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Two of my recent blog posts have been about the need for combustion air ducts and common problems with combustion air ducts, but I never covered the topic of connecting a combustion air duct directly to the plenum on a furnace.  This actually used to be standard practice for a long time; the crude diagram below shows how this worked.

When the combustion air duct is connected directly to the return plenum for a furnace, the blower fan on the furnace pulls outdoor air in to the return plenum.  To supply this air to the furnace room, a supply register is added at the ductwork in the same room, typically right above the furnace.  This is an undesirable setup, for a number of reasons.

Waste of Energy

During the winter, this setup pulls cold outdoor air in to the house, directly in to the return plenum for the furnace.  While a combustion air duct that terminates at the floor will allow cold air to come in to the house, it's not nearly as much air compared to the duct connected to the furnace return plenum.  When it's very cold outside outside, that's a ridiculous amount of cold air being pumped in to the house, but because it gets tempered by the furnace, nobody feels it.  This is a big waste of energy.

During the summer, the opposite happens.  This setup pulls warm, moist outdoor air in to the house just before the air conditioner's evaporator coil, as shown below.  Again, this is a waste of energy.

Pressurizes the House

When a combustion air duct is connected directly to the return plenum on a furnace, the house becomes pressurized when the furnace blower fan starts running.  How does this air leave the house?  Any way it can; plenty of these household air leaks were covered in this post about why houses need combustion air ducts.

The air leaks that really cause problems are the air leaks found in the attic.  Attic air leaks lead to frost in the attic as well as ice dams, and the effect is magnified when a combustion air duct is connected to a furnace return plenum.

Code Violation

As a home inspector I don't get hung up on code requirements, but as long as I'm blogging about this topic, it's worth mentioning that the Minnesota State Mechanical Code no longer allows the combustion air duct to be connected to the return plenum.  Section 708.1, #3 says:

[Combustion air ducts shall:] Terminate in an unobstructed space allowing free movement of combustion air to the appliances.

Frequently Disabled

While it's common for home inspectors to find combustion air ducts stuffed shut with clothing inside the house, it's actually even more common for the combustion air duct to be disabled when it's connected to the return plenum.

Remember, that register above the furnace is what supplies combustion air to the furnace room.  Because this register is located right above the furnace, it's also the most powerful register in the house, which makes the furnace room very warm during the winter.  When well-intentioned homeowners don't understand how their combustion air is connected, they assume that the register above the furnace is only there to supply heat to the room, so they block it off to get more heat to the areas that matter.  This prevents combustion air from getting to the appliances that need it.

Note the cardboard blocking this opening in the photo below, despite the stern warning from the HVAC contractor.

What To Do

If the connection from the combustion air duct to the return plenum is visible inside the house, the fix for this situation is simple: have the combustion air duct disconnected from the furnace return plenum, and have the duct dropped down in to the room.  Also, have the old hole in the return plenum blocked off.

Any HVAC contractor should be able to help make this simple change, which is something we've recommended to countless home inspection clients.

Author: Reuben Saltzman, Structure Tech Home Inspections

The newest version of the Minnesota State Plumbing Code, published electronically on November 13, 2012, now requires a safety device to be installed on bath tub faucets to help reduce the potential for serious scalding injury.  Here's the exact text, which comes from section 4715.1240 of the Minnesota State Plumbing Code:

Subp. 4. Hot water temperature control device for tubs. Bathtubs and whirlpool tubs must be provided with water temperature limiting devices set at a maximum water temperature of 120 degrees Fahrenheit to reduce the risk of scalding, according to ASSE Standard 1070.

This means that the water coming out of a bath tub faucet needs to be tempered so that the hottest water that can come out of the faucet is 120 degrees F. This was already a requirement for single handle faucets, but now it applies to two-handle faucets at bath tubs as well.

So how is this done?  I asked Mark Jerde of RightMark Plumbing how he achieves compliance with this new requirement.  He installs thermostatic mixing valves conforming to ASSE 1070 at the supply lines to bath tub faucets.  

One such mixing valve is the Watts MMV series, pictured at right.  Just don't look for these types of valves at big box home improvement stores; they don't sell them.  Maybe this will change soon. For more info on this new standard, click here: ASSE 1070

Author: Reuben Saltzman, Structure Tech Home Inspections

Last week I wrote about what combustion air ducts are and why houses need them. This week I'm going to follow up with some of the most common problems and solutions related to combustion air duct installation and maintenance.

By far, the most common problem that occurs with combustion air ducts is that they get blocked.  When a combustion air duct is blocked, air needs to 'leak' in to the house through many different undesirable pathways.  I've done a number of home inspections where the windows were completely iced shut throughout the house, and in every case there was a blocked combustion air duct.

Problem: Intentional, ignorant blockage

A combustion air duct brings in fresh outdoor air, which usually means cold outdoor air in Minnesota.  This can create a cold floor where the duct terminates, as well as a cold draft.  I was going to make a nice little drawing of this cold air coming in to the basement around my own combustion air duct, but then I remembered I have an IR camera.  Duh.  Check out the two images below for a nice visual of how the combustion air duct is making my basement floor cold.

To prevent this cold air from dumping in to their home, people sometimes stuff clothes or towels in to the combustion air duct, or the block the intake at the exterior of the home.

Solution: Remove any obstructions.  If you want to help cut down on the amount of cold air that just 'dumps' down in to the basement, try creating a trap at the bottom of the combustion air duct.  Make the air have to rise back up again before coming in to the home.  I don't have any hardcore proof that this makes a big difference, but I've convinced myself that it helps, and it's easy enough to do. The two most common ways of creating a trap are to either make a "J" at the bottom of the duct, or to put a bucket  or box underneath the duct.  With either of these methods, the air will need to rise up before coming in to the home.

Just make sure that the bucket or box you use isn't so small that it restricts air flow.  I've always just eyeballed this, but if you're super anal, you could make your sixth grade math pay off by measuring the inside diameter of the bucket and the outside diameter of the duct, then calculate the surface areas (Πr²) and make sure the bucket's is at least twice that of the duct's.

Also, make sure the duct isn't so long that the opening sits flat on the floor, effectively blocking it.

Problem: Lack of maintenance

The opening at the exterior for the combustion air duct will bring air in to the home, and with that comes dust, dirt, insects, leaves, etc.   I've found that the closer the combustion air duct is located to the ground, the more likely it's going to get blocked with debris.

Solution: take a peek underneath your combustion air duct every year to make sure it stays clean.  If you do this during the summer or fall, watch out for wasps.  They love to make nests in this opening.  If the opening is dirty, vacuum it off.  If you have an HRV, check the HRV intake at the same time.

Problem: Small Mesh at the Exterior

The opening at the exterior of the home needs to be covered with a steel mesh having openings not less than 1/4", and not more than 1/2".   When standard window screen is used here, it will get dirty very quickly.  Click on the photo below for a larger view; you'll see the opening is actually covered with a window screen, which should be removed.

Solution: Remove any restrictive mesh or material, and replace it with 1/4" hardware cloth or something similar if it's not already present.

Problem: Unintentional, ignorant blockage

Every so often, vinyl siding installers will forget which opening was meant for the combustion air intake, and they'll install a damper at this opening instead of a screen.  These dampers allow air out, not in.

Solution: Replace the exterior terminal with a type that is designed for a combustion air intake, or remove the damper and cover the opening with 1/4" hardware cloth.

Problem: Inlet installed too close to the ground

The inlet for the combustion air duct needs to be installed at least 12" above grade.  When it's too close to the ground, it can get dirty very quickly, and can get blocked over with snow.

Solution: When the combustion air inlet is installed this close to the ground, it's usually done because that's where the rim joist was located, so making a higher hole in the side of the house isn't an option.  The solution is to install what Milind calls a 'snorkel'.  I laughed the first time I heard this, but I like this term.  I think the photo below is pretty self-explanatory.

That concludes my list of the most common installation and maintenance defects with combustion air ducts.  If I think of any more, I'll add them to this list in the future.

Author: Reuben Saltzman, Structure Tech Home Inspections

Have you ever noticed a big insulated tube dropping down next to the floor near your furnace or boiler in the basement?

If you trace this duct down, you'll find that it connects to an opening at the exterior of the building.  This is essentially just a hole in the side of the building that brings in fresh outdoor air.  Homeowners, builders, and insulation contractors spend lots of  time trying to seal up every little air leak in to a house, but then the building code requires this big hole that allows cold air to just dump in to the basement.  Silly, right?

I'll try to help make some sense of this.

Houses need air

This opening is a passive intake that provides needed air to the home.  There are several items in a home that remove air - here's a partial list of common items found in Minnesota homes that remove air from the house:

• Furnaces and boilers that are not direct vent / sealed combustion type
• Water heaters that are not direct vent / sealed combustion type (at least 99%)
• Bathroom exhaust fans
• Kitchen exhaust fans
• Clothes dryers
• Wood burning fireplaces

The stack effect in a home, wind, and radon mitigation fans may also remove air.  The most common and obvious problem with too much air being removed from a house is a backdrafting water heater, but there's a lot more to it than just this.

Houses leak

When air is removed from a house, it has to be replaced.  If a house is not built tight, the air will get replaced from every little hole in the envelope in the house; the photos below show a few examples.  These are the things that get corrected to make houses "tighter".   The first photo below shows an outlet box at an exterior wall that hadn't yet been sealed.  Those openings get sealed in new houses today, but this never used to happen.

The photo below shows the furnace vents going through the rim joist.  Daylight is visible around these penetrations, which means air leakage.

The opening around the faucet is obvious.

Of course, windows and doors are also a huge source of air leakage.  Daylight showing through is a dead giveaway.

Unsealed openings in the exterior walls equates to uncontrolled air leakage.  Every time the wind blows, air will leak in or out through these openings.  Even without any air moving at the exterior, the stack effect in a home will cause air to leak in through the lower openings in the envelope of a home, and back out through the upper openings, such as attic bypasses.  The image below, used with permission © 2013 E Source, gives a visual example of the stack effect.

The line of neutral pressure plane will be different in every home.  Some of the factors that affect this are differences in indoor / outdoor temperatures, wind, the height of the home, and how much air is leaking.  For the upper 'positive pressure' leaks, one of the most obvious that can be viewed from inside the house is a loose-fitting attic access panel.

Other attic air leaks, most of which can only be seen from inside the attic, are also major contributors.  These include leaks around furnace vents, electrical cables, plumbing vents, chimneys, etc. When air is allowed to leak through the house uncontrolled like this, the amount of air leakage and energy loss is typically much more than it needs to be, and it doesn't happen where, when, or how it should.  This can lead to condensation and frost at windows, in the attic, and even inside the walls.

The Combustion Air Duct

To help reduce the effects of uncontrolled air leakage, houses get sealed up as tight as possible and a single hole is created to bring outdoor air in to the basement, usually right next to the furnace.  This is the combustion air duct I showed at the beginning of this post.

When a combustion air duct is properly installed, it will help prevent the house from getting depressurized.  The air is allowed to come in to the house as needed through a large opening, and all of those other holes in the walls can be sealed up.  To see how well this works in a new house, try running all of the exhaust fans for about 5 minutes, then put your hand over the end of the combustion air duct; if it's working properly, you'll feel plenty of air pumping in to the house.  Beautiful.

I'll have a follow-up post next week discussing the most common installation and maintenance problems with combustion air ducts, and well as the solutions.  Of course, I'll have photos of everything.

Special thanks to Steve Schirber at Cocoon Insulation for helping to write this post.

Author: Reuben Saltzman, Structure Tech Home Inspections

One of the most common Truth-in-Housing repair items is an unfilled opening in an electrical box or panelboard.  If you walk through the garage or basement of just about any old Minneapolis or Saint Paul home, you'll probably find a few of these.

The national electric code says that unused openings in such equipment need to be closed.  Covering these openings helps to prevent accidental shocks (think of kids), helps to prevent hot sparks from escaping if something goes wrong inside the enclosure, and helps to prevent pests from entering.  It happens.

How to fix a missing knockout plug

First, determine the size of the missing knockout hole and subtract 3/8" to figure out what size knockout plug you'll need.  For example, if you have a 7/8" hole, you'll need a 1/2" knockout plug. Next, go buy some knockout plugs.  They're sold in small quantities at hardware stores and home improvement stores for about a quarter each.

Next, kill the power to any circuits going through the box you're working on.  As GI Joe always thought to himself, "being safe is the other half." Finally, put the plug in the hole.  If you bought the right size, it will be nearly impossible to push the plug in to place with just your hands - you'll need to tap the plug in to place using a hammer.

If you want to make the plug super tight and impossible to remove, you could bend the tabs inside the box with a screwdriver.  I'm sure some electricians would say this should always be done.  The photo below left shows the tabs in their normal position, and the photo below right shows the tabs bent out.

For Truth-in-Housing evaluations, missing knockout plugs are required repair items in Bloomington, Hopkins, Robbinsdale, and South Saint Paul.  They're considered a hazard (H) in Saint Paul, and they're a suggested correction (SC) in Minneapolis.

How to fix a missing twist-out at the front of a panel

First, determine the brand and type of panel.  It will probably say so right on the door.  If it helps, take a photo of the label. Next, go buy some filler plates at a home improvement store or hardware store.  You'll find them next to the circuit breakers.

Finally, shut off the power to the panel and pop the appropriate filler in to place.  This can typically be done without even removing the cover. For Truth-in-Housing evaluations, openings in the front of an electric panel are required repair items in Minneapolis, Bloomington, Hopkins, Robbinsdale, and South Saint Paul.  They're considered a hazard in Saint Paul.

Now go forth and fill your holes with impunity.

Author: Reuben Saltzman, Structure Tech Home Inspections