Comment by jay-saint | Hacker News Reader

Comment by jay-saint | Hacker News Reader

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I have been told by multiple HVAC professionals recently that one should keep their fan on their system always running. They have in fact adopted this practice at our offices and it greatly help with keeping temperatures in consistent from room to room. The arguments they provided for this were: [1] Constant air flow creates more consistent temperatures (less highs and lows from interior to exterior or floor to floor. [2] Allows filtration system to work continuously cleaning the air. [3] (This is the one I am skeptical of) Starting the fan motor has a large current draw, but leaving it running is very low current on modern high efficiency HVAC systems. Also apparently when the fan starts i the most likely time for fan failure due to high current and torque on the motor. These fan motors are typically not repairable in small commercial and residential units and are quite expensive.

We adopted the fan always on with scheduled temperature adjustment here at our office, it is more comfortable for sure and less dusty. I do not know if it affected energy costs. When I tried this at home it was more comfortable but my electric bill went up by ~$20

For offices, it's likely more a matter of keeping pockets of hot/cold air (or other quality issues not related to temperature) from forming. Even just with lighting and office equipment, there's a fairly considerable energy flux into many office spaces, and without mixing you'll tend to get uncomfortable pockets forming.

Your #3 point strikes be as fairly valid: large electric motors do impose a very high load, and the current draw could be harmful to the motor (and switching / control circuits), while the imposed load might also be hard on other equipment. I doubt the energy usage argument (for cycling the motors off) really carries much weight, and it should be possible to idle such systems at low power. For most commercial buildings, you've got systems for heating, chilling, and air handling which are pretty much all operating simultaneously: you have a need for hot and cold air in different places, there are mixers which will deliver what's needed where it's needed (at least in theory), and the fans blow all the time.

Though I've only a pretty glancing familiarity with HVAC in general, not particularly my area of expertise.

I run the circ fan all the time as well, but the bearings on the motor do have a limited lifetime. Ask me how I know this.

A new motor will cost hundreds of dollars, more so if the blower and fan cage are a single assembly. You'll need to amortize that cost into your comfort level.

It will also quit at the least convenient time, following a combination of Tuttle's and Murphy's Laws. Again, ask me how I know this.

About 3, the idea is right, but they're probably overstating the difference between starting power and running power.

Starting a motor is not something you do lightly for bigger motors (but big like a subway or a car) but I doubt they're that heavy for it to be a big concern.

My bet is that it'll spend the starting power in around 5s of running, tops.

[3] is bunk. Those blower motors are immense; if they didn't use a lot of energy as they ran they would be a lot smaller.

In my limited experience the run/start capacitors fail more often than the motor itself.

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