Flue sizing

Dirk Wright

Hi Ron, I have a Fyrite Pro 125, recently calibrated. I ran some tests last night and I think I'm OK. With both appliances off and the flue somewhat warm, I measured -.01" wc draft at the test point for the furnace. I fired up the furnace, and got -.02" to -.04" wc. I then fired up the water heater, and I got 0.00" wc to -.02" wc. It seemed to settle down around -.01" wc. The furnace was running good too, at 82% efficiency, 70 ppm CO, 11% CO2. The outside temperature was about 40 def F. I don't remember the rest of the results. I need to get a printer for this tester. Anyway, I will take your advice and check the calculations to see if I'm undersized for the combined flow for the last 2 feet of metal flue to the chimney. Installing a larger flue in the chimney is not a task to be taken lightly...I am planning on adding 2 feet to the chimney, which may help. I just checked the inside of the chimney in August, so I think it's not clogged or other wise obstructive. Do those metal rain caps reduce the draw? I can remove it easily enough if they do. I also remember hearing the noise from air whistling through the barometric damper when everything was off. I could put my ear to it and hear it. So, I hope I'm good anyway for now anyway. I wonder if the constant draw of the chimney causes depressurization of the house? It would be like have an exhaust fan on constantly I suppose. I wonder what I can do about that?

Dirk Wright

Flue sizing

I know I have this information at home in a book somewhere, but I figured someone here would be able to help me faster. Thanks in advance for any help that may be offered.

I have two oil fired appliances sharing a flue into a chimney. I have not yet measured draft, so I can't answer about that, but I have noticed that when either one is running, the barometric dampers move in slightly as you'd expect. The thing I noticed was that when both are on, the dampers stay fully shut, leading me to think that the flue size is too small.

What I have are a furnace and a water heater sharing a short section of 5" flue. Both appliances have 5" flues at their respective breaches. They are connected by a "T" just before going into the chimney, which is at least 20 feet tall. They both have their own 5" barometric damper near their respective breaches. The firing rate of the water heater is 1.1 gph and for the furnace it's .75 gph. I'm worried that during concurrent operation, flue gases may spill out of the barometric dampers.

Obviously, I need to measure draft during concurrent operation, but I just thought I'd toss this out in hopes that someone would pipe up with a simple answer. Thanks.

bigugh_4

Simple

Chimney is to small. two 5" should go into no less than a 7" chimney. cheap fix, control the heating unit with a system that'd cut it off if the W/H came on-line. Otherwise I'd be replacing or installing another chimney.

Dirk Wright

Chimeny is 8.5\"

I'm sorry if I misspoke, but the chimney is standard 8.5" x 8.5" flue tile. The 5" metal flue is from the appliances to the chimney.

[Deleted User]

Increase the flue pipe

size, between the first & second appliance vent connection, to 7". Run that to the stack. Then get someone w/ a combustion analyzer to set up both units.

Rookie_4

The pipe off each appliance is 5" which is 19.63 sq.in. in area with a total of 39.26 sq. in. for the 2 of them. so your main flue going to the chimney needs to be sized to that. Your choice would be a 7" as Ron said at 38.48 sq. in. or a 8" at 50.27 sq. in.........ROOKIE

Bob Harper

NFPA 31 limitations

Only calculates for one appliance at a time. What I recommend is to install a 5.5" listed liner then put a priority switch btw the two appliaces so only one fires at a time. If you're lucky, you could get a 6" liner down but with one appliance firing you only need a 5" -5.5" liner. Most people put the water heater as the priority.
HTH,

Dirk Wright

Flue sizing

In Audel's they say:
"When more than one appliance is vented into a common flue, the area of the common flue should be equal to the area of the largest flue plus 50 percent of the area of the additional flue."

So, it does look like I screwed up. dang it!

TORCH_2

Might not be the right thread or tell me the right one i'm new here

Hi I have a welding shop in a old shipping container for my self and I am going to be installing a very old heatarola Intency fire air duct coal stove but I'm burning wood in it and would like to use 6in flue instead of 7in since 6 is easier to get at my hardware store does any one know if it is ok in the long run? I have used the stove on 6 before but not vented in to any chimney I did it to warm a outside party I had!! or should I just use 7 if i can find and the parts and pipes? Also when I install it in my shop. I plan on venting the stove out and turn left horizontal into a raised up 35gal drum on it's side with one removable baffle and then out the top of the drum and through the steel container wall in to the next room and turning right in to the I think 14 or 16in 15 foot round concrete chimney ( Made out of old non used sewer pipes) total single wall flue before chimney aprox 10feet and 3 90's and one 45 going into the drum, but I can remove The 45 if I have to and pipe the stove into to back of the drum and out the top front . Will this thing still draft right ?? or will it take a long time to get going?? Thanks to any one or people with tips or help or ideas Thanks BEN AKA TORCH PS. If more info is needed just ask!!!!