HVAC Inspection Guide - InterNACHI House of Horrors

Gas appliances are categorized so that the proper venting system can be selected. There are four categories based upon the pressure produced in the vent and whether or not the flue gas temperatures create condensation.

You'll commonly find Category I and Category IV furnaces and boilers in U.S. houses. Category I does not produce condensate. Category IV operates with a positive vent pressure and condenses flue gases, and their flue vent pipes are made of PVC plastic.

Type B vents are designed for venting non-condensing gas appliances equipped with a draft hood or fan-assisted appliances with a non-positive vent pressure.

Heating and cooling equipment and appliances must be sized in accordance with the Manual S and Manual J of the Air Conditioning Contractors of America.

Combustion air is necessary for the complete combustion of fuel gas, dilution of flue gases, and ventilation of gas-fired appliances and the space in which they are installed. A lack of combustion air can cause the incomplete combustion of fuel, malfunction of an appliance, and spillage of flue gases. Gas-fired appliances often compete with other equipment and systems for available combustion air and makeup air through a house. Supplying makeup air can help natural-draft appliances by compensating the effect of the powered exhaust appliances and equipment in the house, including fireplaces, exhaust fans, ventilation systems, clothes dryers and similar appliances and equipment.

There are five methods of supplying combustion air listed in the 2015 IRC G2407, ranging from simple to complex:

all indoor air,
all outdoor air,
combination indoor and outdoor air,
mechanical combustion air supply, and
engineered design.

These methods of supplying combustion air applies mostly to natural-draft atmospheric-burner-design appliances and Category 1 fan-assisted appliances. Category 2, 3, and 4 appliances must be provided with combustion air as required by the manufacturer's instructions.

METHODS OF COMBUSTION AIR

Indoor Combustion Air

The required volume of indoor air to be used as combustion air is determined in five different ways according to 2017 IRC G2407.5, except when the infiltration rate is known to be less than 0.40 air exchanges per hour (ACH). The terms "confined space", "unconfined space", and "unusually tight construction" are no longer used.

Require volume of indoor air is determined by two methods, unless the known infiltration rate is less than 0.40 ACH:

one is based upon the actual air infiltration rate measured in the house, and
the other is based on the familiar 50 cubic feet per 1,000 Btu/h.

Openings to Connect Indoor Spaces

Where openings are used to connect adjacent indoor spaces on the same floor for the purpose of increasing the available volume of air, the familiar high and low openings may be provided. The openings must be permanently open, and each opening must be at least 100 square inches in size. One opening must be within 12 inches of the top and one opening 12 inches of the bottom of the enclosure.

Outdoor Combustion Air Opening

Openings directly connected to the outdoor air or to spaces that are naturally ventilated with outdoor air, such as crawlspaces or attics, are acceptable. The combustion air openings should be located to prevent blockage. The opening could be one-permanent opening or two-permanent high-low openings. There are various types of opening combinations that may include indoor and outdoor spaces to supply combustion air.

Not Required

Home inspectors are not required to calculate the air for combustion, ventilation, and dilution of flue gases for appliances.

Makeup air is needed where clothes dryers, kitchen exhaust fans, chimneys, and other exhaust and ventilation systems may interfere with the appliance operation. Not enough makeup air may cause negative pressures to develop and may interfere with appliance venting, creating a loss of draft at appliance vents and chimneys or causing combustion byproducts to backdraft into the house. Gas-fired appliances are often in competition with other systems for the available combustion air and makeup air that flows through the house building envelope. And natural draft appliances could starve for air. Supplying makeup air will help prevent negative pressures with a space from developing.

Note the following systems that may deplete combustion air: bathroom exhaust fan at 80 CFM, range hood at 100 CFM, clothes dryer at 200 CFM, and a fireplace chimney at 600 CFM.

Corrugated Stainless Steel Tubing (CSST) is observed in the garage and under the deck.
There is a flexible connector at the HVAC equipment in the furnace room.

CSST should not be confused with natural gas appliance flexible connectors. Flexible connectors are used to attach moveable appliances to the gas piping system. CSST is typically routed beneath, through or alongside floor joists in the basement, inside interior wall cavities and on top of ceiling joists in attic spaces.

CSST piping systems must be installed in accordance with the terms of their approval, the conditions of listing, the manufacturer’s instructions, and the IRC.

CSST is a flexible, stainless steel pipe used to supply natural gas and propane in residential and commercial buildings. It is often coated with a yellow or black plastic coating. CSST should not be confused with flexible gas appliance connectors that join an appliance to the gas supply line of a house.

CSST is safe when properly installed.

For more information about CSST, please refer to the Plumbing Inspection Guide.

Observation

The gas vent pipe from the water heater tank is improperly sloped. There is a disconnection at the elbow. This vent pipe is connected to the shared vent stack with the furnace.

Cloth duct tape is not permitted to be installed on fuel-burning appliance vent pipes. Many municipalities do not allow metal foil tape either.

Single wall vent pipe must be connected with 3 sheet metal screws at each joint.

Single wall flue vents require a six-inch minimum of clearance to combustibles, and B-vent (double wall) requires minimum 1-inch clearance.

The joints, seams and connections in metallic and non-metalllic ducts must be securerly fastened and sealed with welds, gaskets, mastics, liquid sealants, or tapes. Tapes and mastics should be listed and marked with "UL 181 A-P" for tape, "181 A-M" for mastic or "181 A-H" for heat-sensitive tape.

"181 B-FX" is for flexible ducts. Crimp joints for round metallic jucts should have a 1-inch lap and at least 3 sheet-metal screws or rivets at the joint.

Ducts require adequate support at regular intervals.

Nearly all building codes restrict the use of cavity spaces as supply ducts. However, it has been common practice to use cavity spaces as return-air pathways. Building cavities used as return-air plenums is one of the leading causes of duct leakage in homes today.

Still commonly used is the panned floor joist. Using floor joists as return ducts by panning can cause leakage because negative pressure in the cavity will draw air from the outside into the cavity through the construction joints of the rim area at the end of the joist cavity.

Cavities (or interstitial spaces) within walls are also sometimes used as supply- or return-air pathways. These cavities often create a connection of inside air with outside air from an attic or crawlspace. It is very difficult to make such cavity spaces airtight. When cavity spaces are used as return-air pathways or supply-air ducts, a few issues will arise.

Building Cavities Used as Supply or Return Ducts