|by Carol Fey
It’s still a good idea to know how to tune an anticipator.Anticipation
is such an interesting word. It has an implication of excitement,
possibility, maybe even thrills.
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
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
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
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
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
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.