I have a traditional open brick fireplace (42×36) which Im looking to make a bit more energy efficient by adding chimney cap damper. I was wondering if anyone had any experience with these things, and if so, I had a couple questions: 1. the damper inside my fire box fits pretty snug — will I gain that much by using the cap damper instead? 2. I understand a benefit of a cap damper is “pre-heating” the flu, allowing for a more rapid establishment of an up-draft to aid in fire starting, reducing any down draft. But doesn’t that also mean that I will now heat the entire length of my chimney, and open my family room to what I cant imagine is a very well insulated chimney environment? 3. Assuming that the cap damper is the way to go (Im looking at the Lyemance-OPC-Chimney-Cap-Damper), my chimney is topped with 2 side by side 9×13 tile openings. Nothing else runs up the chimney (no furnace etc). Most chimneys seem to have a single 9×13 opening. Can I cap off one permanently, and install the cap damper on the other? Im wondering if a fireplace my size needs both openings almost 30′ up the chimney. Installing 2 cap dampers seems a bit odd. Comments?
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
Few people understand it. Nobody agrees what it is, how to learn about it, or who's responsible for it. It has never been more important
Featured Video
How to Install Cable Rail Around Wood-Post CornersHighlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
I don't have first hand
I don't have first hand experience with cap dampers but
I know fireplaces pretty well.
The cap dampers are supposed to work well and seal the flue
completely when not in use.
The problem with the throat dampers tends to be warpage.
One overfire can permanently warp it, while the cap type is
much farther from the heat source.
If you don't have problems with it drawing properly I'd
suggest tight sealing glass door first.
You will get a more efficient fireplace that way.
It would be very unusual for a fireplace to have two flues.
One is probably for something else.
1. the damper inside my fire box fits pretty snug -- will I gain that much by using the cap damper instead?
Probably not. Flue top dampers are best used when there is no other damper or the firebox damper is warped or rusted out.
2. I understand a benefit of a cap damper is "pre-heating" the flu, allowing for a more rapid establishment of an up-draft to aid in fire starting, reducing any down draft. But doesn’t that also mean that I will now heat the entire length of my chimney, and open my family room to what I cant imagine is a very well insulated chimney environment?
That is correct, especially if the chimney is on an outside wall.
Can I cap off one permanently, and install the cap damper on the other?
No. The rule of thumb for flue cross sectional area is 1/10 of the fireplace opening. Using two 9x13's is a little unorthodox, but I've seem it before. One 9x13 will not carry that size fireplace.
Am I missing something?
42x36=1512
Move the decimal= 151.2
9x13=117
9x13 should handle a pretty big box
Henley: Maybe I'm missing
Henley: Maybe I'm missing something in your post...
You say a 42x36 opening requires 151 sq inches of flew. Then you say a 9x13 tile supplies 117 sq inches... So, wouldn't that mean that the 9x12 tile is (very) roughly 25% too small?
Ah, my dyslexic ways!
it's
Ah, my dyslexic ways!
it's amazing, I sat there scratching my head trying
to figure out what Seeyou was saying.
51 not 15
9x13 should handle a pretty big box
One other consideration: 9x13 is the outside dimension. The inside is closer to 7x11. So 7x11=77x2=154. Barely adequate.
pajor:
Some general comments:
That is not a small fireplace.
If both flue tiles are indeed for the fireplace, I think you would be looking for trouble by blocking one off. The sizing of the various componets is very spefific to make them all work together and produce a fireplace that draws correctly. It is almost an art - maybe even becoming a lost art since so many homes now aday are being built with metal prefab fireplaces due to the cost of a true masonry fireplace.
There are specific building codes for the sizing of fireplace components. I can't recite them off the top of my head but the idea is that they are designed for not only a safe, but also proper functioning fireplace. While you are not normally required to bring an existing house up to current code, it would be worth looking at as it would help you understand what the more standard sizing rules are.
Also the BIA web site (Brick Industry Association) has some information about fireplaces in their technical notes section that may prove informative re this thing about sizing fireplace components. It is more complicated than one might think.
Thx for the comments. In my haste to post (I had been meaning to, but never had a tape measure and computer together in same room) I guesstimated 42 x 36, and figured I could adapt whatever advice I rec'd accordingly. Its actually 36 x 30, so even taking the inside dimension of a 13x9 tile, 77 is not 10% of my 1080" opening.
That said, the current damper is mighty snug, so a chimney topper might be over kill. The biggest draft culprit is the doors to the ash dump.
<>
Yes, indeed.
The 10% we have been talking about is a good example.
That is the proportion put forth by count Rumford in 1795.
That is part of a larger system of proportions specific
to "Rumford Fireplaces".By making the firebox very shallow
with the sides angled towards the room he greatly increased
the overall efficiency.
This made the throat proportions critical as the smoke
wants to come in the room.
(He created the smokeshelf too,which is equally important
in terms of keeping the heat in the house)
While they are coming back into fashion, many (if not most) older units are not Rumfords. They tend to have
much deeper and shorter fireboxes.
That is less efficient but allows it to draw more readily.
So the flue proportions can and have varied from design to
design.
But more to the point, well sealing glass doors are your best and easiest way to get higher efficiency.