grate size for AUF

[racinee]

Hi all,
I'm it the process of building a 2x6 arch and plan on adding AUF. The fire box is 24"x24" and I'll be making the grate out of angle iron. I was wondering if the grate should be the full size of the fire box (24"x24") or if it should be only the center portion of it(maybe 16"x16" or 12"x24" )? The pictures I could find of name brand arch were tapered so not the full 24" wide at the bottom.

[RileySugarbush]

Slightly smaller is OK. Blocking off the edges reduces the probability of air blowing up and around the wood. Especially right in front of the door. 16 x 16 seems a little small. You have probably seen that it is best to have the angles open up so ashes fill them, right? VVVVVV

[racinee]

So I guess block about 4" by the door and maybe 2" on the sides? My 16"x16" was assuming blocking 4" all around. Thanks, yes I will be putting the angles with open end up...been reading a lot lately on this forum :) Probably read more lately then I did through school!

[maple flats]

All I'll go by is my evaporator. It is 3x8. The firebox is old style, meaning the bottom and the grates are narrower than the top. Just guessing, but I think the grates are about 24-26" wide and 32" deep. Most of the newer arches (mine is from 2001) are full width, thus if 2' wide at the pan, maybe 26-27" wide top to bottom. That is because they leave room for firebrick and the pan only sets on 1/4-1/3" on each side.


Back to mine, I have both AOF and AUF. The under fire air enters thru the back, below the grates, at floor level and I have an air duct that extends to the front, with holes every 2". That duct ends about 6" from the front. I aim the air flow by having 3 rows of holes, one top center and one facing each outer edge of the grates. I get a complete burn that way, (with the AOF too). If you are not putting AOF at this time, I'd still think the grates should extend out to the entire width less the space needed for firebricks. The more air you can feed the fire the better the boil.

[racinee]

Thanks...I was wondering because I read a few post comments about air bypassing the fire along the edge. My arch is 26" and pan will be overlapping the edge to the top rails but 1/2". What size are the holes for your AUF?

[red/one]

I'm going to throw in what I did with mine, maybe it will help...:D I have a home built 24x almost 72 made from 1 1/2" angle. The front pan is a 24x30 SL divided pan and the back pan is a cheapish divided 2x3. The arch is insulated with cement board, 1" ceramic, then full brick with the top row being half brick. The pans sit on the brick. Anyway, my grates are made from angle VVVVV. My firebox is roughly 20wx24l. I have 2-2" sch. 40 pipes laying in the ash pan under the grates fed from a single 2" coming from a bounce house fan. The 2 pipes are drilled with 3 rows of 3/8 holes and capped on the ends. I can "aim" my air under the fire by turning the pipes. I get about 15gph.

[maple flats]

My AUF holes are smaller than you will need, because my blower is high pressure. The holes are 5/16", 3 rows, a hole every 2-2.5".

[racinee]

Thanks all, I'll give it a try with having the grate full size and using a pipe/duct to "spread" the air

[RileySugarbush]

Unless you have high pressure air like a bouncehouse blower provides, you definitely do not want to have AUF going through small holes. The normal squirrel cage blower has very little pressure and will not flow through small holes. For my AUF, I simple connect a 4" diameter duct from the blower to the area under the grates. It spreads out and flows through my angle iron grates very evenly.

If you have a high pressure blower feeding under the fire like Flats, you need the small holes or a blast gate or you will get way too much air under the fire.

For reference, my AUF blower is a squirrel cage and puts out 0.5" of water static pressure. My AOF blower is a 1HP bounce house blower and puts out 8" of water static pressure. Totally different animals.

[racinee]

Thanks, I guess I'll make the grates for now until I find which blower I'm going to use before making the AUF. Didn't think the bouncy house blowers were high pressure...I guess for AUF I better look for low pressure

[RileySugarbush]

You can use high pressure but will need to throttle it for AUF. Best to throttle inlet since it uses less power. They cost more than a simple squirrel cage blower though.

[racinee]

would a vent blower from a propane water heater be sufficient for AUF? Can't find CFM rating for it. If not planning on going with inline duct booster fan (240 CFM) from amazon for 32$

[maple flats]

Look for the make and model# of the blower, then go to their website for CFM. Is that blower turning a squirrel cage or is it paddles. If paddles it's high pressure, but likely not very big, not likely good enough for AOF.
My water heater has an exhaust blower. It is low pressure. I don't know what the CFM might be.

[racinee]

Thanks, it is a squirrel cage. Tried to find the CFM using make and model, but no luck. probably better to keep looking for bigger blower or go with the inline duct booster.

[RileySugarbush]

I see you know it's a squirrel cage, so low pressure blower. Just for reference, another clue would be the power rating on the label. A low pressure blower will be 1/6 hp or even lower, with a current rating of 1A or lower on 120Vac. A high pressure blower will be more like 3/4 to 1hp, about 6 Amps.

[maple flats]

Thanks, it is a squirrel cage. Tried to find the CFM using make and model, but no luck. probably better to keep looking for bigger blower or go with the inline duct booster.
An inline duct booster is not the right type. They will not be positive enough movement. Duct boosters are kept low flow so they don't create a breeze in the room it is servicing. Try https://www.grainger.com/product/1TDP3?gclid=CjwKCAiArIH_BRB2EiwALfbH1GveLp8jSYf5TX nE4Op4Mn0rJg37J5SqNWe1BEjrVj2LZdSu1BqW-BoCAOIQAvD_BwE&cm_mmc=PPC:+Google+PLA&ef_id=CjwKCAiArIH_BRB2EiwALfbH1GveLp8jSYf5TXnE4Op4 Mn0rJg37J5SqNWe1BEjrVj2LZdSu1BqW-BoCAOIQAvD_BwE:G:s&s_kwcid=AL!2966!3!264955915919!!!g!437257756459!&gucid=N:N:PS:Paid:GGL:CSM-2295:4P7A1P:20501231 or better https://www.northerntool.com/shop/tools/product_200836979_200836979?cm_mmc=Google-pla&utm_source=Google_PLA&utm_medium=Fans%20%3E%20Air%20Movers&utm_campaign=J%26D%20Mfg.&utm_content=4299476&gclid=CjwKCAiArIH_BRB2EiwALfbH1GV1OQsbyLnKMpxsQL-3lkhyYblnmH4qQGfkPVbLbq2neVlhnzDuERoCn9QQAvD_BwE&gclsrc=aw.ds. The second one moves 2x the CFM and less money. If it moves too much, throttle it using an inlet damper (a tear drop shaped cover over the inlet, you just adjust as needed. It will not hurt the blower.

[racinee]

motor is 1.3 so low pressure for sure! Never though of that for the inline duct booster...I will stay away from that and probably go with the second link. Thanks again guys!