Does lowering the blower speed of a forced-air furnace lower its efficiency?

Question

My gas furnace has two options for the blower speed: MED, and MED-HI. It had been set on the higher speed, but that was too harsh, so I lowered it to the slower speed. Does that have any impact on the efficiency of the unit?

Answer 1

After a bit of research, (translated: "mad google skills") I found an article claiming that lowering the blower speed may increase efficiency. I'm not completely convinced on this, since the claim is made by a company that sells variable speed blowers for retrofitting HVAC systems. I would think adjusting the heating element would save more energy than reducing the blower's electric consumption, but I didn't do a study on it myself. (Heat transfer wasn't one of my favorite classes in college - I focused more on dynamic systems, sorry.)

Article Excerpt:

Many gas-fired, high-efficiency furnaces also save on the electricity required to power the blower motor, though this savings is not factored into the AFUE rating. They do this by coupling a sophisticated, programmable thermostat to a variable-speed motor. Unlike a conventional system—where the furnace goes on, blows hot air into the house at full force for a few minutes, and then shuts off—a variable-speed or “variable-capacity” system runs the blower for longer periods at lower speeds. It provides quieter and more even and comfortable heat than a conventional furnace and doesn’t consume electricity unnecessarily.

For homeowners who want to upgrade rather than replace an existing furnace, Emerson Electric offers an exclusive replacement blower motor called the Mark Z that can be set by the homeowner on a continuous, slower-than-normal speed. It offers three settings: 525 RPM for saving energy, 900 RPM for heating, or 1100 RPM for air conditioning. According to the company, the slower circulation speed reduces energy consumption by 75%.

If you really wanted to be scientific, you could perform an experiment and post the results here. I'd go with logging temperature differential (inside temp - outside temp) vs. energy consumption every day for a month with the blower on high and then for a month with the blower on low. That will give you the most reliable results for your particular system.

Here's what to record, in order of importance:

1. Blower Setting - You'll need a significant time for both MED and MED-HI. I'd say at least two weeks of each, and maybe up to a month. It depends on how much cold weather you normally have during the winter. You could try 8 weeks of alternating 2 week periods.
2. Daily Energy Consumption - You'll have to get this off of your electric meter. If you run any high-power electronics or tools that you don't normally use, you may want to make a note of it on those days that you do.
3. Average Daily Outside Air Temperature - This data will allow you to adjust for climate change which leads to increased/decreased HVAC loads. A high/low for the day wouldn't hurt either.
4. Inside Thermostat Temperature - If you keep this pretty constant (i.e. don't turn the air off/lower during the day) this shouldn't be as big of a factor.
5. Average Daily Wind Speed - High wind speeds increase the convection coefficient between your exterior walls and the outside air.
6. Precipitation - Snow accumulation on the roof and humidity from rainfall can also effect heat transfer rates.

The last two (5 and 6) aren't as important, but more data is always better. (Trust me, the worst thing that happens to me on the job is having inconclusive results due to insufficient data collection during an expensive and time-intensive study.) It shouldn't take more than a few minutes a day to record the data anyway. I'm not sure exactly the formula I'd use to normalize the results, but I'm pretty confident I could come up with some correlation if I had the data to play with. If a clear distinction between the blower speeds isn't obvious in the data, then you're probably not going to save much money either way, so go with what's most comfortable for you.

Answer 2

If the furnace manufacturer supplies different burners for that unit you may be risking premature failure. The heat exchanger can be overheated and burned out if the blower does not remove enough heat. Proper duct sizing can resolve the issue you are having. Sometimes you have to add more ducts to achieve the proper pressure and velocity.

Answer 3

Empirical Engineering Answers:

Yes: you will save electricity costs by running your blower on low.

No: you will not save on gas/oil costs by running your blower on low. Because the slower air velocity over the heat element is transferring fewer BTU units into the circulated air and they simply go up the exhaust pipe.

The NET difference of the above two figures is your savings or cost.

I am currently heavily invested in studying and learning the subject as I commit myself to reducing my winter energy costs this year.

With heating oil at $4.50/gal I can assure you without measuring anything that running my blower on low is NOT more efficient from a $$ perspective. I'm in the process of re-wiring my blower to run on high with the furnace. In support of this: I found the mfr owner's manual for my furnace and in it, it states, "if you have an air-conditioning coil installed, you should switch the furnace blower speed to high to accommodate the decreased air-flow caused by the cooling coil in the plenum."

I have also measured that on low speed my furnace requires AT-LEAST 1 hour of constant running to raise my home temp by 6 deg F. On high speed it takes 30-40 min. With a one-gallon-per-hour burner nozzle that saves me AT-LEAST 30% on fuel oil plus another 20-30 min of blower run time or at least 30-50% run time savings. Now, I haven't done the measurement to learn how much electricity my blower uses on low vs high. But I'm reasonably sure it uses less to run on high vs low then it uses to run on low vs OFF. I do know that my entire furnace runs safely on a 15amp breaker with the blower on high, the burner running, the humidifier running, and the water pump to empty the humidifier running. Thus, my blower can't be using that much electricity on high.

So clearly, running your furnace blower on low is only more efficient IF-and-ONLY-IF the cost of the heating fuel is much much less then the cost of the electricity to run the blower that little bit faster.

Answer 4

Yes--the efficiency of the heat exchanger will decrease. The blower energy draw at a lower speed will be offset by the loss in fan wheel inertia. But you will have a tough time measuring either on your basement Trane.

If you want to solve the air noise problem, move the return air filters out of their hallway locations back to the upstream side of the return air duct at the furnace.