Did you know your HVAC blower speed is easily adjustable?

[gorcorps]

I had no idea. Apparently there's four different colored wires that are each for a different blower speed. From what I read, going slowest to fastest is: red, blue, yellow, black

You connect whatever speed you want to the cooling jumper, and another one to the heat jumper, and the other two just go unused. My system had the black one on heat, and yellow on cool which seemed backwards to me so I flipped them. I've never been able to reliably get below 73 degrees in the summer and I wonder if this will help.

 

[Raizinman]

HVAC systems are normally designed for a specific speed fan for best heating and cooling. Changing or tweaking this is not normally recommended. Too slow a fan speed can waste energy and too fast a fan speed can also prevent proper cooling or heating.

 

[NutBucket]

If the fan is too fast/slow it won't properly transfer energy from the hvac to the passing air.

 

[stormkroe]

3rded

CFM and btu are mated by the engineer. Changing one without the other can either cause too-cold air or burnt out plenums.

 

[Red Squirrel]

It won't really "Waste" energy but making it slower might cause the heat exchanger to get too hot. It might also reduce the amount of airflow throughout the house so some places will feel hotter than others. You could experiment, but I would keep track of the plenum temperature as well as various rooms just to compare.

For AC, too slow might cause the coil to freeze up.

 

[gorcorps]

It won't really "Waste" energy but making it slower might cause the heat exchanger to get too hot. It might also reduce the amount of airflow throughout the house so some places will feel hotter than others. You could experiment, but I would keep track of the plenum temperature as well as various rooms just to compare.

For AC, too slow might cause the coil to freeze up.

They recommend to not hook up the lowest speed (red) to the heater as it can easily overheat, and that typically the cool side would be set faster than the heat. Other than that, any changes I do is to fix what I consider to be an already sub-par HVAC system. The more likely scenario is the system is undersized for the house, or starting to suffer being 16 years old, but I want to try fixes that are free before spending. Based on what I've seen in the house already, the previous owner may have messed with these speeds before me anyway.

 

[Greenman]

HVAC systems are normally designed for a specific speed fan for best heating and cooling. Changing or tweaking this is not normally recommended. Too slow a fan speed can waste energy and too fast a fan speed can also prevent proper cooling or heating.

This is correct as far as it goes, but most HVAC systems are grossly over sized, and not so much designed as guesstamated.

 

[Raizinman]

This is correct as far as it goes, but most HVAC systems are grossly over sized, and not so much designed as guesstamated.

I would disagree with that, being that most HVAC systems (residential in the last 25 years) are grossly under sized. HVAC building contractors will normally put a 10 or 15 percent smaller unit on the belief that a smaller more efficient unit running 80% of the time is better than a larger unit running 70% of the time going on and off more frequently. Most home owners tend to replace their original HVAC units with a unit about 20% larger or more for better comfort.

 

[NutBucket]

"Grossly" oversized doesn't work because if there isn't enough heat load the evaporator will freeze and/or the system will cycle off and on too frequently.

 

[WackyWRZ]

The purpose of not having the fan speed on high for cooling has a lot to do with de-humidification. Often engineering and setting up the systems takes location and climate into account as well. A temperature a couple degrees higher with lower humidity is much more comfortable than a low temperature with high humidity (RH). Also when heating the burner and heat exchangers work most efficiently at a certain temperature range.

From ASHRAE: "In dry climates such as the Southwest United States, the same issues happen with regard to the indoor (evaporator) coil: higher airflow helps, lower airflow hurts. In humid climates, the situation is much more complex. At higher airflows, there will be less dehumidification, leading to high indoor humidities. If the airflow gets too low, however, the evaporator coil may freeze."

 

[Greenman]

I would disagree with that, being that most HVAC systems (residential in the last 25 years) are grossly under sized. HVAC building contractors will normally put a 10 or 15 percent smaller unit on the belief that a smaller more efficient unit running 80% of the time is better than a larger unit running 70% of the time going on and off more frequently. Most home owners tend to replace their original HVAC units with a unit about 20% larger or more for better comfort.

Most of the places I remodel get a 50% increase in conditioned space, and a 30 to 40% smaller furnace. We run them at a low level for longer periods of time. This improves comfort because there is less stratification, and far less system cycling. There is generally little to no increase in fuel usage even with the added square footage due to upgrading the existing structure. The systems are also very well balanced, a heat load calc is done for each room, and ducting sized accordingly.
Over sized systems are generally used to compensate for leakage in the structure and the ducting, they also prevent call backs, that's why HVAC contractors oversize them. I test my systems and the structure. System leakage by code can't be more than 4%, my last project was at 2%. Also on my last project, I met the envelop leakage goal before drywall, on completion I was so far below the limit that the HERS tester didn't bother checking any of the exterior seals.
Build tight, air seal everything, and size the systems for actual conditions. That results in a very efficient and comfortable home.

 

[Red Squirrel]

They recommend to not hook up the lowest speed (red) to the heater as it can easily overheat, and that typically the cool side would be set faster than the heat. Other than that, any changes I do is to fix what I consider to be an already sub-par HVAC system. The more likely scenario is the system is undersized for the house, or starting to suffer being 16 years old, but I want to try fixes that are free before spending. Based on what I've seen in the house already, the previous owner may have messed with these speeds before me anyway.

Yeah would not hurt to experiment. Just keep track of how it was connected before, take some base line temperature measurements of the plenum and maybe in areas of the house that you feel are too hot or too cold. Then compare this for a bit, then once you have data you can try to change it, try to test on a day where the outside temp is about the same.

If you really want to get fancy you could try to find someone with a thermal imaging gun. I kinda want to buy one as they are coming down in price actually... but they're still quite expensive for what would just be a toy for me.