Peculiar Air

Sucking machines

Your furnace sucks

Most modern furnaces use oil or gas.

Gas leaves no combustion residue.

Oil, in contrast, leaves a lot.

I visited an acquaintance, and descending into the basement I smelled oil fumes. I asked where the oil furnace was. I was told it was removed some 20 years prior!

When conventional furnaces start a heating cycle, the flame ignites after a certain amount of fuel is available. It’s common to hear a Woof when the flame starts. When this happens a bubble of combustion gases exceeds the dimensions of the furnace. If the fuel is gas, the combustion fumes bubble simply gets sucked back into the furnace and vented up the stack. If the fuel is oil, however, the combustion fumes bubble permeates into nearby porous material, such as concrete blocks or structural lumber, leaving a smelly residue, and what is left is then sucked into the furnace and vented up the stack.

Once vented up the stack, it’s all said and done. Right? Well, not _ so _ fast.

Combustion fumes are vented up the stack because the combustion effect produces gases and other products that are hot and lighter than ambient air. So they rise up the stack. As they rise and exit the stack, they cool down, significantly.

While living in town, when our gas furnace would fire, we would smell oil fumes from our neighbor’s oil furnace, some 50 feet distant. That’s because the venting of the exhaust gases cause the house to experience a vacuum and sucks air in at all door, window, and other boundary imperfections. While this brings in a lot of fresh air, it is also cold and usually very dry, so you sit by a window and feel a draft, and need a humidifier for proper comfort. Although the exhaust stacks may be 20+ feet high, and above the roofline, once the gases exit they cool and meander whichever way the wind blows. So 50 feet is too close for town homes to be located. Those living in apartments may have the heating system fully separated from the living spaces, so their experience may be fully divorced from the above.

An acquaintance with an oil furnace in the basement and a fireplace in the living room always smelled oil fumes whenever the furnace was on. That’s because the furnace sucks, and creating a vacuum, if the fireplace exhaust stack is right near the furnace exhaust stack, one will blow out while the other sucks in. Try to light a fireplace while the furnace is operating and you may have difficulty getting an updraft started, possibly bringing smoke indoors directly.

But your fireplace sucks too

Fireplace design makes no bones about it. Having an open access for depositing new wood, that same open access is what provides oxygen for its combustion. While it’s cozy to sit by a fireplace with a fire, you will also be windswept as the air rushes past you to feed the fire. Doing some research, I found the minimum stack velocity for a fireplace to be somewhere between 1 and 10 feet per second. There is no standard minimum because, depending on whether you burn hardwood or cardboard, you’ll have a slow fire with slow exhaust velocity or a raging fireball with very high exhaust velocity. And depending on what type of fuel you burn, the exhaust gases may coat the exhaust stack with residues that can ignite at higher temperature, such as those produced by cardboard, and cause a stack explosion. It happens.

While living in the country, I light a log or two in the fireplace, and my wife says she smells burning wood (or garbage) at the other end of the house (making her eyes water). In the country, homes are usually more separate, like perhaps 300 to 500 feet, but country dwellers can also burn garbage. Garbage (or leaves) is (are) usually burned at ground level, and the virtual exhaust stack varies in height depending on whether it is a smoldering or raging fire. As the fumes cool down they waft about and meander whichever way the wind blows. Start a fire in the fireplace, and suck in the smell of burning wood from other homeowners’ fireplaces. While there is certain ambiance with the aromas of an open fire, they can be a nuisance when they occur throughout the home. So 300 to 500 feet is too close for country homes to be located. An added caution is that before lighting a fireplace, you should go outside and walk around the home, to see (and smell) if any neighbor is burning garbage.

High efficiency furnaces have solved all these problems.

Almost.

A high efficiency furnace has a fan to develop an exhaust air stream before the fuel ignites, so the exhaust fumes bubble is not created at the moment of ignition. So while the previous paragraphs dealt with what is known as Open Combustion, high efficiency furnaces are considered Sealed Combustion, in that they have a dedicated path (a pipe) for combustion air, so they do not use the home’s air space for their oxygen needs. Since combustion is more complete, and the exhaust air cooler, supply and exhaust piping may be plastic. But where they are located can cause peculiar effects. If the furnace is located at one end of the home, that end of the home will experience a low pressure as the machine sucks air from its return ducting, which may consist of a main air filter and an air duct perhaps 2 feet by 2 feet or bigger. This suction will be exerted on all nearby exterior boundary walls, and suck in air from outside. A kitchen exhaust vent placed near this area will display an inbound stream of cold air, unless provided with a flow check valve. Other areas of the home will experience higher pressure and blow conditioned air (heated or cooled) towards and through any exterior wall imperfection. So for “sealed combustion” to function as per design, the unit (or its central return duct opening) must be located central to the home. How many homes follow that design? Zero. Or at least, none of those I’ve visited.

While the above dealt with staying warm in winter, how that warmth is propagated about is another matter altogether.

Some heating systems use water, which is heated, and then circulated about to various radiators.

Some heating systems use water, which is heated above boiling, and the steam circulated about to various radiators.

The radiators warm the air, causing it to rise and circulate about the room they are in. This flow aggregates dust, which gets deposited where 1) the air stream slows down, or 2) where there is some obstruction to flow. If you have a bed suspended above the floor, you should look under there sometimes (where the air slows down). Acquiring a home with radiant baseboards I used pressurized can air to blow air into the radiators opposite from the normal flow, with a vacuum cleaner sucking at the bottom, where the air would normally enter, extracting loads of collected dust. An acquaintance alerted me about having an adult (30) daughter with airway difficulty whenever the heat was on. Visiting, I noted about 1/2 inch of accumulated dust under the radiators. When they were in operation, this dust would partially become liberated, heated, and then inhaled. It can be embarrassing to inquire about a problem, and be told you’re not keeping a clean house.

Forced air systems can combine heating and cooling in a single unit. The internal fan will impact and partially collect any particulates in the air it pushes around. For anyone owning a ceiling fan, or any other more accessible air mover, you know the fan blades get dirty / filthy. I have visited many acquaintances whose central air system had a poor air filter, allowing a gradual buildup on the fan blades, and eventual carryover, when the blade dust loading becomes excessive. One had a filter that had not been changed for the life of the unit, about 5 years, because they did not know the filter needed to be changed. Another had so many uncontrolled openings besides the filter that the fan was fouled with plastic and paper bags. Really!

And then there is the multimillion dollar home where, when the heating system is offline, a local fan is needed for user comfort.

Chestnuts roasting on an open fire,

Jack Frost nipping at your nose . . .

(Well, not really, inside)

In conclusion . . .

Your car has a fire within it, or rather many small ones, that in sequence make it move, or keep you warm. It sucks air in, and blows exhaust out.

Your gas stove has a fire on it, or in it, to cook something. Maybe not as obvious, but it too sucks air in, and blows exhaust out, which you (may, or Should) vent outdoors, somehow.

Your electric stove has a fire located miles away, somewhere, to generate the needed electricity. The related suction and exhaust happen far, far away.

Your furnace has a fire in it. It sucks air in, and blows exhaust out. Simple, yeah?

Well, maybe so by design, but not usually so in practice.

I had lots of nifty graphics I wanted to insert, but was chased by a lazy streak. Sorry.

Brought to you by the letter D, Quicker Oats (from farm to table, with no in-betweens), and the Atlantic Ocean.

You’re welcome, & Happy New Year

© Sal La Duca, www.emfrelief.com

PS. Since I don’t get out much, probably most homes’ systems operate perfectly. I know none of you have these issues, so you might want to share with an acquaintance who’s ignorant. I suspect we all have a few.