Everything you ever wanted to know about tankless water heaters, whole house, point of use, how to select one, and how they work.
Tankless water heaters – how they work
Tankless water heaters, often referred to as instant or instantaneous water heaters, do not instantly heat water. Heating water takes time whether it’s a big tank full, or a long piece of tubing coiled up over a natural gas burner.
Tankless water heaters have pros and cons, just like everything else. They can supply an endless supply of hot water, and can save you energy because they do not have standby losses like storage type water heaters.
However, they are limited in the amount of hot water that can be produced at one time and they are more expensive to purchase and install than a conventional storage type water heater.
They also make it take longer for you to get your hot water, since they don’t start heating the water until you turn on the faucet. Typically they add 10 to 20 seconds to the time it takes to get hot water to the fixture.
There is a minimum flow rate required to turn on the heater, typically between .5 gallons per minute and .75 gallons per minute. This means letting the faucet run with just a trickle of hot or warm water will not work. You will need at least 1/2 gallon per minute of flow or more to turn on the unit.
Some dishwashers and washing machines do not have enough flow to turn on the heater.
Tankless Water Heaters – Energy Savings
Tankless hot water heaters save energy and thus money. Water heating accounts for up to 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. This places the monthly savings at about $3.00 to $9.00 a month.
Storage tank-type water heaters raise and maintain the water temperature to the temperature setting on the tank (usually between 120 degrees -140 degrees F (49 degrees -60 degrees 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.
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 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.
Point of use tankless water heaters
In the past tankless water heaters were usually used as “point of use” heaters. Point of use water heaters operate more efficiently than whole house water heaters since there is only a short distance between the heater and the fixture. This eliminates the piping losses and reduces the hot water used each time a use occurs. With a long pipe, when you finish using hot water you have also “used” all the hot water left to cool in the pipe. You paid for the energy to heat that water.
Point of use installations use smaller heaters since they don’t have to supply more than one fixture at a time. The smaller installations are much less expensive than whole house water heaters.
Whole house tankless water heaters
Saving money with a whole house tankless water heater is much more difficult due to the higher cost of installation. Typically whole house gas tankless heaters need substantially larger flues, and often they must be stainless steel. Chances are you will also need to have a larger than normal gas pipe. Even though they burn gas to heat the water, they have electronic controls, so in the event of a power outage you will not be able to get hot water…something to think about.
Electric whole house tankless water heaters require a huge amount of electricity to operate. Standard house wiring will not usually be acceptable, and you will have to run a dedicated electrical service line to the heater, and install dedicated circuit breakers. This can be quite expensive.
Selecting a Tankless Water Heater
Select a tankless water heater based on the maximum amount of hot water 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.
Unless you know otherwise, assume that the incoming potable water temperature is 50 degrees F (10 degrees C). You will want your water heated to 120 degrees F (49 degrees C) for most uses, or 140 degrees F (60 degrees 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 degrees F (39 degrees 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 degrees F (10 degrees 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.