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Forced Air Heating in the North

Technical Series 94-209
Key Messages

  • Forced air is the most common heating system in the North.
  • Advantages:
      - less expensive and lower repair costs;
      - skilled personnel and replacement parts are more available;
      - no danger of water damage from freeze up; and
      - air circulation provides more even heat distribution.
  • Disadvantages:
      - electricity costs for fan motor can be high;
      - because there is no basement, ducting must be routed through floors and walls, reducing system performance.
  • Caution:
      - there is a potential for backdrafting in today's tighter houses with both hydronic and forced air systems.

  • Introduction

    Forced air furnaces (Figure 1) are a common heating system installed in the North. Oil is the predominant heating fuel. Natural gas is generally unavailable, propane has limited use and is only available in urban areas, and electricity rates can exceed $0.50 per kWh. Forced air heating technology is mature and trained technicians and installers are generally available. Forced air systems offer an effective means of heating, ventilating, humidifying, and distributing house air.

    Typical Oil-fired Air Furnace
    FIGURE 1. Typical Oil-fired Forced Air Furnace Enlarged Image

    The American Society for Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) states that an average oil-fired furnace should have a useful life of about 18 years. This lifetime would be somewhat less in the Arctic where the heating season is long and the heating system cycles on and off more frequently than in other climates.

    Design Factors

    Arctic houses are typically in the order of 95 m2 (1000 sq. ft.) of floor area and are constructed above grade. Relatively few houses have basements and those that do are mostly in urban areas. Recently constructed houses are reasonably airtight and require some form of mechanical ventilation. Ventilation rates are typically 0.5 air changes per hour or less. These factors result in design space heating loads for these units that are typically in the range of nine to twelve kW (30,000-40,000 Btu/h).

    Performance

    Advantages

    Forced air systems are reasonably responsive to thermostat control and, when properly balanced, can distribute heat to living spaces uniformly. A central humidifier can easily be added to these systems to provide humidification control.

    It has been stated that forced air systems have a slight performance advantage over hydronic systems for sub-Arctic and Arctic housing. It has also been stated that forced air systems are less expensive (capital cost and maintenance cost) than hydronic systems. Other advantages of forced air systems include the following.

  • Because of more widespread use, more skilled maintenance personnel and replacement parts should be available than for other systems.
  • Window condensation can be reduced by locating supply grills under windows.
  • Repair costs can be less than for hydronic systems.
  • Freeze up does not damage the system.
  • Air circulation reduces temperature stratification and space temperature variations, especially if a two speed fan is used.

    Disadvantages

    Unfortunately, because electricity is expensive in the Arctic, the cost of operating the furnace fan becomes a significant addition to the operating costs of the system.

    Conventional oil-fired furnaces available on the market are rated from about 18-30 kW (60,000-100,000 Btu/h). The low space heating loads required in Arctic housing are difficult to match with commercially available oil-fired furnaces. Generally, conventional oil-fired systems are significantly oversized relative to the space heating load of the house throughout the heating season. The result is less efficient operation of the furnace due to short cycling. The seasonal efficiency of conventional oil-fired systems in the Arctic would be less than the industry norm of 60-70%. To compound this problem, the unit never reaches steady state operation which results in lower stack temperatures, colder chimneys, increased potential for condensation and ice accumulation in the chimneys, and in the long term, accelerated deterioration of the chimney liner.

    The lack of basements in remote Arctic housing requires ducting to be installed through walls, above ceilings, or boxed in and passed down to floor level through interior walls. These added lengths increase pressure drops and duct leakage, lowering the overall performance of the system. Ductwork also requires special design for optimum performance, especially considering the lengths that have to be used. This places an added burden on the installer; thus, it is difficult in practice to achieve good design flows.

    Lastly, the potential for backdrafting in flued systems is increased substantially in tighter houses. This problem is common with both forced air and hydronic heating systems.

    System Components

    A forced air heating system (Figure 2) in the North typically consists of an oil-fired forced air furnace, ducts for air distribution, air balancing dampers in each major duct branch and dampered registers in each room. Return air ducts return room air to the furnace and a single room thermostat controls the operation of the furnace burner and fan. Return air grilles are generally located close to the furnace (rather than ducting from each room) due to cost and space considerations. As mentioned earlier, the ductwork for single-storey houses may be run above the ceiling and down in partitions, or be boxed in. In two-storey houses or houses with basements, the ducts are run between floor joists. Less frequently, with newer, single-storey houses, false floors are constructed to act as a plenum or to house the ductwork from a counterflow furnace. Although this is not a recommended solution, ventilation is typically provided by natural air leakage which increases as kitchen and bathroom exhaust fans are operated.

    Forced Air Heating Schematic
    FIGURE 2. Forced Air Heating System Schematic Enlarged Image

    Installation

    Skilled Labour Requirements

    A qualified burner technician is required to properly install and set up the heating system and controls. Sheet metal workers are required to install the heating system ductwork. Much of the sheet metal ductwork is prefabricated but certain sections must be custom formed, usually on-site.

    Furnaces

    Most furnaces can be installed on combustible floors.

    Ductwork

    Some compromise may be required in the layout of internal walls in order to provide for simple and efficient duct runs in forced air systems. Special framing, such as a built-up floor structure or boxes in the ceilings and walls, is required to accommodate the ductwork.

    Until relatively recently, most Arctic housing used prefabricated, stressed skin panel construction. In this type of construction, incorporating ductwork is difficult. More recently, stick-building or open panel construction has become quite common. It is much easier to incorporate a duct system into open framing and it can be roughed in along with electrical and plumbing services.

    Duct losses are a significant source of wasted energy. In terms of transferring energy from the furnace to the registers, the ducts are about 70% efficient. It is therefore important that duct losses be minimized by ensuring that vertical duct runs are on inside walls or above ceilings (on the warm side of the insulation), and that outside wall (boxed-in) duct runs are minimized. Ductwork joints should be taped and made as tight as possible.

    An almost universal feature of Arctic housing is insulated floors above unheated crawlspace. This prohibits the option of putting the heating distribution system in the floor. Using a sub-floor to provide duct space is expensive and creates construction scheduling problems, but it is one way of solving the problem of cold floors.

    Oil Storage

    To save valuable floor space, heating oil for a house is usually stored in an outdoor, above-ground oil tank. To ensure that the oil will flow freely to the burner, the pipe from the tank to the inside of the building should be a minimum of 50 mm in diameter. At low temperatures, oil does not atomize easily, affecting the combustion efficiency of the burner. With outdoor storage, oil seldom reaches the burner at the optimum temperature. In some cases, the 50 mm pipe is run with a few additional elbows inside the building so that the oil has a chance to warm before it is delivered to the burner.

    With an oil tank located inside a building, the cold fuel oil expands quickly as it warms and may overflow through the vent pipe. This same phenomenon can occur with full outdoor tanks on sunny days in the spring.

    Maintenance

    The furnace heat exchanger, burner and filters require regular maintenance to maximize energy efficiency and minimize additional energy costs. The furnace filter has to be replaced regularly, the heat exchanger in the furnace has to be cleaned and the burner has to be maintained and adjusted.

    Advanced Maintenance Controls

    Recent experimental work has been aimed at developing and evaluating a broad range of sensing options that could be used to signal problems in oil-fired heating appliances. By determining performance deficiencies and aiding in the furnace servicing, these devices can increase energy efficiency and therefore reduce operating costs.

    Prototype sensing devices have been developed in three areas: heat exchanger fouling, fuel/air ratio, and flame quality. It was found that the rate of performance degradation due to heat exchanger fouling can be measured using the peak flue gas temperature during heating cycles. Tests on low-cost oxygen sensors based on zirconium probes showed that these sensors can be very useful for setting and evaluating fuel/air ratios. The use of flame optical emissions was found to be a very useful indicator of flame quality. Output or control options under consideration include local or remote indicators, tools to aid rapid and accurate air/fuel ratio setting, and automatic excess air trim.

    Cost

    The cost of installing a conventional oil-fired heating system in an average northern house is approximately $7,000 (including DHW system, in 1986). The fuel oil costs are roughly $580 per year (at $0.27/litre).

    Resources

    Oil-Fired Heating Technology for the Arctic, by Robin Sinha.

    Seminar - Heating Systems for Arctic and Sub-Arctic Canada, by Buchan, Lawton, Parent Ltd., published by Canada Mortgage and Housing Corporation, June 1985.

    Alternate Heating Strategies - Data Collection and Report, by Ferguson, Simek, Clark Engineers and Architects, published by Canada Mortgage and Housing Corporation, May 1988.

    Examples of Housing Construction in the North, by Burdett-Moulton Architects and Engineers, Don Jossa and Associates, Wayne Wilkinson, April 1987.

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