This article was originally published in the July/August 1997 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online July/August 1997
Dampers, Reclaimers, and Pumps--Oh My!
by Doug Johnson and Edward Wyatt
Doug Johnson is a freelance writer based in San Francisco.
Ed Wyatt is an energy consultant in Marin County, California and Home Energy's assistant technical editor.
Residential energy retrofitters often hear much hype about new products designed to increase the efficiency of domestic water heating. But how much energy do these devices really save, and how do they save it?
Next to space conditioning, water heating is the largest energy user in most homes. However, opportunities for savings aren't always obvious. Today's fossil-fueled storage water heaters are now available with federal Energy Factor (EF) ratings as high as 0.64, and heat pumps can provide electric water heating with an EF above 2. The lowest-efficiency water heaters currently on the market are more efficient than most units sold in 1980.
Changing a low-efficiency water heater for a high efficiency model is one of the best things you can do to save water heating energy. Changing behaviors to save water also helps. But residents will only go so far; they don't want to start using water before it warms up; they consider replacing a low-efficiency water heater before it fails to be a costly inconvenience; and they don't want to reuse hot water once it's dirty. Current gadgets allow residents to increase the efficiency of an old water heater, or to recapture energy as water goes down the drain. The gadgets can also save water.
Water heater flue dampers aim to reduce standby losses in low-efficiency water heaters. The outside of a water heater tank is insulated, but the flue is a thin-walled metal tube up the middle of the tank, opening into ambient air. When a burner is on, the highly conductive flue heats the water, but with the burner off, the water loses heat through the metal and up the flue. Flue dampers reduce standby losses by sealing heat inside the flue when the burner is off.
Flue dampers typically use an electric sensor that activates an electromechanical vent, requiring a reliable power source. A new system from Advanced Conservation Technologies (ACT) is a residential damper activated solely by the convective force of the flue gas. This system could reduce standby losses in low-efficiency water heaters. However, it may not function with the low vapor pressures of high-efficiency water heaters.
ACT's damper has an ultra-light metal diaphragm. When the burner is firing, hot exhaust gases lift the diaphragm up a vertical rod, opening the vents and allowing dilution air to enter the flue. When the heater turns off and the gases cool a bit, the diaphragm settles back down. Any exhaust gases left in the flue, including exhaust from the pilot, can still escape through intentional gaps around the edge of the damper. The entire assembly simply replaces the existing draft diverter unit on top of the water heater.
Tests so far show that the damper reduces standby losses. The American Gas Association (AGA) tested a water heater with no hot-water demand that simply maintained a set water temperature. The damper reduced heat loss per hour by 45%. Another AGA test cycled the damper to simulate about 14 years of average water heater use. It found that the damper saved 29% of standby losses. In cases where 40% of water heater energy was being lost to standby, this would mean that the damper could save 12% of water heater energy.
This residential flue damper has a suggested retail price of under $50. If it saves 5% of water heater energy without increasing tank maintenance (and assuming that installation is quick and simple), the devices will pay for themselves in a few years in most homes.
Storage tanks are problematic. They are difficult to install and their efficiency is low in real-world situations. They also have not been effective in extracting much heat from wastewater.
The GFX system, introduced to the market in 1995, uses no tanks or pumps, and its operation does not require electricity, so materials and operating costs are reduced considerably. Six models are currently available, ranging in price from $180 to $410.
A GFX heat recovery unit is a counterflow heat exchanger that extracts heat from drain water. Using a coil-and-tube design, it can easily be installed in a normal plumbing main in a basement or large crawlspace. The standard device is 5 ft tall, consisting of a 3-inch copper pipe tightly wrapped with 1/2-inch copper tubing. Larger models, using a 4-inch pipe with 3/4-inch tubing, are also available. The pipe connects to the drain line, and the tubing connects to the cold-water supply line.
Incoming pressurized cold water flows upward through the coiled tubing that surrounds the central drainpipe. Because the GFX heat exchanger is installed vertically, the drain water falls downward and spreads around the inner walls of the vertical pipe. The recovered heat preheats the inlet water.
A high heat transfer coefficient means that the GFX transfers heat to the inlet water more efficiently than other hot water recovery systems. The smooth copper surface prevents organic growth, and soapy films pass through in less than two seconds, leaving little time for suspended grease and debris to accumulate. In various installations and tests, the unit has not clogged.
As warm water passes down the pipe, it clings to the side of the pipe, warming the wall. Cold water rising up the coil around the pipe readily absorbs this heat. However, the system works best when the drain and supply lines are being used at the same time, reaching maximum efficiency when drain and supply flow rates are equal. The Department of Energy test procedure for Energy Factors (EF) uses several large draws of hot water. When this test is done so that the water drains through the GFX heat exchanger, with the supply rate equaling the drain rate, the GFX increases the EF of the water heating system by about 34% and can triple first hour ratings. Baths, laundry machines, dishwashers, and other batch users would receive little benefit from the unit, but showers would do well.
The GFX system has been through five technical evaluations in different studies. Of these, the most extensive and most recent was done by Old Dominion University in Virginia. The system was tested with three types of electric water heaters. Under three different usage schedules, GFX preheated supply water by 20°F-30°F, depending on inlet temperature, at flows of 2.5 to 3.0 gallons per minute (gpm). Total energy savings by the GFX ranged from 47% to 64%, and averaged 57%. The GFX increased the EF on each tested heater by 57%-73%.
Researchers caution that actual savings depend highly on whether the home tends to use batches of hot water that are drained later, as in the case of washing machines, or whether the big users drain continuously, as do showers. Other factors that can affect savings are the cold-water inlet temperature and the difference between supply and drain flow rates.
The GFX system is a good complement to tankless water heaters, which often have limited water flow. Old Dominion research suggests that for one shower, a GFX can provide savings of 13-34 kBtu per hour. Thus for a daily ten-minute shower, a reduction in energy use of 240-600 kWh per person per year is possible; for the average domestic cost of 8.6¢/kWh, these savings total $20-$50 annually per person. Use of GFX is most effective with electric water heaters; cost savings are not as dramatic with oil- and gas-powered units. On electric water heaters, the study showed that with GFX the heaters never turned on their upper resistance elements. Electric water heaters could thus be cheaper to install, as they would require less peak demand capacity.
One drawback of the GFX is its effect on drinking water. In typical installations, the main cold-water inlet passes through the unit, so cold water to all taps is delivered at 75°F-85°F when warm water is in the drain. This can be avoided by installing the GFX downstream of any branches to taps used for drinking water, but this might complicate retrofit installations.
Two technologies are available to reduce the wasted water and the problem of heating cold pipes (see Water Heaters and Energy Conservation--Choices, Choices! HE May/June '96, p. 15). Recirculating hot water systems continually pump water through the hot water lines and back to the water heater, so there is always hot water available at the tap. However, the hot water lines throughout the house act as radiators year-round, increasing cooling loads and draining significant amounts of energy from the water heater. And a full recirculation system requires an extra run of pipes, which entails a significant expense, especially as a retrofit. Another alternative is a tankless, or instantaneous, water heater, but these suffer from low peak capacity, and they call for unusually big electric or gas lines.
To improve on continuous recirculation, ACT has developed a demand pump that circulates water from the hot-water line back to the water heater when there is a demand for hot water, rather than continuously. There is a button at each hot-water tap, either wired to the central pump or connected by remote control. Users press the button to activate the pump, which then shunts tepid water from the hot-water line into the cold-water line and back to the hot-water heater. Thus, unlike recirculation systems with a hot-water return line, this system requires no extra pipes. According to ACT President Larry Acker, the pump forces water through the pipes at 6 to 10 gallons per minute (gpm), so it brings hot water to the taps much faster than a 1.5 gpm typical faucet, a 2.5 gpm low-flow showerhead, or a 4 gpm bathtub spigot.
ACT claims the system can save 20 to 50 gallons of water per day in typical single-family homes. However, the U.S. Department of Energy estimates a typical household uses 63 gallons of water per day and a committee is currently considering lowering that estimate. In light of DOE's estimate, projected savings as high as 50 gallons per day seems less likely.
In addition to saving water and offering greater convenience, the demand pump may save energy. An ACT survey of users shows that residents reduced their water tank thermostat setpoints from 10°F to 22°F after installing the pump. If the survey was accurate and the behavior could be counted on, that would account for significant energy savings. ACT claims that faster delivery of hot water to the faucets is the reason residents were able to lower their water heater temperatures. Acker claims that the demand pump's fast-moving hot water does not have a chance to cool as much on its way to the tap; instead it fills the pipes with hot water and heats the whole pipe almost simultaneously.
Acker also claims that energy savings arise from warm water being returned to the water heater. However, it is unclear how much warm water gets returned to the water heater. If the warm water line is the same length as the cold water line, the cold water line would start returning tepid water just as fully heated water reached the pump.
There are as yet no independent monitoring studies to provide a reliable estimate of energy savings. The 85W pump runs for only 20 to 45 seconds at a time, so if it is used 30 times per day, it will consume about 10 kWh per year.
The potential for energy savings depends mostly on individual behavior. If a resident normally turns on the hot water and then walks away, the demand pump could prevent gallons of hot water from escaping down the drain. Gary Klein, an energy specialist at the CEC, uses a demand pump at home. Klein points out that the waste from waiting for hot water is more significant than people realize. When you run the tap full blast until it heats up, then turn it down and mix in some cold water, every minute it was running full blast was drawing twice or three times as much water as when you're actually using the hot water. Thus, if water runs for two minutes to heat up, and is then used for six minutes to wash dishes, as much energy and water may have been consumed in waiting for hot water as was used washing dishes.
The demand pump retails for around $400. According to Acker, it can be installed by a homeowner in an hour or two and rarely requires a professional plumber.
If the pump actually allows residents to reduce the water heater setpoint by 10°F, and they are using 62 gallons of hot water per day, it will save them about $0.95 per month at average natural gas rates (60¢/therm) or $3.70 per month at average electric rates (8¢/kWh). In other words, a cost-effective installation would have to be in a home with many hot-water draws per day, long pipe runs, and/or high-priced water (see How Much Does a Shower Cost?). Most customers will probably buy this system for its added convenience, and any energy or water savings will be incidental.
The demand pump's performance requires more in-depth analysis, both in a controlled benchtop environment and in real homes. The City of San Jose, California is considering using demand pumps in 100 single-family homes and plans to conduct a monitoring study to record the effects on water and energy use.
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