Backdrafting is the reverse flow of gas in the flues of fuel-fired appliances that results in the intrusion of combustion byproducts into the living space. Many fuel-fired water heaters and boilers use household air and lack an induced draft, which makes them especially vulnerable to backdrafting when indoor air pressure becomes unusually low. Inspectors should try to spot evidence of backdrafting in homes.
How does backdrafting happen?
Fuel-fired water heaters, boilers, wall heaters, and furnaces are designed to exhaust the byproducts of combustion to the outdoors through a flue. These hot gases rise through the flue and exit the home because they are not as dense as indoor air. The pressure differential that allows for the release of combustion gases can be overcome by unusually low indoor air pressure caused by a high rate of expulsion of air into the outdoors through exhaust fans, fireplaces and dryers. When this happens, combustion gases can be sucked back into the house and may potentially harm or kill building occupants. Improperly configured flues or flue blockages can also cause backdrafting.
 
How can InterNACHI inspectors test for backdrafting?

• An inspector can release smoke or powder into the draft diverter to see whether it gets sucked into the duct or if it spills back into the room. A smoke pencil or a chemical puffer can be used to safely simulate smoke.   
  
  
• An inspector can hold a lighter beside the draft diverter to see whether there is sufficient draft to pull the flame in the direction of the flue.
  
  
• Combustion gases that back-draft into a house may leave a dark residue on the top of the water heater. The presence of soot is an indication of backdrafting, although its absence does not guarantee that backdrafting has not happened.
  
  
• A carbon monoxide analyzer can be used to test for backdrafting of that gas. Inspectors should be properly trained to use these before they attempt to use one during an actual inspection, primarily to avoid false negatives.
  
  While performing the above-noted tests, it is helpful if inspectors ask their clients to turn on all devices that vent air into the outdoors in order to simulate worst-case conditions. Such devices may be dryers, or bathroom and kitchen fans.

Types of fuel-fired water heaters:

• Atmospheric DraftMost backdrafting is the result of the characteristics of this type of water heater. Combustion gases rise through the ventilation duct solely by the force of convection, which might not be strong enough to counter the pull from dips in indoor air pressure.

• Induced Draft
  This system incorporates a fan that creates a controlled draft. The potential for backdrafting is reduced because the induced draft is usually strong enough to overcome any competing pull from an indoor air-pressure drop.  

• Sealed Combustion
  The combustion and venting systems are completely sealed off from household air. Combustion air is drawn in from the outdoors through a pipe that is designed for that purpose. The potential for backdrafting is nearly eliminated because the rate of ventilation is not influenced by indoor air pressure, and the vented gas has no pathway into the home.

• Water Heater Location
   The installation of fuel-fired water heaters in particular household locations can increase the chances of personal harm caused by backdrafting. The 2006 edition of the International Residential Code (IRC) states the following concerning improper location: 

Fuel-fired water heaters shall not be installed in a room used as a storage closet. Water heaters located in a bedroom or bathroom shall be installed in a sealed enclosure so that combustion air will not be taken from the living space.
In summary, inspectors should try to spot evidence of backdrafting. 

​by Nick Gromicko, CMI® and Kenton Shepard

According to the InterNACHI Residential Standards of Practice, a home inspection is a non-invasive, visual examination of a residential dwelling that is designed to identify observed material defects within specific components of that dwelling. Part of the home inspection includes the inspection, identification and description of the heating system, which includes heat pumps.
The inspector is required to inspect the heating systems using normal operating controls, and describe the energy source and heating method. The inspector’s report shall describe and identify, in written format, the inspected heating system, and shall identify material defects observed.
In order to perform an inspection according to the Standards of Practice, an inspector must apply the knowledge of what he/she understands about the different types of residential heating systems. To inspect and identify a particular heating system, describe its heating method, and identify any material defects observed, an inspector should be able to explain and discuss with their client: 

• the heating system; 
• its heating method; 
• its type or identification; 
• how the heating system operates; 
• how to maintain it; and 
• the common problems that may be found.  

The inspector must be able to thoroughly examine a heating system, understand how a particular heating system operates, and analyze and draw conclusions as to its apparent condition. An inspector should also be able to justify his/her observations, opinions and recommendations that were written in the inspection report. 
Here, we cover some fundamentals of a particular heating system called a heat pump using non-invasive, visual-only inspection techniques. We also discuss its defrost cycle. It is up to the inspector’s judgment as to how detailed the inspection and report will be. For example, the inspector is not required to determine the capacity or BTU of the inspected heating system, but many inspectors record that detailed information in their reports. 

How it Operates
When a heat pump is operating in the heating mode or heat cycle, the outdoor air is relatively cool and the outdoor coil acts as an evaporator. Under certain conditions of temperature and relative humidity, frost might form on the surface of the outdoor coil. The layer of frost will interfere with the operation of the heat pump by making the pump work harder and, therefore, inefficiently. The frost must be removed. A heat pump has a cycle called a defrost cycle, which removes the frost from the outdoor coil.
A heat pump unit will defrost regularly when frost conditions occur. The defrost cycle should be long enough to melt the ice, and short enough to be energy-efficient.
In the defrost cycle, the heat pump is automatically operated in reverse, for a moment, in the cooling cycle. This action temporarily warms up the outdoor coil and melts the frost from the coil. In this defrost cycle, the outdoor fan is prevented from turning on when the heat pump switches over, and the temperature rise of the outdoor coil is accelerated and increased.
The heat pump will operate in the defrost cycle until the outdoor coil temperature reaches around 57° F. The time it takes to melt and remove accumulated frost from an outdoor coil will vary, depending on the amount of frost and the internal timing device of the system.
 
Interior Heating Element 
During this defrost cycle with older heat pumps, the indoor unit might be operating with the fan blowing cool air. To prevent cool air from being produced and distributed inside the house, an electric heating element can be installed and engaged at the same time as the defrost cycle. In defrost mode, this heating element will automatically turn on, or the interior blower fan will turn off. The heating component is wired up to the second stage of a two-stage thermostat.
 
The Typical Cycle 
The components that make up the defrost cycle system includes a thermostat, timer and a relay. There is a special thermostat or sensor of the defrost cycle system, often referred to as the frost thermostat. It is located on the bottom of the outdoor coil where it can detect the temperature of the coil.
 
When the outdoor coil temperature drops to around 32° F, the thermostat closes the circuit and makes the system respond. This causes an internal timer to start. Many heat pumps have a generic timer that energizes the defrost relays at certain intervals of time. Some generic timers will energize the defrost cycle every 30, 60 and 90 minutes.
 
The defrost relays turn on the compressor, switch the reversing valve of the heat pump, turn on the interior electric heating element, and stop the fan at the outdoor coil from spinning. The unit is now in the defrost cycle.
 
The unit remains in the defrost cycle (or cooling cycle) until the thermostat on the bottom of the outdoor coil senses that the outdoor coil temperature has reached about 57° F. At that temperature, the outdoor coil should be free of frost. The frost thermostat opens the circuit, stops the timer, then the defrost cycle stops, the internal heater turns off, the valve reverses, and the unit returns to the heating cycle. A typical defrost cycle might run from 30 seconds to a few minutes. The defrost cycles should repeat regularly at timed intervals. An inspector should not observe a rapid cycling of the defrost operation.
 
In summary, certain conditions can force a heat pump into a defrost cycle (or cooling cycle) where the fan in the outdoor coil is stopped, the indoor fan is stopped or electric heat is turned on, the frost melts and is removed from the outdoor coils. When the frost thermostat is satisfied or a certain pre-set time period elapses, the outdoor fan comes back on, and the heat pump goes back into the heating cycle.
 
One problem of many older heat pump systems is that the unit will operate in the defrost cycle regardless of whether ice is present. On these systems, if it’s cold outside, the defrost cycle might turn on when it is not needed.
 
If the defrost cycle is not functioning properly, the outdoor coil will appear like a big block of ice, making the unit non-functional. Damage could result if the heat pump operates without a functional, normal-operating defrost cycle.
 
Causes of Frost
There are many reasons why an inspector might find frost and ice stuck on an outdoor coil of a heat pump that is not properly defrosting.  

The cause of the frost and ice problem may include: 

• a bad reversing valve;
• a damaged outdoor coil;
• a wiring problem;
• a bad thermostat;
• a leak in the refrigerant;
• a dirty outdoor coil covered with grass, dirt, debris and/or pet hair; 
• a fan that won’t turn on; 
• a fan installed backwards with the blades running in the wrong direction; 
• a motor operating in the incorrect direction; and/or 
• a replacement fan motor spinning at a very low rpm.  

Diagnosing apparent problems with the defrost cycle of a heat pump is beyond the scope of a home inspection.


The Best Techniques
There are three cycles of a heat pump:  heating, cooling and defrost cycles. We learned about the defrost cycle of a heat pump. According to the InterNACHI Standards of Practice, the inspector is required to inspect the heating systems using normal operating controls and describe the heating method. The inspector’s report shall describe and identify, in written format, the inspected heating system, and shall identify material defects observed.
 
To learn how to properly inspect a heat pump system using the best non-invasive, visual-only inspection techniques as required by the Standards of Practice, an inspector should be professionally trained.   

​by Nick Gromicko, CMI® and Ben Gromicko