Go Big Or Go Home:How Many CFM in a 2x6

[Johnjenner]

I have had multiple question answered on this site and truly appreciate it!!! Next question. Want to put a AUF squirrel cage blower on my Leader 2x6. How many CFM do i go to get maximum output. I will be putting a reostat on to be able to control the speed. Again, Thanks again for the replies.

[Haynes Forest Products]

Don't be fooled just by CFM's boc fan will deliver a lot of CFM's but not at high pressure. Some pans wont do well restricted so choose wisely. Some motors will over heat if a rheostat is used. I just did a kitchen tear out and removed the below cabinet Jenair exhaust fan and that thing has power and delivers massive air. They come with a speed controller.

I also recommend a blast gate to be able to turn the air on and off quickly to fire. Waiting for a blower to slow down and then restart adds time to your firing time. You can use the rehstst as a way to keep the blower at a sweet spot so when you shut the blast gate and fire your right back to you setting.

https://www.ebay.com/itm/3-CAST-ALUMINUM-BLAST-GATE-DUST-COLLECTORS-BLAST-GATES/351683761436?hash=item51e1fc551c:g:t74AAOxyPLpRcZE 8

[Tweegs]

Don’t use a rheostat for motor speed control. Just don’t.

Rheostats limit current.

In order to control motor speed you need to adjust voltage.
Use a variable frequency drive (VFD), assuming single phase power.

The cheaper option is to let the motor run full speed and restrict inlet airflow.

[RileySugarbush]

Tweegs and haynes have it right. A couple of adjustments to how I have been doing it...

High pressure blowers are needed for AOF. For AUF only, a low pressure fan is fine. There is little restriction in the path of the air going up through the grate. 1/6 hp squirrel cage blower should work. Typically used on furnace draft inducers. You may be able to salvage one from a HVAC shop.


I used a flapper plate on the inlet to my fan. So it quickly shuts off the air flow for firing. I never turn the blower off.

Use a blast gate between the arch and the blower to regulate the flow if you want or need to. That can be set and forgotten. So kind of the opposite of what Haynes is suggesting:). On and off at the inlet. Tune flow on the outlet. Let the blower run full on.

[crzypete]

Don’t use a rheostat for motor speed control. Just don’t.

Rheostats limit current.

In order to control motor speed you need to adjust voltage.
Use a variable frequency drive (VFD), assuming single phase power.

The cheaper option is to let the motor run full speed and restrict inlet airflow.

Just to clarify, a VFD needs a three Phase motor, they do not work on single phase motors. You can however create 3Ø electricity with some VFD's, so you could potentially take 220v 1Ø (two hot legs) and output 220v 3Ø (three hot legs) with variable hertz that will allow varying speeds. I believe there are a few units which can actually create 3Ø from standard 110v electricity, but I am less familiar with the scenario.

Peter

[maple flats]

Any VFD will cost far more than a simple air flow cover. I have had both in the past, a homemade blast gate and an inlet air flow cover. In use both were effective, the inlet cover was easiest. I did not close it to add wood, as long as you have good draft on your stack (and I did) I just opened the doors and filled the wood. I only used the air flow adjustment to get the sweet spot where the fire burned best, then I left it there.
Even now with my combination high pressure air over fire (AOF) and air under fire (AUF) once I found the right setting for the air flow on both (off one blower) I have not changed it, and I start the blower as soon as a fire is lit and it stays on until about 30-45 minutes after last fueling (as the coals are almost all burned up).

[Johnjenner]

100, 200, 300,400???

[maple flats]

According to Leader's 2018 catalog, any unit from 2x4 up to 3x10 does well using a 273cfm blower. My thoughts on that is that they are trying to sell one size fits all. I would go for something in the 125-175 cfm range and make a damper set up on the inlet to regulate and find the sweet spot.