CFM Per Room

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Old 04-01-14, 12:37 PM
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CFM Per Room

Hello. Here's my project, and your help would be appreciated.

I have a fireplace, the ICC-RSF Opel 3.
Opel 3 Fireplace | ICC Chimney - RSF Fireplaces

It's installed in my house, and the vents above the mantle pump out 70,000 BTU.
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The supplied fan for these vents is inadequate. Besides, it burned out during my second season. The fireplace has an 8" opening on the side for an air inlet. This air circulates around the hot firebox (not into the firebox), then out into the room. I'm in the process of connecting this inlet with ducting and an inline booster fan.

When this fan is running, room air will be drawn from 2 bedrooms to feed the fireplace. There will be about 40 feet of duct with a wye to the bedroom vents. Then warm air from the living room will be drawn down the hall and into these bedrooms, effectively heating the bedrooms.

My questions are about the CFM required. The 8" duct booster fan outputs 420 CFM and will be connected to a variable speed controller.
Diversitech 625-AF6 8" Round Duct Fan - 420 CFM
If it delivers too much CFM, the controller is turned down to reduce it, so the ability to exceed what's optimal is desired.

1 - Is 420 CFM enough/too much flow for 2 bedrooms?

2 - If so, maybe I can draw out of 3 bedrooms?

3 - How much CFM is generally supplied by A/C or furnace to your average bedroom? This number is my target CFM per bedroom.

4 - Regarding airflow resistance, where should I locate the fan? Center, closer to the bedroom vent, or closer to the fireplace?

Thank you in advance.

Carl
 
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Old 04-01-14, 02:40 PM
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that 420 cfm is with no load. Once you put 40ft of duct on it, joints, Y's its not going to do much. Then you will have the temp drop in the duct. Id want a few hundred CFM per small room
 
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Old 04-01-14, 03:02 PM
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If I put 2 of them in the 40' x 8" duct, maybe that would do it?
Temp drop isn't a factor, drawing room air INTO the fireplace.

Another choice is a 12" fan with reducers to 8", which pushes 675 CFM no load?
 
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Old 04-01-14, 03:35 PM
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I found this site:
FANTECH Inline Centrifugal Duct Fan,8-1/4 In. L - Inline Duct Fans - 4YM45|FG8XL - Grainger Industrial Supply

It's advertised fan lists a chart depicting resistance numbers like:
CFM @ Duct502
CFM @ 0.000-In. SP 502
CFM @ 0.125-In. SP 487
CFM @ 0.200-In. SP 470
CFM @ 0.250-In. SP 465
CFM @ 0.375-In. SP 438
CFM @ 0.400-In. SP 428
CFM @ 0.500-In. SP 411
CFM @ 0.600-In. SP 388
CFM @ 0.750-In. SP 364
CFM @ 0.800-In. SP 351
CFM @ 1.000-In. SP 313
CFM @ 1.200-In. SP 266
CFM @ 1.500-In. SP 218

Having a 90 at the inlet vent above the bedrooms, and a 90 at the fireplace, and all ducting between (turning a bit, with a lazy 90 at the ceiling by just routing and hanging the flexible duct pipe), how do I determine the resistance of this configuration?
 
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Old 04-02-14, 10:35 AM
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Dont think you have the correct fan Think you need something more like Centrifugal Inline Fans - Greenheck Fan Corporation

SQ and BSQ these fans will deliver the cfm you need


8in flex is normally rated for 160cfm on supply duct
 
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Old 04-02-14, 12:34 PM
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That's a big fan. Way too big for my application. I might pick up a more robust fan that delivers in the 400 cfm range, and with the variable speed controller, set it to the right volume of air. That should sufficiently overcome the resistance.
 
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Old 04-02-14, 01:18 PM
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those fans go from 241 cfm to 1948 so they can be even smaller than what you have. but they are made to move air
 
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Old 04-02-14, 05:26 PM
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They certainly look like units that will outlive the house.

I just learned from the rep at RSF Fireplaces, that the knock-out panel that opened up an 8" vent inlet to the fireplace is designed NOT to have a fan forcing air into it. I'm calling FOUL!

1 - Since the previous (and worthless) fan that came with the unit wasn't worth it's weight in used beer, a new fan with a definite air flow will be on order.
2 - Closing up the front air inlet (I glued sheet metal to the grate's inside), and opening the side 8" knock-out panel inlet is the manufacturer's design. Yeah, convection flow only. But the convection flow is soooo slow, the temperature of the air at the outlet vents are 40 hotter. I worry about building up too much heat inside the wall.
3 - Since the thermal switch for the old fan is still installed and operational, and controlled through a variable speed rheostat switch, this switch can operate a different fan located in the attic.
4 - With the front vent blocked off, the firebox will get hot and the thermal switch will turn on the fan. Therefore, the firebox shouldn't get so hot that I should worry about burning the house down.

5 - Should I design this for a possible failure? If the fan doesn't turn on, or burns out, I need convection flow, or the fireplace gets too hot. So, when the fan is running, a valve closes off the convection opening. When the fan is off, the valve/damper opens, allowing a free flow of ambient air to enter the fireplace, aided only by convection. I wonder if a damper is made to perform this task?
 
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Old 04-02-14, 06:29 PM
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Where does this fireplace take it's combustion air from?

If it's from the same intake (shared), installing a powerful fan could mess up the fuel to air ratio. The last thing you want in a high efficiency fireplace is too much excess air.
 
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Old 04-02-14, 07:38 PM
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The combustion air is through a port on the other side, and it's piped from the outside wall. It has a control arm that regulates the combustion air intake. This fireplace is a very well designed unit. But I want to safely pump air into the "room-air-inlet" to increase the airflow out of it, and draw warm air to the bedrooms.
 
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Old 04-02-14, 07:55 PM
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You can calculate required airflow using this formula:

CFM = Rated BTU output / (1.08 x desired delta-t)

I'm not sure if the 70 000 btu rating is input or output -> if input, take off 30-40%.

You might have a delta-t of 100F or so. (lower is better but causes more noise)


5 - Should I design this for a possible failure? If the fan doesn't turn on, or burns out, I need convection flow, or the fireplace gets too hot. So, when the fan is running, a valve closes off the convection opening. When the fan is off, the valve/damper opens, allowing a free flow of ambient air to enter the fireplace, aided only by convection. I wonder if a damper is made to perform this task?
Will there be much convention if you have 40 ft of pipe connected to the outlet?

In gas and oil furnaces, when the fan fails, a high temperature limit shuts off the burners.

If your fan were to fail, the fire would keep on burning -> might want to check with your insurance company before connecting ducts to the fireplace.

There might be a much safer way to distribute heat produced by the fireplace.
 
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Old 04-02-14, 08:57 PM
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You mean convection? I seriously doubt I'll have any with a sealed pipe going 8' vertical, then 32 feet to the bedrooms. I need a system that will automatically open the hole for convection air flow close to the fireplace when the fan turns off. And being behind the wall, it's probably like junction boxes, can't do it where it's not accessible.

To heat the bedrooms, my next choice is to create a system that is not connected to the fireplace. Like a vent drawing the hot air from the ceiling area of the living room and blowing it into the bedrooms. No fireplace mods on that one at all.

But there's still not enough air flow through the fireplace.

Now, if I mount the fan to the 90 elbow, and make sure the pipes are horizontal (not vertical), and leave it at that, I think the convection could still draw air through the fan housing into the fireplace. The fan will not be hooked up to any duct pipes. It'll just be open on one side.

This might be my answer. The sales rep at ICC-RSF (Dan Bonar), says "The Opel is not approved to draw circulating air from more than 5 away from the fireplace." So the total length of pipe mounted would be about 18" which includes the 90 elbow and the fan housing. No where near 5', but there's a fan blade in the way. I think air will flow through it fine if the fan fails. But I need to check this out with Dan.
 
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Old 04-02-14, 10:12 PM
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To heat the bedrooms, my next choice is to create a system that is not connected to the fireplace. Like a vent drawing the hot air from the ceiling area of the living room and blowing it into the bedrooms. No fireplace mods on that one at all.
I think that's the way to go. I'm leery of ducting a fireplace much more so than using a fan - any fan you use to enhance convection should be specifically designed for fireplaces.

Out of curiosity, is your propane heating system forced air? With return air vents in the right locations (pulling air from ceiling), you might be able to use it to distribute heat from the fireplace.
 
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Old 04-02-14, 10:15 PM
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Wow! If that idea works, then the fireplace would actually be drawing air from outside. (Inside the enclosure behind the bricks, but through the vent in the wall panel). So when it's pumping air into the living room, the house is effectively getting pressured. So I open up a window in the bedroom, and the warm air will flow to that open window.

Similar to operating a swamp cooler in a dry climate. And I don't have to cut holes in the ceiling. I wonder if it will be enough.
 
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Old 04-03-14, 12:08 PM
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Let me start with a comment about ICC-RSF, specifically Dan Bonar, a Sales Rep. He's answered all my questions and continues to. I'm very impressed with the support from him and ICC-RSF Fireplaces.
ICC-RSF - Home | ICC Chimney - RSF Fireplaces

His latest comment is the Opel 3 cannot be sealed up, without any inlet vents. Also the 8" inlet vent on the side should be connected to draw air from inside the house, without any more than 5' of ducting. But, as Dan said: "If you have temperatures above 180 coming out of the gravity vents, then you might want to consider increasing the air flow through the fireplace which could be done by allowing the bottom louver to be opened." I commonly have more than 180 exiting these vents, and excessively more with just the side 8" inlet feeding it. That's one supporting reason to have a fan feeding it. But if the fan fails, I risk an overheat which could mean a house on fire. Dan said "You cannot block off all air flow as you suggested..."

I'm trying to find a way to increase air flow through this fireplace safely, including maintaining a safe convective air flow if the fan fails. So far I'm stumped.
 
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Old 04-03-14, 02:21 PM
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Maybe high temperature rated (if they exist) axial fans installed in both outlets?

Axial fans have a lot of free area when off, wheres centrifugal fans don't allow for much air movement.
 
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Old 04-06-14, 02:46 PM
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If I could find a fan that could withstand 250f, I'll buy 2 of them and the problem is solved. I'll just close up the hole on the side of the fireplace, and open up the front plate.

But....

This fan:
Diversitech 625-AF6 8" Round Duct Fan - 420 CFM
is an axial fan, right? It's 8" in diameter, and 7 1/4" long.

Now, if that fan was connected to the inlet:
http://Photo.Sagerquist.net/Files/Fi...ll3-23-14d.jpg
- and the inlet was turned horizontal to reduce any resistance,
- and no other ducting connected to it,
if the fan failed, air could flow past the blades into the fireplace, therefore complying with the manufacturer's recommendations of not more than a 5 foot duct connected to this opening.

Let me continue...

The inside compartment where the fireplace is located is rather well sealed off. So air to this hole comes from the vents around the exit vents, and the vent in the outside wall. Although that spells resistance, I think the fireplace will be vented enough for a broken fan in this configuration to still feed the fireplace adequately through convection.

Next step is to feed that enclosed area with a ducting system that starts from the bedrooms. Terminating at the ceiling of the enclosure, and NOT connected to the fan mounted to the fireplace, the ducted system will effectively pressurize the fireplace enclosure to feed that fan. That way, the fireplace is fed with air mostly from inside the house. Maybe some will escape the vent on the outside wall, such is life.

But with this config, if the fan fails, there's still airflow into the fireplace complying with the manufacturer's guidelines.

Thoughts?
 
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Old 04-06-14, 03:01 PM
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BTW, that Diversetech fan can withstand a 266 operating temp, but that's not free air temp. If the air is flowing fast enough, I doubt I'll have more then 160 flowing out of the vents. I think this is my answer. Feeding the bedrooms will be a separate project.
 
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Old 04-10-14, 02:04 PM
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I haven't though about it much, but this is what comes to mind now:

a) You don't need to worry about high temperature if the fan is on the intake. With the intake fan off, that area doesn't get hot, right?

b) High cfm fans can be pretty loud, especially axial fans. You might want to buy one of those from a place with a good return policy to check for noise.

How much airflow do you need?

c) On second thought, I wouldn't subject anything with a motor in the airstream to the supply air from a fireplace. (direct drive may not be an option) You'll be better off with a fan on the intake.
 
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Old 04-11-14, 10:00 AM
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Another idea:

Experiment with pressurizing the cavity where the intake is with a centrifugal fan, rather than connecting a noisy axial fan to the intake.
 
 

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