Furnace Blower Fan on 100% of the time - Good or Bad or it depends?

I was at a dinner party last night and two guests described how their HVAC reps told them to run their furnace blowers 100% of the time.  The HVAC reps reasons for the recommendation, as relayed by the dinner guests, were:
1. continuous running reduces the wear and tear on the blower motor that cycling on and off causes, thereby avoiding an earlier motor replacement
2. circulating the air around the home provides better balanced temperatures throughout the home and, 
3. continuous air flow would provide cleaner air due to the continuous filtering.

We live in northern Michigan and are a heating dominate climate.  I am curious to hear from the HVAC professionals about what they recommend to their clients and why.  At the moment I don't buy the above arguments and feel those customers end up paying more in energy costs for little benefit.  However I do understand there is no disputing tastes when talking about perceived comfort.  Also if anyone knows of a good source or any research done on this issue I would like to see a more in depth analysis (if its out there).

[NOTE: Neither home has an ERV, HRV, or other mechanical ventilation other than intermittent bath and kitchen fans.  Both are natural gas, forced air furnaces.]

Any discussion on this subject?
Thanks,
TJ

Tags: HVAC, IAQ, energy, motors

Views: 128709

Replies to This Discussion

TJ,

Prior to my 'Repotting Career Change' to HERS Rater, I served as Administrator of a 36,000 sf skilled nursing facility, from 1983 - 2009. I purchased a lot of new furnaces over the years. We were heated and cooled with residential style hvac units. I had 21 units ranging from 2 tons to 5 tons of AC.

I have replaced units due to cracked heat exchangers, and bad compressors. I have replaced squirrel cage fans that got out of balance. I have replaced units due to age and due to Hail Storms.

I have not experienced a blower fan motor failure, or heard of them among my fellow administrators. I have experienced motor failure of the ventilation fans that are required by state and federal law to run 24/7. These routinely fail, and must be replaced.

The ventilation fans meet a requirement to maintain a depressurized difference from common areas to patient rooms, to bathrooms. This is an infection control issue, and secondarily a odor control issue.

I think running a 400 CFM (per ton - so 1200 - 1800 CFM) fan to provide fresh air instead of a 60 or 100 CFM balanced ventilation system is not good building science. it is certainly not good energy efficiency and energy management.

I've found most blower motor failures are due to unbalanced blower wheels (bearings go bad) or overheating (windings burn up). Both caused by excess dirt in motor/blower wheel.

Thanks for giving your furnace/motor experience John. One issue I have with the 100% furnace fan run-time is that it does not provide 'fresh air' but recirculates the indoor air. This practice would certainly require a increase in air filter replacement. And yes seems to me to be an energy waster. Anyone hear the argument that motor start up amps are huge when cycling, so better to leave the motor running and you could 'save energy'? Sounds like a similar argument we've heard about lighting. Any electricians want to chime in on this one?



John Nicholas said:
TJ,

Prior to my 'Repotting Career Change' to HERS Rater, I served as Administrator of a 36,000 sf skilled nursing facility, from 1983 - 2009. I purchased a lot of new furnaces over the years. We were heated and cooled with residential style hvac units. I had 21 units ranging from 2 tons to 5 tons of AC.

I have replaced units due to cracked heat exchangers, and bad compressors. I have replaced squirrel cage fans that got out of balance. I have replaced units due to age and due to Hail Storms.

I have not experienced a blower fan motor failure, or heard of them among my fellow administrators. I have experienced motor failure of the ventilation fans that are required by state and federal law to run 24/7. These routinely fail, and must be replaced.

The ventilation fans meet a requirement to maintain a depressurized difference from common areas to patient rooms, to bathrooms. This is an infection control issue, and secondarily a odor control issue.

I think running a 400 CFM (per ton - so 1200 - 1800 CFM) fan to provide fresh air instead of a 60 or 100 CFM balanced ventilation system is not good building science. it is certainly not good energy efficiency and energy management.
Great question and what I understand thru a building science aspect that keeping something running all the time is the best. Didn't make much sence to me either untill someone explained it to me (below). I am not a HVAC person so I don't know what should be on always, if the blower or A/C or HEAT is on I don't know which one. It also lets the home stay at a comfortable temp. It takes alot of energy to get a hot house cool and vice versa
Here are the things I was thought thru building science in re to energy The analagy I was told is let's say you take your car on a roap trip, if you went straight without a stop of any kind and cruised there at 55 mph you would use a lot less gas, wear and tear on your car and less overall time vs getting on the road to do the same trip, in couple off ramps you need to get your coffee for the trip, you stop your car, turn off the engine, open the door, close the door, take 15 mins in the store get back in your car, start the engine step on the brakes at lights, excel up the freeway ramp get to steady state now you need to go to the bathroom, get off, step on the breaks..... on the fwy again get up to speed... its lunch time ... you know where this is going right. Your car, mpg, brakes...deceleration, exceleration. You can see the wear and tear your putting on your car and the time it took is much longer. Same with how a proper HVAC unit should work and it is important to do your twice a year service. Most HVAC units are oversized due to our homes leaking as much as they do. I am in Southern Cal and since our temp is so tepid, our construction is just slapped together and very leaky. We are learning more about this because now we are getting hotter summers and cooler winters and our utility bills keep rising. So people are starting to listen cuz in this economy every penny counts and they are no longer comfortable with their indoor air and their homes that have no insulations in their attics or walls.
I would be interested to hear more and their thought because John (other commentor) who had a great explanation the way I could understand, is different than what I was taught. I think this is all real new to us and us is So Cal and we need to keep records of homes performance in different regions and talk about it in 2 years and 5 years...
This is why these forums are exceptional knowleadge and a learning arena. Thanks BIG for putting this together.
Judi Lyall with SHE BUILDS "GREEN" GC, BPI, HERS, GPR...
The right answer is "it Depends" Constant air movement give the percieved effect that moving air is more comfortable than still air, it in effect does the same function for the home that a ceiling fan does for a room. It affects the person not the room temp. there are two problems that can occur. In winter air movement can create wind chill if the fan speed is to fast or blowing directly on a person, in summer with air condioning, it can reduce the amount of humidity that a properly operating a/c can remove from the air. When the a/c cycles off the fan will continue to operate and raise the evap coil temp until it is above the dew point of the home and the moisture removed can be reintroduced into the home off the coil or drain pan. If the humidity seems high in summer, turn the fan back to auto. Newer furnaces with variable speed fans (ecm) will drop down to a lower speed when just moving air and they operate at a reduced cost compared to standard motors Some can run all year long at a lower cost that a regular motor will running on auto. They can save enough electricity to pay for the funace over its lifetime. Filters work better with continuous fan, they can filter only when there is air movement ( next question is what kind of filter do they have) Constant air movement can help reduce hot and cold spots but the root cause of this is poor duct design and installation. The poorer the system design the worse the problem - turning the fan on is easy and while it may not help it doesnt actually hurt anything and is an easy thing to try. While assumptions about all homes can be made the devil is in the details and each house and how it performs is as individual as the people who live in them. Just to let you know, I am a HVAC contractor with 35 year experience and training in enrgy conservation and auditing. That being said I enjoy learning every day (its the monkey in me LOL) Anyone with additional info or ideas, Im all ears
Steven,
Thanks for commenting and offering your experience. I'm with you about constantly learning something new. It's never ending in our fields! I also agree with you about poor duct design and installation. It is a root problem for many issues seen.

As for running the fan on 'auto' or continuously 'on', from an energy perspective I tend to believe that 'auto' would be more energy efficient over time regardless of whether an individual fan is variable or not (I'm not comparing a variable speed vs a conventional motor here just the fan setting). Wisconsin's Focus on Energy recommends setting fans to 'auto', see link here. They offer commentary on common assumptions about running blower fans continuously. But Focus on Energy's take contradicts what my friends were told by their HVAC reps. Focus on Energy's assumption #3 really speaks to your point that running the fan for temperature stability really masks other issues such as poor duct design, leaky envelope areas, inadequate insulation, etc.
Thanks for the link. very nice information. I agree with you that in regards to energy use the best way to conserve energy is to run the fan on auto. I went back and reread your original post and the HVAC reps recommendations for operation actually had little to with energy use as a main consideration and mostly concerned comfort/health issues. The first suggestion as to wear is one that I would like to see data on. The others regard operation to hopefully improve comfort with little additional costs.

If we only look at energy usage, the best solution is to not run anything. Being human (lol) doesnt really allow for that option but it is an part of the equation that we all should use when we evaluate our own personal choices as we balance comfort and conservation.

information is important but understanding is key. People want to do the right think but may not understand what they do or why they do it other than someone recommended it. as an example - a customer told me that they were saving energy by putting ceiling fans in every room and ran them all day long to make the house cooler. The problem was that there was no one home all day. Had to explain that moving air help the human body cool but has no effect on the house temp except to raise it due to the heat generated by the 9 ceiling fans.
I think the ideal situation would be if you had a high efficeincy ECM variable speed fan motor, and the system was integrated with an HRV or ERV to provide whole house mechanical ventilation. Then running the furnace fan blower on a low setting 24/7 would be a good thing to do, for continuous air circulation and filtration, as well as fresh air circulation.

I know a lot of HVAC installers who recommend keeping the fan blower on all the time even when it is not hooked up to an HRV, just for circulating air to better balance temperatures and for better air filtration. But usually this is only when the furnace air handler is a variable speed ECM.

So basically, I buy arguments 2 and 3 regarding air circulation and filtration, but on the condition that the air handler is a high efficiency variable speed ECM and the fan is running on a low setting.

I don't buy running the fan all the time on a high setting, and/or if it isn't a high efficiency fan. And I don't really buy the energy saving argument in any case. I think even if it is technically true, it is probably minimal.
Most furnace blower fan motors are only 3/4HP to 1.5HP and only draw .5 amp to 2 amps at most. The coast of running that 24/7 a year is only $50 for the year if your electric company charged you $.09 a kilowatt hour. By turning it off and on with it in on auto you might save $5.00 a year. The coast is minimal.
This is just plain wrong. Standard furnace blowers use about 500 Watts and setting them to "ON" for the whole year vs. AUTO wastes about 3,000 kWh/yr. This is a tremendous waste of energy. In addition, in cooling season the fan adds significant heat to the home directly and indirectly if the ducts run outside the conditioned space.

Jeremy Kays said:
Most furnace blower fan motors are only 3/4HP to 1.5HP and only draw .5 amp to 2 amps at most. The coast of running that 24/7 a year is only $50 for the year if your electric company charged you $.09 a kilowatt hour. By turning it off and on with it in on auto you might save $5.00 a year. The coast is minimal.

+1

Great topic for discussion here, TJ. The answer is that you should NOT leave the fan in the ON position. It will cost a lot more in electricity, as Michael Blasnik said. If the house has unbalanced duct leakage, it will also cost extra in heating and cooling because of the added air leakage.

Yes, having an HVAC system that runs longer has benefits, but you want to achieve that by having a properly sized system and, if you can afford it, a variable speed blower with an ECM. As has been mentioned by others, running the fan in the cooling season in a humid climate will increase the humidity in the house because it will evaporate moisture from the coil and drain pan.

If you want cleaner air, filtration is the last step you take, so your point number 3 isn't valid either. First you get bad stuff out of the house and seal the air leaks and duct leakage so you don't bring more bad stuff in.

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