venting a 93% efficiency furnace
All~ (sorry in advance, this is lengthy. I can post photos later)
I am re-locating the installation of a 93% efficiency furnace, and am re-worked a botched HVAC installation.
The utility room/furnace room is 3/4 below grade. The below grade foundation is standard 8″ concrete block. Above grade, the block has sandstone fascia attached.Therefore, there is a relatively narrow band of block, essentially at-grade, to cut through the concrete block for flue gas venting and combustion air inlet.
The exterior wall at-grade and where the 3″ PVC vent and inlet piping exits the home is free of shrubs, driveways, and walkways. Operable windows are approx 4′ distant. The PVC piping exits behind, and to the left of a heat pump air conditioner that is on stilts above grade approximately 6-8″. The PVC piping is, however, UGLY.
The total vent distance from the furnace to the openings in the concrete block wall is extremely short – less than 28″ to exterior air, including field-supplied fittings. The combustion air distance is 24″ longer, across the width of the furnace.
That side of the residence faces northeast, and the snowload is always light – there is never any drifting snow in that area of the building.
Sooo, here’s the questions for alternate, non-standard installations of venting/inlet piping…..
1) The conventional installation of a concentric fitting horizontal through a sidewall would place the rain cap more-or-less at grade. Would it be possible to excavate slightly alongside the foundation, and install a corrugated galvanized steel window well to effectively lower the grade? Why or why not? If so, how large of a free air space around the window well would be required? As that part of the foundation would be exposed, what exterior treatment(s) to the block would be recommended (tar, insulation, etc.)?
2) Could a concentric vent termination be installed vertically with sidewall terminations? In other words, could the concentric vent be installed as if on an attic installation, but entirely in ambient outside air? The concentric vent can be supported to the sidewall of the residence appropriately, and with field-supplied 90º elbows at the concentric wye end and routed through existing holes in the concrete block?
3) Same question as #1, but instead of using a concentric fitting horizontally, could a BAYVENT200B Side Wall Vent Kit be used (http://www.myhvacparts.com/Catalogue/Accessories/Termination%20Kit.htm)?
I’d welcome any opinions and comments that you can provide.
Thanks in advance, Jim
Edited 10/24/2009 4:24 pm ET by coastalcg
Replies
Go to the manufacturer's web site and get the installation instructions.
Anything else is likely to be wrong, as there is are variations
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Any venting is supposed to terminate 2' above the local snow line. If you don't have snow, you should terminate it 2' above grade.
Terminating the venting at grade level leaves an open invitation for critters.
If the pipes come out of the foundation at grade level, you can elbow up the wall and then elbow horizontally at a point 24" above grade. Any exhaust piping exposed to outdoor ambient is supposed to be insulated with R8 Armaflex.
There is a possibility that your piece of equipment has specific venting instructions, and those should be followed.
As the others have suggested, you should always follow the manufacturers specs for venting. These new boilers and furnaces are very sensitive to exhaust and air intake issues and it can easily affect the way they perform if the piping isn't done right.
You definitely do not want either of those pipes at or near the ground. There are problems with snow, water, and ice that forms from dripping. They can be plugged off from leaves and other stuff accumulating.
I would use two separate pipes, elbow them up where they belong and either paint them or enclose the stack up to the elbows with something that looks good.
Also pay close attention to the offsets between the pipes. That can make a big difference in performance.