WAHX blower speed / temp of air and efficiency

[hartkem]

I have been thinking about lowering my blower speed on my furnace so its not so loud. Currently I use the fan on terminal (G) to activate my blower. I also use the w terminal for my propane backup with a seperate thermostat. I know changing the heating mode speed could cause the heat exchanger to over heat when using propane but I think I can change the (G) terminal speed sperate from the propane(w) terminal speed. Im aware I would have to change it back in the summer since A/C uses the G terminal to activate the fan. Also, how much heating ability would I lose by lowering my fan speed? Would the increased air temperature offset the reduced air volume? I know lots of questions. Bottom line is I simply want to reduce the drafts in the house and get a more even slow distribution of heat. But I also don't want the blower to run 24/7 because its to slow to heat properly, a happy medium i guess.

 

[Bob Rohr]

you need sufficient air flow to get the heat energy at the farthest register. That will be determined by the flow resistance in the duct work. Also if the airflow is too low you can really scortch the air and get an un-healthy burned dust smell.

With the newest high efficient forced air equipment they vary the speed of the blower as well as the burner output. That provides an ideal match of energy in and energy out, aka thermal equiulbreum. You can do this manually but the heat load of a building is ever-changing. Colder outdoor, more drafts, or air infiltration would require more energy moved by the blower and the heat source be it gas, wood, geo, etc.

It's not a bad idea to have near continious air movement as long as the noise is not excessive. This allows you to constantly filter the air, add or subtract humidity, and prevent room temperature swings. Variable speed ECM motors allow this with very minimum energy consumption when compared to bang- bang, stop- start fan operation.

The same concept applies to hydronics, keep the fluid moving and regulate the burner and pump speed to the load. adding outdoor reset controls allows the system to adjust as the outdoor conditions change.

 

[Bob Rohr]

you need sufficient air flow to get the heat energy at the farthest register. That will be determined by the flow resistance in the duct work. Also if the airflow is too low you can really scortch the air and get an un-healthy burned dust smell.

With the newest high efficient forced air equipment they vary the speed of the blower as well as the burner output. That provides an ideal match of energy in and energy out, aka thermal equiulbreum. You can do this manually but the heat load of a building is ever-changing. Colder outdoor, more drafts, or air infiltration would require more energy moved by the blower and the heat source be it gas, wood, geo, etc.

It's not a bad idea to have near continious air movement as long as the noise is not excessive. This allows you to constantly filter the air, add or subtract humidity, and prevent room temperature swings. Variable speed ECM motors allow this with very minimum energy consumption when compared to bang- bang, stop- start fan operation.

The same concept applies to hydronics, keep the fluid moving and regulate the burner and pump speed to the load. adding outdoor reset controls allows the system to adjust as the outdoor conditions change.

Here is a typical HW coil output chart. This coil was designed for 1 GPM flow. Notice that increasing the flow rate does not change the output so much, even tripling the flow to 3 gpm. Notice also how quickly output drops below 1 GPM. So careful and accurate coil sizing is important.

Coil manufacturers will have a similar graph showing output at different air flow rates. It will show a similar curve, with not a huge difference in output as air flow changes.

With low operating temperature coils a large surface area is better. But with low temperature coil operation you do not want the airflow blowing directly on you, allow it to mix with the room air or it may feel too cool.

Also increasing fan speed (air velocity) tends to feel uncomfortable (drafty)even if the discharge air is warmer than the room air temperature. Outdoors we call this chill factor, inside it called a draft. It's about air movement across your moist skin.

So you are correct it is a delicate balancing act. A pro would have a draft hood to measure and adjust air flow at the various registers, you may need to play around and adjust to your comfort level, or your spouses With wood fired heat it is a tougher go as the supply temperature to the coil would have a wider temperature range compared to a gas fired burner.

This is why radiant or hydronic heat is prefered by homeowners as ideal comfort systems, little to no air movement to contend with.