“Our house overheats in the daytime and is too cool at night.”

“Our house won’t keep up when it is cold outside.”

“Our utility bills are way too high and the system is doing a poor job of heating.”

“Our boiler seems to run all the time.”

These are not the comments I want to hear from radiant floor heating customers. Unfortunately, I hear them a lot this time of year.

These people are frustrated with their local heating contractors and have somehow found me at the Radiant Panel Association (RPA). Some are even radiant heating installers themselves. The interesting thing is that many of these problems have the same simple solution: a small adjustment to the thermostat.

Little thermostat on the wall ...

In most heating systems, the thermostat on the wall has the final say as to when and how much heat is allowed into the space it controls. Some of the more sophisticated control strategies have variable temperature mixing, outdoor setback, multiple temperature delivery, and even flow rate controls, but they all have some sort of direct feedback from the space they are trying to heat.

The most common thermostat used in radiant heating systems is still the conventional bimetallic spring and heat anticipator type. When properly installed and adjusted, it is hard to beat for air temperature control. When out of whack, it can cause all kinds of problems.

Every heating contractor should have a good foundational knowledge of how and why a thermostat heat anticipator works.

I would estimate that there are hundreds, if not thousands, of radiantly heated homes that are not operating anywhere near their potential energy efficiency or comfort levels simply because the anticipator is improperly set.

The more mass in the system (like concrete, gypsum, stone, or brick), the greater the problems.

Often times the owner doesn’t complain because even a poorly operating radiant system can be more comfortable than a forced- air system.

Electronic thermostats attempt to use computer logic in place of the heat anticipator to achieve constant air temperature in the space. Again, the greater the mass in the system, the more difficult it is for “fuzzy logic” to predict what is going to happen and adjust for it.

For the operation and adjustment of electronic thermostats, check with the thermostat manufacturer. It may not be suitable for use with a high-mass radiant floor.

Figure 1. Without anticipation the system will tend to run for hours and overheat the space during the day.

The amazing heat anticipator

Let’s examine how the conventional heat anticipator works and its effect on a high-mass radiant floor system. To do that, we should first look at what happens when there is no heat anticipation (see Figure 1).

Assume that the thermostat mounted on the wall turns the system on when the air temperature drops below 70°F and off again when it reaches 70°.

The concrete slab starts out at room temperature. As the room drops below 70°, the system comes on and begins pumping heat into the concrete floor. Concrete has a large capacity to absorb heat so it takes a while before the temperature of the concrete begins to increase. While it is absorbing the heat, the room temperature continues to drop.

Slowly the floor begins to heat up and starts to contribute heat to the space — only a small amount at first, but as it increases in temperature, it sends more heat to the space.

The room temperature begins to rise towards the 70° setpoint. All this time, heat is being pumped into the concrete. When the room temperature finally reaches the 70° setpoint and turns off the thermostat, all the heat that was pumped into the slab just keeps coming and heating the room.

The air temperature keeps on rising and rising until all that extra heat in the slab has entered the space or the occupants open the windows. Eventually the slab runs out of heat and starts to cool down; another slow process. Because the space is so overheated, the floor has plenty of time to cool down to room temperature before the cycle starts over again.

This scenario is aggravated by the fact that most thermostats have a differential or “hysteresis” built in of 2° to 5°. That means our thermostat without anticipation would turn on at 70° and off at 73°, prolonging the upward climb of slab and room temperature.

The net result is seen in many homes where the anticipator is set to be ineffective. The system doesn’t come on until late at night and runs for hours, pumping heat into the slab. About the time the sun comes up, the thermostat is finally satisfied and turns off for the rest of the day while the heat stored in the slab, as well as heat from the sun, lights, and household appliances, drive the room temperature sky high.

Opening and closing windows becomes the means of temperature control. Homeowners call and complain about drastic temperature overshoot, high utility bills, and chilly evenings.

Here is where the simple anticipator can do amazing things.

Think of the anticipator as a miniature electric space heater inside the thermostat housing. When the room drops below 70° and the thermostat turns on the system, it also turns on the mini-space heater (anticipator). The anticipator heats up the inside of the thermostat housing (the thermostat housing must be in place for the anticipator to work).

When the thermostat housing reaches 73° (remember the hysteresis), the thermostat shuts the system and the anticipator off even if the room has not reached 70°. The housing and anticipator cool down to room temperature.

Figure 2. A proper anticipator setting will produce a consistent "on" cycle of between 10 and 14 min, with the "off" cycle varying in length depending on the outdoor temperature.
If the room is still below 70°, the thermostat will turn on and the cycle will begin again and repeat over and over until the room reaches 70°. The anticipator actually allows shots of heat to the slab and then waits to see what affect it has on the room (see Figure 2).

The anticipator has an adjustment on it that controls how fast it will get hot. It is usually a small lever with a pointer and a scale. The scale is marked in amps and often has the words “longer” and “shorter” at each end. With the pointer at the “shorter” end, the anticipator will heat up faster and produce a shorter “on” timer for the thermostat. At the “longer” end of the scale, the anticipator will heat up slowly, thereby causing the thermostat to stay on longer.

We know what will happen if the indicator is set too far to the “longer” end of the scale. On the other side of the scale, if the pointer is set to far to the “shorter” end, the system will short cycle.

The thermostat will turn on, the anticipator will heat up rapidly, and the thermostat will turn off the system too soon. This will result in small, short bursts of heat to the slab, which aren’t sufficient to keep up with the heat loss of the space.

How to determine anticipator settings

There are two ways to determine the initial setting of the anticipator.

1. Check the amperage draw printed on the device the thermostat is controlling (relay, valve, controller). Set the anticipator indicator to that amperage value.

If the amp value is not available, disconnect one leg of the thermostat wires and place an ammeter between the disconnected wire and its terminal. Read the amp draw when the thermostat is turned on. Use this value to set the anticipator.

2. Once the initial setting is made, check the operation of the system. Set the thermostat and let the system cycle naturally several times.

3. Time the “on” or “run” cycles. The thermostat should turn the system on for 10 to 14 min and then turn off. This should be fairly constant.

The length of time the system is off will vary, but the on times should be fairly constant. For example, on cold days you may have four or five on cycles per hour, whereas on mild days there may be only one cycle per hour. Systems with outdoor reset controls that regulate delivery water temperature tend to even out the run cycles and produce more even floor temperatures.

When you get a call that a system isn’t keeping up, or the house is over heating, or the utility bills are higher than they should be, or the boiler seems to run constantly . . . don’t start replacing parts or redesigning.

Check the anticipator. You and your customer may be in for a pleasant surprise.

Reprinted from the Radiant Panel Report with permission from the RPA.