Using hot water from a tankless water heater is not the same as a using hot water from a tank type water heater. If you haven’t experienced living with a tankless water heater, it may take some getting used to.
Hot Water Temperature Characteristics of Storage Water Heaters
With storage water heaters when you turn on the tap the hot water begins flowing through the hot water piping on its way to the faucet. As the hot water flows toward the fixture it gives up some of its heat to the piping material. The faster the water flows the less heat it gives up on the way.
Depending on the ambient temperature, pipe run length, and piping material it can take up to 40% or more gallons of water to get hot water at the fixture than the pipe holds.
Temperature Stability
The temperature of the hot water in the tank is always changing. When the burner is on the water temperature is increasing, and when the burners are turned off the water temperature will be decreasing since heat is being lost through standby losses. To obtain the desired hot water temperature the user mixes hot and cold water together at the fixture.
The burners aren’t big enough to keep up with flowing water, so when you use the hot water the temperature drops fairly quickly at the bottom of the tank, and as the hotter water at the top of the tank exits the tank and the colder water begins rising to the top to replace the consumed hot water, the temperature begins to drop rapidly . At this point you have to begin re-adjusting the hot and cold water valves to keep the temperature where you want it and not long after there is no more hot water.
You never really know what the actual hot water temperature will be coming from a tank type water heater since the temperature is constantly going up or down depending on whether the burners are on or not.
Storage water heater reliability
Storage tank type water heaters are simple devices. A tank of course, a burner with pilot light and thermocouple, a combination thermostat gas valve, anode rode, PTR valve, drain valve, and that’s about it. No electronics or sensors other than the thermostat. Parts are easy to get and inexpensive. Any plumber can fix or replace them.
Tankless Water Heaters
Tankless water heaters are highly complex with many more parts than a storage heater. They have sensors to monitor water flow rate, gas pressure, exhaust temperature and more. They have computers built into them to control the burners and modulating gas valves to control the flame size. More parts means that there are more things that can fail. More complexity means more trouble fixing them.
You can think of a tankless water heater, (gas), as a coil of copper piping in the flames over a burner. If you run water through the tubing with the flames on, the water coming out of the other end will be hotter than the water going in the inlet.
A longer coiled pipe will result in hotter water and a shorter coil will provide lower temperature water at the outlet. That is because a longer pipe will keep the water the flames for a longer time.
Higher flow rates will result in less time in the flames and the water won’t be heated as much and lower flow rates expose the water to the heat longer and thus the water will get hotter. Any change in inlet temperature will result in a corresponding change in outlet temperature as well.
With storage hot water heaters a change in inlet temperature does not result in a change in the outlet temperature, nor will a change in flow change the outlet temperature.
With a tankless water heater you set the hot water outlet temperature the water heater controls the size of the flames to keep the outlet temperature at the set point.
Tankless water heaters have a minimum flow rate that must be achieved to turn on and keep the heater on. They also have a maximum flow rate that if exceeded causes the outlet temperature to drop as flow further increases.
Bathroom sink faucets are often have a flow rate of one gallon per minute. That means that to just keep the tankless heater on you need to run the hot water faucet at almost full blast. If you want only warm water you have to add more cold water to the mix and run the risk of reducing the flow rate enough that the heater shuts off.
In order to supply continuous hot water tankless hot water heaters have to have much bigger burners than storage heaters and need larger gas supply lines. If the tankless heater is electric it will probably need its own extra heavy wiring and possibly require a service upgrade from your utility company.
With gas tankless water heaters the exhaust vent will need to be substantially larger than the vent for a storage water heater.
Sediment Vs Scale buildup
If you live in a hard water area then you probably get sediment buildup in your water heater. Over a few years you can get quite a bit of sediment and it will begin to reduce the amount of hot water available from the heater. At some point you may have to drain the heater and flush out the sediment.
With a tankless water heater the hard water will cause scale buildup in the heat exchanger. The scale buildup increases the pressure drop through the heater and causes the flow rate to lower which leads to overheating and heat exchanger damage. Tankless water heaters in hard water areas usually need to be flushed out and de-scaled once a year.
Operating a tankless water heater with very hard water can void the warranty. In extremely hard water areas tankless water heaters can only be used if they have a water softener supplying the cold inlet water. Check the owner’s manual for the maximum water hardness can be without voiding the warranty.
Trouble Shooting Electric Water Heaters
Trouble shooting gas water heaters
Comparing Tankless Water Heaters
Comparing Storage Heaters to Tankless Heaters
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