Chimney / furnace flue

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Chimney / furnace flue

AuthorMessage
David Mack

Keystone Home Inspections
NACHI Member: Yes
(as of 3/25/07)
NACHI Member
Posts: 9
User: dmack
Posted: Dec 22, 2005 9:57 PM       Post Subject:
Some infromation on the distance of a furnace flue to the chimney flue would be appreciated. Question is I remember reading that a chimney flue should be 5' above the furnace flue, in a different BB, is this true? Additionally, Will the cap also increase the chance that furnace exhaust may be sucked douwn the chimney flue? Sorry abut the pictures. The first picture shows both flues (very poorly) Flue on left is chimney, flue to right is furnace. The second picture shows the clearance from furnce flue to the cap.

[ Image: https://www.nachi.org/bbsystem/usrimages/1/11-01-05_004.jpg ]
[ Image: https://www.nachi.org/bbsystem/usrimages/1/11-01-05_0071.jpg ]
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John Rupert

Solitaire Heating & Cooling HVAC Mechanical Inspection Service
NACHI Member: Yes
(as of 3/25/07)
NACHI Member
Posts: 25
User: jrupert
Posted: Dec 27, 2005 5:15 PM       Post Subject:
The lateral run from the furnace to the chimney is based the BTUH of the furnace, the distance to the chimney, the furnace type or efficiency, the venting material type (single wall or double wall) is the furnace the only appliance venting to the same location, how many elbows in the flue run etc?all plugged in to the AGA/GAMA venting tables, etc? What I trying to say this is not a simple calculation and no rule of thumbs should be applied.

Depending on how old the furnace the venting may not meet the new GAMA table but this does not necessarily mean that it?s wrong, it may have complied with the manufactures guidelines at the time of installation

Generally 60% efficient furnaces with a venting over 10ft lateral with more then 2 elbows may be an issue, newer 80% efficient furnaces should be under 10ft

The vent connector from the furnace to the chimney should slope a minimum of ? inch per lineal foot towards the furnace

With the proper flue cap, the furnace flue particulates should not back draft down the chimney providing the house is not under a negative pressure

John Rupert
Accurate Property Inspections
Solitaire Heating & Cooling
CSG Training (HVAC Training & Consulting)
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Marcel Cyr

Cyr Home Inspections
NACHI Member: Yes
(as of 3/25/07)
NACHI Member
Posts: 795
User: mcyr
Posted: Dec 27, 2005 7:07 PM       Post Subject:
icon_smile.gif icon_smile.gif

I will agree with John on this one, because I have no idea what he is talking about.

I did notice that the chimney mason that constructed this chimney and the roofer that installed the flashing need more educated experience in the installation.
I am not a strong advocate of silicone caulking for sealing exterior counter flashing. JMO.

Marcel

P.S. No, there is not enough space from the top of the flue and the rain sheild cap.
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