Anticipation and Cycle Rate

In the world of heating controls, do you know what an anticipator is? Most of us have heard the word. Few of us know what it does or how to set it. Do you know what cycle rate is? Of course you do — that’s how many times an hour the heat can come on. Happy surprise, they’re really about the same thing — keeping the temperature constant and “comfortable.” Ah, yes, this again. We’re all in the comfort business.

One of the definitions of comfort is that the room or building temperature always stays at controller set point. When the heat is on, the room doesn’t get too hot. When it’s been awhile, the room doesn’t get too cold. It’s pretty amazing that the temperature doesn’t “overshoot” every time the burner comes on. Credit goes to the anticipator in mercury-bulb thermostats, and to the cycle rate adjustment in electronic thermostats. We’ll talk about both.

An anticipator is simply a way to allow mercury-bulb (non-electronic) thermostats to control the cycle rate.

Every thermostat has a cycle rate. The purpose of cycle rate is to keep a room’s temperature even — never noticeably hot or cold, even though sometimes the equipment is on and sometimes off.

Electronic thermostats have a cycle rate adjustment built into them. That’s easier, but less fun, than setting the anticipator. The classic round thermostat and many rectangular non-electronic thermostats use a mercury bulb to respond to temperature change. It’s an old technology, but it still does a great job. There’s usually no reason other than appearance to replace one. The temperature control is quite satisfactory, and the mercury isn’t going to cause any environmental problem so long as it’s in the bulb.

Since plenty of mercury-bulb thermostats still remain in place, let’s make sure we know how to control cycle rate by setting the anticipator. But first, let’s talk about cycle rate.

Most thermostats come out of the box set for five or six cycles per hour. That’s ideal for most forced-air systems. But it may not be best for hydronic systems, and certainly not for electric. More on that in a moment...

A cycle is burner on-time plus off-time. Divide a 60-minute hour by six cycles and you get 10 minutes for each cycle. That means the equipment has the opportunity to come on every 10 minutes if there’s a need for heat.

The furnace or boiler will run for as much of the cycle as is needed to reach the thermostat set point. The heat will then turn off for the remainder of the cycle. If there’s no need for heat at the beginning of the cycle, the equipment doesn’t come on.

This standard cycle rate applies to hydronic baseboard heating and forced air furnaces of less than 90% efficiency. The standard for cast-iron radiators and high-efficiency furnaces is three cycles per hour — 20 minutes per cycle. The standard for electric heat is nine cycles per hour.

The longer it takes the system to emit all its heat, the longer a cycle should be. Of course the longer the cycles, the fewer there are in an hour.

Cycle rate adjustment with electronic thermostats is very simple. For forced air, you don’t change anything because the thermostat comes set for five or six cycles per hour. If you have cast-iron radiators, a 90-plus furnace or electric heat, then go into installer setup and adjust the cycle rate according to instructions.

Now back to mercury-bulb thermostats. This requires understanding what to do with the anticipator. Exactly what does an anticipator look like? When you open a mercury-bulb thermostat, you can see a strip of metal or a disk with tiny numbers on it. The numbers are in decimals: 0.2, 0.3, etc. These numbers are tenths of amps. Behind the numbers is a winding of wire thin as thread. This winding is a resistor. It creates a small amount of heat inside whenever there’s electricity going through the thermostat. A call for heat closes the thermostat switch. That allows electricity to flow through the thermostat. Sometimes the anticipator is called a heater.

Heat created by the anticipator makes the thermostat think the room is a little warmer than it really is. So the thermostat turns off the burner just a little early. Then the residual (left over) heat is used to reach set point. If the anticipator didn’t trick the thermostat into turning off early, residual heat would cause the room to overheat every cycle. Then the people would notice temperature swing, and there would be wasted energy and a lack of comfort.

So how do you set cycle rate with the anticipator? And what are those little numbers for? Remember, the numbers are tenths of amps. To get six cycles per hour, you set the pointer on the anticipator to match the amp rating (also called amp draw) of the load.

Uh-oh, what’s an amp rating, and what’s the load? The load is the device that the thermostat controls. In a hydronic system, it’s the zone valve or zone pump. In a furnace, the load is the gas valve or oil primary, plus the ignition system.

The amp rating is printed on the load. Most mercury-bulb thermostats come out of the box with the anticipator set to match the amp rating of a gas valve since that’s the most common load for forced air. But if your load is a zone valve, for example, you should set the anticipator to match the amp rating of the particular zone valve. Depending upon the manufacturer, that amp rating can range from 0.3 amps to 0.8 amps. You might think that tenths are too small to matter, but they do for the anticipator.

Again, you get six cycles per hour when you set the anticipator to match the amp rating of the load. In hydronics, six cycles per hour is probably good for fin tube radiators since the heat emits fairly rapidly.

But six cycles per hour isn’t best for every emitter. Cast-iron radiators, which give off heat slowly, may do better with a cycle rate of three cycles per hour. Sixty minutes in an hour divided by three cycles per hour is 20-minute cycles. Recall that a cycle is heat on-time plus off time. To get three cycles per hour with an anticipator, multiply the amp draw of the load (zone valve or zone pump) by a factor of 1.2.

So if your load is a 0.3 amp zone valve, to get three cycles per hour instead of six, simply multiply the 0.3 amp load by the factor of 1.2. That gives you an anticipator setting of 0.36. Even though 0.36 doesn’t seem very different from 0.3, to the thermostat it determines whether the cycle (on-time plus off-time) is 10 minutes or 20.

Cycle rate for electric heat is recommended to be nine cycles per hour. To get that, multiply the amp draw of the load by a factor of 0.8. That brings on the heat every six to seven minutes.

The answer for the best cycle rate for infloor radiant heating is “it depends.” There isn’t one rule because of differences in the ability to transfer heat of various floor materials. The mass of a poured floor varies depending upon its thickness. Completely different still is staple-up tubing under carpet and a pad. You’ll need to experiment to find what cycle rate is best for the particular combination of system and floor material.

Yes, cooling systems have cycle rate, too. But it’s typically not adjustable. What you get is three cycles per hour, whether that’s controlled by electronic cycle rate or a good old-fashioned anticipator. Get your stop watch and graph paper ready to test if the AC comes on every 20 minutes.

Would you like to learn more from Carol Fey? Check out her books and classes.