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posted: 11/18/2012 5:36 AM

Why is HVAC system emitting sewer smell?

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Q. I am a single lady living in my own one-story house for 25 years. My problem is that for the past six months, my house smells like a sewer every time the heat (or air conditioner) kicks on. My gas, forced-air furnace draws from the crawl space, which is fully vented to the outside.

My neighbor went down into the crawl space and reported no leaks. My plumber rodded the sewer line and said that would do it. Nope. Now he said to wait a month or so and see how it is then. This doesn't sound good to me.

The only other thing my friends say to do is have it Perma-Sealed. But I know that costs thousands and thousands. I have been a faithful follower of yours, and I have nowhere else to turn for help or suggestions. Whatever you say, I'll do!

A. When you say that your furnace "draws from the crawl space," I assume you mean this is where the furnace is located. Did the plumber or your neighbor notice any unpleasant smell when they went down in the crawl space? Is the floor of your crawl space bare dirt, is it dirt that is thoroughly covered with plastic (in good shape), or does it have a concrete floor? If the soil is bare, it should be covered with a minimum of 6-mil plastic, and if the soil is rough, it should have two layers of plastic or a much heavier agricultural-grade plastic to resist punctures. (You can buy it in an agricultural store.)

Since the plumber found no leaks in the waste lines, and since the bad smell started only recently, I wonder if a small animal got in and died there. But the smell from a dead animal should not last this long. It is also possible that mold has developed recently in the soil from excessive soil moisture, causing the bad smell.

I suggest you have an experienced general contractor check the crawl space to make sure no rot has developed on beams, plates or joists, which would generate an unpleasant smell.

There should be no need to have the crawl space lined at great expense. I have concerns about this step, because any moisture permeating from the outside through the foundation walls becomes trapped between the walls and the liner, with no way to escape. This may lead to the development of unhealthy mold.

Today's recommendations are to make sure the bare floor of a crawl space is thoroughly covered and that all vents are closed. Open vents let in warm, moist air in the summer, which creates an undesirable moisture condition, and in the winter, it makes the crawl space cold.

If your furnace is relatively new and gets its makeup air from the outside, there is no problem with shutting the vents. But if it needs makeup air from the crawl space, you should ask your HVAC contractor -- who should inspect the furnace yearly for safety anyway -- if you need to keep one vent open to provide it.

Q. You give excellent advice. My question is whether you would recommend refinishing or replacing a fiberglass tub/shower unit that has been in our bathroom since our house was built in the 1970s. The unit is in good condition; however, it is yellow. I'm tired of the color and would like to refinish or replace it with white.

What are the advantages and disadvantages to each option? Which would be most cost-effective?

A. I suggest you get a price from tub refinishers (Bath Fitter, etc.) and reglazers (find them in your Yellow Pages under "Bathtubs & Sinks -- Repair & Refinish" or a similar heading) and one or two quotes from plumbing or general contractors for replacing your 40-year-old unit. It may be less costly to replace it than refinish it, depending on how difficult it is to do so. Whichever way you decide to go is really a question of price.

Q. I have a newly built house. During the first three months, we experienced a dip in our morning room floor, and the 2-by-10-inch beams below showed some cupping. The builder went ahead and sistered in new beams on a couple of the beams.

The floor still was dipping, and the builder brought in an engineer. The engineer noticed the floor dipped about three-eighths of an inch and wrote up a fix. They went ahead and cut six of the beams and sister beams right through and sistered in new 2-by-10 beams. They ended up sistering three in a row, skipping one and then sistering one more this way. The beams that they sistered on are about 8 feet in length, so they do not rest on the metal beam or on the outside wall. The morning room is 10-by-14 feet, with the beams running on the 10 foot from a metal beam in the basement to outside wall, which is wood (walkout basement).

I have the engineer coming back out since we developed another low spot in our dinette area. I am debating whether to have them pull the whole floor out of both rooms, rebeam the whole thing and install a new floor.

My question: How many beams can you sister like this in a row without causing a dimensional stability issue? Also, in our first-floor bathroom, the plans had a beam going right through where the toilet went. The builder decided to make the beams under the bathroom set 20.5 inches apart from each other. Is this code?

A. I know of no limit to the number of joists that can be sistered (the placement of a similar framing member against an existing one to reinforce it). It can be all of them or only some, depending on what needs to be accomplished.

Since an engineer is involved, I would follow his or her specifications for remediation of your floor. The engineer and builder will probably determine which is most cost-effective: repair or replace.

Standard spacing for joists and rafters sometimes needs to be adjusted, such as when plumbing is in the way. There are tables that specify allowable spans and spacing between different widths and species of wood.

Q. A contractor told me he can't install handicap bars on our fiberglass shower unit because there is not a solid wood surface behind it. Is there a solution?

A. Yes, there is. Moen's Home Care has developed the SecureMount anchor for just such a situation.

I have installed a handicap bar on a fiberglass tub unit using this type of anchor. It was easy to install, but care must be taken in drilling the holes to avoid damaging the fiberglass beyond the escutcheons.

You should be able to get SecureMount at a plumbing supply house.

Q. I have been to your website and read a lot of the Q-and-A, but nothing seems to cover my problem.

I have a circa 1929 house. The winter before last, I had to have my roof cleaned off. I lost all the soffit, fascia and gutters on the south side of the house from tons of ice, so I have had the house resided and roofed. And my insurance company didn't cancel me!

I am about to put more insulation in the attic. I bought several rolls of unfaced 23-by-6-inch batting. Right now there is a poured-in, very soft, white (like cotton) insulation about 4 inches thick. My plan was to put the other insulation over it, but a contractor told me I should make sure I block the top of the studs, so the warm air won't cause ice dams. There is no insulation on the outside walls, only the wrap that was put under the new siding.

If I block the tops of the studs, won't that cause moisture to form in the outside walls?

The other thing he suggested was to scoop up the poured-in insulation and drop it down the walls. That way I would have insulation in there. Then put the EcoTouch in place of it.

A. From your description, your house is balloon-framed. In this type of construction, replaced decades ago by what is called Western platform framing, the outside walls are framed with full-length studs from the foundation to the roof. The attic floor joists are set on a ribbon (usually a 1-by-6) that is let in (notched into) the studs, which are capped by a plate supporting the rafters. This means that the uninsulated outside walls are accessible from the attic. This allowed warm air from the conditioned spaces to flow unimpeded to the attic and cause the havoc you suffered.

I suspect that the destruction of your eaves was caused by condensation from the warm air convecting to the eaves, which rotted their components.

This type of construction presents a great fire risk. If a fire starts in the basement, it will engulf the house in seconds, as the open stud cavities are flues from the basement to the roof. It is very important that these cavities be filled with insulation and sealed top and bottom.

Your contractor is right to some extent, but it is more important to insulate the walls, both for energy efficiency and for fire protection.

I would not recommend dropping what is probably rockwool insulation from the attic floor into the walls. It cannot be successfully done because the insulation will be caught by all kinds of rough spots and other obstacles, and it will not be able to fill the wall cavities tightly and fully.

Instead, have an experienced contractor check to see if the bases of the stud cavities are also opened, as is usually the case in balloon-frame construction. If so, they should be sealed as a fire barrier and to keep the blown-in insulation from falling into the basement. Then have dense-pack cellulose blown in the exterior walls, which should be easily done from the attic.

Once the cellulose is in, the top of the stud cavities should also be sealed. Follow this by laying the fiberglass insulation over the existing insulation.

Too bad you didn't have 1-inch-thick rigid insulation installed under the new siding; it is an opportunity lost to make your house much more energy efficient. The new wrap the siding contractor installed is not worth much as an insulator, although it helps as an air barrier.

To retard the permeation of interior moisture through the finish of the exterior walls, consider painting them with a coat of B-I-N to provide a vapor retarder, followed by your choice of finish paint, or select a low-perm paint as a finish paint.

An endorsement: A dear friend who read my suggestion for catching fruit flies (place ripe banana slices in a jar covered with plastic wrap in which a few holes are punched) called me to say that she did just that. The flies can get in the jar, but they can't get out. Within 48 hours, she said, her jar had caught lots of fruit flies, and she was just about free of them. She asked me what to do with them.

I suggested she take them outside some distance from her house and release them to fend for themselves. There are other ways to dispose of them, which I will leave to your imagination in order not to be deluged with angry comments from bug lovers.

She also said a neighbor told her that putting vinegar in the jar works just as well. I'd love to hear from someone who did this successfully.

• Henri de Marne was a remodeling contractor in Washington, D.C., for many years, and is now a consultant. Write to him in care of the Daily Herald, P.O. Box 280, Arlington Heights, IL 60006, or via email at henridemarne@gmavt.net.

2012, United Feature Syndicate Inc.

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