I thought I would address the hottest thing in plumbing these days, the tankless water heater. These devices have received a lot of press lately. Unfortunately most of the articles that I have read only tell about half of the story.
Now don’t get me wrong, I do like these units but I won’t sell one unless I know the customer fully understands and can live with the limitations and differences of these units. It helps to understand where the technology originated and the lifestyle of that region. Tankless water heaters originated in Japan then moved across Europe. No doubt about it, a tankless water heater requires a fraction of the space of a traditional tank-style water heater. Space is at an ultra-premium in Japan. Also at a premium is fresh drinking water. Japan is surrounded by salt water, not sitting next to the largest body of fresh water on our planet like Chicago. The average showerhead in Japan produces 1.0 gallon per minute. In the good ‘ole USA, shower heads produce a maximum of 2.5 gallons per minute by law but many customers remove the restrictor for an increased flow, sometimes up to 10 gallons per minute.
Water Flow Rate Limitations:
The biggest limitation of a tankless water heater is the flow rate of water the unit can produce. If water flows too quickly through the tankless heater, it does not stay in the unit long enough to be brought up to a usable temperature. Most tankless manufacturers rate their product to produce between 4 to 9 gallons per minute at a 40-degree rise in temperature. That means in the summer, if the incoming cold-water temperature is 70 degrees Fahrenheit, the outlet will be 110oF. A little low still, but not too bad. In Chicago, we have a little thing called winter. The incoming cold water temperature during winter months is about 40oF, resulting in an outlet temperature of 80oF. All tank style heaters are rated at a 90oF temperature rise which under the same conditions will produce 130oF water. You can still get hot water from a tankless unit but the stated flow rate (usually between 4 to 9 gallons per minute to begin with) will be cut by about 55% for our winter months.
Energy Factor vs. Thermal Efficiency for Tankless Water Heaters:
Thermal Efficiency is a measure of how efficiently a water heater is turning fuel into heat. The Energy Factor examines the total efficiency of the water heater. The Energy Factor is always equal to or less than the thermal efficiency.
The Energy Factor is the portion of the energy going into the water heater that gets turned into usable hot water under average conditions. It takes into account heat loss through the walls of the tank, up the exhaust vent piping, and fuel combustion efficiency. The higher the Energy Factor, the less you pay to operate the heater.
Because tankless water heaters don’t have the losses associated with tanks, their Energy Factors are normally higher (although well-insulated, ultra-efficient tank heaters also have high Energy Factors). Energy Factors for gas-fired tankless water heaters range from 0.69 to 0.84, compared with 0.55 for a conventional tank and 0.86 for an ultra-efficient tank heaters.
Conventional electric tank water heaters have an Energy Factor of about 0.87 compared with 0.91 for an ultra-efficient tank and 0.98 for electric tankless heaters.
In terms of dollars paid to the gas utility, for every $1.00 of gas purchased, a standard water heater will cost $0.55 to operate versus $0.69 for a standard tankless unit.
When converting from a tank-style to a tankless for the first time, there are usually some pretty steep costs associated with running electricity and a larger gas line to the tankless unit. Also, tankless units cannot exhaust up the chimney like most tank-style water heaters. A tankless unit must vent through a high grade stainless steel exhaust line directly out the side of the house that must terminate at least 4’ away from any door or window to prevent the unit’s exhaust from contaminating the air in the home.
Finally, manufacturer’s neglect to inform consumers that a tankless water heater must be de-limed every 2 to 4 years to maintain it’s efficiency and lifespan at a current cost of around $300.00 to $500.00.
Time to Get Hot Water:
It is common to have to wait a little longer to get hot water from a tankless water heater than a tank type. When you turn on your shower, a tank type water immediately starts providing hot water. The time you wait is the time it takes to move the hot water through the piping from the water heater to the shower valve. With a tankless water heater, when you turn on the same shower valve, the tankless unit has to sense that water is flowing through the unit before it turns on its burner and starts to heat water. For this reason it typically takes an extra 30 seconds of water use before getting hot water.
Larger homes are equipped with a hot water return line to decrease the time you have to wait to get hot water. Due to the intentionally low flow rate of a hot water return system, a return line will not operate with a tankless unit without installing a small electric water heater to act a as a buffer. This will solve the problem but adds cost to the installation.