New water heaters balance pressure in the flue with that in the room to prevent gasses from spilling into the room.
Whenever your project includes replacing a gas water heater, you probably don’t spend much time mulling over the features of competing units. Maybe you should. Options that improve safety, indoor air quality and efficiency can be sold as benefits.
Water heaters and fires
The relatively open flame in a water heater can cause a house fire if it comes in contact with flammable vapors. During an average year in the early 1990s, ignition of flammable vapors by gas-fired water heaters caused nearly 2,000 fires, 320 injuries, 20 deaths and more than $26 million in property damage. In response to this problem, the U.S. Consumer Product Safety Commission pushed for safer appliances. Manufacturers formed a consortium in 1995 to research solutions.
As of late 2001, all major water heater manufacturers offered products that meet the new vapor-resistant standard. The designs include a new type of gas valve and a flame-arrester plate below the tank. The initial price bump is $75 and up, but that should drop as the new designs become standard.
Higher efficiency standards
New water heater efficiency standards ordered by the Clinton administration in 2000 take effect in January 2004. Gas-fired units must improve from an energy factor of 0.54 (54% efficient) to 0.59, and electric units must increase from 0.86 to 0.90.
Gas-fired water heater efficiency is measured by the combination of combustion efficiency (usually about 76%) and losses from the tank into the room when the water heater is in standby mode. A number of units already meet the higher standards. Changes usually involve the use of heat traps (preventing hot water from being lost from pipes above the tank), a more efficient flue baffle (to improve heat transfer during combustion) and more foam around the tank. Most builders report the price difference to be $50 to $100. Once these water heaters become standard, expect the efficiency price bump to be $25.
Backdrafting and spillage
The draft of warm air rising up a water heater flue is measured in small units of air pressure called pascals. You typically can count on 5 to 10 pascals of positive draft pressure drawing in surrounding room air. When the space where the water heater is located is put under a negative pressure equivalent to the positive pressure in the flue, the combustion byproducts rising up that flue might backdraft — spilling back into the room and potentially causing health and safety problems. In a random sampling of relatively new homes by Fort Collins Utilities (a Colorado municipal utility), one-third of the homes had negative pressures sufficient to be considered a safety hazard.
Your options? You can reduce or eliminate the negative-pressure problem, or you can install a water heater that won’t spill byproducts of combustion back into the home.
There are excellent reasons for eliminating the negative-pressure situation. Because the problem is caused primarily by leaking ductwork, reducing or eliminating those leaks addresses several problems. The house is made safer, the sizing of HVAC equipment (especially air conditioning) can be reduced, equipment durability should improve, and comfort will increase.
But just because you specify tight ductwork doesn’t mean it will happen. Given the slow pace of industry change, your best short-term option is to buy a more foolproof water heater. Buy one that isn’t atmospherically vented and that doesn’t passively draw house air up the flue past an open, dish-shaped draft hood.
Your four primary choices? First, buy a water heater with an induced-draft fan, the type installed on 80%-efficient furnaces. Second, buy a PVC-vented appliance, with one vent that brings in outside air and another that fan-forces away flue gases. Third, buy a direct-vent appliance; it must go next to the exterior wall and vented through double-walled pipe out the rim joist. Fourth, buy an indirect or side-arm water heater fired by a boiler. These options cost more — at least $200. But such a water heater should be part of any systems-engineered retrofit package.
Writer and consultant Steve Andrews is a board member with the Energy & Environmental Building Association.