Image credit: Shaylor, used under Creative Commons license.
Dear Pablo: I am considering getting a hot water recirculation pump. They claim to save a lot of water and I am wondering if this makes up for their energy use. Should I get a hot water recirculation pump?
Hot water recirculation pumps are a convenient way to ensure that you have immediate hot water from the tap. These systems slowly pump hot water through your hot water pipes and back to the water heater through either a dedicated line or through the cold water line. Several models are available and some claim to save "10,000 gallons or more of water per year" and "up to 15,000 gallons per year" while using "less energy than a 25 watt light bulb." First I will examine these claims, then I will contrast the water saved with the energy used, and finally I will discuss some alternatives.
How Much Does A Hot Water Recirculation Pump Really Save?
Let's look at the calculations on one website:
An average home has approximately 125 ft of 3/4 inch piping.
125 feet of 3/4" pipe holds 3.14 gallons of water.
10 draws per day wastes over 31 gallons of water waiting for the water to get hot.
Over a year, the wasted water equals 11,461 gallons.
It may be true that the average home holds 125 ft of 3/4-inch piping, although no source was provided for this factoid. But when you turn on the tap, the water doesn't run through all 125 feet. The water runs the most direct course from the water heater to your tap. Also, I would think that half of that pipe is dedicated to cold water. In my house the distance from the water heater to the farther faucet is less than 50 feet. Using their assumptions, the amount of water in the pipe does not come out to 3.14 gallons, but rather 2.8687 gallons.
The next assumption is that you draw water ten times per day. This assumes that the water in the pipes cools completely between each draw. In most households however, there are two periods of the day when hot water is being drawn; for the morning shower and the evening dishes. During these periods the water in the pipes would probably not cool down very much so you would realistically only need to wait for hot water two or three times per day.
Using the assumptions and calculations from our source, we can confirm that 11,461 gallons would be wasted each year. Using my corrected assumptions, I would put that number closer to 838 gallons. Of course some homes are occupied all day, have a more spread out floorplan, and have a lot more hands to wash. Still, the 11,461 gallons saved are very optimistic. How much money does the pump save on the water bill? Using their savings and California's high water prices you would save around $50 per year, but the reality is probably closer to $4.
How Much Does A Hot Water Recirculation Pump Really Cost?The price tag of a hot water recirculation pump is around $200 and most can be installed by the consumer but some require a plumber. In addition to this fixed cost, there are two variable costs to consider, the energy used by the pump, and the additional water heating required. The 25 Watt pump would use 219 kWh per year, costing about $32 (depending on your local electrical rates). Many models feature timers so that they only run during set times of the day. Setting the pump timer to run two hours in the morning and two hours at night would cut the electricity use down to 36 kWh, or $5.50 per year.
Next we need to estimate the heat loss from the pipe as the hot water is circulated. Assuming that your hot water is 120°F and the air surrounding the pipe is 52°F you will lose 45 Btu (British Thermal Units) per hour per foot. If you use the 125-foot assumption, this means you will lose 49,275,000 Btu, while using my 50 foot assumption you would only lose 19,710,000 Btu. One therm, the common unit of measure for natural gas, in the US is equivalent to 100,000 Btu, so you will burn through an extra 493 therms (or 197 using my assumptions), costing you an extra $400 per year (or $160 using my assumptions).
Should You Get A Hot Water Recirculation Pump?The pump will cost you $200 to install, $5.50-$32 to operate, will waste $160-$400 per year and will save you $4-$50 on your water bill. This gives you a negative return on investment (ROI), so it certainly doesn't make sense from a cost saving or environmental perspective. But don't take my word for it, there are actual case studies out there with empirical data.
Why Would Anyone Want A Hot Water Recirculation PumpThe reason for installing a hot water circulation pump is pure convenience. If you can't stand waiting a minute for hot water to emerge from the faucet and the operating cost is of no concern to you, then this is the solution for you. For the rest of us there are a few simple things that we can do to gain some convenience and save some money.
- Insulate: By insulating your hot water pipes you will cut down on the heat lost from the water while it is travelling to your faucet and the water in the pipes will remain hot much longer for the next time you need it. If you already have a recirculation pump, insulating your pipes will significantly reduce the heat loss and will have an immediate ROI.
- ShowerStart Technology: Showerheads using ShowerStart Technology have a temperature-sensitive switch that turns off the water when the hot water has arrived. While this doesn't stop the cold water in the pipe from going down the drain you can easily collect it in a bucket for watering plants or filling the toilet tank.
- Go Tankless: Tankless, or instant water heaters create hot water on demand. Since they are usually located very close to the faucet there is almost not wait for hot water. This solution works well in homes that have very few faucets, or where all the faucets are located in close proximity.