Attic Furnace

As home space became more and more at a premium and fewer homes were built with a basement, architects and home builders turned to putting the furnace and/or HVAC air handling unit in the attic. While this is a seemingly good answer to solving the problem of where to put the unit, it has its pros and cons. Current design is actually moving back away from this option, as the tides of thought are focusing more on the cons of such an installation, rather than the pros.

Before making an installation on a new home, it’s a good idea to weigh those pros and cons, looking for what will work best for the home. While the trend is moving back away from attic furnace installations, that doesn’t mean that there aren’t cases when it still makes sense.

Attic furnaces and HVAC units are different than those intended for use in the basement. They are designed to be mounted horizontally, rather than vertically. This is especially important for gas-fired furnaces, as the gas burner must be mounted correctly. A basement unit installed horizontally in the attic may not work at all or may be more susceptible to causing a fire.

Of course, if a furnace is already installed in the home, it makes more sense to replace that existing furnace with something similar. Making a switch from a basement furnace installation to an attic one or from an attic to basement is considerably more expensive, due to the cost of installing all new ductwork throughout the home.

Benefits to an Attic Installation

The most obvious reason to install the furnace in the attic is to save space. Most people don’t effectively utilize the space in their attics anyway, so putting the furnace up there, along with the necessary ductwork, doesn’t take up otherwise usable space. But it’s also usually cheaper to install a furnace in the attic, especially on new construction, because a shorter chimney can be used. Installing ductwork in the attic can also be easier than installing it in the basement.

There really isn’t a home design where an attic mounted furnace won’t work, unless there is no attic. The downflow air design of an attic furnace works in all homes. There are many installation options in the attic, where it is possible to install both gas and electric furnaces.

Disadvantages to an Attic Installation

All is not perfect in the attic installation world though. The biggest drawback to installing an attic furnace is that they tend to provide lower efficiency. This isn’t the fault of the furnace design, but rather simple physics. Some of this can be overcome by using a high-efficiency furnace.

Everyone learns that heat rises and cold drops. This is part of how a home furnace in the basement works. Warm air from the furnace is pumped through ductwork to the upstairs, under pressure, where it warms the home. As it does, the air loses its heat and gradually makes its way back down to the basement, where it can be warmed again.

With an attic mounted furnace, the air that is being brought into the furnace intake comes from the upper story of the home or the ceiling of a one-story home. Either way, it’s the warmest air in the house, being reheated and then circulated back into the living areas. It is not uncommon for the floor level of a home to be a few degrees than the ceiling level.

This same problem which makes an attic mounted furnace less-efficient will make an attic mounted air conditioner more efficient, as the air which will be brought into the HVAC unit will be the warmest air in the living quarters of the home.

Attic installations can cause some maintenance problems as well. Being away from the family’s activities, any problems occurring with the unit can go undetected longer, until they are causing a problem. While these are usually minor problems, they are a nuisance nonetheless.

Is an Attic Installation Safe?

Attic installations of both gas and electric furnaces are perfectly safe, when professionally installed. This is not the sort of project a homeowner should take on themselves. If the unit is a combination heating and air conditioning system, the homeowner cannot legally remove the Freon from the old unit and install it in the new. Special equipment is required for this, as well as training and certification.

It is necessary to maintain a minimal amount of space around the furnace, in which flammable materials, other than the home’s framing, cannot be introduced. This is actually no different than for a furnace installed in a basement, but it is easier to make a mistake in the attic, especially with insulation.

Before a furnace is installed in an attic, the attic should be insulated. It is normal for insulation to go under the HVAC air handling unit, but almost impossible to put it there once the unit is in place. The same goes for the ductwork, to a lesser extent, as the ducts are generally of a smaller dimension than the HVAC unit itself. Insulating beneath these will help hold the heating and cooling in the living portion of the home.

But insulation shouldn’t stop there, since the bulk of the attic space is not usually insulated. The ducts themselves need to be insulated, so that the air passing through them is not heated or cooled by the ambient air in the attic. Insulated duct material, made of either a material similar to bubble pack or a fibrous fiberglass matt are the most common materials used. Even then, it is a good idea to add another layer of thicker insulating blanket around the ducts, as the insulating value of the ducts themselves isn’t all that high.

When installing blown-in insulation in an attic, the HVAC unit itself should be avoided, especially access panels and air intake grilles. While it is fine to pile blown-in insulation around the ductwork, the air handling unit needs to be given free access for maintenance and to prevent the risk of fire.

attic furnance
Attic furnance, Mark Doliner

Installing a Gas Furnace in the Attic

Gas furnaces in the attic require that a gas line be brought up to the attic from the ground floor of the home and that a furnace flue or chimney be installed. The gas line will normally be installed by the plumber, as part of the home’s construction. It should be pressurized and checked for leaks, using soapy water, before being connected to the gas and the gas turned on.

A double-wall chimney pipe is required to meet building code. This provides an air space between the active part of the flue, where the hot exhaust gases are passing, and the outer skin of the chimney. The space in-between the two walls is left open at both ends, allowing air to pass through by natural convection. This helps prevent the danger of any heat buildup. If the chimney pipe is enclosed in any way, a minimum of two inches of space is required between the outer wall of the pipe and the enclosure.

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