This article was originally published in the November/December 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online November/December 1996
Take It Outside: Water Heaters Get Fired Up
In much of the United States, converting electric water heaters to gas can reduce household energy costs. However, many properties-particularly multifamily homes-have space limitations or venting problems that make such a retrofit impossible. A retrofit system called the Seahorse gets around these obstacles by being mounted outdoors.
The Seahorse outdoor gas water-heating system was introduced by Gas-Fired Products Incorporated (GFP) of Charlotte, North Carolina, in 1992. The Seahorse is a gas-fired heat exchanger unit with a pump placed on an outside wall of the home and plumbed into the existing electric water heater storage tank.
While the majority of the several thousand Seahorse systems installed to date have been in single-family homes, the conversion of 44 units in the University Village retirement community in Tulsa, Oklahoma, demonstrated the Seahorse's potential for multiunit housing. Another recent retrofit at an Army residence at Fort Stewart, Georgia, was evaluated for energy cost savings.
When hot water is drawn from the storage tank for use, it is replaced by cooler water. When tank temperature drops below the thermostat setting, the thermostatically controlled pump draws the cooler water from the bottom of the tank into the Seahorse heat exchanger, where it is heated rapidly by a gas burner. It is then returned to the storage tank, available for use or reheating. The thermostat used can either be the original tank thermostat, or a submersible thermostat, or aquastat, available from GFP. When the tank's water temperature is raised to the thermostat setpoint, the pump automatically stops, and the flow of natural gas to the burner is shut off. To prevent freeze damage in power failures, the unit can be equipped with a non-electric freeze protection valve, which will constantly circulate water as long as the Seahorse inlet temperature is below 38oF.
The Seahorse has a nominal gas input of 60,000 Btu per hour. Based on the manufacturer's measurements under laboratory conditions, the Seahorse has a steady-state efficiency of 86% at its standard flow rate of 5 gallons per minute. According to its manufacturer, the Seahorse has three times the water-heating capacity of a standard electric water heater, providing 66 gallons of water each hour at a 90oF rise in temperature, compared to 21 gallons at the same temperature rise for a 9 kilowatt 40-gallon electric water heater. This rapid recovery capacity can make the Seahorse an effective heat source for both water- and space-heating in some installations.
Because of the Seahorse's recovery capacity, hot water was also used for space heating by adding an Aqua Therm air handler from First Company in Dallas, Texas. In this system, hot water is circulated from the storage tank of the original electric water heater through a separate coil-type heat exchanger located in the air handler, and was then returned to the storage tank. The fan in the air handler blows air across the coil and into the existing duct system. A 24-volt wall thermostat triggers a small pump that circulates hot water from the storage tank to the air handler. The same thermostat controls cooling, for which a conventional condensing unit is installed outside each residence and connected to the air handler.
The Seahorse conversion gave residents individual control of their heating and cooling needs, and it reduced the number of complaints. It significantly reduced load on the central plant's boilers and chillers. The facility's management also believes that the Seahorse, by frequently circulating water through the water heater storage tanks, reduces sediment buildup and corrosion (see Water Heaters and Energy Conservation-Choices, Choices! HE May/June '96, p. 15).
Oklahoma Natural Gas added distribution lines to each cottage, and gas consumption in each fourplex can now be monitored, which was not possible in the past. Conversion of the remaining cottages is expected in 1997.
According to the report, the Seahorse easily met the residence's hot water load at all times. Energy costs at the base were very low-2.5¢/kWh for electricity and 30¢/therm for natural gas, and warm inlet water at Fort Stewart is likely to lower residential hot-water energy consumption there. As a result, the calculated life cycle energy cost savings of $778 did not offset the typical installation cost of $835. However, marginally higher hot water energy requirements or higher electricity costs (both of which exist in most residential retrofits) would make Seahorse a cost-effective retrofit over its life cycle. Four out of eight federal sites in the United States have energy costs that would probably make it cost-effective to convert electric resistance water heaters to Seahorses. PNNL also compared Seahorse performance with that of a typical 40-gallon tank-type gas water heater (with an energy factor of 0.54) and found roughly the same level of efficiency for each.
The Fort Stewart test led the researchers to suggest some minor modifications of Seahorse technology and installation in order to improve performance and cost-effectiveness. GFP has incorporated these improvements, such as a check valve in the flow loop, into newer Seahorse models.
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