Fireplace Renovation

sellears

I have a 2 story 1890 farmhouse. The chimney and fireplace were rebuilt in the 1960s with CMU and firebrick, basement to roof with a pair of clay lined chimneys, one of which the fireplace uses. the fireplace faces a 15x15 living room and the back is to the large kitchen and dining room (15x30). I cant put in a modern fireplace without removing all the CMU above the firebox to meet the "open space" code to ceiling. Inserts drastically reduce the viewing area of the fire, which is what enjoy. Chimney will need to be lined. Fireplace used 1-2x week Oct-March. I want the following in my design

-efficiency increase (not to EPA standards - but enough that the fire doesnt kick on my furnace)

-heat dump - would love to dump heat into larger room behind fireplace, maybe even plumb into my forced air return

-low(er) emissions - if i can get a cat converter in there im game

-outside air - exterior wall is close by with easy access to plumb air into bottom of fireplace

-long(er) burn time

-at least 20x30 viewing

My impression is that there isn't an off the shelf product for this. I am looking at welding up a custom box (insert) to fit into the existing fireplace opening to get much of what I would like. Thought i would reach out here to see if this is the case and if anyone has tried this before and has some design lessons.

Jamie Hall

What is the overall geometry of the original firebox, or what you might be able to get back to? Or is nothing left of it (which, sadly, is very likely)?

I don't know of any more or less otff the shelf units which will do what you would like, particularly in terms of viewing angle — as you have observed.

As far as geometry goes, look up Rumford fireplaces. They, and later designs (not as high — the two I have are lower; one is about the same height as the front width, while the other is eight feet wide at the front, four feet wide at the back, two feet depth front to back, and only four feet high at the front opening — and both of them heat their spaces spectacularly well). The key to the Rumford design is the angled sides and the smooth, but not overly large, throat to the flue (with a wind shelf at the back — often overlooked).

If you can find tempered glass doors which can be closed on a going fire, with an automatic temperature controlled damper at the bottom, they work wonders. Almost impossible to find today.

Bob Harper

Forget about getting outside air into the firebox- it doesn't work and will hinder combustion.

Forget about an insert if you want a big open hearth fire with some heat. You have two effective choices if you have to keep the masonry fireplace: Ahren Fire and PriorFire retro kits. These require a full length listed liner, parged smoke chamber and kit. They mitigate clearances to combustibles, put out appreciable heat and typically draw like Da Vinci. Both draw on Rumford/ Rosin concepts. They'll incorporate a top mounted damper system Chris Prior's PriorFire design is about 45 yrs in the making. He lives in the Adirondacks building nothing but masonry fireplace to his own designs. Nobody knows more about masonry fireplace construction than Chris. He partnered with SaverSystems to test and build a design that preheats ambient room air injecting it and the ideal place. His firebox is insulating vermiculite-based whereas the Ahren Fire German system uses cast refractories and insulation.
As for building a Rummy, check out Jim Buckley's Rumford.com site. He's actually almost as knowledgeable about fireplaces having done testing with Nerbert Senf from the Masonry Heater Institute and Lopez Labs.
If you can locate a copy, Prof. Peter Rosin's 1929 Fluid Dynamics of an Open Hearth is a classic demonstrating the fallacy of the smoke shelf and how various shapes hinder with turbulence or help with laminar airflow measuring Reynolds Numbers. Cool stuff but it explains why certain fireplaces spill smoke while others don't and it has very little to do with the flue. It also shows improved rear walls featuring a parabolic curve so the heat and smoke hug it, transferring heat then shooting up the throat entraining room air like a venturi (Bernoulli Effect). Lots of science in fireplaces. Then again, if you have a copy of Count Rumford's 1795 treatise you can read things like (paraphrased: " The whole mystery of smoking fireplaces can be summed up by removing the local hindrances thereof or, more specifically those that inhibit the cooler room air from displacing the warmer flue gases." He figured that out about 234 yrs ago as he ran all over New England then London fixing fireplaces.
HTH

Mad Dog_2

CMU?? Mad 🐕 Dog

bburd

https://forum.heatinghelp.com/discussion/comment/1805796#Comment_1805796

Concrete Masonry Unit, a.k.a. concrete block.

mattmia2

https://forum.heatinghelp.com/discussion/comment/1805796#Comment_1805796

Cheaper and faster than brick for the structure if you're rebuilding the whole thing.

Cuppetorn

To improve heat efficiency, consider adding a heat exchanger to help distribute warmth more effectively. Bringing in outside air for combustion can make the fire burn more efficiently and reduce drafts. Using refractory materials inside the firebox can help extend burn times, and installing a catalytic converter can reduce emissions and improve efficiency. For more professional advice, definitely check out some renovation companies like https://topgradeenterprises.com/. They can provide expert guidance, help with design, and make sure everything is up to code.