Comparing a Tankless Water Heater and a Tank Type Hot Water Heater – A Product Review & Comparison.
In the market for a tankless hot water heater? If you are, and you are not familiar with the operation of a tankless water heater, then there are a few things you should think about before you make that purchase.
Hot water piping — the basics
There are major differences between the way a tank type water heater provides you with hot water, and the way the tankless unit does. With a tank type hot water heater you turn on the faucet, and the hot water begins traveling through the hot water pipe to your fixture. Since the pipe is cold, it absorbs some of the heat, gradually warming up to the temperature of the water.
This is why your hot water gradually goes from cold to hot if you leave your hand under the running water. If your pipe run is very short, the warm-up won’t be very gradual. The longer the pipes and the heavier the piping material, the more gradual will be the warm-up since the pipe material will be able to absorb more heat from the moving water.
Another factor in the warm-up is the speed at which the water is flowing. The faster the water travels the faster the warm-up. This is because the faster the water flows, the less time it is exposed to the cold pipe. Obviously the ambient temperature is a major factor. If your pipes are in the attic, and you are in Phoenix Arizona in mid summer, your pipes will already be so hot they will absorb little if any heat from the water.
However, if you are in Truckee California in January, your pipe warm-up will take far longer, be more gradual, and with very long un-insulated pipes, the high temperature might never be reached due to the heat loss from the pipes to the surrounding air.
Within a few seconds of the hot water reaching the fixture the temperature stabilizes. Since you have a big tank full of hot water that is pretty much at one temperature, the temperature at your faucet should not vary much, even when you change the flow rate. A trickle of water is pretty much the same temperature as with the faucet turned full-on.
Tankless hot water heaters
Now let’s suppose you have a tankless hot water heater. All of the previous warm-up factors apply plus a few more things come into play. When you first turn on the water heater there is no big tank of hot water to start flowing. There is instead a tankless water heater full of cold water that immediately begins flowing into the hot water pipe. In order to heat the water to full temperature, the incoming water must flow through the entire heater since it takes time to heat the water. This will cause a longer delay in getting the hot water to the fixture, and results in running more water down the drain while waiting.
A simple way to picture the workings of a tankless water heater is to picture a coil of copper tubing with a gas burner in the middle of it. As the water travels through the tubing it absorbs heat from the flames and gets hotter and hotter.
Outlet temperature depends on starting temperature and flow rate
The temperature of the water coming out of the tube depends on how fast the water is flowing and what the starting temperature is. If you are starting with 45 degree water, and the heater is capable of heating the water 90 degrees at a flow of 1 gallon per minute, then you will get 135 degree water out. But if for some reason the water temperature coming into the heater rises to 55 degrees, then outlet temperature will rise to 145 degrees. So be sure that your heater is powerful enough to handle the maximum amount of flow you will need, or you will see temperature fluctuations in the outlet temperature.
With electric tankless water heaters the water temperature is difficult to hold constant with changes in flow rate. The electric heating elements don’t respond quickly enough to compensate rapidly for sudden changes in the flow rate.
Turning on the tankless unit
To further complicate things, a tankless hot water heater does not turn on until a sufficient flow rate is achieved. Most tankless water heaters require between 1/2″ gallon and 3/4 gallons per minute to turn on.
So if you are in the habit of running the hot water tap full blast to get the hot water to the sink, and then throttling it back while you brush your teeth or wash your hands, then the tankless water heater will change your behavior. When you throttle it back, the water heater shuts off, and cold water is on its way. No more using just a trickle of hot water.
Hot water re-circ and circulating systems
Tankless water heaters won’t work with most hot water circulating systems. If the circulating pump has enough power to turn on the heater, it cause the water heater to run continuously until some safety mechanism shuts it down, or your fixtures melt. Most don’t have enough power to turn the heater on and will just become cold water circulators.
Hot water demand systems
Demand type hot water pumps will however work with the tankless heaters, and will get the hot water to the fixture more quickly and without running any water down the drain. Two such pumps are the Metlund D’mand pump and the Chilipepper CP6000. Unfortunately they don’t solve the other problems.
Tankless water heaters do have their place. They do eliminate standby losses that tank type units have, the loss of heat through the walls of the tank, and they do provide unlimited amounts of hot water.
Make sure your heater will heat the water to a high enough temperature at the flow rate you desire and the lowest expected cold water inlet temperature.