Add Combustion Air Intake to Gas Furnace

[Mark_F]

I recently had my older 80% efficient gas furnace replaced with a new 98% efficient furnace. The furnace is located in an unheated 18' x 30' crawl space. The crawl space is vented to the outside with two 6 x 14 vents at opposite corners of the crawl space. The old furnace used combustion air from the crawl space and exhausted through a type B vent routed up through a chase, thru the attic and out of the roof.

The new furnace was installed and, I think, unfortunately was set up to continue to draw it's combustion air from the crawl space. It seems to me that drawing cold air from the outside, into the crawl space, is not as good as drawing only the cold air needed, thru a PVC pipe connected directly to the furnace. IOW for overall home energy efficiency and heat conservation I think drawing cold air into the entire crawl space to supply the furnace is not as desirable as having the furnace intake air piped directly to the outside.

I have included some photos of the furnace and piping. The HAVC contractor ran the new PVC exhaust thru the existing type B vent. There is plenty of room inside the type B vent for another equal size pvc pipe. The photos show that, due to how the exhaust PVC was attached to the furnace, there is not enough room for an intake PVC 90 degree elbow to attach.

My questions are:
1) Am I correct that piping the intake to the outside will help enough with overall energy efficiency to make it worthwhile to do?
2) If I add the fittings needed to make room for a 90 elbow to attach to the furnace for intake, would those additional fittings unduly compromise the exhaust flow?

I have a lot of home building and repair experience. Expertise as carpenter and ok with simple plumbing, electrician, tile setting, drywall etc work.

Thanks in advance for any advice or comments.

 

[WorthFlorida]

You are saying that the furnace is in the crawl space and it gets it fresh combustion air from the attic space? Is the vent covered over just for the non heating season? At the furnace it is 3" pipe but to the roof it is 2"? The Lennox installation manual has everything you need to know on venting & pipe sizing. You should read through it to see where your install sits to the manufacturer's recommendation.

 

[Sponsor]

 

[Mark_F]

No, the furnace gets it’s combustion air from the crawl space. The crawl space is vented as stated above. I show the attic photo so you can see where the combustion air pipe would come up to and turn out thru the wall and exit as shown in outside photo.

The pipe is 3” all the way, not sure where you see it change to 2”

The attic vent is covered up because I am using a continuous ridge vent and soffit vents for the attic. Attic venting is not pertinent to my questions or situation with this.

I have read the Lennox install manual and truthfully cannot be certain of how to apply it to my situation. Also my question #1 above is not answered by the manual.

 

[Stuff]

Usually the issue without a dedicated intake is that the furnace is taking inside air. This slightly depressurizes the house and sucks in outside air through all leaks in your envelope to make up for it. In your case you are drawing air from the unconditioned crawlspace so has minimal affect on efficiency. Only issue I can see is if you had snow levels that can block the crawl space vents.

Putting the intake vent through the wall could possibly cause other issues as it would be in a different pressure zone than the exhaust. Better to re-route both pipes through the side.

The manual should state number of elbows allowed per pipe size, length, and furnace BTU.

 

[Dana]

The total rate air drawn in for combustion is tiny compared to the average amount of air coming through the crawlspace vents, which is where your REAL inefficiency problem is.

Drawing combustion air from a vented attic would be fine, or out the side. But air-seal both the bottom and top of the B-vent to block stack effect 24/365 draw that's depressurizing the crawlspace relative to the outdoors. The stack effect draw into the crawl space is probably on the same order of magnitude as the air motion induced by a light wind. Be sure to air seal ALL plumbing stack and electrical chases (not just the flue chases) that extend from the crawlspace to the attic &/or beyond!

Putting down a ground vapor barrier, closing off the crawlspace vents, and insulating the foundation walls to a code-min continuous R15 is likely to cut your heating fuel use down by more than 15%, maybe even 25%.

Changing the fittings to the exhaust adds "equivalent length" for ever ell, but a pair of 45s is "shorter" than a single sharp-throated 90, and a long radius 90 is shorter still. Measure up the full linear length, and add all the equivalent lengths of the fittings to see how close you are to the manufacturer's specified limits, or just go with the " 4501-10,000 ft. " elevation chart for vent lengths on p11 of the manual. If it's 3" the whole way I don't think you're even close to hitting the limits in a 1 story house.

 

[Mark_F]

Dana, you rock! All the info I wanted to know and more. Very helpful, Thanks!

I’m not sure what stack effect is, but I’m going to learn more about it and will seal off the B vent at both open ends in the attic.

The attic is a “cold roof” so the attic isn’t actually vented. The vent space is in the bays between the 2x12 rafters, above the R30 insulation batts. A 2” space between top of insulation and bottom of roof sheathing with continuous ridge vent and soffit vents in each rafter bay.

So, I have to route the intake thru the wall to the outside. I don't think venting from the attic would be viable with my situation.

 

[Dana]

Dana, you rock! All the info I wanted to know and more. Very helpful, Thanks!

I’m not sure what stack effect is, but I’m going to learn more about it and will seal off the B vent at both open ends in the attic.

The attic is a “cold roof” so the attic isn’t actually vented. The vent space is in the bays between the 2x12 rafters, above the R30 insulation batts. A 2” space between top of insulation and bottom of roof sheathing with continuous ridge vent and soffit vents in each rafter bay.

So, I have to route the intake thru the wall to the outside. I don't think venting from the attic would be viable with my situation.

Stack effect is the pressure difference created by lower density warmer air rising to the top of the enclosed space (in your case the house, not an actual smoke stack). As the warm air rises and leaves through the leaks at the top of the stack, it depressurized the bottom of the stack relative to the outdoors, drawing more air into the bottom. This creates a continuous convection loop that has it's strongest drive during the coldest weather, since that's when the density difference between the warm air and cold outdoor (or crawlspace) air is the greatest.

To limit this effect the most important air leakage points are at the bottom and top of the house. Right now you have a B-vent conduit doing a full short-circuit for that loop, but you likely have other leaks as well. The annular ring space between the walls of the B-vent, and between the PVC vent and B-vent both need to be air sealed with something like expanding foam at both ends of the section that goes from the crawlspace to the attic, and the bottom of the section that goes through the insulated roof. It's hard to tell how well (if at all) the B-vent is air sealed to the polyethylene vapor barrier where it goes through the roof. Even if you don't do it elsewhere it's probably worth installing a piece of wallboard or plywood to the underside of the rafters around the B-vent, and seal it to the vapor barrier and B-vent with can-foam, and sealing the cut in the wallboard/plywood with duct mastic.

The BIG leak at the bottom of the house is the crawlspace vents, but until you have a ground vapor barrier closing the vents off could result in soil gases (including water vapor and radon) entering the house via the crawlspace. Any plumbing, duct, or vent penetrations of the subfloor would also need to be sealed, as well as the seams and joints of the ducts and furnace cabinet. Foil HVAC tape works fine on shiny-clean galvanized duct and painted furnace cabinet, but other joints, especially curved joints on ells or register boots would be better sealed using duct mastic. Seal the register boots to the subfloor with polyurethane caulk for any gaps up to 1/2" wide. Any gaps wider than that use housewrap tape, covering over the edges with duct mastic.

It's also worth air sealing any plumbing, flue, and electrical penetrations of the attic floor too, even though the vapor barrier at the insulated roof is nominally the pressure boundary of the house. It's actually pretty hard to make the house TOO tight in retrofit situations. Where polyethylene sheeting is used as a primary air barrier it's important to overlap and tape where different sheets join at locations where the taped seam can be stapled to framing, and seal over the staples with another patch of tape. The instructions for Certainteed MemBrain vapor retarder have a lot of tips on air sealing flexible sheet goods such as 6 mil polyethylene. (MemBrain is only 2-mil nylon, but a bit tougher than polyethylene at any given thickness.)

R30 is way below the IRC code minimum R49 for US climate zone 7. (Telluride averages over 9000 annual heating degree days, which makes it zone 7, even if most of the county is zone 6.). At some point it may be worth installing 2" rigid foil faced polyisocyanurate foam board to the underside of the rafters, which would more than double the R value of the thermal bridging rafters, and bring it pretty close to the current code-max U0.026 performance, despite being less than R49 total at center cavity. Cap nails or cap screws that penetrate the rafters by at least 1.5" would be preferred. The seams of foil facers can be taped with foil HVAC tape. If you used fire rated Dow Thermax you could just leave it, but fire codes would normally call for a timed thermal barrier against ignition between the room space and the foam board. Running 1x4 furring 24" on center perpendicular to the rafters, through screwed to the rafters with a 4" pancake head timber screw (eg FastenMaster HeadLok) would allow you to mount half-inch wallboard on the ceiling as that thermal barrier. Either taped rigid foam board or gypsum board are each a more reliable long term air barrier than exposed polyethylene detailed as an air barrier.

BTW: Couldn't help but notice the Dynafits- been getting any summer turns in?

 

[Mark_F]

Wow Dana. Very thorough discussion.

When I built the addition in 1986 the code was for R30 in roofs. Now everyone uses spray in foam and I don't know what r-value they achieve but it's high. Also eliminates the need for venting the rafter spaces.

Like you , I like to be thorough and if I'm gonna do it I want to do it "right". If I had it to do over I would follow your advice for sealing up the crawl and vapor barriers and etc. At this point it's kind of too late for a lot of that.

Where the type B vent goes through the insulation I have it sealed with metal tape. It used to be operational so it need 1" clearance to combustibles and I have it insulated with rockwool and the VB taped with metal/foil tape. I had taped all the attic VB joints with some reinforced tape that is failing. What tape you recommend for polyethylene VB?

not sure if your aware of how much snow we had in Colorado this spring. Historic. Skiing from the top of the ski area to town at the end of May was like skiing on a good powder day in February in BC. A lot of those days in May. Come summer I'm ready to switch over to other things so...kayaking on the snow melt and etc. Others are still skiing, it looks pretty sun cupped at this point from what I've seen.

Thanks again for all your helpful info! I'm going to implement a lot of your suggestions.

 

[Dana]

If there's enough space in the crawlspace for a furnace, there's enough room to work with to put down a vapor barrier and insulate the foundation walls. It looks like there is concrete slab under the furnace- does it cover the full crawlspace floor? If yes, there are decent enough epoxy coatings for vapor-sealing the floor, after filling any cracks with purpose-made self-leveling polyurethane caulk. A mere 2.5 inches of foil faced polyiso on the crawl space walls would hit R15. (I did my basement walls with 3" of reclaimed fiber faced roofing polyiso, which has a lower R/inch than the foil faced stuff.)

Housewrap tapes seem to hang on to polyethylene pretty good over time. Certainteed recommends sheathing tapes (designed for sealing OSB & plywood seams) for their 2-mil nylon, which should also work for polyethylene.

Late July into August skiing tends to be pretty suncupped & runneled out where I originally hail from in the Pacific Northwest, and probably not "worth it" for most locals used to skiing the better stuff, but for New Englanders who haven't skied since late April suncups and runnels sound pretty good! Seems like you got spoiled with a pretty good season this year! I usually visit family and ski the glaciers and permanent snowfields in WA in July, but have missed out for a couple years running. This was taken with a hand-held still camera in video mode (note the double-poled arm waving) on the Muir Snowfield on Mt. Rainier just above the bergschrund of the Paradise Glacier a handful of Julys back, about three days after they had a few inches of freshies (pretty sweet light corn!) I've skied lift-served at Crystal Mt. WA as late as July 27th during one epic year when I was a kid, but conditions are usually better higher up.

This is what local "skiing" in MA usually looks like in May. We call it the "Silly-Skiing Season". One year I was challenged to a silly-slalom race on the last day of May, but the other guy didn't show, not believing there would be enough "snow", so I shot a few videos as evidence.

 

[Mark_F]

The foundation walls are insulted with rigid foam, but I think it's only like 1.5" or maybe 2" can't remember. R maybe 10. It was a long time ago. I could go look and verify but you get the idea.

Crawl space floor is unfortunately dirt and very uneven with lots of big rocks. Wish I had done a better job of prepping that but I was young, it was a very low budget deal, etc.

My house has increased in value over almost 20x since purchase in 1985. Any and all work I do to it is only for my comfort and etc. Nothing I do can increase the value even 1cent. Upon sale, when I'm too old to live up here, the addition will be torn off, the historic part will be lifted and a full basement will be put under the house and it will expanded to the greatest extent allowable and probably for 1 million dollars or more cost. that's what happens to all of the houses here. I have just finished, over a 5 year period and out of pocket, remodeling everything I did 30 years ago. Gutted the bath down to sub-floor and studs, new recess LED throughout, New flooring throughout, all drywall repair and new paint, new kitchen finished last fall on the day before I turned 60. all work done by me,carpentry, plumbing, wiring, tile, drywall, cabinet install etc, etc. Only way I can afford it.

Had to have some HAVAC guys put in the new furnace. Got a great, unbelievable, deal from local eco-action group. They basically paid for the whole thing...because I "qualified". Put in new Nest thermostat to go with it and got $100 rebate on that from the gas company.

those videos are priceless!

 

[Dana]

Adding a ground vapor barrier sealed with tape/caulk/mastic to the existing foundation insulation and tight fitting (or taped) 2" polyiso hatches/plugs for the vents would be pretty cheap, and would pay for itself in lower heating costs in under 2 heating seasons, maybe less than one. (It won't be as cheap as a practically free condensing furnace though!) Using EPDM membrane roofing material for the vapor barrier is more rugged than cheaper polyethylene sheeting, but 10-12 mil polyethylene is still rugged enough if you're not walking /crawling on it all the time.

Glad you liked the videos- not everybody appreciates the humor.

 

[Mark_F]

If I do pipe the furnace intake to the outside as planned, so I can close off my crawl space vents, I am concerned about the effect of that on the crawl space. I don't want to create a condensation/mold type issue. This is a very dry climate and the crawl space is very dry. Radon may also be a concern. I have tested very low for radon in the bedroom above the crawl, but the crawl is currently vented.

It is a 4' high space from dirt to bottom of truss joist floor joists. The furnace keeps it somewhat warmish. I have insulated between the floor joists but no vapor barrier on the bottom the joists. Lots of ducting and plumbing and stuff running through various areas between joists and also the large outflow and return trunk lines coming to and from the furnace are hung under the joists.

I'm thinking if I do close off the crawl space vents then I may need to add the vapor barrier to the dirt floor. I don't go down there every day or anything but I have some stuff that can't freeze stored down there like paint, spackle, caulk, adhesives, etc. that I get at every so often. I also need to open/close the back flow preventer for my irrigation twice a year, change furnace filter. and occasional other stuff. The floor is so rough I think putting a vapor barrier of any type down there as is may not work.

Some photos are attached to show the existing conditions. That's 1.5" R7.5 insulation on the walls. R19 unfaced batts in the joist bays and a lot of rocky mountain rocks on the floor. what looks like dirt is actually 90% rocks of varying size.

Thanks for your helpful advice!

 

[Dana]

Condensation requires surfaces colder than the dew point of the proximate air. Most crawlspaces with condensation/mold issues are getting the moisture from the outdoor air, in houses that are air conditioned in climates with high outdoor dew points. The summertime outdoor dew points in Telluride are quite low- it's very hard to create condensation conditions in the crawl space even if air conditioning, even if your dirt floor is actively wet a lot of the time AND you skip the ground vapor barrier. Most unvented crawlspaces on well drained glacial till soils won't have moisture condensation problems in climates such as yours, but could raise the radon levels. In your case the primary moisture source would be the ground, and in a higher than average radon zone it's prudent to put down the ground vapor barrier (even if the crawlspace remained vented.)

You may need to roll the bigger rocks out of the way as you go along then roll/stack them back onto the vapor barrier. A cubic yard of sand can go a long way to smoothing over glacial-till type rounded stone. EPDM roofing can handle some pretty sharp stone, and it's fine to drape it over fairly big rocks if need be. If polyethylene, overlap seams in the sheets by a foot or so with a thin bead of caulk between them, and tape the edges with house wrap tape. Making yourself a storage pad with cheap concrete patio pavers to protect the vapor barrier from punctures would be reasonable, and a lot cheaper than pouring a 2" rat-slab.

R7.5 is fully half the current R15 code minimum for crawlspace walls, which isn't terrible. Whether it's "worth it" to add another 1-1.5" of polyiso over the pink XPS is your call. Taping the seams of the XPS with housewrap tape at the same time the vapor barrier goes down makes it more air-tight against soil gases. XPS is labeled R7.5, but if it was manufactured since the Montreal Protocol it's HFC blowing agents will eventually dissipate over a few decades to about R6.8 or less (not that you'd notice.) If it was installed in 1986 it's probably blown with CFCs, which take much longer to bleed out, and it may still be performing at that level. Whatever it's actual performance is, beefing up the wall-R is much lower priority (and much longer payback) than blocking the air flow through the vents & air-sealing the space.

 

[Mark_F]

Thanks again for your expert and knowledgable discussion of this! You clearly possess a lot of detailed and specific knowledge about these products, systems and more. Are you an engineer of some type?

 

[Dana]

Thanks again for your expert and knowledgable discussion of this! You clearly possess a lot of detailed and specific knowledge about these products, systems and more. Are you an engineer of some type?

What, is the "Engi-NERD" stamp on my forehead beginning to show? Or maybe it's my (lack of) skiing style?

I'm currently working as an electrical engineer, but come from a family history steeped in the construction biz (mostly commercial, some residential), and having studied up a bit on the building-science end of residential construction end up advising on all sorts of construction projects (within the family and elsewhere, including a couple of web forums.)