Fireplace design - air inlet question

[Aristo]

Hi all,

I am new to this forum, great info sofar!
For the sake of nostalgia, I am building a traditional fireplace (not a Rumford model). Rough dimensions: front opening of 25in width and 29in height. Flue liner from the top of the smoke chamber 27ft with an 8in inside diameter.
It is safe to say that the air consumption will be around 12,000 cu/ft per hour.
The living room (5,000 cu ft) is isolated quite well and depends on floor heating. I’d like to avoid sucking all the warm air out, especially as floor heating will take a considerable time to compensate.
(A glass door panel is no option, open fire is the objective).

I am considering an external (18 sq in) air supply (with valve regulator) from the crawl space. The crawl space is ventilated naturally by ventilation holes.

Your opinions are highly appreciated on:
1. the extend to which an external air supply will partly overcome exhausting all the warm living room air. Will it make a difference?
2. the location of the fire grate (air supply): located near the back-wall under the fire -or- located more towards the front opening? (pls, see attached drawing)

Thank you in advance!

Aristo

 

[jatoxico]

I'm not the best person to answer this but not sure how the external air will act as the combustion air unless the open face of the fireplace is sealed. Before adding my insert my open fireplace had an external air source. It was woefully undersized but even so I don't think it worked at all. Instead the fire was fed by air taking the path of least resistance. Open fire consumed copious air.

 

[Aristo]

I'm not the best person to answer this but not sure how the external air will act as the combustion air unless the open face of the fireplace is sealed. Before adding my insert my open fireplace had an external air source. It was woefully undersized but even so I don't think it worked at all. Instead the fire was fed by air taking the path of least resistance. Open fire consumed copious air.

Hi Jatoxico,
Thanks for your reply. I agree that sealing would be the most efficient option, however esthetically I prefer an open face. Before deciding, I hope more members can share their experiences with air inlets and open face fire places.
Aristo.

 

[jhjove]

When we bought our home the item we loved was the centrally built masonry fireplace. The previous owner, who
built the home, had an outside air system integrated. The unit is called an Air-a-lator, where the outside air is
presented at the front-bottom entrance of the fireplace (your option A in the drawing).
With a metal eyehook you can easily adjust the amount of outside air introduced into the fireplace.

The fires were great but in our experience very little heat was radiated into the house. And even with this outside
source of air there was still heat loss from the remainder of the house. Let me say the fireplace did not reduce the
usage of the oil furnace. I think the previous owner would burn HUGE fires and use up much wood to get any heat contributed to the house.
Albeit, the masonry structure got warm....the previous owner had to put a humidistat on the attic ventilator. This was due to condensation build-up on the mansonry chimney as it passed through the attic and out through the roof (cold air in the attic and a warm chimney produced moisture on the masonry).

We only had a metal fire screen, no doors. So had to keep the flue damper open even on smoldering fire
which adds to heat loss, otherwise smoke and CO/CO2 stay in the house.
I did think about replacing the metal screen with a screen with doors and ceramic glass, so we could run the
fire with the doors closed, hence only using outside air and reducing heat loss from the house. But cost of
ceramic glass lead us to the purchase of a woodstove.

So the short and sweet, if you are sticking with a traditional fireplace, I would go with a fire screen made of
ceramic glass that can be closed with a roaring fire.

As for the air intake position....I think front is the premium position for the fire and best air flow.
Surely the control of the air flow is easier from the front...although I would guess that a rear position air intake would have some sort of lever available for use by the user. The air-a-lator is only front mounted I believe.

 

[Destructor]

To cut down on the amount of oil being used while burning an open fire shut off the furnace. That’s what I do. I too have a masonry fireplace and I love using it. I crack open a nearby window. I use a lot of wood on weekends, maybe a week’s worth of wood if I had a small wood stove instead. I spend my time in the living room which gets cozy warm from the fire. The farther reaches of my 1950’s cape do get chilly but not cold. Once I have a well established hardwood fire going which doesn’t spark I keep the screen open while I’m in the room. The screen blocks a surprising amount of radiant heat. When the fire has died down and is mostly coals at the end of the night I close the glass doors, turn the furnace back on then go to bed. I also use a quality grate and a steel plate for a fireback. My goal isn’t to save oil but to enjoy an open fire, it warms the living room but is very inefficient. I get my hardwood from the nearby woods so I don’t pay for it. I do it mostly for fun.

 

[Aristo]

@jhjove and destructor, thanks for your comments.
I've made a few inquiries on lift doors with ceramic glass, but the quotes made me look for alternatives. Hence my questions.
From your experiences my first conclusion is, that an external air supply in combination with an open fire place will not keep the warm living room air in.
So I guess it all boils down to accepting a colder back but enjoying the look, sound and scent of an open fire -vs- the efficiency benefits of a closed system. Food for thought..

 

[SteveKG]

I built my own fireplace, and though it is a Rumford design, this applies to your post. For outside air, I left a couple of bricks out of the floor of the fireplace, so it left a hole in the floor the size of two firebricks laid end-to-end. I had planned for this when I poured the base of the fireplace and had a tunnel leading to the outside. [To create the tunnel, I used some spare galvanized pipe I had lying around in my junk pile.]

I constructed a door of steel plate and angle iron to cover the outside outlet, and I did the same with the hole in the floor of the fireplace. That one is hinged [angle iron frame with a flat plate and hinges]. I welded a ring to the top of the plate so I can pull it open. This door/opening is toward the front of the fireplace so that the grate sits back behind it. I can crack that little door if and when I want air from outside. When the fireplace isn't in use, I close the door and prevent outside [cold] air from leaking inside.

 

[Aristo]

Steve,

Thanks for sharing!
I assume your Rumford is an open face? If so, can you share your experiences when opening and closing the air inlet? Do you notice any differences/does the fire actually benefit from the external air supply?

For my application, I was planning to run a galvanized pipe down into the crawlspace. The bottom of the pipe will be closed with a screw cap. A T-junction (sloped a bit down to drain any condensation) will be provided with an electric valve to regulate airflow and avoid radon gasses from entering the living room when the application is not in use. (Imo another benefit is that crawlspace air is replaced with fresh outside air more frequently). The top of the pipe at the base of the fireplace will be offset to the grade, to avoid ashes falling down directly. Looking at your design, I think I will reconsider using a grade and instead create a metal door/valve that can be raised from the base, with its opening directly facing the fire.

Now in respect to Rumford; in my previous house I built a small Rumford on top of my kitchen counter (see pics). Reason for choosing this design, was the limited surface depth, but more importantly a flue diameter of only 12,5 cm. With a traditional fireplace the front opening would have been too small for my taste. As Rumford is basically not known in the Netherlands, I decided to build my own; firebricks for the base and walls. The “wing” I made of sand cement mortar, poured in a wooden mold and polished to a super smooth surface. The firebox was welded with metal sheets and a simple manual valve allowed to open the flue. A mesh screen was added later, to add some safety at night.
I was surprised how well it performed. With high fires, the airflow was like a curtain keeping the wing cool; you could easily place your hand on it. Even with the adjacent kitchen extractor in highest gear, there was enough flue draft to keep the fumes in. However 1 minor design flaw; as I had to add a metal sheet just above the base, as the underlying cabinet became a bit to hot. Radiant heat directly in front was amazing. The only drawback was cleaning the chimney. Due to the shallow opening between the top of the wing and the firebox, the only way to clean the chimney was from the top; a pretty scary undertaking. In addition it was basically impossible to get all the residues out of the firebox. I enjoyed the build and figuring out the proper wing dimensions and slopes.

 

[SteveKG]

Yes, open-faced fireplace. The air inlet definitely provides lots of air and gets the fire burning rapidly. I don't always want it burning that way and will close the inlet. Or, I will raise the door a tiny bit and stick a splinter of wood in the crack to keep it open. I have the ring welded onto the lid so I can use a poker to open and close it. In other words, I can fine-tune how much air I'm allowing to come in from outside. There would certainly be any number of ways to come up with a more elegant solution [to partially opening the lid] but this works great for us.

This Rumford is about 3 meter by 3 meter opening plus an arch. It radiates a lot of heat [as you've experienced] into the room. There have been times I opened the door to the outdoor deck [in the same room] and we pushed our chairs and etc. back farther, simply too hot. It is simple to control this after a bit of experience by building slightly smaller fires. Again, as you likely have seen.

I, too, enjoyed working out the angles and dimensions for this; I purchased a book about Rumford's after reading about them in a source book we used to have over here called The Whole Earth Catalogue. Guests are always impressed with the radiant heat from the fireplace.

 

[SteveKG]

My Rumford:

 

[SteveKG]

The air inlet door:

 

[jhjove]

When I did use my fireplace (it has an Air-A-lator) my real issue was the after-the-burn time....when the fire goes from hot coals to embers to ashes. The entire time I had to leave the chimney flue damper open otherwise I'd get smoke/CO/CO2 in the house. All the while the heat generated from the fire is sucked up the flue.

Do you guys do anything different? Do you put a solid screen in front of the fireplace to allow for normal fire burnout without sacrificing the heat in the house?

I did look into a screen with ceramic doors....that I could keep closed during a burn, hence be on the positive side
of heat generation....but the cost for the size of the ceramic glass was crazy....almost same price for installed woodstove.

Just curious.