly drained it can cause serious problems years down the road after the condensate finally eats through the heat exchanger or something.  That type of problem is not covered by the warranty.

Unlike tank type storage water heaters, tankless water heaters have a minimum flow rate that will keep the unit turned on. Typically it's 1/2" gallon per minute and for larger units it's 3/4 gallons per minute of water.  This can be an inconvenience at times.

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 heaters don't typically work with hot water circulating systems, although recently they have begun to appear.  Navien for example has a hot water circulating system friendly model.  I've seen hot water circulating pumps being advertised for tankless water heaters, and they just have a higher flow rate than traditional recirc pumps.  Check the water heater warranty before using such a system, as I mention previously, many tankless water heaters are not compatible with hot water circulating systems.

Hot water demand systems such as the Metlund hot water pumps and the Chilipepper are compatible with tankless water heaters.

Bosch gas tankless water heaters have a built in generator. The flow of water when you turn on the tap generates the electricity needed for the brain.  It doesn't need to be hooked up to electrical service and so during a power outage it will still provide hot water.

Here is a brief comparison between several brands of tankless water heaters. Compare Tankless Water Heaters.
 

Tankless Water Heater Maintenance

Tankless hot water heater manufacturers typically recommend some annual maintenance such as flushing out filters and de-scaling the heat exchanger.  In areas where the water is very hard it is important to clean the scale out of the tankless heater regularly. A build up of scale on the interior of the heat exchanger can cause the heater to malfunction and can even damage the heat exchanger.

Some owner's manuals call for periodically inspecting other parts of the water heater that requires some disassembly of the heater. 


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 water heaters are generally small units not intended to supply enough hot water for say a shower.  Just hand washing and cooking basically. 

The reason is that it takes so much electricity to run them that the wiring has to be much heavier than standard house wiring, and will probably need it's own breakers etc.  Often the electrical service from the utility pole to you house will need to be upgraded.  And even with all that they aren't capable of heating water as fast a gas unit.

They are excellent for point-of-use water heating systems if all you need is a small amount of hot water at a time.


Water Conservation with a Tankless Heater

The incoming cold water must pass through the heat exchanger to get hot.  The water sitting in the heat exchanger when the heater comes on is cold.  It takes time for the water to pass completely through the exchanger to become fully heated.

This means that tankless water heaters take longer to get hot water to the fixtures.  Longer waits for hot water translates into more water run down the drain, which does not conserve water to say the least.

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.