Hydronic Heating in the North
A hydronic heating system provides heat to one or more thermostatically controlled spaces in a house by circulating a hot water and glycol mixture to room radiators (fin-coil units). Valves at the room radiators open and close in response to the thermostat setting. The water and glycol mixture is typically heated by an oil-fired water boiler.
Hydronic heating is becoming commonplace in sub-Arctic and Arctic housing, especially in larger houses, such as two-storey houses, with more than one heating zone. Oil is the predominant heating fuel for hydronic heating systems in the North. Natural gas is generally unavailable, propane has limited use and only in urban areas, and electricity rates can exceed $0.50 per kWh.
Arctic houses are typically in the order of 95 m2 (1000 sq. ft.) of floor area and 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 in the range of nine to twelve kW (30,000-40,000 Btu/h).
One of the reasons for the trend towards hydronic heating is the elimination of the electrical operating cost of the fan system associated with forced air heating. A second reason is that piping can be installed in the house with relatively smaller space requirements than ducting. Piping can also be run under floors which can help reduce cold floors.
Another advantage of hydronic systems is the ability to integrate the domestic hot water (DHW) system into the space heating system (Figure 1). In fact, it has been argued that in energy efficient housing, DHW may in fact be the dominant load. Integrating heating and DHW reduces maintenance costs since there is only one boiler/burner. Integrated heating/DHW systems also have lower stack losses (upward warm air movement through openings in the building envelope) than separate systems because only one chimney is involved.
Through the use of thermostatically controlled valves, hydronic systems allow easier thermal control of multiple house zones than forced air systems.
The boilers of hydronic systems generally take up more space than forced air furnaces because of code-required clearances. It has been argued that hydronic systems have higher maintenance costs than warm air systems but, if properly maintained, a boiler can last 25 to 30 years.
There are fewer properly trained installers and service technicians available for hydronic systems than for forced air systems. The presence of more controls on a hydronic system adds to the burden of the proper maintenance of these systems.
Correct sizing of the heat distribution system is critical since balancing capabilities (the ability to distribute heat uniformly) are more limited than with forced air systems. Placement of the thermostat is another critical issue for optimum performance of the system.
One of the complaints with hydronic systems is the perception of stuffiness in the house. This is because distribution of fresh air must be achieved by a separate ventilation system. In leakier houses dedicated air distribution systems may not be present or needed. In more energy efficient houses, air distribution is handled through a separate ventilation system.
Conventional 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 percent. To compound this problem, the unit never reaches steady state operation which results in lower stack temperatures, a colder chimney, increased potential for condensation and ice accumulation in the chimney and, in the long term, accelerated deterioration of the chimney liner. If the space heating system is integrated with the DHW system, over-sizing is somewhat relieved by the extra DHW heating load on hydronic systems. However, this does not occur in milder weather when space heating requirements are reduced or eliminated.
Lastly, the potential for backdrafting in flued systems is increased substantially in tighter houses. This problem is common to both warm air and hydronic systems with chimneys.
Glycol is an antifreeze that is mixed with water to prevent freezing and the damage that results from frozen pipes. The liquids are mixed in a storage tank and then pumped into a standard system which consists of a boiler, circulator, expansion tank, piping, fin-coil units (radiators), drain valves at low points, air vents at high points, a pressure gauge, and a pressure relief valve. The glycol/water fluid circulates from the boiler to the fin-coil units and then back to the boiler. Room thermostats are used either to open and close zone control valves or to switch the circulator on and off. The burner is operated by a controller that maintains the boiler fluid temperature within specified limits.
DHW heating and space heating can be provided from one common boiler in houses with hydronic heating. A separate water circuit is piped to the DHW tank and controlled from the thermostat in the DHW tank. Although this is an extra load on the boiler, it seldom peaks at the same time that the heating load peaks, i.e. at night.
Most boilers are not approved for installation on combustible floors, but since a good percentage of houses do not have concrete basements, the boiler is often placed on top of a metal pan and hollow core concrete blocks.
The space requirements of the glycol/water tank and the boiler require a large floor area in the basement or mechanical room.
With maintenance being a continuous problem, there are benefits from using pre-charged bladder type expansion tanks.
The coefficient of expansion for a 50/50 glycol/water solution is greater than that of water and the expansion tank should be sized to reflect this difference.
The pressure relief valve from the boiler should be piped to the glycol/water storage tank so that valuable glycol is not lost. If not, then it should be piped to a floor drain and to the underside of the house (not to the sewer system). The loss of the glycol/water mixture through the relief valve can cause a great deal of damage to the floor insulation. Quite often the floor drain provides a dual function; it acts as a drain for the boiler pressure relief valve and is used as a combustion air intake for the oil-fired burner.
Glycol is toxic if consumed. The mixing of glycol and water must be performed carefully in a manner that meets local health codes and such that the potable water supply will not be contaminated. The glycol/water make-up fluid is mixed and stored in a tank. To avoid contamination, an accepted method for adding water to the tank is to ensure that there is at least a 150 mm space between the bottom of the water supply pipe and the top of the storage tank. Propylene glycol is the standard antifreeze used in these systems. The recommended mixture is 50/50 glycol/water by volume.
Ventilation and Combustion Air Requirements
An opening from the outdoors to an area near the oil-fired boiler is required in newer, more energy efficient houses. The Canadian Standards Association indicates that in a confined space, ventilation and combustion air should be provided by two openings to the outdoors (Figure 2). In the North however, the cold winter air blows in through these openings and it is not uncommon to find them blocked off by the residents.
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 can warm before it is delivered to the burner.
When cold fuel oil is delivered to an oil tank located inside a building, the oil expands quickly as it warms. The expanded oil may overflow through the vent pipe. This same phenomenon may occur with full outdoor tanks on sunny days in the spring.
Maintenance is very important with these systems because there are many pieces of equipment that can cause problems. The boiler heat exchanger has to be cleaned and the burner has to be maintained and adjusted. The heating fluid should be tested periodically to ensure that the proper glycol/water ratio is maintained. The mixture should not freeze but, at low temperatures, it can get thick and difficult to pump. Standard type expansion tanks (without a bladder) have to be checked to ensure that they don't lose their air pocket. A break in a pipe can cause water damage to the building. In remote northern communities, replacement glycol can be difficult and costly to obtain. But, with proper maintenance, the system should last as long as the building.
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 performance problems in oil-fired boilers. By determining performance deficiencies and aiding in the furnace servicing, these devices can increase energy efficiency and 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 were found to be very useful indicators 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.
The cost of installing a conventional oil-fired hydronic heating system in an average northern house is slightly more than a forced air heating system ($7,000-$7,500 including DHW system, in 1986). The fuel oil costs are roughly $580 per year (at $0.27/litre).
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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