Fireplace chimney draft question

[ironhat]

Our cabin is a three year double-wide mobile with a fireplace which is giving me a fit. The manual states that due to the airtight nature of manufactured homes it is necessary to open a window for proper draft. In fact, our eyes will burn if we don’t do that. The unit has a relatively large external air supply duct – if I recall correctly it’s 4" in diameter. So, my questions are these; 1) Is it possible that the chimney isn’t tall enough for proper draft or is it more likely that it is properly engineered for the application (the later is my thought). 2) Would it be useful to add a duct fan to the air supply so that we can eliminate the open window or is it probable that I won’t be able to control the burn rate (a rheostat might be indicated on this fan).
I’m thinking that these fireplaces really aren’t meant to be used much. The grates are so widely spaced that it won’t allow the build up of a nice bed of coals. It also cautions you not to build a fire directly on the firebrick floor of the firebox. OK, have at the opinions and suggestions.
As always, TIA!

[LCHIEN]

the heat from the fire is supposed to direct the air flow up the flue.

One of the problems THat can occur when starting a fire is that the cold air in the chimney before you start the fire will tend to fall down into the house and displace air thru the window opening. Then as you start the fire the smoke goes into the house from this down draft until the fire warms up enough to start upward circulation.

What exactly, is your problem?

[Slik Geek]

Ironhat: Are you saying that there is an outside air duct built into the fireplace to supply the oxygen needs of the fire? But you suspect that it isn't doing the job?

[ironhat]

the heat from the fire is supposed to direct the air flow up the flue.

One of the problems THat can occur when starting a fire is that the cold air in the chimney before you start the fire will tend to fall down into the house and displace air thru the window opening. Then as you start the fire the smoke goes into the house from this down draft until the fire warms up enough to start upward circulation.


Thanks for the input, Loring. I always preheat the box by holding some lit newspaper up toward the damper. I've been doing this procedure successfully at home for years. This unit fails to draft after an hour of continuous fire. It's actually not until about an hour that we begin to feel the stinging in our eyes. Opening a window seems to help prevent this but we're certainly losing heat when doing so.

What exactly, is your problem?

Ironhat: Are you saying that there is an outside air duct built into the fireplace to supply the oxygen needs of the fire? But you suspect that it isn't doing the job?

Ditto to you, SG and, yes, that's exactly my thoughts. I have crawled under the house and looked up the intake with a flashlight and it's definitely clear. In fact, there's a factory installed screen to prevent critters from gaining access. It's either a lack of intake or a short chimney but I doubt the chimney angle.

[panthony]

Fireplace chimney draft question

Ironhat,

Rule of thumb...hot air is always attracted to cold. When you begin to start a fire the flue is cold and the house air is warm so until the fire can get hot enough to begin drawing up the flue you may get some backdraft into the house.

Second...the reason to crack a window open is that the fireplace is open to the living area and needs oxygen to burn efficiently. Without the open window the chances of depleting the oxygen in the room (and there not alot of room in a double wide) making it an unsafe environment for the occupants. Fireplace manufactures are covering thier butts on that one but it is a good practice to slightly crack a window maybe 1/4" or so.

Third...unless the fireplace has an alternate air supply vent directly into the firebox for fresh air you will need to keep a window cracked during burn times. Many new flue vents are double walled with the inner flue used to exhaust the hot gasses while the outer flue is used for fresh air. If you have this type of set up the manf. will still reccomend a secondary vent location if the firebox is open to the living space.

Pete A.

[ironhat]

Ironhat,

Rule of thumb...hot air is always attracted to cold. When you begin to start a fire the flue is cold and the house air is warm so until the fire can get hot enough to begin drawing up the flue you may get some backdraft into the house.

Solid on that. Hence the reason that I preheat the flue before lighting the fire. This has been successful for years - shouldn't be different in this application I wouldn't think.

Second...the reason to crack a window open is that the fireplace is open to the living area and needs oxygen to burn efficiently. Without the open window the chances of depleting the oxygen in the room (and there not alot of room in a double wide) making it an unsafe environment for the occupants. Fireplace manufactures are covering thier butts on that one but it is a good practice to slightly crack a window maybe 1/4" or so.

Third...unless the fireplace has an alternate air supply vent directly into the firebox for fresh air you will need to keep a window cracked during burn times. Many new flue vents are double walled with the inner flue used to exhaust the hot gasses while the outer flue is used for fresh air. If you have this type of set up the manf. will still reccomend a secondary vent location if the firebox is open to the living space.

Exactly what I was trying to say. Maybe my terminology was off but you will note in a previous post that I'm thinking of turbocharging the vent and controlling the fan speen with a rheostat.


Pete A.

Thanks for the response!

[panthony]

Fireplace chimney draft question

Ironhat,

Did you ever check the draft into the firebox by taking something that smolders giving off some smoke to visually see if there is some place around the firebox where there is an opposite draft condition.

Sounds like the fireplace is gasping for more air and modifying the air intake may be the wrong thing to do. Most fireplace systems are designed to give an efficient balanced burn with the built-in air intake.

Does the eye-burning time period take place when you have an open window?

When your eyes begin to burn does is smell like smoke is in the house?

Have you checked to see if there are any leaks in the flue system"

Is there a flue termination cap on the top of the flue supplied with the fireplace...or is it an aftermarket one installed later?

Are there any obstructions within 10' of the flue on the outside and is the flue termination at least 30" above the nearest obstruction within 10'?

This is in accordance with International Building Code. If there is an improper clearance condition on the exterior such as a tree to close or a roof that encroached the 10' - 30" design rule you will have improper draft issues.

I would check those specific items prior to modifying the air intake. Fireplace regulations are very specific and come with extremely high liabilities to the original manufacturer and if you modify something that can cause harm to the next home-owner.

Check also with the original manufacturer to get the installation specs and see if everything is properly installed.

Pete A.

[Cheeky]

i only fire up the fireplace once or twice a year, and when i do, smoke loves to move inwards. what i've found that prevents that from happening is to heat up the flue.

roll up some newspaper tightly and light it. then hold it up to the flue. works for me.

[ironhat]

Ironhat, (I'll try not to be long winded this time)

Did you ever check the draft into the firebox by taking something that smolders giving off some smoke to visually see if there is some place around the firebox where there is an opposite draft condition. No.

Sounds like the fireplace is gasping for more air and modifying the air intake may be the wrong thing to do. Most fireplace systems are designed to give an efficient balanced burn with the built-in air intake.
Even by trying to increase it but moderating the fan with a rheostat?
Does the eye-burning time period take place when you have an open window? No.

When your eyes begin to burn does is smell like smoke is in the house? No

Have you checked to see if there are any leaks in the flue system" In a manufactured home the flue goes directly through the ceiling, the 12" of insulated space and outside.

Is there a flue termination cap on the top of the flue supplied with the fireplace...or is it an aftermarket one installed later? Supplied with unit.

Are there any obstructions within 10' of the flue on the outside and is the flue termination at least 30" above the nearest obstruction within 10'? Hmmm, 30" higher than the ridge line of the roof in this case and I don't know it that's true here. I wondered what that number is. I'll have to check the next time it's safe to mount the roof.

This is in accordance with International Building Code. If there is an improper clearance condition on the exterior such as a tree to close or a roof that encroached the 10' - 30" design rule you will have improper draft issues.

I would check those specific items prior to modifying the air intake. Fireplace regulations are very specific and come with extremely high liabilities to the original manufacturer and if you modify something that can cause harm to the next home-owner. ...or nullify my home-owners insurance policy.

Check also with the original manufacturer to get the installation specs and see if everything is properly installed. Good point as the dealer of the home who also installed the fireplace was not very bright. They actually installed the house 180* wrong orientation and got snooty that I was angry and made them re-erect the house. It wasn't pretty!

Pete A.

I have to type here in order for the forum's program to accept this.

[Slik Geek]

Where does the intake vent enter the firebox? Could it be getting obstructed by ash???? (I'm grasping at straws).

[ironhat]

The air from the vent enters the airbox from behind some sheetmetal to the one side and forward of the fire. There's never much ash in there whatsoever. A very poor set up so I'm trying to optimize it. After I check the height of the chimney stack I may just have a better grate welded up so that I can build a better fire.

[panthony]

Fireplace chimney draft question

Ironhat,

OK....two last things that need to be approached.

Do you have a carbon monoxide detector installed close to or on the ceiling of your home? Not just a smoke detector but a CO detector and it needs to be as high to the ceiling as possible because CO is less dense than ambient air.
If the fireplace is causing or lowering the levels of oxygen in the home chances are you may have a CO leak...just check to make sure. And by all means do not go to bed with the fireplace in operation which is just a good rule to go by.

There is one more thing...
Do you experience the eye burning sensation when the temperature in your home is elevated due to the fireplace? This is a critical issue because you live in a manufactured home, much of which is probably made with products that contain high level of urea-formaldehyde. When the temperature increases the formaldehyde off gasses more readily causing your eyes to be irritated. This is common during periods of high temp dry air conditions. The effects of this off-gassing will become less and less as time goes by because most of the gas will dissipate from the materials in your home. It's in everything from the sheathing to the carpet and cabinets....there is no way around it. Best to keep a window cracked during when the fireplace is operating. Open a window in a room far enough from the fireplace for the cold air to warm a little before it gets to the main room.

Carbon monoxide will not cause your eyes to burn it will, however make you feel like you have flu-like symptoms and fell tired due to the body's intake of CO to the bloodstream.
Formaldehyde is an eye irritant as well as making your throat feel dry.

Pete A.

[ironhat]

Roger the CO detector installed and *not* going to bed while the fireplace is active. As far as the elevated temp it has never gotten higher than 72* mostly due to the lousy fireplace grate and teh open window. We have never had any eye burning or respiratory distress at any other time.

Thanks to all, especially the code info from you, Pete, for the suggestions and cautions. This won't get much attention until the weather breaks and the roof clears.

[LinuxRandal]

From experience with a friends firebox fiasco. Still check up top at the exit pipe. His was capped off (which I am sure yours isn't from what you have typed), but they also had damaged a section (cracked) so that it wasn't double walled (exhaust would heat and expand the crack and enter the intake section of the pipe).

Just one more idea.

[ironhat]

From experience with a friends firebox fiasco. Still check up top at the exit pipe. His was capped off (which I am sure yours isn't from what you have typed), but they also had damaged a section (cracked) so that it wasn't double walled (exhaust would heat and expand the crack and enter the intake section of the pipe).

Just one more idea.

Just to keep this clear, the double walled chimney pipe isn't the type that you see with LP installations where it's intake + exhaust. This is just double walled for the insulating value to keep the pipe cool as it goes through flammable materials. The intake that I'm referring to is coming into the firebox from under the house. Thanks for the input, though. Poor installation would be easy to overlook and I'll keep my eyes open for such things when I can get up there.