Venting 2 furnaces and WH

[piersonjc]

I am putting in 2 new furnaces and a new water heater into the same utility room and need help understanding a safe and efficient way to vent all 3 appliances.
Furnaces are both 96% efficient category IV. 80K and 120K BTUs.
WH is a Rheem with power vent, 0.67 efficiency.
I have an existing opening to the outside that is 9"x15" that is at ground level outside.
I will have to run 5 separate 3" PVC pipes through the opening.
I need to get separation between the pipes on the exterior of the home.
The exterior of the home is brick so I have no ledger board to drill through unless I remove brick.

Can I run the PVC horizontally then vertically along the exterior to get them the minimum distances apart?
Can you recommend a layout on the exterior for the pipes?

Thanks for any help.

--Jason

 

[Dana]

Given that 19 out of 20 houses in the US have design heat loads under 50,000 BTU/hr I'm wondering whether you've sized the furnaces correctly to the loads. Oversizing by 2x or even 5x doesn't affect efficiency with hot air furnaces, but it does impact comfort. AFUE is tested at a presumed oversizing factor of 1.7x, ASHRAE recommends no more than 1.4x oversizing, even for multi-stage furnaces. For a point of reference, a 80K furnace would be more than 2x oversized for my ~2400' 2x4 framed sub-code insulated 1920s antique, and a 120K unit would be pushing 4x oversized.

Even at ridiculous oversizing levels, if you haven't already taken delivery on the equipment, you can probably run the whole thing on a 100-150K BTU/hr condensing hot water heater and a couple of hydronic air handlers with lower standby loss and higher net efficiency, and it would mean only one vent/intake pair to worry about.

If by "furnace" you're actually referring to hydronic boilers sizing it correctly is critical for condensing efficiency without short cycling. Read this first.

 

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[piersonjc]

I am looking for information on how to route the pvc on the exterior. These are standard hot air furnaces, 96%. Thanks.

 

[Dana]

I'm sure the venting lengths & other parameters are spelled out in the installation manuals, but without so much as a model number it's hard to say whether you'd be risking hitting some limit. When looking at length limits, be sure to include the "equivalent length" of all the ells, not just the straight pipe lengths. Horizontal runs will have minimum slope requirements to deal with condensation disposal. In cold climates (say at 10,000' in the Wasatch) you may run into ice-up problems if very much of the venting is exterior to the building envelope, though that may not be spelled out in the manuals.

I'm still skeptical about the oversizing factors- a 120K furnace can typically heat an 8000 square foot code-min McMansion at 0F with sufficient margin for those nights when it drops to -15F, yet for some reason people often install them in sub-3000' houses, with a noticeable step down in comfort. Is this going into some cavernous building like a church or something?

 

[piersonjc]

Thanks for the info.