The Greek Goddess of Open Toed Sandals

Let’s get hysterical about hysteresis for a moment. If you’re new to this HVAC game, you might think hysteresis was the Greek goddess of open toed sandals or something. Or maybe hysteresis is that problem that sends your wife to the pharmacy at 11 PM on Saturday night, wearing sweatpants, because, you know.

No, hysteresis is a somewhat less interesting subject than that. Technically, hysteresis can apply to all sorts of things, like magnetism or the gas pedal in your car. In short, it means a lag between what you want, and what you get. The only place you’ll probably ever encounter it as a topic of conversation in the wild is when you’re talking to the dirty HVAC installer who leaves handprints all over your wallpaper while adjusting your thermostat. He’ll say, “How much hysteresis to you want, anyway?”, and you’ll mumble and dissemble and try to remember Greek legends to no avail.

I’ll try to explain it. Let’s say you want your house to be 72-degrees. With the panache of Canute, you command your furnace, if it’s winter, to make it so. Of course you’re a bit of a coward, and are justifiably afraid of confronting your furnace face to dial (it’s dark and scary down there), so you send an envoy instead. This envoy, your thermostat, sends a strongly worded communique via a twelve-volt DC tingle through a tiny wire or two to your furnace. But your furnace isn’t afraid of you. It knows that if given a Sophie’s Choice between the furnace and her husband, any sensible woman would choose central heating every time. So the furnace tells you to wait, tough guy, until it’s ready to give you some heat.

That’s a form of hysteresis. It’s related to inertia. In the same way your teenage boys on the couch respond to stimuli, your furnace doesn’t jump to it immediately when ordered to do things. It’s a smart policy on their part. People fumble around with the settings on the thermostat constantly, and hysteresis helps them sit shiva on the current settings until you make up your mind, or your thumb, if you have a touchpad. Some time passes, and it finally decides to trust you, and the furnace clicks on.

But you’re not done with hysteresis. Not by a long shot. Let’s say it’s 66-degrees in your dining room, and you want 72 in there. With most furnaces, they’ll hit 72 and keep going. They figure you’re an excitable sort, and if they halt the heating proceedings exactly at 72, the temperature will drop to 71 pretty quick, and you’ll start banging on the thermostat again right away. So the furnace is set to over shoot the temp you want. That’s hysteresis. By the same token, if the low temp is set for something like 66, so you can sleep at night with more than a sheet over you, it likely will not rouse itself until the temp reaches 64, and then click on. Coming or going, that’s hysteresis.

Of course the modern electronic thermostat has many, many settings that adjust hysteresis. They involve calling up fourteen menus on a tiny screen and interpreting and entering more codes than it takes to launch missiles from a Trident sub, but it can be done. In practice, whatever comes from the factory is good enough, whether it is or not. Our thermostat stops a few minutes after reaching the desired temp, before it adds an additional degree, and won’t come on again until a short while after the temp is two degrees below the desired. It actually tells you to WAIT on the screen when it hits a set point. Mighty haughty, your thermostat is. Your mileage might vary.

So hysteresis is smart and stupid at the same time. It’s smart, because the furnace hates cycling on and off over and over over minor changes in temp, so it overshoots a bit when it runs, and waits a little before it runs, or doesn’t run. That last sentence is kinda confusing, I admit, and doesn’t explain a lot, but how many chances does an author get to write “on and off over and over over” and still make the sentence parse? I couldn’t resist.

So that’s the smart part. The dumb part is that the ideal heating appliance would run continuously, so hysteresis would be out of a job. It would adjust itself continuously to the conditions and keep everything steady-state. The Amalgamated Brotherhood of United Hysteresis is a very powerful union, though, so this ideal heating device is rarely discussed, never mind invented.

Modern houses are usually much better insulated than in the past, and weatherstipped until a firefly in a peanut butter jar with holes punched in the lid has better ventilation. The heating (and cooling, of course) devices are better at doing their thing, and the thermostats are more accurate than the big clunky dial on the wall of our utes. But there’s still a fair bit of hysteresis involved, especially if you have too much money, and are the impatient sort.

Let’s say you’re building a new snouthouse and you’re determined to finally have things your way and you’re not scrimping on anything this time around. You inform the General Contractor to inform the HVAC guy to put in the HeatSmiter Megaladon furnace or the the COOLpro Ice Station Zebra model AC blaster. You’re not going to wait for the house to warm up, or cool off, ever again. You tell him you want your hair to be blown back like a biplane pilot when the thing comes on.

This is dumb. HVAC doesn’t work (well) that way. You may want a machine that does the trick, but a machine that does the trick+10 is a worse machine, not a better one. If you select a bigger air conditioner than you need, the condenser outside becomes a block of ice. The heat inside your house that you’re trying to get rid of isn’t enough to warm up the circulating refrigerant to keep it from freezing up. By the same token, if you have a ginormous furnace, the hysteresis will be notable, because it will blast way past what you actually require before it can stop. It will run a lot, just for very short periods of time. If you know anything about machinery, you’ll know that starting and stopping is what wears them out quick. And you’ll be too hot and too cold over and over anyway.

So we couldn’t afford a bigger heat pump, or to run it even if we could. Good. The bigger one would have frozen up solid when we ran the A/C, because we don’t need very much here in Maine. And the bigger heating capacity would have been nice, but bankrupting yourself to run a bigger thing less often wouldn’t help us. We were going to achieve the closest thing to steady state HVAC we could cook up.

[To be continued]