direct outside air to oil-fired furnace?

[jacksono]

I have a big, old oil-fired boiler furnace in basement (with 4 zones - it's a large, 3-family house) and was wondering if I piped cold air from outside in winter (New Hampshire) directly to as close to the air intake on the furnace as possible, would this be more efficient than just allowing ambient air from the basement which stays around 55 °F or so all winter to feed the furnace? I was thinking of running a 3 inch sheet metal pipe with a simple pressure damper from the wall to the furnace, about a 12 foot run.
thanks. (oreilly.jackson@gmail.com)

[jadnashua]

To be effective, you might need a larger duct. If I remember correctly, a combustion device should have at least 1sqin/1K BTU or so. Now, it is currently pulling that air through cracks in the house, and yes, pulling it from outside directly or closer to where it is being used would make the whole house feel more comfortable IF you can prevent that cold air from chilling off the basement when the thing isn't running. A 3" duct is only about 7sqin, and every elbow or other restriction cuts its effective ability down. A closed combustion system can get by with a small pipe because it is fan assisted and the outlet is directly coupled, which helps to pull it in as well. A naturally vented device can't handle much restriction, so the air must be provided very freely.

[Dana]

Adding a duct would not increase the efficiency of the system unless you made it truly sealed-combustion and insulated the duct. I've seen some real Rube Goldberg retrofits to achieve this by building a tiny air-tight room around the boiler to which large air-intake ducts with hot-air zone-valve type gate valves that get opened only on calls for heat and "proven" as part of the boiler start up sequence, but that's a lot of retro-engineering and cobbling for fairly small gains.

A better way to boost the efficiency of an old-school boiler running a 4-zone system is installing an economizer such as the Beckett Heat Manager or Intellicon 3250 HW+ and programming the low-limit of the economizer to 140F. This is a DIY project for the electrically-handy. Internet pricing on these units vary, but $180-$250 is the typical range. Installed by pros it's on the order of ~$500. In multi-zoned systems you can expect double-digit percentage savings with this type of control, and at $3.50 oil in a NH climate it would usually pay for itself in less than a single heating season.

An average basement temp of 55F is an indication that the foundation is uninsulated, and the outdoor infiltration rate is high, and/or that there is substantial insulation between the first floor and the basement(?). Rather than cutting new holes in the basement to let more air in, the building & boiler as a system will work more efficiently if you limit the air infiltration as much as possible. A 55F basement isn't that far above the deep subsoil temps for most of NH, and the standby of a big-old boiler alone would usually keep the basement into the 60s if it wasn't sucking in huge amounts of outdoor air already. (What vintage house & boiler are we talking here?).

By air sealing and insulating foundation and band joists to the point that the mean winter temp in the basement was 65F from boiler jacket losses instead of 55F would reduce jacket losses 5-10%, but it would also take a significant load off the first floor heating zone, using less fuel, and make it more comfortable to boot. Insulating all of the heating plumbing in the basement to at least R6 would also benefit system-efficiency by reducing distribution losses. Insulating the plumbing may take a couple degrees of the basement temps, but that's OK too- it's still a net improvement in the overall picture.