HVAC Question about running fan continuously VS setting on Auto so it runs when AC or furnace runs

I get a lot of confusion from homeowners who are told by their HVAC contractor to run the fan "on" all the time.  I realize that these high-efficiency furnaces now have the DC motors that take less electric cost but should they run all the time? 

I want to give my customer the correct answer and not which is correct or if there is a correct answer.

I know the older alternating current motors take a lot of electricity to run for a year.

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There is more than fan energy to be saved by setting fans to run on "auto". Most forced air heating/cooling systems in homes do not have individual zone return air ducts. Air returning from a bedroom typically has to pass beneath the gap at the bottom of the bedroom door. Running the fan with the door closed pressurizes the bedroom and forces air in the bedroom out of gaps in the windows, light fixture openings and other gaps to the outside. Air leaving the pressurized room causes the rest of the house to go negative and requires bringing in additional outside air that then needs to be conditioned.
There are also duct leaks in the supply and return air ducts that can lead to additional air infiltration.
Running the fans continuously creates even temperatures throughout the house, but it will typically use more energy, especially if the doors are closed throughout the house.

Qualified "It Depends" kind of answer.

1. Temperature Perception:
There is an intangible comfort advantage to "Fan-On" in that it obviously provides continuous circulation. In a tight, modern house, there is a certain "deadness" and stagnation that some people experience when the fan is off. I know that I do. So with constant circulation, a universal swizzle stick, is always stirring and reducing, narrowing, the temperature differential. Where the thermostat is near the return, it is more likely to be sampled and thus more responsive.

2. Outside Air Ventilation (especially via ERVs/HRVs):
When the central air system is the final delivery method, the fan should always be on to give greatest benefit. If ducted to and from the return (as is a common method), when the fan is off, the pre and post-recovery air simply short circuit, "play catch" if you will, in the return duct. This does nothing for the occupied space.
If your HRV/ERV is ducted in parallel or is interlocked to your prime mover, none of the above is of concern.

3. Fan Energy: In commercial work at least, the cost of moving air in our climate (Boston), exceeds the cost of heating and cooling the air combined. Granted this is a function of economizer operation and deep buildings with lots of interior space, where free cooling in winter is the way to go.
In a typical residence, say you have a 1/2 HP ECM fan responding to a constant volume need, maybe responding to filter loading or a wet vs. dry cooling coil. Say that fan draws 200 Watts (a shade over 1/4 HP) and runs 8760 hours per year. That is 1,752 kWH per year. At 18 cents per, that is $315 per year, practically a dollar per day allowing for weekends and vacations away. So it is something on all fronts.

I think my side of conservation would be to use the fan in constant-on when occupied and go to auto when away.

It is understood that the system will still cycle as needed of course but the time between cycles and deviation from set-point between cycles would be much larger. But if you are not home is that OK?

Your mileage may vary.

EDIT: I just read some of the other comments which covered a few of my points, I did not mean to just repeat but concurrence is good too. 


I have found that ceiling fans are the simplest way to control room stratification 

Agreed! True, if an open plan. Simple is best. But away spaces seem to be making a come-back. And as others noted, return air pathways, let alone directly-ducted return air to each room, is rare indeed.

Jerry, as an energy consultant I imagine you are well versed in solar systems. Have you seen how valuable whole house fans are when combined with solar? Here is a white paper by an expert, Jonathan Hill - 


Jerry,  Luke below has also suggested the first thing that came to my mind.  An ERV/HRV is installed and the furnace fan at low circulation speeds is being used to mix the fresh air from the ERV/HRV.   It should be easy to check,  look to see if there is one installed.

The fan only runs when the furnace tells it to run. On cooling, the fan will only work when the air conditioner is running. Advantage. The "auto" setting means less power consumption because it keeps the fan running for the least amount of time with the lowest speed required for heating and cooling. If you are looking to know more on pros and cons of furnace duct cleaning then follow the blog link to read more.

Keeping the fan on creates a more even distribution of heating and cooling, circulating the air if you have cold or hot spots in your house, like a bedroom above a garage. Less frequent starts and stops of the fan can reduce the stress from startup, and potentially help extend it’s lifespan. Leaving the fan on 24/7 ensures cleaner air, as the air is pulled through the filtration or UV light system. This can be better for people with allergies.

It could cost you hundreds of dollars a year. The average unit has a 500 watt fan, which uses almost as much energy over the course of the year as a refrigerator, resulting in a $50/month bill. During the summer months, the ducts in your attic and close to your walls can fill with warm air, resulting in the fan blowing hot air into your house. Therefore, you may end up running your AC even more, to counterbalance the extra heat. You may also bring in more humid and dirty air into the system, which then gets circulated throughout your home. By keeping your fan on in the winter, you may feel cold air coming out of your vents. Although the air temperature will be similar to the space temperature, your house will feel cold.

It all depends on the house - attic ducts, older unit, poor return capability - but I like the thermostats that have the "CIRC" circulate mode that operates the fan for only a portion of the time.  In my experience, it does the job for stratification and for evening out rooms once the sun has heated a portion of the house.  Problem is, if it takes any occupant input, it probably won't happen the way you envision it.

The fan's auto setting works with your HVAC system so your fan is running at least amount of time needed and even at the slowest speed. Your furnace filter will last longer, better dehumidification, when you set your fan is on auto. We've also received several questions, where people keep on asking if they run their furnace fan continuously in fan on mode or not? Actually running the fan continuously does have several benefits. This can recirculate the cold air throughout the house which will help in maintaining air conditioners desired temperature.

The group ROCIS (Reducing Outdoor Contaminants in Indoor Spaces), found at ROCIS.org, has been playing with improved furnace ducting and efficient fan use for a couple of years. We have found that the majority of indoor particles can be eliminated by using an efficient furnace filter and continuous fan usage. The ducting, the fan motor, and the fan speed are all optimized by a contractor so that there is only a small incremental energy cost due to continuous circulation. Here is an article on the fan-filter work being done by ROCIS:  http://greenhvacrmag.com/2018/rocis/

It is true that a furnace fan running continuously will increase energy costs. The advantages of cleaner air and good indoor circulation (e.g. in closed-door bedrooms) may make that small cost increase acceptable.

The fan only works when the furnace tells it to run. The fan is controlled by a timer circuit or at a boundary circuit on the device. The fan starts to run for approx. 40 seconds after the burners are switched on and off for approx. one minute after the temperature is satisfactory. When cooled, the fan will only work when the air conditioner is running. The "auto" setting means less energy usage because it keeps the fan running the least amount of time at the slowest speed needed for heating and cooling. Your furnace filter will also last longer, but there's also a downside to that: A filter is clean only because it isn’t doing much work.


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