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HVAC Introduction

The Old House Web

Parts of this story: Fuel burning units, general >> Forced warm air heating systems >> Forced hot water (hydronic) heating systems >> Steam heating systems >> Electric resistance heating >>Air conditioning systems


Most HVAC (heating, ventilating, air conditioning) systems in small residential buildings are relatively simple in design and operation.

They consist of four components:

  • Controls
  • Fuel supply
  • Heating or cooling unit
  • Distribution system

Each component must be evaluated for its physical and functional condition and its adequacy in terms of the buildings planned reuse. The adequacy of heating and cooling is often quite subjective and depends upon occupant perceptions that are affected by the distribution of air, the location of return air vents, air velocity, the sound of the system in operation, and similar characteristics. For this reason, past energy use should not be used as the basis for estimating future energy use.

This series of describes inspection procedures for oil- and gas-fired warm air, hot water, and steam heating systems; electric resistance heaters; chilled air and evaporative systems; humidifiers; unit air conditioners; and attic fans.

When inspecting the HVAC system, look for equipment service records and read all equipment data plates. Whenever possible, ask building occupants about the HVAC systems history of performance. Always try to observe equipment in actual operation.

When universal design is a part of a rehabilitation, consult HUD publication Residential Remodeling and Universal Design for detailed information about HVAC controls. HVAC systems have used asbestos bearing insulation on piping, ducts, and equipment, and may have lead-based paint on piping and equipment such as radiators. When inspecting the HVAC system, pay particular attention to the presence of these hazardous materials.

Assess heating and cooling capacity as described below:

Assessing Heating and Cooling Capacity

The capacity of an existing heating or cooling system, as measured by its ability to heat or cool a specific building or space, can be determined in either of two ways:

Field test. Properly sized heating and cooling systems should operate at full capacity at normal yearly outside temperature extremes and should be slightly undersized for unusual outside temperature extremes. It is rare, however, that they can be checked under such conditions.

Operate the heating system on the coolest possible day and the cooling system on the warmest possible day (within the limitations of the inspection period). Note how hard the system is working to maintain the preset indoor temperature, as indicated by how often the system cycles on and off, and compare this to outside temperatures. This procedure, while inexact, may provide some idea of the systems potential capacity.

When the system has a history of continuous use, maintenance, and repair, it can be assumed to have sufficient capacity. However, check with present or former building tenants on this matter. Of more concern is the fuel efficiency of the system. Ask the local utility company or fuel distributor for records of past fuel consumption and consider this in the overall assessment of the HVAC system.

Design calculation. An HVAC systems capacity can be more accurately determined by noting its heating or cooling output (in tons or BTUs) from information on the manufacturers data plate and comparing it to the buildings heating and cooling loads. These loads can be calculated using the Air Conditioning Contractors of Americas Manual J or similar load calculation guide.

A rough estimate of a buildings required heating equipment size in BTUs per hour (BTUH) can be obtained by using the following formula:

BTUH = .33 x [square footage of building to be heated] x [difference between outside and inside design temperatures]

The factor of .33 in this formula is based on R11 exterior walls, an R19 ceiling at the top floor or roof, and double-glazed windows.

A rough estimate of a buildings required cooling equipment size, in tons, can be made by dividing the floor area by 550 (each ton equals 12,000 BTUH).

Tonnage is not an adequate measure of cooling capacity in a dwelling of three or more floors with the air handling unit located on the lowest floor, with such a layout, the top floor can never be properly cooled.

These estimates should be followed by a complete load calculation after rehabilitation needs are firmly established.

Thermostatic controls

Residential HVAC controls consist of one or more thermostats and a master switch for the heating or cooling unit. Inspect them as follows:

  • Thermostats. Thermostats are temperature-sensitive switches that automatically control the heating or cooling system. They normally operate at 24 volts. Thermostats should be located in areas with average temperature conditions and away from heat sources such as windows, water pipes, or ducts. For a thermostat that controls both heating and cooling, a location near the return air grille is ideal.


Check each thermostat by adjusting it to activate the HVAC equipment. Then match the temperature setting at which activation occurs with the room temperature as shown on the thermostats thermometer. Take off the thermostat cover and check for dust on the spring coil and dirty or corroded electrical contact points.

Newer thermostats have a mercury switch in lieu of electrical contacts. Plan to replace worn or defective thermostats. There may be more than one thermostat in each living unit. Sometimes two thermostats separately control the heating and cooling system, and sometimes the living unit is divided into zones, each with its own thermostat.

Multi-family buildings with a central HVAC system will be divided into at least one zone per living unit and buildings with electric baseboard heat may have a thermostat in every room or on every heating unit.


Check the functioning of multi-zone systems by operating the HVAC system in all its modes and noting whether distribution is adequate in each zone. Consider the zoning needs for the planned rehabilitation of the building.

Refer to the National Environmental Balancing Bureaus Procedural Standards for Testing, Adjusting, and Balancing of Environmental Systems or the Associated Air Balance Councils MN-4, ABBC Test and Balance Procedures. 

Master switch
Every gas- and oil-burning system should have a master switch that serves as an emergency shutoff for the burner. Master switches are usually located near the burner unit or, if there is a basement, near the top of the stairs.

Cooling system controls also may include a master switch, which in the off position will not allow the compressor to start, as well as a switch allowing only the circulating fan to operate.


Operate all master and emergency shut-off switches when the burner is in operation to see whether they deactivate the unit.

In hot water heating systems that also are used to generate domestic hot water, the thermostat controls the circulating pump rather than the burner (see Section 7.4).

Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.Click here for other stories in this series.

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