On demand water heaters heat water as it passes through them. Opening a hot water faucet starts water flowing through the unit, turning its burners on. Once flow stops, the burners stop. The unit stores no hot water. These facts account for the efficiency and fuel savings found in on demand heaters, and also for the differences in their behavior. Following is a list of pros and cons, then a longer explanation follows.
- More compact, wall hung, space-saving
- No stand-by loss. Saves energy.
- Option available to those who have hot air furnaces.
- Provides an unlimited source of hot water at a limited rate.
- Unit is made of high quality material. Less likely to end up leaking and sent to a landfill.
- Most models qualify for rebates from the gas company.
- Needs large gas feed.
- It takes time to get hot water to the tap. The column of cold water between the unit and the opened tap must be exhausted before hot water reaches tap. There is no gravity “creep” in the hot water line bringing hot water closer.
- A minimum flow is required to start burners- you cannot get hot water at a very low flow.
- There is a limit to the volume of hot water you can run at once; there is no “dump load”. You cannot open your tub faucet wide and fill the tub.
- You need to accept that to get hotter water, you have to run the water more slowly.
Because the unit must make the hot water needed the moment the water is passing by its “doorway”, the engine required to make that energy transfer is very large. By large we do not mean dimensionally, we mean large energy output. The more hot water it must make at one time, the larger the energy input has to be, to keep up. Burners will modulate with change in flow up to the maximum rate the unit can achieve. The units are rated for gallons per minute (gpm) at a listed temperature rise. Temperature rise means the difference between the incoming water temperature and the temperature at which you would like your hot water to be. The listed temperature rise is important to pay attention to because we live in a climate where the incoming water can be colder than 40° F. in the winter. Manufacturers of many on demand heater brands have southern headquarters and factories and at first advertised their outputs with 60° temperature rises. We need to see at least a 90° rise for hot enough water from an on demand system. When comparing performance of indirect heater brands, look for the 90° rise column to see how much water the unit can make at any moment in our climate.
The maximum output of the on demand unit will determine how many taps can be opened at once. To calculate the number of gallons per minute your heater must supply, you should know that in Massachusetts, shower heads are supposed to operate with no more than 2.5 gpm flow, and new sink aerators hold flow to 1.25 gpm. An open tub spout can draw 5 gpm or more, as can a top loading clothes washer. An on demand heater with a 5 gpm rating at a 90° temperature rise would allow you to run 2 showers at once, or to wash dishes and shower simultaneously, but you could not start the top loading washer while someone was in the shower. Well, you could, but one of you would be sad.
On demand heaters store no hot water, and therefore cannot waste energy as the stored water loses its heat either up the chimney, or through the pipes. A negative effect of this efficiency is the amount of cold water which has to run out of the tap before the hot water reaches you. In Europe, where indirect water heaters are common, the units are put as close to the fixtures they serve as possible, usually in the same room, which alleviates the foot tapping, water waste problem. Also, because the unit shuts down when the tap is shut, if you want to run small amounts of water repeatedly, as in washing and then rinsing dishes, you will get slugs of cold water mixed in with the hot.
Some new on demand heaters are very efficient and well made. (Some are literally disposable- they are designed to be unrepairable [!] and must be completely replaced if any part breaks). A modern, condensing on demand heater can be much like a high efficiency boiler both in energy savings and in complexity and installation cost. They are vented through the side wall of the building, have acidic condensate that needs to be neutralized and disposed of, and need proper combustion set up and periodic maintenance.