Understanding furnaces

I had to have my furnace replaced recently, and some of the friends I’ve talked to about it have said that they only turn their furnaces on when it gets cold. This puzzles me - I thought that was one purpose of the thermostat: the furnace doesn’t come on until the temperature drops below some minimum that you set. With a modern thermostat, you can also set it to have different minimum temperatures at night, on weekdays and on weekends. If you have a pilotless furnace it won’t even burn any fuel while it’s not producing heat.

Given this, why do people think they save energy by turning their furnaces on and off manually. Am I missing something?

I notice you’re posting from relatively mild California. I can see this perhaps being the case if A) the climate is generally mild in winter and B) the furnace control knob is in a relatively easy-to-get-to location. Other than that, the behavior seems odd in some sort of perhaps misinformed way on their part.

Umm, so their furnace doesn’t inadvertently kick on in July when they have the AC blasting a little too much? I manually switch off my heat too. There is absolutely no scenario where I want the heat to be on during the summer, so why would I leave it up to the thermostat when I can flick a tiny switch that ensures it won’t go on?

I understood the OP to mean that his friends turn the furnace on and off as they feel cold in the house during the winter, not “turning on once as winter nears and then off after cold weather is over.”

In addition to having programmable temperature setpoints for morning/day/evening/night, my thermostat has a switch that gets flipped from “HEAT” to “COOL” depending on which system you want to run. Because of this switch, in my house it’s not possible for the heating and cooling systems to go to war with each other as you describe.

The switch also has an “OFF” position. During transitional periods in spring and fall, I’ll set it to “OFF” so neither the A/C or furnace can turn on. If the house dips a degree or two below 72F at night, I don’t want the furnace to come on in the morning (when the setpoint jumps back up to 72), since I know the house will warm back up as soon as the sun rises. This is a pretty small energy savings, true, but it seems wasteful to pay for natural gas heating when the indoor temperature is still tolerable and the sun is about to heat it up for free.

My thermostat is like Machine Elf’s except that it’s set to come on if the temp drops below 67° during the day or below 62° at night. In summer, we keep it around 78°. If we have nice enough weather that we can open the windows, I turn it off, just in case.

Plus we have a circulator function that turns on the furnace fan every 15 minutes or so - it keeps the house from developing hot spots or cold spots.

That’s kind of the point of a thermostat. Your AC shouldn’t be “blasting a little too much” if you set your thermostat right. If you want your house at 70 you set it at 70 and the AC shuts off at 70.
I’ve got the typical thermostat also with the “Heat/Off/Cool” which I find annoying to have to switch over during certain times of the year.
If I want the house to stay between 68 and 71 I would like the AC to kick on when it hits 72 and the heat to turn on when it hits 67.

Exactly. I turn the A/C and furnace off (I have separate systems with separate thermostats) for weeks at a time in the spring and fall. If it’s a cool morning, I don’t want the furnace running, because it is likely to heat up later in the day, and I don’t want to waste money heating the house when it’s going to warm up anyway. Same thing if it gets a little toasty in the afternoon, but is supposed to cool down in the evening and at night. I let the thermostats do their jobs in the summer and winter, but I provide a bit of manual control in the spring and fall.

That’s why a decent programmable thermostat is such a good idea. You can tailor your set points so that you don’t have to muck about with turning the furnace/AC on and off even on those interim days you speak of.

My reasoning, too. I live in Tucson, so we have very mild winters, and I tolerate colder temperatures very well. I have my thermostat turned off during the day, the temperature inside hardly drops below 70°. At night when it gets chilly, I’ll turn it on for awhile, then turn it off when I go to bed. I don’t have a programmable thermostat, so I can’t set it for more than one temperature.

Doesn’t quite work that way… every spring and fall, we’ll end up with some stretch where we have wild temp swings. For example, it was something like 78 degrees last Monday (AC weather) and then we had these cold fronts come through, and it was 22 degrees (heater weather) something like 26 hours later. And now a week later, we’re back up into the low 60s. That’s an extreme example, but it’s not at all uncommon to go from mid-70s to lower 40s over the course of a day, and on into the upper 20s overnight if a strong cold front comes through. Then it warms right back up a few days later.

So we basically keep tabs on the expected temps and flip the switch from “HEAT” to “COOL” and back as needed. At some point, it stays at “HEAT” during the winter and “COOL” during the summer, but there are definitely times when the temp swings are drastic enough to need to manually switch it.

In much of the Bay Area people don’t have air conditioning. In the central Bay Area we typically have only a week of really hot weather, and it wouldn’t make sense to invest in something with so little utility. The climate pattern changes in areas where there’s a ridge between you and the bay - the summers are hotter and the winters are colder. So, for example, houses in Walnut Creek have air conditioners, but not houses in Berkeley.

It seems to me that a furnace and an air conditioner won’t “fight it out” as long as there’s a gap between the minimum and maximum temperatures set on the thermostat. So, for example, if the furnace is set to kick in if the temperature falls below 68, and the air conditioner is set to kick in if the temperature rises above 78, they won’t work against each other.

Why not just turn the thermostat down to a temp where you would like it to kick on? Set it and forget it. Why waste time turing the furnace off and on manually?

I don’t really waste time, as I typically go for weeks in the spring and summer without using any heat or AC. I don’t want to have the heat kick on just because the temp overnight dipped, when I know it will be warming up the next day. I don’t turn the furnace “off and on” manually, or at least not very often. If I waste any time, it’s a matter of seconds. I’d rather do that than waste money heating or cooling when I really don’t need to.

As someone with an old heating system… let’s rephrase that… ancient heating system, turning on the furnace is a ‘thing’. It has a naked pilot light, which uses about $20 of gas every month all by itself. It also needs to be drained and refilled with clean water, the sight glass checked for clogs, etc. Then, once you fire the puppy up, you let it get nice and hot that first day to check for leaks in the radiators, or bad air vents.

On the plus side, if my power goes out, I still have heat and hot water, no electricity needed.