Most of the thermostats in today's hot water heating systems have evolved from a long history of baseboard, fan coil or radiator heating systems. Using the right thermostat is key to optimal radiant performance and zone control.

Most of the thermostats in today’s hot water heating systems have evolved from a long history of baseboard, fan coil or radiator heating systems. These thermostats work quite well and have a proven track record for their intended use. Some “sophisticated” thermostats even include anticipators and other performance-enhancing devices. However, these thermostats are now being used with radiant floor systems—in some cases, not with the best of results.

How many installers have been called back to jobs because the radiant system gets too hot and then too cool? It never seems to maintain a steady and comfortable temperature. How many homeowners have complained that their radiant floors are not as warm as they had anticipated?

Most of these problems are related to the type of thermostat being used to control the radiant floor. Most likely, the thermostat was not designed for radiant floors; instead, it was designed for baseboard or fan coil zones. The use of a proper thermostat can solve most of these problems.

Air Sensing or Slab Sensing

There are two ways a thermostat can be used to control a radiant floor. Either an air sensor can be used to control the air temperature or a slab sensor can be used to control the floor temperature.

An air sensor measures the air temperature in the space where the radiant floor is located. The thermostat operates the radiant floor to maintain the air temperature. The result of using an air sensor is that although the floor may feel warm under certain conditions and may feel cool under other conditions, the air temperature stays at the desired temperature. Typical uses for air sensors include most living spaces such as dining rooms, living rooms and kitchens. In these areas, a steady air temperature is much more comfortable than a steady floor temperature.

A slab sensor measures the floor temperature in the area where the radiant floor is located. The thermostat operates the radiant floor to maintain the floor temperature. The result of using a slab sensor is that although the air temperature may overheat under certain conditions and may underheat under other conditions, the floor temperature stays at the desired temperature. Typical uses for slab sensors include bathroom floors and tile entrances. In these areas, a warm floor is more desirable than a steady air temperature.

Slab-sensing thermostats are also used in systems where limiting the maximum floor temperature is necessary either to ensure occupant comfort or to help protect surface coverings, such as hardwood and vinyl, or other flooring materials.

Ideally, a thermostat used with a radiant floor should be capable of sensing both the air temperature and the floor temperature in the same space. A thermostat that senses both the air and floor temperatures would be capable of satisfying many more requirements of a radiant floor than would one that sensed only one or the other.

The ideal thermostat would have settings for the air temperature, the minimum floor temperature and the maximum floor temperature. This thermostat would also be capable of handling the common requirements of today’s radiant floor industry. These include, but are not limited to, one-stage heating, two-stage heating and heating/cooling.

One-Stage Space Heating with Slab Maximum

The most common use of a thermostat for radiant applications is a one-stage heating zone with a maximum allowed floor temperature. The thermostat maintains the air temperature with the ability to limit the floor’s surface temperature. This is achieved by setting the air temperature and a maximum allowed floor temperature.

In this situation, the radiant floor is operated by the thermostat to maintain the air temperature in the space. As the heating load increases, the floor temperature also increases. This operation continues until the slab sensor indicates that the radiant floor is operating at its maximum allowed temperature. At this point, the thermostat continues to operate the floor to maintain the maximum allowed floor temperature, and the air temperature is allowed to drop.

Floor Warming with Slab Minimum

Floor warming is another common application for the thermostat designed for radiant floors. In floor warming, the thermostat maintains a minimum floor temperature. The air temperature in the zone does not affect the operation of the thermostat. The radiant floor is operated by the thermostat to maintain the floor at the minimum floor temperature. This provides the homeowner with the feeling of a warm floor all of the time.

One-Stage Space Heating with Slab Minimum and Slab Maximum

There is often a desire to combine the preceding two situations into a one-stage heating application with a minimum and maximum floor temperature. The thermostat maintains the minimum floor temperature during mild outdoor conditions. This can give a warm sensation to a tile floor during mild conditions. However, overheating of the space can occur because the floor is being maintained at its minimum temperature. As the outdoor temperature begins to drop, the air temperature in the room begins to control the radiant floor. The floor temperature increases to keep the room warm until the floor reaches the maximum allowed temperature. This maximum temperature may be needed to protect a surface covering such as a hardwood floor.

Two-Stage Space Heating with Slab Maximum

In many cases, a radiant floor may need additional heat to keep the room warm during cold winter days. This becomes a two-stage heating zone. In a two-stage heating zone, the radiant floor is combined with a secondary source of heat such as a baseboard or a fan coil. If hardwood is used as the floor covering, a maximum floor temperature may also be used. The thermostat operates the radiant floor during mild conditions to maintain the air temperature.

As the load increases, the temperature of the floor increases to maintain the space temperature. Under extreme load conditions, the floor temperature may increase to the point where the floor coverings or flooring materials may sustain damage. In systems where this is a concern, a slab sensor should be used and the thermostat should be programmed with a maximum allowable floor temperature. Once the maximum allowed floor temperature is reached, the second stage begins to operate to maintain the desired space temperature.

One-Stage Space Heating with Slab Minimum and Space Cooling

A unique scenario with radiant floors is the desire to heat the floor and cool the air at the same time. An example of this would be an entrance in which the homeowner desires to have a warm floor to welcome people’s feet whenever they remove their shoes, whether during summer or during winter. However, during the spring, fall and summer, this warm floor will often overheat the space.

In this situation, the homeowner may want to operate the cooling system at the same time to maintain a comfortable air temperature. To operate this system, the thermostat must have a slab sensor and a minimum floor temperature setting. At the same time, this thermostat must be a cooling thermostat that operates based on air temperature. Only a thermostat capable of sensing both the air and floor temperatures at the same time is capable of controlling this type of system.

Scenarios Unique to Radiant Floor Heating Systems

There are several situations unique to radiant floor heating systems. These situations can be extremely difficult to address unless a proper thermostat, capable of sensing both the air and floor temperatures, is used to control the system.

Some of the most difficult types of radiant heating systems to control properly are those that experience large internal heat gains. An example of such a system is a radiant zone that has a large glass area with a southern exposure. If this zone were operated with a thermostat that sensed only air temperature, there could be wide temperature swings.

During the morning, before there were any significant solar gains, the radiant floor would operate to maintain the proper air temperature. During the day, as the solar gains increased, the radiant floor would shut off as the zone began to overheat. During the evening, the solar gains would stop once the sun was below the horizon. At this point, the room would begin to cool off and the radiant floor would turn on. However, because of the slow response time, it could take several hours before the radiant floor started to stabilize the air temperature. This would result in a period in the evening when the room would become cool and feel uncomfortable to its occupants.

To compensate for the long response time of the radiant floor, the system should maintain either a minimum floor temperature during the entire day or a minimum floor temperature just prior to the solar gains stopping. To achieve this, the thermostat must have a slab sensor. The ideal thermostat for this situation would incorporate air heating, air cooling and slab minimum combined with a scheduling system. During the night, the thermostat should operate to maintain a heated air temperature. During the day, the thermostat should automatically switch between heating and cooling as the air temperature dictates. A few hours prior to sunset each evening, a minimum floor temperature should come into effect. At this point, the floor would begin to heat. When the sun sets and the air temperature begins to cool in the space, the floor is already pre-charged with heat. This minimizes the time during the evening where the zone may be cool because the radiant floor has such a slow response time.

Using Schedules with Air and Slab Sensors

Adding a scheduling system to a thermostat capable of sensing slab and air temperatures adds the flexibility needed to accomplish all of the applications required from a radiant system.

A proper schedule would allow for heating and/or cooling for each period as well as a minimum floor temperature for each scheduled period. This schedule would allow for alternate periods of heating and cooling as well as for floor warming.

An example would be a bathroom. A user may desire a warm floor in the bathroom during the morning while he or she is showering and may desire a comfortable air temperature to be maintained for the remainder of the day.

To meet all of the expectations of radiant floor customers, your systems need a proper thermostat that is able to sense both the air and slab temperatures. Know how to give your customers the warm floors or comfortable rooms they are looking for. Better yet, give them both.

"This article was originally posted on ww.reevesjournal.com."