Thermostat Heat Anticipator Operation How the heat anticipator works
HEAT ANTICIPATOR OPERATION - CONTENTS: explanation of how the thermostat heat anticipator actually works. Difference Between a Heat Anticipator and a Heating Control Device Differential? A heat anticipator will not work accurately unless these conditions are met
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Thermostat heat anticipator function & adjustment: how a thermostat heat anticipator works.
We explain how adjusting the heat anticipator pointer changes the heat output of the anticipator that in turn changes the behavior of the room thermostat to turn the burner off sooner or later with respect to the actual room temperature.
Our page top photo illustrates key parts of a traditional room thermostat including the temperature sensing device, thermostat switch, and the heat anticipator assembly.
What is a Thermostat Heat Anticipator & How Does it Work?
A heat anticipator is used to avoid room temperature overshoot
For some building heating systems, particularly hot water and steam heat, because the radiators retain and continue to give off heat after the thermostat is satisfied, and thus after the actual heating system has turned off, a room might become warmer than the temperature called for by the thermostat setting.
Heating technicians call this over-heating problem room temperature overshoot.
To prevent heating overshoot or making the room warmer than wanted, a heat anticipator is added to some thermostats to "anticipate" that additional heat output (after the thermostat is satisfied) and to stop the call for heat a little early, thus turning off the heating system early to avoid that overheating.
In sum, stopping the call for heat a little early allows for the delivery of residual heat that is already in the boiler or furnace but that has not yet reached the living space.
[Click to enlarge any image]
In my photo above the green arrow and dots show the path of electricity through the heat anticipator.
Current flows through a tan wire to the heat anticipator's mounting screw at the left side of the photo, from there through the flat-coiled nichrome wire, up throug the black contact button on the movable adjuster, through the adjuster's copper arm, and back into the thermostat.
The green dots show the electrical current's path through the small left end of the heater that is mostly hidden by copper pointer.
The red arrow, which I regret as it clutters the photos, is a pointer to the wider portion of the body of the nichrome wire heating element. Notice that the little nichrome wire heater is located conveniently just above the coiled bi-metallic spring that turns the thermostat's control wire on and off to turn the heating system on and off.
A Nano-Sized History of the Thermostat Heat Anticipator
The first heat anticipator was produced in 1924 (Walker 2008 cited at REFERENCES) in a design that, like the heat anticipator described in Honeywell's T87, used the heat output of an electric current passing through a tiny heating device (a coil of nichrome wire) to warm the coiled bimetallic spring that in turn was used to turn off the thermostat ahead of the time when the room would reach the set temperature called for on the thermostat control.
This was an improvement to a much-older device, the electric room thermostat patented in 1883 the U.S. by Warren Johnson (Johnson Controls).
Just two years later in 1885 Albert Butz patented a furnace heat output regulator that used a moving flap to adjust air entry into a heating furnace.
Butz's company, The Electric Heat Regulator Company ultimately became the Honeywell Corporation whose 1960 first round wall thermostat is shown above. This classic 1960 Honeywell T8332A round day-night thermostat is illustrated and discussed further at HEAT ANTICIPATOR FAQs.
A heat anticipator is actually a tiny electric heater inside of the room thermostat
The heat anticipator, not found on all thermostats, is a tiny little electric heater that will, depending on its setting, warm up the inside of the themostat, thus warming the thermostat's room temperature sensor, thus fooling the thermostat into thinking the room is a little warmer than it actually is.
That little heat anticipating joke played on the thermostat's room temperature sensor causes it to "open" its contacts (stop calling for heat) a little before the room temperature actually reaches the thermostat's "set" temperature.
The heat anticipator is anticipating the additional heat that is going to arrive and regulating the thermostat accordingly.
Thus we avoid overshooting: we avoid making the room warmer than the thermostat's set temperature.
You won't find a heat anticipator in all thermostats. In fact most newer room thermostats use a thermistor to sense room temperature and they do not usually include a heat anticipator device.
In our photgraph above you can see the critical components of a thermostat heat anticipator such as in the Honeywell T87.
How does a heat anticipator work inside of a wall thermostat?
The components of the heat anticipator are shown in the photo and are explained in more detail below. They include:
A scale showing Amperes or current ranging from 1.2 at the left to 0.10 at the right end of the scale.
What the heck? The scale also includes the word LONGER imprinted at its right end where we see the lowest amps numbers.
Honeywell, a company I admire, ought nonetheless to apologize to everyone for the countless hours of confusion caused by this unfortunate detail.
If you click to enlarge the photo you'll see an arrow below the word LONGER that points to the left, to the larger Amps numbers at the other end of the scale.
This means that the larger current or Amps numbers will result in a longer heating system "on" cycle. I think some dopey engineer put the word "LONGER" at the right end because that's where there was plenty of room to show it.
A brass pointer indicating the heat anticipator setting along the Amps scale.
A resistance heating wire, probably nichrome wire, wound around a triangular base.
An electrical contact that slides to different positions on the resistance wire coil.
The Nichrome Wire in the Heat Anticipator is a Tiny Electric Heater
The heat anticipator in an electromechanical thermostat includes a tiny heating coil of nichrome wire which gets warm as elecricity (current) flows through the wire.
Moving the pointer along the Amps scale moves the position of the contact that in turn changes the length of nichrome wire that will be used, making the in-use wire shorter or longer.
Meaning of the Numbers on the Heat Anticipator Scale
The heat anticipator numbers along its scale are measurements of current or Amps that will flow through the heat anticipator's little heater at different settings.
When we move the heat anticipator adjusting arm we are moving an electrical contact (blue arrow) along the flat-wound resistor wire, effectively increasing (to the higher current numbers on the left) or decreasing (to the lower current numbers on the right) the amps or current flow through this tiny heating device by lengthening or shortening the total length of wire included in the heater circuit.
Increasing or decreasing the wire length included in the circuit changes how hot the wire gets as current flows through it.
Above: the heat anticipator pointer is set to about 0.45 Amps.
Longest heat anticipator nichrome wire length = most resistance (Ohms) = least current flow ( 0.1 Amps) = least heat from the heat anticipator = heating system stays on longer
If we move the arm and contact fully to the left to the 1.2A position that's using the maximum length of the nichrome wire or heater length - that's the circuit with the most resistance, the least current flow, so the least in-thermostat-heating, so we turn the actual heating system off later.
Shortest heat anticipator nichrome wire length = least resistance (Ohms) = most current flow ( 1.2 Amps) = most heat from the heat anticipator = heating turns off sooner
If we move the arm and contact fully to the right to the 0.1A position, that's using the shortest length of the nichrome wire heater length - that's the circuit with the least resistance, and thus the highest current flow through the wire, so the wire gets hotter, so it produces the most in-thermostat-heating, so the thermostat inside warms up faster than the room air, so the thermostat will turn off the actual heating system off sooner.
How Adjusting the Heat Anticipator Changes Its Behavior - Variable Resistor
That tiny resistance wire or on older thermostats, a wire wound into a flat coil, is a tiny electrical resistance heater that puts some heat into the interior of the thermostat, fooling it into thinking the room is just a little warmer than it is.
The heating thermostat manufacturer's instructions & heat anticipator operation explanation can be a bit confusing, but significantly, as we detail
at HEAT ANTICIPATOR ADJUSTMENT
The shortest burner-on time will be when the heat anticipator puts out the most heat. This warms up the thermostat's room temperature sensor and therefore tells the thermostat the room is up to set temperature earliest.
The longest burner-on time will be when the heat anticipator puts out the least heat, thus does not turn off heat early, thus lets the burner keep running longer.
The graph at above left, adapted from one provided by Carrier, illustrates how a different device, a thermistor, translates temperature changes into a change in electrical resistance that in turn can be used by a room thermostat to control building heating equipment. That's how newer thermostats work.
Changing the Heat Anticipator's Pointer Changes the Wire Length, Current Amps, Heat Output
More heat output from our teensy electrical resistance heater inside the wall thermostat means more room heat anticipation (more pre-heating of the room thermostat's sensor) and thus a shorter heat-on cycle.
Here we explain heat anticipators a second time to offer another way to understand what's happening in a thermostat that uses a heat anticipator circuit.
Moving the heat anticipator pointer (the open triangle at the top of the copper arm) changes the effective wire length and thus the electrical resistance and thus the heat output of the heat anticipator inside the thermostat.
When we move the adjuster we are essentially using more (longer) or less (shorter) lengths of wire, thus causing more or less resistance through the device.
Key Concepts: Relationship of Wire Length, Resistance (Ohms), Current flow (Amps), Heat Generated, Heat Anticipator Warming, Heating System On-Cycle
If we assume, as is the case for a typical heating system thermostat, that the thermostat voltage level remains fixed (typically 24VAC) then moving the heat anticipator position changes the length of the nichrome wire in the active circuit.
Longer Wire = More electrical resistance = less current flow = less heat generated by the heat anticipator = less pre-warming of the thermostat = longer heat on cycle.
Shorter Wire = Less electrical resistance = more current flow = more heat generated by the heat anticipator = more pre-warming of the thermostat's sensor = shorter burner on time
Below: the heat anticipator pointer has been moved down towards the right end of the scale, and is set to about 0.15 Amps.
LONGER Heat-ON Cycles: Moving the heat anticipator adjusting arm to the RIGHT to LOWER Amps numbers (in the sketch and in the photo above) towards the 0.10A end of the scale uses more of the total length of the nichrome wire heating element, increasing the resistance of the device and decreasing the current flow.
Longer Wire = More electrical resistance = less current flow = less heat generated by the heat anticipator = less pre-warming of the thermostat = longer heat on cycle.
SHORTER Heat-ON Cycles: Moving the heat anticipator arm to the LEFT to HIGHER Amps numbers (in the photo and sketch just above) towards the the 1.2A end of the scale uses less of the heating element's nichrome wire, decreasing resistance and giving more current flow, more heating of the heat anticipator heater element and thus a shorter burner on time.
Shorter Wire = Less electrical resistance = more current flows, more heat is generated by the heat anticipator = more pre-warming of the thermostat's sensor = shorter burner on time.
Watch out: before changing the heat anticipator setting you might want to read both the manufacturer's actual recommended settings and our explanation about matching the heat anticipator to the actual installed-thermostat circuit's current draw in the rest of this article.
Many of us have found the heat anticipator in thermostats confusing because we think intuitively but incorrectly that more resistance means more heat is generated. No.
More resistance (higher Amps numbers) = less current flow = less heat is generated = the heat anticipator heats up less = the thermostat turns off the heat later (or it runs "longer").
Why Lower Electrical Resistance Means More Heat is Generated and Why Higher Electrical Resistance Means Less Heat is Generated in a Heat Anticipator Circuit
A heat anticipator will not work accurately unless the following conditions are met:
The heating or air conditioning system itself has been adjusted to match the electrical current of the valve or relay which the thermostat is controlling. This is a technical problem for your heating or air conditioning service technician, not something a homeowner can address.
The thermostat is installed at a location where voltage and current (amps) fall on or within the limits of the adjustable heat anticipator
The thermostat is not installed on a power pile system (don't ask).
The heat anticipator on the thermostat should be set to match the requirement & electrical characteristics of the particular heating or air conditioning control circuit that it is switching on and off.
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