Text size:

Sensors Boost Boilers' Efficiency

House Doctor | Gary Dymski

March 8, 2001

YOUR COOKTOP would be inefficient if it operated at its highest setting all the time. And it would not be a practical thing for hot water to gush from a faucet at its boiling point.

And so for safety, comfort and efficiency, range tops and faucets have temperature controls. Shouldn't the home's largest user of energy - the heating system - have the same type of control feature?

But a furnace already has a thermostat to control temperature, right? Yes, but while a thermostat senses the temperature in the dwelling, it doesn't control the temperature of the flow of heat from the furnace.

In hydronic heating systems -- also known as hot-water systems -- a single-stage reset control device can sense changes in outdoor temperature and adjust the temperature of the hot water that circulates through the system. The result increases the efficiency of the furnace without sacrificing comfort.

House Doctor Recent columns

These control devices, also called indoor-outdoor reset controls, cost from about $500 installed, and studies show that they can reduce annual fuel costs anywhere from 5 to about 25 percent. The most common range of savings, however, is in the 12 to 15 percent range.

Reset controls have been popular in Europe, which has experienced high fuel costs for more than 30 years. The technology has been common in commercial heating in the United States for nearly as long. With the cost of home heating fuel skyrocketing, these controls are becoming more popular in residential applications.

"They're excellent," says Ed Schoen of Prestige Heating Service Inc. in Massapequa. "Most of the time, I find people are buying them with the installation of a new boiler. ...Really, the control is part of an energy-efficient package, which includes everything from a high-efficiency boiler and a digital, programmable thermostat to a well-insulated home."

Richard Levi, part owner of Comfort Specialists in Oceanside, also is a fan of the indoor-outdoor reset controls. "In a new boiler installation, it's a no- brainer," Levi says. "Oil and gas, the prices are not going down, and over the long haul this saves on fuel."

Levi, however, says recent technology is the reason the controls are beginning to make a dent in the residential market. "Years ago, these controls were not very reliable because they were made of big boxes with mechanical relays that broke down frequently," he says. For the past 10 years, Comfort Specialists has been installing reset controls for commercial heating systems. "I've never had one go bad on me yet," Levi says.

The reset control devices are manufactured by companies such as Tekmar Controls, Honeywell and Johnson Controls. Here's how they work:

The size of a heating system is calculated, in part, based on the coldest day of the year in a given climate. "Since every day is not the coldest day of the year, a heating system often is running 'oversized' and inefficiently," says Don Rathe, owner of Rathe Associates Inc., a Syosset company that distributes heating and cooling components, including reset controls by Tekmar.

So whether the thermostat is set at 68 degrees or 78 degrees, a hydronic heating system circulates water at a maximum temperature of between 180 degrees and 190 degrees. In mild or moderate temperatures, this output is excessive. The boiler heats and circulates water to the hottest temperature, even though there isn't a maximum demand for heat.

With a reset device installed, an air sensor mounted on the exterior of the north side of the building (commonly the coldest part of the exterior) senses any change in outside air temperature. The air sensor communicates this outside temperature change to a control box mounted on the boiler. Another sensor is installed on the pipe that supplies water to the boiler. The control box is programmed to heat the water that circulates through the boiler based on the outside temperature. So on a 40-degree day, water might be circulated at 150 degrees instead of 180 degrees or 190 degrees.

"The boiler doesn't have to work as hard to heat the water, so there's a savings in fuel," Rathe says. "The thermostat remains the brains of the heating system, but the reset control provides more support-that's the perfect word - for the thermostat and the system to work more efficiently."

Although the control box is factory programmed, it also can be re-programmed to a homeowner's comfort and demands, Rathe says.

Levi says the bigger the heating system, the better the savings. "Systems with large quantities of water, bigger homes, offices and apartment complexes, this is where you see the most benefit," Levi says. "On a smaller system, you get savings, but I think you also get a lot more comfort. You don't have these bursts of heat coming at you and then have the system shutting down.

"There's a more consistent output of heat."

And while installation starts at about $500, Levi says high-end control devices that control water in gas water heaters and radiant floor heating can run as high as $1,500.

Rathe says the controls are available only for gas- or oil-fired hydronic heating systems. Installation inquiries should be made through a plumber, a heating specialist or a homeowner's fuel provider.