Contractor Talk - Professional Construction and Remodeling Forum banner
1 - 1 of 1 Posts

mason contractors
487 Posts
Discussion Starter · #1 ·
Most know I advocate a chokable fireplace if it's draft will tolerate it.

The Rumford 4'' throat rule was prescibed by him 200 years back for the straight backed firebox with it's rear positioned throat which was about half the length of the opening lintel and breast.
He did say 3'' may be used but it would make the fp tempermental... Exactly Jim Buckley's words too when I asked him on the phone a few weeks back, why his throats are so large in area. He rebuttled with a 1:20 when I mentioned his terribly bad ratios use a 1:17 ratio.
I did read that in one of his posts and an engineer whom this past Dec. contacted me about my theory, also recalls it.
He has been follwing my internet posts and agrees with me in that a throat must be up front, ie. long and thin.

I perhaps mentioned some of this already but because the fp industry has been so badly mislead all these years by appuratus manufacturers
making up mad science to bolster sales of what they prescribe and or sell.

A firebox solely built for looks is fine on Fifth Ave or at a condo where one would pick up a bottle of wine and a bag of logs, a dura log, or perhaps a gas fire just for quick recreation.
These fixed throat fireplaces never heat up completely and run on very poor efficiency unless they close the glass doors.

Very poor draft is the one reason for not being able to burn a properly built fireplace at a high choke. This can be due to a cold exterior chimney with just a 4'' brick around the flue or a bad draft via a low lying area where enough pressure difference from chimney height won't suffice draft due to the homes low roofs with high neighboring structures etc.

This is not too bad because in these situations a fully opened damper is already at a low velocity and volume hence the importance for the adjustable damper to governor high drafting chimneys.
The difference is that with a deep, wide open throat, especially one with sluggish draft, inner breast eddys will descend easier,.. the reason being low velocity thru a wide throat.
The thin throat can and does ward off these eddys much these easier although they still come in through ocassionally during draft pause.

What matters most weather in a srtong or weak drafted fireplace is to build a firebox that isn't dynamically offensive.
Highly angled side angles especially coupled with over pitching of the rear wall ie."building it too deep" or not having the finished throat high enough, or even worse, having both fit this descripton, especially without a rounded breast....Kudos to Buckley on the breast rounding as I never before did see it's vast importance when getting high throat ratios, and yes on a slant!
Contrast between me and Jim liking the rounded breast exist. He uses it in high voulme while I use it to lessen the velocity, as both do benefit by the same air foil via it's Bernoulli Principal.

It should kept in mind that a low air volumed non restricitve throat damper with laminar flow, outfitted with a full length throat with a round breast and a vortex facilitating ''inner throat breast chamber" cannot overcome an over agressivly leaned firebox. In fact nothing will, not even a wide open throat especially if it's set back and short.

One cannot fit a slanted fp, conventional style or orton style with a throat of a straight back by setting the throat back from the front...especially with widely angled side walls coupled with too deep a firebox, resulting in a over agressive rear slanted wall.
And if you add in a 8'' deep breast which has a plumb inside wall, straight up to the throat, it's disasterously destined for high turbulence and leakage from a consortium of causes.

When Buckley told me he tried to close the throats years ago when he and superior designed them. He said they smoked so they stopped. I did ask him if he studied the smoke currents inside at that point? He said no!

I asked a mason Kieth Childs on the Houzz if he could choke his Straight Back Rumford? He said no, If you try they'll smoke the house out!

I know the best guidline in building a fireplace is to plan on low velocity and low air volume because unless outdoors that's the environment it has to perform well in!
This was never done without slight eddys as far as I know, and I don't mean for an hour or two, but rather 8 and 12 hours burns and 24 and 48.

Here's a look at my 50'' reverse blade damper [the dirty one from last years burns] that burns regularly at 1-3/4'' or 1:30....I even do a 1-1/2'' at a 1;37 or so....Drives Buckley But it does get tempermental requiring care in wood loading. It now has used 4 smokeless chords.

The others are a cut off vestal and a few of other fabricated lintel dampers. I have moe but the upload limit is prohibiting.


1 - 1 of 1 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.