How much energy is wasted?
I wanted to pass on my experience with our hot water recirculation system, and some measurements on how much energy it wastes.
These systems are designed to get hot water instantly even at faucets distant from the hot water tank, but they can be VERY wasteful if not very carefully designed and implemented.
Homes, Hotels Hospitals with long plumbing runs from the hot water tank to the places where the water is used can have a long wait for hot water. You turn the hot water tap on, and a minute later it’s still running cold. This wastes both water and energy, and is an inconvenience.
To overcome this problem, a water recirculation system can be added. These systems overcome the problem by keeping the water in the hot water supply pipe hot, so that when you turn the tap on, you get instant hot water.
There are several kinds of systems. The mostly used systems are described hereunder.
One type runs an extra water pipe out to the farthest water using fixture you have. This extra pipe allows water to be circulated from the hot water tank, out through the hot water supply pipe, and then back the extra pipe to the water heater. In this way the water in the supply pipe is always hot.
Most of these systems use a pump to circulate water, but passive circulation is also possible.
The pumps are controlled in various ways:
- They can run 24/7
- They can be on a timer so that the pump is only running when people might want hot water.
- They can be run on a thermostat that only runs the pump when the water temperature in the pipe cools below a certain point.
- They can be set up to only run the pump when the person who wants water “requests” it. The “request” can be pushing a button to indicate that someone is going to use hot water soon,
- They can be a motion sensors that turn the pumps on when people are in the bathroom.
As expected, these systems use (waste) different amounts of energy. The energy waste is of two types:
- Electricity used to run the pump. For a modest sized pump, this might be 400 to 800 KWH a year if the pump runs all the time.It depends on pump size & plumbing layout.
- Heat loss from the pipe. Whenever the two pipes are hot, they will be losing heat. This heat comes from the hot water tank, and the water heater must use more fuel to provide the heat. The heat loss is significant even if the pipes are insulated. During the winter, this heat may help to heat house. Or, it may not, depending on how the pipe is routed. This added heat may be a benefit in the winter, but it will also add heat to house in the summer, which will just make it hotter, and may result in higher AC bills.How Much Energy is Wasted? It is vary system to system.
- The ones that run a pump 24/7 obviously waste more heat and electricity. The ones that work off a timer or thermostat would save some of this waste, but not nearly all of it.
- The “demand” systems would be much more efficient, since they only circulate water when its actually needed.
- The estimated the energy waste for recirculation by measuring the amount of energy water heater uses with and without the recirculation loop enabled. With the recirculation system by thumb rule 50 % extra energy wasted per year
- To calculate the heat wasted ,measure the lengths of pipe with sizes. Calculate the volume of water in pipe lines. Calculate number of cycles set through timer per day or check the recirculation startup & shunt down time in case of thermostat based system. Then wastage of heat is =Volume of water in pipeline X number of cycles per day X Temp diff
If it is a new house, try and avoid any need for recirculation systems at all. Try to locate the hot water using fixtures as close to the hot water heater as possible. Using a “Home Run” plumbing system will also help greatly to minimize the wait for hot water and the waste of water.
If you are going to use a recirculation system, use an efficient one. The demand style systems will waste far less energy than the other types of systems. The demand systems may cost a bit more, and may be a little less convenient to use, but the saving in energy and greenhouse gas emissions are well worth it. While these systems still lose all the energy used to heat the water left in the supply pipe after using hot water, they at least don’t keep reheating the water in the pipes constantly, and they don’t constantly waste electricity to run the circulation pump.
Even if you don’t use a recirculation system, or use the more efficient on demand system, you will still waste the energy that went into heating the water in the line out to the far fixture every time you use hot water. This water just gets left in the pipe and cools down to house or crawl space temperature
The only way to avoid this energy waste is to locate the hot water tank near the fixtures, and use the Home Run system mentioned to minimize the water volume in the pipes.