Masonry Fireplace Modifications

[Ripper62]

Hello all. This is my first post on this forum. I have been looking at posts for a few weeks now and have been getting some great ideas to improve the performance of my open masonry fireplace. If the answers to my questions have already been answered on this forum, if you could please direct me to the right place I would appreciate it. Basically what I am trying to do is install air tight doors, fresh air intake, top damper, and a heat exchanger onto my open masonry fireplace. I have access to a full metal shop so this metal work stuff is doable for me. I could buy/install a FP insert, but I have a corner fireplace and have not been able to find an insert that is two sided. From what I can tell one doesn't exist. Also, I think I can make these components for cheaper and I love DIY projects. My fireplace measures 40"W x 18"D x 26"H with a 12"sq flue at the roof. The fireplace also has a rather large as dump. Here as some of the questions I have with the various components.

Sizing the fresh air intake: I am having a hard time finding information on sizing fresh air intakes. I find it hard to believe a passive 4" intake can provide enough air for my fireplace looking at some of the consumption rates I've seen on this forum. I definitely plan on installing a backdraft damper but I am wondering if a 4" variable speed Inline fan would be worth it. I have found 110 and 180 CFM fans that look like they would work.

Metal for the heat exchanger: I am a bit concerned about the durability of the metal in the heat exchanger. I am thinking about using 3/8" steel plate and schedule 40 pipe. Would coating the metal in a heat resistant powder coating or enamel be an overkill or totally worth it?

Top Damper: While I haven't done as much research on this as I have the other parts, I haven't yet been able to find any adjustable dampers. They all seem to be either opened or closed. My hope is to slow down the flue gases and the combustion rate with an adjustable damper like in a wood stove. My fireplace can go through a lot of wood. I am a little concerned with retarding the hot gasses in my 65 year old masonry chimney. I tuckpointed the portion of the chimney that extends above the roof this summer. The rest of the chimney appears to be in good shape. I am hoping to not have to install a flue liner if I can.

Thanks in advance for everyones help.

Ripper

 

[webbie]

I think I can answer a bit of this....

When it comes to the amount of air needed, this is in relation to the BTU level which you want to burn at. Some basic common sense (looking at an air intake on various wood burners) should show you some idea of what's up.

Let me hazard some guesses here. A woodstove like the Vermont Castings Vigilant probably had a 2" square air inlet, which was rarely open all the way - and the stove could probably max out at 60,000 BTU or more. So we know 4 square inches can support quite a bit of heat output.

Some furnaces and boilers I sold probably has 3" x 4" air inlets - or 12 square inches, and some of these went up to 150,000 BTU plus.

So let's backtrack a bit. Say you were OK with burning 10 lbs of wood each hour - that would be about 70,000 BTU input. 4 or 5 square inches of input should support that. A 4" round pipe is about 12+ square inches which would, in theory, support up to 20+ pounds of wood burned per hour!

Summary - a 4" pipe should be plenty!

In terms of a top sealing damper, the Chimalator (among others) have adjustability:
(broken link removed)

I see no need to coat any of the metal. If desired you could paint it with stove paint and it would be likely to stay black on a lot of areas.

 

[webby3650]

How will you get an air tight door to fit that corner opening? If you have something custom made, make sure it has ceramic glass. If not, it can't be used with the doors shut.

 

[webbie]

Stoll fireplace company would likely make the doors for you....in fact, they'd make the whole deal probably....but that would be no fun.

 

[webby3650]

Stoll fireplace company would likely make the doors for you....in fact, they'd make the whole deal probably....but that would be no fun.

They do make a heat grate/airtight door set-up. We have done a few and people are happy!

 

[webby3650]

In fact, you could get a High Valley insert and have Stoll make a custom surround panel for that opening! They do some amazing Old World and Blacksmith forge work!

 

[xtrordinair]

stoll makes some nice looking stuff

 

[weatherguy]

Wilkening also has an airtight door and a heat grate combo.

 

[Ripper62]

I think I can answer a bit of this.....

Thanks for taking the time to go through all my questions individually.

If I am able to pull this off and install a fresh air intake, air tight doors, heat grate, and top down damper to my open masonry fireplace, what kind of efficiency do you think I could expect? Also, if you can think of anything else I could do to improve its efficiency short of buying and insert I am all ears. I know an insert would the most efficient option for me but a few things are keeping me from wanting to go that direction. The two at the top of the list being cost and viewing area.

 

[rwhite]

What style of doors are you looking at that allow you burn with the doors closed and will work with a corner unit? Wilkening insert grates look well made (although expensive) and their airtight doors (as well as Stoll's) require that they be open during use. Which would negate their operating effenciency. I'm confused about not having to reline a chimney when using a heat exchanger/grate with doors as well since you are essentially installing an insert. Seems to me that it would be no more than a slammer install.

End the end I'm thinking you will improve the overall heat loss with airtight doors but maybe not the operating effenciency of the fireplace. But I am interested to see what you come up with. I chased this rabbit around a tree for several years before I finally installed an insert.