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Furnace Fan Setting: On vs Auto

If you’ve ever looked closely at your thermostat, you’ve probably noticed two options for the fan: on and auto. Or maybe you’ve wondered why your fan runs some times and not others. What’s the difference between auto and on for a furnace fan, and which one is better?

Keeping your furnace fan set to auto or on is a choice you’ll have to make depending on your priorities and needs. Read on to learn more about these two options and the benefits and downfalls each can offer.


What Is Auto?

The auto setting on your furnace fan relies on your thermostat to determine when the fan needs to be running. You may hear it kick on and off as the temperature in the house fluctuates. If you have a smart thermostat with a schedule on it, the auto setting may mean you never have to touch your thermostat again.

Your thermostat will monitor the temperature of your house and compare it to the temperature you have it set on. When the house gets warmer or colder than your set temperature, the thermostat will send a signal to your furnace fan to kick on. When the house reaches your desired temperature again, the thermostat will signal your furnace fan to shut off again.


What Is On?

The on setting on your thermostat is more or less what it sounds like. Your fan runs all the time no matter what the temperature in the house is. But this doesn’t mean your house will always be the Sahara or a freezer.

Although your fan runs all the time, that doesn’t mean your furnace or compressor does. Your thermostat still monitors the temperature in your house and switches those on and off as appropriate. But even when no hot or cold air is being produced, the fan is still running, circulating air through the house.


Better Air Quality

Leaving your furnace fan on all the time can have some limited benefits. The biggest one of these is air quality; running your fan all the time means your air gets run through your filter all the time. It never has time to stagnate, and any unfiltered air that comes into the house through doors, windows, or leakage spots gets run through the filter almost immediately.

If you have bad allergies or a severe case of asthma, having this improved air quality can be critical to your health. This can also be important if you live somewhere very smoky or polluted. But the difference is negligible, so unless you’re in one of these extreme situations, you won’t notice much of a difference from this change.


Even Temperature Distribution

The fan running all the time also means air gets circulated through your house more completely. You may notice that there are hot or cold spots in your house. These usually happen around doors and windows, where air leakage tends to be the worst.

These hot and cold spots happen because outside air leaks in at those spots, cooling them off or warming them up depending on what the weather outside is. But if you have a fan running all the time, those spots don’t have time to build up warm or cool air. This can help keep the temperature inside your home more consistent all the way through.


More Filter Usage

You might not be surprised to learn that more fan use means more filter use. Remember that improved air quality we were talking about that comes as a result of filtering air constantly? Well, that constant filtering also means the filter fills up with particulates more often.

In a house that’s running the furnace fan on auto, you want to aim to change your air filter out every month or so. But in polluted areas or houses that run the air filter all the time, you need to swap them out more often. It’s a good idea to plan on switching filters every two to three weeks, which can start to add up in filter costs.


Higher Energy Costs

On the subject of higher costs, one of the biggest drawbacks of running the fan all the time is the high energy costs. A standard furnace fan motor uses about 400 watts of electricity every hour. Assuming a national average of 12 cents per kilowatt-hour, let’s see how much that would cost you in the long run.

If you run your furnace fan twenty-four hours a day, it will cost you a little more than $1.15 every day. Over the course of a year, you’ll spend $420 running that fan. But if you reduced that running time to twelve hours a day with the auto setting, you could save more than $200 every year.


More Frequent Repairs

All equipment has a lifespan, and how long a piece of equipment lasts depends on how long it takes for that lifespan to get used up. With use, parts wear out, pieces break, and you have to replace them. That can start to add up to a lot of money in air conditioner repair costs over the years.

It may seem like starting and stopping the fan would put more wear and tear on it than running it at a constant speed. But the amount of extra time it’s running puts more strain on those pieces than starts and stops do. If you run your fan all the time, you can expect to have to repair it more frequently.


Higher Humidity

One of the unseen functions of an air conditioner in your home is to remove humidity. As your air conditioner cools your house down, it’s also removing moisture from the air. This is why you don’t want to get a bigger unit than you need; the quick cool-down times don’t leave time for humidity control.

The same principle applies to running the fan all the time. Because the fan is already running and the air is already moving, when your air conditioner kicks on, it doesn’t have to run long before the house is cooled down. But if you’ve ever been to a coastal city during winter, you’ll know that a cool humid environment is no fun to be in.


Worse Leakage

No matter how tightly you try to close up a system, HVAC systems always leak a little. Most of this happens in the ductwork, where energy is lost when that air slips out through tiny holes and gaps. Most of the time, this is a negligible enough amount that it doesn’t cause any real problems.

But imagine how much more air will seep out through those gaps if it has a fan moving behind it instead of just sitting still. When you run your fan all the time, you’re constantly driving air out through those gaps. And that means you’re losing energy and money every hour of the day.


Colder in Winter

If you’re a cold-natured person, you definitely do not want to keep your furnace fan on all the time during the winter. Heat is essentially energy from air particles bouncing around, and the more they bounce, the more energy they lose. Fans moving air particles around strip them of that energy and make them colder.

If your fan is running all the time but your heater isn’t, you’re losing the residual heat from those particles all the time. This means your home is going to get colder between those bouts of heating than a house with their fan on auto would. You may wind up cranking the heat up further to compensate, wasting energy and landing yourself in a continually changing temperature environment.


How to Improve Your Air Quality

Let’s say you’re one of those people with allergies or asthma that we discussed earlier. You need the extra air purity, but you don’t want to sacrifice all these other downsides that come from running your fan all the time. Don’t worry – there are ways to improve your air quality that don’t involve leaving your fan on.

First of all, try changing your air filter out more frequently. You will spend a little more money on filters, but it won’t be much compared to the money you’d lose running the fan twenty-four hours a day. Taking steps to weatherproof your home and reducing allergens and triggers inside your home can also help.


Save Your Furnace Fan

Running your furnace fan all the time is a good way to waste energy and money. If you set your fan to auto, your system will last longer, your energy bills will be lower, and you won’t go through filters as much. Use other methods to deal with air purity and save yourself the costs of leaving the fan on.

If you’d like to make sure your air conditioning is in top-notch shape, reach out to us at Kaiser Air Conditioning. We have been providing high-quality heating and air conditioning services since 1981. Contact us today to get your free home estimate for heating, air conditioning, and/or duct replacement.

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