December, 2007
Inspection News and Views from the American Society of Home Inspectors

Identifying Heating Systems


Commercial Inspection Tips

Expand your business with commercial building information from Carson Dunlop Weldon

Welcome to Commercial Inspection Tips, a monthly feature providing technical and business information on commercial building inspections. We’re contributing to the ASHI Reporter with the hope of stimulating your interest in diversifying into the field of commercial building inspections as a way to expand and grow your business.

When describing the heating systems in a building, there are a number of variables to consider, including the following:

  • The type of fuel used
  • The medium used for heating (air, water, steam)
  • Open vs. closed flame
  • Direct vs. indirect fired
  • Convection vs. radiant
  • Fuel type

The most common fuels used are natural gas, oil and electricity. In areas where available, natural gas is by far
the most popular because it is the least expensive. When determining the type of fuel used in a system, here are a few points to consider.

Gas-fired systems will always have gas piping (typically black steel or copper) leading to the equipment. For other than steel gas piping, the requirement for identification is a yellow label marked ‘Gas’ in black letters. The maximum spacing for these labels is five feet; however, the markings are not required on pipe located in the same room as the equipment served. Gas meters, usually located outside the building, also suggest gas heat.

For oil-fired systems, an oil tank in the building is an obvious indication of this type of fuel. Oil tanks that are buried outside the building are less obvious, but can be identified by the presence of an oil fill and vent line outside or a small, black rectangular fuel gauge inside the main boiler room. See Figure 1 below. There are obviously environmental implications associated with buried oil tanks. Depending on the type of oil used, there may be a preheating section to aid in getting the oil to flow more readily.


Figure 1: Oil tank fuel gauge on interior wall.
This typically signifies the previous or current presence of an underground oil storage tank.   

Electric heating systems are harder to identify, as electric radiant systems can be embedded in floors and ceilings, and electric heating elements or heat strips can be concealed inside air-handling units of various descriptions. Electric baseboard heaters are obviously easy to identify, but don’t mistake hot water convectors for electric baseboard heaters. One way to identify an electric heating system in the building is to review the labeling on the main electrical equipment at the service entrance.

If the building is heated by package rooftop units, the building can be gas or electrically heated. The presence or absence of gas lines above the roof can tell you which it is.

Heat pumps use electricity for the compressor; however, a gas-fired boiler may be providing hot water to the heat pump or, if the heat pump is an air source, electric duct heaters may also provide backup heat.

Heat distribution medium

Differentiating between forced air and hot water or steam is usually straightforward. Forced-air systems use ductwork, and hot water or steam systems use pipes and radiators, convectors or fan coil units.

Some systems use both hot water or steam and forced air: a boiler generates hot water or steam, which is pumped through pipes to a fan coil unit or large air handler. The fan blows air across the hot coil and warm air is delivered to the space. Depending on the application, there may or may not be a duct system.

Hot water vs. steam

It is more difficult to differentiate between hot water and steam than between these systems and forced air. One clue is to look at the piping going to the radiators. In some old steam systems, there is only one pipe.

This pipe brings steam to the radiator and also returns condensate back to the boiler. A single-pipe system is definitely steam, but a two-pipe system could be steam or hot water.

The numerous ways to differentiate between steam and hot water heating systems will have to wait for another discussion. 

Open flame vs. closed flame

Open flame or closed flame describes where the combustion air comes from. These types of heating systems are either natural gas or oil fired. With an open flame system you can see the flame inside the building. The air in the building can come in contact with the flame.

A closed flame system, also referred to as a closed combustion system, does not allow the air in the building to interact with the flame at the heating equipment. See Figure 2.


Open flame systems are by far the most common. These would include some types of interior furnaces and most types of boilers, radiant heaters or unit heaters. Closed flame systems are those that use outside air for combustion and do not draw air from within the building. These would include most types of high-efficiency furnaces, some boilers, radiant heaters or unit heaters and all rooftop package units.

The circumstances where closed flamed systems would be required typically involve buildings that contain explosive dust or vapors. For example, some types of woodworking or wood manufacturing plants would require closed flame heating systems because of the amount of sawdust in the air inside. Spray paint booths are another example of where closed flame heating systems would be expected.   

Direct fired vs. indirect fired

Most heating systems are indirect fired. This means that the products of combustion are vented to the building
exterior and do not enter the building. Any heating system with a chimney is an indirect system.

There are, however, direct-fired systems, where the products of combustion are delivered inside the building. Common examples of direct-fired heating systems include facilities where there are large volumes of air being exhausted due to processes such as commercial kitchens with large exhaust fans. See Figure 3 below.


Because there is so much air being exhausted from these buildings, there is a requirement for air to be brought back in. Direct-fired heating units in this case, act as makeup air units, as well and will bring 100 percent outside air into the building, heating the air with an open flame. There is no heat exchanger or chimney. Since there is such a large volume of air being exhausted, there is little risk of carbon monoxide, carbon dioxide or water-vapor buildup in the building.

There are special requirements for direct-fired heating systems. The heated air brought into the building must be balanced by a mechanical exhaust system, relief openings or natural air leakage. For recirculating systems, outdoor air must also be brought into the building at a rate of not less than 4 cubic feet per minute per 1,000 Btu-per-hour-rated input of the heater. Practically, direct-fired heating systems have to be interlocked so they can only work if an exhaust system is functioning.

Convection vs. radiant heat

Convection or forced-air systems heat the air within the building and blow the air around to heat the space. Radiant heating systems work in a similar fashion to the sun (on a much smaller scale, obviously!). Objects in the building are heated by electromagnetic waves emitted by a hot source. Radiant systems move heat through the air without heating the air. Radiant heating systems can be embedded in the floor or ceiling (electric or hot water radiant heat), or can be suspended from the ceiling (electric or gas-fired infrared radiant heaters).

Forced air systems with ducts have certain advantages over radiant and hot water systems, including the opportunity to provide:

  • cooling
  • air filtration
  • humidification
  • fresh air into the building

The advantages of the radian t heating systems include:

1. Even temperature distribution
Since the radiant heaters are directed downward or embedded in the floor, the floor slab tends to get warm and there is a more even temperature over the height of the building.

2. Lower heating bills
Since the people and objects in the building are warmed directly, the actual air temperature is somewhat lower (65°F vs. 70°F). As the temperature differential between the building interior and exterior is less, there is less heat loss from the building.

There are generally three intensities of infrared radiant heating systems. Most systems are typically low-intensity. This means the source temperature would be below 1200°F. High-intensity systems have source temperatures as high as 5000°F.

So, the next time you are writing a report on a warehouse, remember — it’s not a unit heater in there, it’s a ceiling-mounted, gas-fired, open flame, indirect, convection heating unit!

Carson Dunlop Weldon & Associates is a leading provider of commercial inspections and commercial inspection training, author of the Technical Reference Guide and the CommQuotTM Commercial Fee Quoting and Proposal Writing System. This article and accompanying diagrams have been taken from a new, not-yet-released Commercial Building Inspection Training Module written by Carson Dunlop Weldon & Associates Ltd. Visit