draft and chimney flue size

Joe_13

I currently have a boiler with a 6" flue going into a 8"x8"
clay pipe lined chimney. Most specs on boilers with 6 to 8"
flues would probably be OK, but what about low mass/high efficency boilers with 4 and 5" flues? Would such a difference effect draft and require a liner to reduce flue size?

Rich Bruno

Is the Chimney interior or exterior. Oil or Gas? NFPA 54 will give gas liner sizes, NFPA 31 for oil. Interior chimneys you have a fighting chance of gewtting away with it, exteriors will be a problem.

Rich

Jack Smith

Liners

Always read the installation guidelines on the appliance. Most will indicate that they require a liner. I drop a liner everytime it is possible, doesn't matter inside or outside chimney. You have a large tile, liner should be a piece of cake, drop the liner.

John Pilger

Liners

The manufacturers instruction will usually tell you to install the appliance into a chimney that complies with NFPA 211, Standard for Chimneys, Vents, Fireplaces and Solid Fuel Burning Appliances. If the appliance required a chimney lining systen the manufacturers instructions will spec the size.
If a high efficiency boiler is installed in an oversized chimney, problems will occur. Draft may be poor, for example, if you vent into a round chimney or liner of 5" it will have an area of 19.64 sq.inches. If the appliance is vented into a 8x8 clay tiled chimney it will have an area of 64 sq. inches. Notice that the area of the 8x8 chimney is 3.25 times larger that the 5" round chimney or liner. This larger volume of area will reduce the pressure needed (draft)to drive the flue gages through the venting system. The larger area will also reducue the already low stack tempertures and increase flue gas residence time. This will cause the flue gases to reach its dew point and condensate. This condensate is highly corrosive and will rapidly deteriorate clay flue tiles or aluminum.
If the chimney is installed on an exterior wall of the home and a liner is needed you will want to insulate it with ether ceramic blanket or thermix chimney lining insulation. This will keep the flue gases from cooling before it exits the system. If you don't want to get involved with chimneys and installation of lining systems you can partner up with a Chimney Sweep certified by the CSIA (www.csia.org.)
If the manufacturer does not specify the size liner you can size it per NFPA 31 Appendix E (oil) or NFPA 54 (National Fuel Gas Code)

Hope this helped,
John

Joe_13

Thanks for all the replies

The boiler is oil fired and the chimney is brick on the exterior of the home. It sounds like the extra cost of a high efficency boiler replacement might now not be worth the cost when so much must be spent on chimney modifications. The last maintenance check tested the flue gas @ 780 degrees. Thanks again.

Rich Bruno

780 stack temp? If your service company is telling you to change the boiler and/or reline the chimnet they are giving you good advice. Your heating dollars are going up the chimney. A properly sized boiler with a properly working chimney will pay for it self in savings and comfort.

Joe_13

analyser stats

This comes right off the Fyrite Pro Analyser printout:
Efficency 75.8%
Excess air 35.6%
Primary Temp 85.7 degrees F.
Stack Temp 787 degrees F.
O2 5.8%
CO2 11.3%
CO 42 ppm
CO air free 58 ppm

Draft -0.25 mB

This was after a cleaning, do these numbers look OK?
140K input BTU 100K net output BTU with 1GPH Beckett gun
also has a tankless coil.

Rich Bruno

Stack temp seems high.

Joe_13

fuel company services the boiler

So I don't think they're looking for you to use less fuel. 75% efficency isn't bad for a 13 yr old Thermodynamics ATF steel boiler. Probably a freebie the last home owner got if contracted with the fuel company for 5 years. I thought the stack temp seemed high also. I would think you'd want the heat going into the water, not to heat the chimney. This last tough winter made me think about getting a replacement configuration ready in case something major breaks rather then fixing it. Replacement is probably a year down the road. This was a 900 gal. winter on Long Island. Past few years I barely got over 700 (1800 sq. ft house)

Steve Ebels

Compare them to these

Numbers are from a Vitorond firing at 197,000.
87.8% eff., CO2 12.1%, O2 5.2%, net flue temp 328*F, 3PPM CO. I get similar number from Buderus 115 and 215 series boilers with Riello burners. Even the Thermopride furnaces we install with Riello's run in the mid to upper 80% range with stack temps rarely above 400* if the old ductwork is up to the task.