The Good Old Heat Anticipator
As a technician you most likely know some customers who still have an oldie thermostat (you know, those old mercury bulb things, like the round Honeywell CT87 and such). Keep in mind those usually have an adjustable heat anticipator. If you’re newer in the field you may not have seen or worked with those very much, or even not at all. They can seem confusing at first (why is it set with amperage? What amperage? How am I actually adjusting this???) but actually are quite simple to work with.
First of all, I hear you thinking “do you actually need to adjust that? I mean, is it going to make that much of a difference?” Honestly, in most cases, no, it won’t make a big difference. But it’s no reason to ignore it. And when it actually does make a difference, you will want to know how to adjust it properly.
Here’s a hypothetical story: you just changed a system, let’s say converted from a 30 year old oil furnace to a brand new condensing gas furnace. The homeowner just loves their old, simple, ‘’I-just-have-to-turn-it-up-or-down’’ thermostat and won’t upgrade it to a modern digital one. And hey, it still works fine, so why bother. Then, a few days later, you get this service call: ‘’that new furnace you guys just put in, it doesn’t work right! It keeps starting and stopping every 5 minutes! (or) It stays on for too long and overshoots the set temperature by a whole degree!’’ (and the line everybody loves to hear): ‘’It didn’t do that with my old furnace! It’s that new one, you sold me defective garbage equipment!!!’’
Okay, it doesn’t happen like that all the time, but I’m sure you’ve heard of similar stories. Now, to focus on the problem. I’m writing this tip about heat anticipators, but please don’t assume that’s going to be the issue whenever you get this kind of service call. I am merely reminding you that it is one of the many possible problems. So let’s say everything else is normal, no faults occurring during furnace cycles, no airflow issues, proper system sizing, etc. There’s a chance a very poorly adjusted heat anticipator will make a significant difference in cycle time. After all, it’s what it’s designed to do.
In short, the anticipator is simply a resistor built in the thermostat that is in series with the heat call low voltage circuit, i.e. the “W” terminal. That resistor generates a tiny amount of heat to preheat the bimetal and end the furnace cycle a little bit earlier, anticipating the residual heat from the furnace and fan off delay to cover the gap in temperature and avoid overheating the space. Now, even though it’s a resistor, you don’t set it by ohms. You set it by amperage. The amperage drawn by the heat control circuit.
Now it takes a little bit of effort to get that measurement properly, but it is quite simple. First of all, you need to remove the anticipator itself from the circuit when checking the control’s amp draw! All this means is you need to remove the thermostat from the circuit by twisting together the R and W wires at the thermostat. This will, obviously, give you a constant call for heat.
Now the amperage you need to measure is typically very low, no more than half an amp in most cases, sometimes much lower. So, in order to get a more precise reading (unless you have a super sweet meter that gives you precise readings in the tenths to hundredths of an amp range, this would be done in series instead of with a clamp) you should proceed as follows: get a nice very long piece of thermostat wire, which you will repeatedly wrap around your meter clamp, so it goes through it 10 times. Then connect that wire to your W wire from the thermostat on one end and to the W terminal on your furnace control on the other end. Simply put, just extend the W wire so that you have enough to wrap it around the clamp ten times. Then turn the power on and let the furnace cycle begin. Wait until all the relays and components are energized (on a typical gas furnace you will see the greatest amp draw coming when the gas valve is energized), then take your reading. Divide it by 10, and you have your heat anticipator adjustment value. Simple as that. For example, you might read (completely arbitrarily) 2.40 Amps on your meter with ten wraps of wire. Which means the control actually draws 0.24 Amps, so you will need to set your heat anticipator to 0.24. It is recommended to insert the tip of a pen or something similar in the slot to gently slide the needle to the desired setting. And this procedure, by the way, is still explained in modern install manuals.
Honeywall also gives a basic guideline for different heat types
To further adjust cycle times if the actual setting doesn’t seem to work quite right, you may change it accordingly: higher amperage setting = longer cycle time lower setting = shorter run time. I wouldn’t stray too much from the ‘’proper’’ setting, however.
— Ben Mongeau
Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade