Thermostats That Feel the Heat
 
Thermostats That Feel the HeatMost American thermostats measure only air temperature. But there are numerous other elements that affect thermal comfort, and some of these can be used to turn HVAC equipment on and off. Many air conditioner thermostats come with a humidistat, so the air conditioner can double as a dehumidifier. In Japan, advanced thermostats have gone another step--each wall-mounted heating and cooling unit has a remote control. The occupant can either leave the remote in a control location or carry it around, so the unit can respond to the temperature, humidity, radiant heat, and even air speed at the person's exact location.

However, advanced controls have yet to catch on in the United States. This is largely because the heaters and air conditioners that could benefit most from advanced thermostats have not been packaged with such controls. A new thermostat from the Solid State Heating Corporation (SSHC), the TSSHC-2-D Diaphragm Line Voltage Thermostat, breaks from this tradition. It uses an operative temperature measurement to control ceiling-mounted electric radiant heating panels. It helps electric radiant panels provide quick comfort, while saving energy compared to other electric resistance systems.

Operative temperature is a combination of radiant and air temperature. Radiant temperature is the average temperature of the objects and surfaces that surround us, which radiate heat to (and absorb radiant heat from) our bodies. It is affected by nearby surface temperatures, surface emissivity, the geometry of a space, and an occupant's location in it. Air temperature, on the other hand, is simply the energy in ambient air, without regard to radiant heat sources or sinks.

In the past, radiant heating systems have been controlled by thermostats that sense only air temperature, under the assumption that radiant and air temperatures would be equal. This assumption is rarely if ever accurate--it would require a superinsulated home with super low-e windows and a heating system that provides uniform delivery temperatures and that runs the same temperature all the time. Older homes have uninsulated building surfaces; large expanses of single- or double-glazed windows; and scattered heat delivery registers, baseboards, radiators, or other heaters--all of which create differences between air and radiant temperatures. Thermostat setbacks also lower radiant temperature--as the house heats up after an extended setback, the air temperature may be at the prescribed temperature, while building surfaces and objects in the space remain colder. Cold surfaces reflect a low mean radiant temperature, resulting in inadequate comfort levels. Now ASHRAE guidelines reflect the new understanding of the role radiant heat plays in comfort.

While thermostats have ignored radiant temperature, research by P.O. Fanger and others shows that radiant temperature affects comfort at least as much as air temperature. A room may feel comfortable at a low air temperature if the objects are radiating a lot of heat, and a room where the air temperature is within the normal range may still feel cool if the surfaces are cold.

A radiant heater, combined with a control mechanism that senses both air and radiant temperature, can make a room comfortable by raising the radiant heat without needing to heat much air. Based on this understanding, SSHC developed the new thermostat to complement Enerjoy, its line of radiant heating panels. The thermostat has a vapor-filled dual diaphragm inside its knob. This diaphragm absorbs both radiant and ambient heat, causing it to respond to both ambient air temperature and the mean radiant temperature of the surrounding space. It has a 130¡ã field of "view" and a quick response time to changing conditions. The sensor also has a mere 1¡ãF differential to keep thermal comfort ranges within a very narrow band. This is particularly important with overhead radiant heating since extended exposure to high radiant temperatures can lead to discomfort.
Savings through ControlThe careful control allows the Enerjoy to provide comfort at lower air temperatures. The Enerjoy is a low-mass heater, meaning that it can be turned off whenever a resident leaves the room and turned back on when she or he returns. It takes only four or five minutes to heat up enough to provide comfort. The Enerjoy has two advantages over central heat pumps: it has no duct losses and works very well with severe setbacks. Unlike electric baseboards, it doesn't expend much energy heating air, and has a very quick response time. A study performed by the National Association of Home Builders (NAHB) Research Center showed a 33% savings over an air-to-air heat pump operating in the same structure and a 52% savings over electric baseboards, despite the fact that the radiant panels use typically expensive electric resistance for heat. This makes the panels a reasonable option for homes without access to gas, or where occupants are willing to control heat on a room-by-room basis.

The operative temperature thermostat is crucial for such a system. With a conventional thermostat, a ceiling radiator could make people quite uncomfortable before it got the air up to a typical setpoint of 70¡ãF. With a motion sensor control, the panel could turn off every time the resident watched television, and turn on every time a pet walked through the room.

The diaphragm thermostat is available through SSHC, with the purchase of the Enerjoy system.