The classic mercury-bulb thermostat comes with an anticipator. Hardly anyone in the business really knows what to do with it, but at least you could count on anticipation if you ever wanted to figure out what an anticipator did.

An anticipator simply is a not-so-straightforward means to get non-electronic - such as mercury-bulb, bi-metal, electromechanical or analog - 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 option built into them. That’s much better, but less fun for the few who know what to do with an anticipator.

For example, this recently appeared on “The Wall” at the HeatingHelp.com Web site: “I’m going to sorely miss analog ’stats like the T87. I could tune the anticipator to match the characteristics of individual heating systems. Once the heating system is cleaned and balanced, tuning the anticipator to the individual system creates minimum cycle times for good heat distribution.”

Classic Round: The T87 thermostat the contractor mentions is the classic round thermostat. It, like many rectangular non-electronic thermostats, uses a mercury bulb to respond to temperature change. It’s an old technology, but the temperature control is quite satisfactory. However, the industry isn’t losing a function by losing anticipators. Let me assure you that in terms of function and appearance, “analog ’stats like the T87” have not changed. On the outside they look the same. On the inside they still control the same.

The difference is that what was mechanical is now electronic. But since plenty of mercury-bulb thermostats still remain in place, let’s make sure we know, like our contractor friend on The Wall, how to tune the anticipator.

Most thermostats come out of the box set for six cycles per hour. That’s ideal for the majority of heating systems because it keeps the temperature within the industry comfort standard of no more than two degrees of temperature swing.

A cycle is 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 boiler will run for as much of the 10-minute 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 percent 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 idea is that the longer it takes the equipment to emit all its heat, the longer a cycle should be. Of course the longer the cycles, the fewer there are in an hour.

Adjusting Cycles: Cycle rate adjustment with electronic thermostats is very simple. In most cases, you don’t change anything because the thermostat comes set for 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 accordingly to three.

But cycle rate adjustment for a non-electronic thermostat 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 small device with tiny numbers on it. The numbers are in decimals: 0.2, 0.3, etc. These numbers are tenths of amps. Behind the numbers, you can see a winding of wire that’s thin as thread. This winding is a resistor that creates a small amount of heat inside the thermostat whenever there’s electricity going through the thermostat. In fact, sometimes an anticipator is called a heater.

The heat from the anticipator makes the thermostat think the room is a little warmer than it really is. Because of that, the thermostat turns off the heat just a little early. Then the heat left over in the fin tube or in the furnace blower compartment can be used to reach set point. If the anticipator didn’t trick the thermostat into turning off early, the leftover heat would cause the room to overheat every cycle. Then the people would notice temperature swing and there would be wasted energy.

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 the standard six cycles per hour, you set the pointer on the anticipator to match the amp rating of the load.

The load is the device that the thermostat controls. In a hydronic system, it’s usually the zone valve or zone pump. In a furnace, it’s the gas valve or oil primary, plus the ignition system.

Amps And Loads: Most mercury-bulb thermostats come out of the box with the anticipator set to match the amp draw of a gas valve since that’s the most common load. But if your load is a zone valve, for example, you should set the anticipator to match the amp draw of the zone valve. Depending upon the manufacturer, that amp draw can range from 0.3 amps to 0.8 amps. Find the amp draw printed on the load itself.

You might think tenths don’t matter, but they do for the anticipator. Here’s why: If you have cast-iron radiators, you may want to set the cycle rate to three cycles per hour for greater comfort. To get three cycles per hour with an anticipator, multiply the amp draw of the load by a factor of 1.2.

So if your load were a 0.32 amp zone valve, to get three cycles per hour instead of six, you’d set the anticipator for 0.38 rather than 0.32. To get the nine cycles per hour recommended for electric heat, use a factor of 0.8 times the amp draw of the load.

An area of debate is what the cycle rate should be for infloor radiant heating. As I suggested above, recommended cycle rate is related to how quickly the heat is dissipated. Because of the large differences in the mass and heat resistance of various floors, there isn’t one rule. The mass of a poured floor varies depending upon its thickness. Completely different still would be staple-up under carpeting.

So, unless you have experience that says otherwise, I suggest you start out with the thermostat at six cycles per hour. That means if it’s an electronic thermostat, don’t mess with the cycle rate adjustment. And again, if it’s a mercury-bulb stat, to get six cycles per hour, set the anticipator to match the amp draw of the load.

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