Fireplace xtrordinair FPX 36 Air Control slider bar function?

[Darren418]

Hi all. New member here. Joined so I could ask a question and learn about my new woodstove.

My family just bought a vacation house up in the mountains north of Boise. It's a fairly well-insulated 2,600 sq ft single story structure built in 1998. The previous owner (and original owner who built the house) was an electrical engineer who was a perfectionist with how the house was designed and built, researching every last detail. He selected an FPX 36 to heat the house, a great choice by most accounts.

We've been up at the house several weeks this winter and have been using wood heat almost exclusively as propane is much more expensive. The wood heat works great and is able to carry the whole heating load even down to below zero temperatures .. as long as you keep the fresh wood coming and the fire burning hot...

I have a few questions about the operation of the stove (or should I call this a "zero-clearance stove or fireplace?), the Air Control in particular, and whether it controls outside combustion air supply or not. I don't really understand the design and function of the combustion air supply. From reading the manual, it seems clear the stove uses outside air for its combustion. The blower duct supplies outside air to not only the area around the firebox that is heated and blown into the room, but this same duct also supplies the combustion air. The manual is unclear on this, but I think that's right. There are also "cooling vents" but those just cool the outside of the whole fireplace and the chimney and aren't connected to any of the air supplies that are actually used by the stove.

But then the confusion is: if the blower supplies the combustion air, then what does the Air Control do on the front of the fireplace? It is connected to a long thin metal bar that when moved to the left, slides down and exposes a series of little holes, and when slid to the right, covers them up most of the way but not all the way. The manual goes on to say the Air Control is what controls the combustion air and the burn rate, which when I have experimented, is what happens. The Air Control lever doesn't seem to be connected to anything else inside, it's just the think metal slide bar that flops around when you jiggle the control. It does jam easily, but I'll figure that out later.

So what exactly does the Air Control lever do, and where exactly does the combustion air come from? If I close the Air Control completely, does that force the stove to use outside air via the blower duct? And when it's open, it allows a much greater quantity of air to be sucked directly in via the little holes in the front of the stove? I've also seen posts that say the Air Control valve just controls the air wash air that keeps the glass clean and the little orifice that is at the bottom center of the firebox. When I slide the control to the right, it definitely does lower the burn, so does this mean that I am starving the fire for combustion air? And when the Air Control is closed, is the stove still sucking what it can through the blower duct, thus the blower duct acts like a constant minimum combustion air level and the Air Control simply adds more combustion air directly from the room? If so, then wouldn't running with the Air Control wide open, like with a hot fire, serve to defeat the whole "positive pressure" aspect of this stove?

I just want to understand the functioning of the combustion air supply so that I can better operate my stove in the most efficient and comfortable way possible. I've got other questions, but this one is the one that I am really anxious to get figured out first.

Great website and forum. Very impressed with the level of knowledge here and the spirit of helpfulness.

Darren

 

[FoxTrotBoogie]

Your combustion air is not brought in through your blower or the front of the fireplace. It is brought in through the cold air intakes outside that are also used to cool the box and chimney. You're correct in that all the slider bar does is let more or less of that air into the firebox, and doesn't quite shut all the way allowing a good hot fire with lots of draw to burn very slowly. My slider also sticks when slid all the way over to the low burn position, usually I can lift up on the slide handle and then it moves freely again. I also used a fine marker and marked full open, middle, and low burn with lines on the surround above the slider. I dont usually go all the way closed, but leave it a touch open.

 

[Darren418]

Thanks for the response. I'm still not totally convinced the combustion air is supplied by the cooling vents, although it would make sense and would be the best engineering solution. The FPX manuals (confusing) don't say one way or the other. I sure wish they had a flow diagram showing how the stove draws combustion air.

You said: "[combustion air] is brought in through the cold air intakes outside that are also used to cool the box and chimney. You're correct in that all the slider bar does is let more or less of that air into the firebox"

So you are saying that the Air Control controls the amount of combustion air by controlling the air coming in via the cooling vents and does NOT draw the extra air in from the room?

However, the FPX sales brochure contradicts this and is contributing to my confusion. It clearly says the outside combustion air is supplied by the blower:



"The Posi-Pressure System

Fresh outside air is drawn through a remote, quiet 388 cfm blower (13) to

pressurize the house, and provide outside air for combustion. A washable

air filter (14) keeps incoming air clean."



But yet, the operator manual seems to indicate that the combustion air is supplied through the Air Control. Here is an excerpt from the operation manual:


"Primary Combustion: This is the combustion (fire) that takes place directly on the wood. Primary combustion

determines how fast the fire burns. Air for primary combustion is supplied through the air control. When you

adjust the air control you control the amount of air that reaches the fire and creates primary combustion. The air

control supplies air to the air wash (the air holes above the door opening – used to help clean the glass) and

through the pilot orifice (center bottom of the door opening). By using the air control, and supplying air through

these two openings, you control primary combustion."


Finally, the installation manual describes the Cooling Vents and makes no mention of supplying combustion air:

"Cooling Vents

Located on the top rear corners of the fireplace are two 6" diameter air duct collars that must

be connected to 6" diameter air ducts that terminate outside. The purpose of the these air

ducts is to provide cooling air for the zero clearance fireplace and the air cooled chimney

system. Air is drawn into the ducts and circulated around the zero clearance fireplace. It then

exits through the outer liner of the chimney system, keeping the zero clearance fireplace and

chimney enclosure cool."



So I'm still confused by FPX's own documentation, but I will take you word for it that the combustion air is supplied by the cooling vents. Makes sense that it would be via the cooling vents, since there's no need to filter the combustion air that goes up the chimney like there is with the blower air that is sent to the interior of the house.


Thanks for the tip on marking the Air Control positions with a fine marker. I had been struggling with getting the control set correctly and consistently, your suggestion should solve that problem.

 

[FoxTrotBoogie]

I see what you're saying about the manual being confusing. I don't think the blower is providing combustion air because A.) The blower has a baffle in it preventing air from coming in when it is not on. That would make for some hard fire starting. B.) If the blower was providing combustion air there would be no way to control it. You would be adding pressure to the combustion air when turning the blower up, and decreasing air when turning it down therefore changing the burn rate. You'd have to adjust the combustion air everytime you adjusted the blower.

 

[Darren418]

Ah yes, that makes even more sense now that you point out the baffle in the blower duct. I forgot about that. The draft would probably be enough to keep it open once the fire was going, but it would make it hard to start a fire and get it going at first. Plus, the blower doesn't even come on until 15-30 minutes after start-up. The only definitive statement from FPX regarding the combustion air source comes out of their sales brochure not the actual operating or installation manuals. Wouldn't be the first time the marketing guys got something wrong.

So the Cooling Vents provide the baseline constant low-level combustion air with the Air Control adding more air when necessary or desired. Only remaining question is whether the Air Control lets in its additional air from inside the room or somehow (via a linkage?) from the Cooling Vent supply. It looks like it would just let it in from the room, but it's hard to tell. Maybe I will try a smoke test with the door closed and see whether air is getting sucked in through the little holes behind the Air Control slider.

Thanks for the answers! I've been wondering about this ever since I first used the stove several months ago.

 

[FoxTrotBoogie]

I believe all combustion air is provided from outside as to keep with there positive pressure selling point. I don't see why they would use two different methods.

 

[Darren418]

I would agree that sucking in a portion of the combustion air would defeat the purpose of the positive pressure system. By the way, the Positive Pressure system works VERY well. It pushes heat all the way into the furthest reaches of the house. Considering it is 2,600 square feet and single level in a big U-shape, that's quite a feat. I really love this fireplace. Thanks again.