Undersized Flue?

[snaple4]

Hello,

To start I work as an HVAC technician and I thank you for all the great info on this forum. We don't work on fireplaces much unless it is a separate system from the house integrated into the HVAC system. I purchased a house (fixer-upper) this year and was told when the chimneys were cleaned that there are gaps in the clay flue. The company didn't red-tag them but said he wouldn't use them in his house. It is a 1970's house with all original parts exept for the downstairs had a new damper installed at the cap.

So to my questions. Looking through our mechanical code book it has a chapter for fireplaces but simply refers to the fuel and gas code. Everything in the gas code is for an appliance. I can find online calculations but not sure which one to use for proper flue size. 1/8, 1/10, 1/12... Both fireplaces are 24"Wx36"H at entrance, have a 12x12 OD Terra cotta flue system. Downstairs is roughly 20' tall, Upstairs is 12' tall.

From what I found the flue system is undersized? If I decide to redo the liner (after double checking the integrity) would going to a round (how would you transition from the firebox) or square be best?

Thank you for helping me.

Pictures are of the downstairs.

View attachment 218140 View attachment 218141 View attachment 218142

 

[armanidog]

I was curious and looked it up. I found the flue sizing chart for fireplaces.
Is your fireplace 2 feet wide by 3 feet high? That size is off the chart.
If the fireplace is 3 feet wide by 2 feet high, then you would need a 10 inch diameter round flue. I'm guessing there would need to be a shroud collecting the fireplace smoke attached to the bottom of the pipe.
Would you be able to fit a 10 inch pipe inside the chimney?
This is a pdf link:
http://www.mason-lite.com/wp-content/uploads/2013/05/ChimneySizeChart-new.pdf

 

[snaple4]

Did you find it in your code book? I know ours may be different but could you give me the reference number or a picture of it? It is bugging me that I am unable to find a code requirement...

The flue is 11x11 ID so a 10 would technically fit. Not sure insulation would though. There is a slight offset right out of the firebox. Still confused on how to transition. Do I just attempt to point the round pipe to the firebox then use refractory cement to seal it?

 

[begreen]

I think the dimensions got reversed. The fireplace is wider than tall so I am guessing it is 36"w x 24"h.

If the goal is heat, have you considered dropping down a 6" liner and installing an insert? That would still provide a nice fireview, but would be dramatically more efficient.

 

[bholler]

You need to maintain atleast a 10 to 1 ratio between the fireplace opening and the liner. To blend the new liner into the smoke chamber you will need to use wire mesh and chambertech to mortar it in place