Tankless hot water heaters have
their good and bad points
Tankless water heaters have their good points and bad
points just like
everything else. They can provide you with an unlimited supply of hot water, and
can save you some energy.
However, they are limited in the amount of hot water that
can be produced at one time and they are more expensive to purchase than a
conventional storage type water heater.
The amount of energy they save is relatively small, and you won't save much over
the life of the heater.
They also take longer to deliver your hot
water, since they have to heat the water in addition to delivering it to you
unlike a tank type system. This problem can be solved by using a specialized
pump, which when used with the tankless hot water heater can deliver your hot water
to you in about half the time it would take running the faucet full
blast. More about the pump shortly.
Increased water usage
The tankless water heaters also cause an increase in water
wastage since you have to let the water run longer to get your hot
water. This problem is also solved when using the specialized pumping
system. Water conservation is an important advantage to the pumping
system. One such system is the
Chilipepper hot
water pump. With it you get both water conservation and the convenience
benefits of faster hot water, with an un-limited amount of hot
water.
Another potential problem with tankless units is that they
require typically a half-gallon per minute or more flow rate to turn on. So if
you want just a trickle of hot water for something you are out of luck. It can
also be difficult to get a bathtub full of hot water.
It's more expensive to install a tankless water heater since if
it's gas it needs a bigger flue than a tank type unit, and if it's electric it
probably needs much heavier electric wiring than a tank type heater.
Tankless water heaters save money and energy...well a little
anyway
Tankless hot water heaters save energy and thus money.
Water heating accounts for 20% or more of an average household’s annual energy
expenditures. The yearly operating costs for conventional gas or electric
storage tank water heaters average $200 or $450, respectively.
Storage tank-type water heaters raise and maintain the water temperature to the temperature setting on the tank (usually between 120° -140° F (49° -60° C). Even if no hot water is drawn from the tank (and cold water enters the tank), the heater will operate periodically to maintain the water temperature.
This is due to "standby losses": the heat conducted and radiated from the walls of the tank—and in gas-fired water heaters—through the flue pipe. These standby losses represent 10% to 20% of a household’s annual water heating costs.
So if you have an old water heater with poor insulation you may save 20% of that
yearly operating costs of $200 to $450 per year. About $40 to $90 per year on
the high side. Lets hope your tankless unit lasts a long time to make up the
difference in price.
Tankless units are common in Japan and Europe. They began appearing in the United States about
30 years ago. Unlike "conventional"
storage tank water heaters, tankless hot water heaters heat water only as it is used, or on demand. A tankless
hot water heater has a heating device that is activated by the flow of water when a hot water valve is opened. Once activated, the heater delivers a constant supply of hot water. The output, however, limits the rate of the heated water flow.
Selecting a Tankless Water Heater
Select a tankless water heater based on the maximum amount of hot water you need to meet your peak demand. Use the following assumptions on water flow for various appliances to find the size of unit that is right for your purposes:
Faucets: 0.75 gallons (2.84 liters) to 2.5 gallons (9.46 liters) per minute.
Low-flow showerheads: 1.2 gallons (4.54 liters) to 2 gallons (7.57 liters) per minute.
Older standard shower heads: 2.5 gallons (9.46 liters) to 3.5 gallons (13.25 liters) per minute.
Clothes washers and dishwashers: 1 gallon (3.79 liters) to 2 gallons (7.57 liters) per minute.
Bathtub faucets: up to 10 gallons or even more per minute
Unless you know otherwise, assume that the incoming potable water temperature is 50° F (10° C). You will want your water heated to 120° F (49° C) for most uses, or 140° F (60° C) for dishwashers without internal heaters. To determine how much of a temperature rise your
need, subtract the incoming water temperature from the desired output temperature. In this example, the needed rise is 70° F (39° C).
List the number of hot water devices you expect to have open at any one time, and add up their flow rates. This is the desired flow rate for the water heater. Select a manufacturer that makes such a unit. Most
tankless hot water heaters are rated for a variety of inlet water temperatures. Choose the model
that is closest to your needs.
As an example, assume the following conditions: One hot water faucet open with a flow rate of 0.75 gallons (2.84 liters) per minute. One person bathing using a shower head with a flow rate of 2.5 gallons (9.46 liters) per minute. Add the two flow rates together. If the inlet water temperature is 50° F (10° C), the needed flow rate through the heater would need to be no greater than 3.25 gallons (12.3 liters) per minute. Faster flow rates or cooler inlet temperatures will reduce the water temperature at the most distant faucet. Using low-flow showerheads and water-conserving faucets are a good idea.
Some types are thermostatically controlled. They can vary their output temperature according to the water flow rate and the inlet water temperature. This is useful when using a solar water heater for preheating the inlet water. If, using the above example, you connect this same
tankless water heater to the outlet of a solar system, it only has to raise the water temperature a few degrees more, if at all, depending on the amount of solar gain that day.
Gas and electric
tankless water heaters More information about tankless heaters.
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