Ideally, every heating system would continuously adjust its rate of heat delivery to match its building’s current rate of heat loss.
This would allow inside air temperature to remain constant regardless of outside conditions. Outdoor reset control was developed to do this by changing the temperature of the water supplied to the system in response to outdoor temperature.
Here are some benefits offered by outdoor reset control. Some are specific to systems supplied by renewable energy heat sources while others apply to any hydronic system.
Optimal use of renewable energy:
Outdoor reset control allows a renewable energy heat source to operate at the lowest possible water temperature that can satisfy the heating load of the building based on current outdoor conditions. Reduced water temperatures improve the thermal efficiency and heating capacity of most thermally-based renewable energy heat sources such as solar thermal collectors, heat pumps and thermal storage tanks supplied by biomass boilers.
Outdoor reset control also can provide the logic necessary to turn on an auxiliary heat source when a thermal storage tank heated by a renewable energy heat source can no longer supply the space heating load. With proper design, this transition occurs at the lowest possible water temperature and seamlessly sustains thermal comfort within the building. The specifics of how this works are discussed later in this article.
Stable indoor temperature:
Outdoor reset control reduces fluctuation of indoor temperature. When the reset control is properly adjusted, the water temperature supplied to the heat emitters is just high enough for the prevailing heating load. The rate of heat delivery is always maintained very close to the rate of building heat loss.
This yields very stable indoor temperature compared to less sophisticated hydronic systems that deliver water to the heat emitters as if it were always the coldest day of winter. In the latter case, the flow of heated water to the heat emitters must be turned on and off to prevent overheating under partial load conditions. This often creates an easily detected and undesirable sensation that the heat delivery system is on vs. off. With properly adjusted outdoor reset control, building occupants should have no sensation that heat delivery is on or off, just a sensation of continuous comfort.
Because outdoor reset control supplies water just hot enough to meet the current heating load, the distribution circulator remains on most of the time. Flow through the distribution system helps prevent overheating when internal heat gains from sunlight, equipment and interior lighting are present.
Near-continuous flow also can redistribute heat within heated floor slabs. Heat stored within interior portions of such slabs can be absorbed by the circulating water and carried to cooler perimeter areas of the slab. This can occur when there is no heat input to the slab from the system’s heat source. It is especially helpful when the slab has one or more heat sinks, such as the floor area just inside an overhead door where there is an insufficient thermal break between the interior slab and exterior pavement. The redistributed heat helps prevent freezing in such an area during times when the heat source is off for several hours.
Reduced expansion noise:
The combination of near-continuous circulation and very gradual changes in water temperature minimizes expansion noises from the distribution piping and heat emitters. This is especially important when PEX tubing is used with metal heat transfer plates in radiant panel heating systems. During a typical heating season, the piping and heat emitters will experience thermal expansion movement similar to that in systems not using outdoor reset control. However, when outdoor reset controls are used, the expansion movement takes place over days, even weeks, compared to what might only be seconds in systems that simply turn the flow of hot water on and off. Piping expansion noise is much more noticeable in systems where rapid changes in water temperature occur.
Indoor temperature limiting:
When water is supplied to the heat emitters at design temperature regardless of the load, occupants can choose to set the thermostat to a high temperature and open windows and doors to control overheating. Although this sounds like a foolish way to control comfort, it is often done in situations where tenants do not pay for their heat.
However, if supply water temperature is regulated by outdoor reset control, it is just warm enough to meet the heating load with the windows and doors closed. Wasteful use of energy is discouraged.
Reduced energy consumption:
Outdoor reset control has demonstrated its ability to reduce fuel consumption in hydronic heating systems. The savings are a combination of reduced heat loss from boilers, reduced heat loss from distribution piping and in the case of condensing boilers, increased time in condensing mode operation. Exact savings will vary from one project to another and conservative estimates of 10-15% are often cited.
An outdoor reset controller continuously calculates the ideal “target” temperature of the fluid supplied to a hydronic distribution system. This target temperature depends on the type of heat emitters used in the system, as well as the current outdoor temperature. It therefore has the potential to change from one moment to the next.
Outdoor reset controllers use Formula 1 to determine the target water temperature.
Formula 1(see above)
Ttarget = the “ideal” target supply water temperature to the system
Tindoor = desired indoor air temperature
RR = reset ratio (slope of reset line)
The graph in Figure 1 is a good way to visualize these relationships.
In this case, the desired indoor temperature is assumed to be 70° F and the reset ratio (RR) is assumed to be 0.5.
The red dot in the upper right portion of the graph represents design load conditions (e.g., the coldest day of winter). For the graph as shown, the red dot indicates the target supply water temperature should be 110° when the outdoor temperature is -10°.
The blue dot in the lower left corner represents no load conditions (e.g., where no heat output is needed from the heat emitters). Thus, the target supply water temperature would be 70° when the outdoor temperature is 70°.
The sloping line that connects these two dots is called a reset line. Every hydronic distribution system can be thought of as having its own reset line. The slope of that line depends on the type of heat emitters used, how they are sized and the heating load characteristics of the building.
The mathematical slope of a reset line is called the reset ratio. It can be calculated as the change in supply water temperature divided by the change in outdoor temperature between any two points on the reset line. This is represented by Formula 2. The end points of the reset line are typically used to make this calculation.
Formula 2(see above)
RR = reset ratio
?Tsupply water = change in supply water temperature between design load and no load conditions (° F)
?Toutdoor = change in outdoor temperature between design load and no load condition (° F)
Here is an example: A building has a design heat loss of 80,000 Btu/hr. when the indoor temperature is 70° and the outdoor temperature is –10°. The heat distribution system for this building has been designed so it can release 80,000 Btu/hr. when the supply water temperature is 110° and the inside air temperature is 70°. Determine the reset ratio for this system.
The change in water temperature between the no-load and design- load conditions is 110 – 70 = 40°. The corresponding change in outdoor temperature is 70 – (–10) = 80°. Thus, the necessary reset ratio when this heat distribution system is used in this building is: (See Formula 3 above)
Every hydronic distribution system, in combination with the building it is installed in, yields a unique reset line. A building equipped with slab-type floor heating will have a different reset line compared to the same building using fin-tube baseboard convectors. This is because of the difference in supply water temperatures commonly used for these types of heat emitters.
Figure 2 shows some representative reset lines and their associated reset ratios for several types of heat emitters that have been sized based on “customary” water temperatures. However, these water temperatures are not necessarily optimal or recommended when the system will be supplied by a renewable energy heat source.
According to this graph the fin-tube baseboard distribution system has been sized so it can provide design load heat output when the outdoor temperature is -10° if supplied with water at 180°. Likewise, the panel radiator distribution system has been sized so it can provide design load output when the outdoor temperature is -10° if supplied with water at 150°.
The lowest (e.g., “shallowest”) reset line on the graph is for a radiant floor panel. It has been sized to provide design load output when the outdoor temperature is -10° if supplied with water at 105°.
Keep in mind the sizing of a given type of heat emitter, rather than choosing a different type of heat emitter, can have a significant effect on the reset ratio. For example, if a designer chooses to use larger panel radiators, which can meet the design heating load at lower supply water temperatures, the distribution system will have a lower reset ratio as illustrated in Figure 3.
This also is true for other types of heat emitters such as fan coils, fin-tube baseboard or site-built radiant panels.
When the heating distribution system is supplied by renewable energy heat sources, I suggest heat emitters be selected, sized and piped so the distribution system can meet the building’s design heating load with a supply water temperature no higher than 120°.
Outdoor reset controllers can be used in systems supplied by renewable energy heat sources to determine if the water in the thermal storage tank is able to supply a heating distribution system.
The logic is simple: A properly adjusted reset controller calculates a “target” water temperature just high enough to supply the current space heating load (based on the system as well as the current outdoor temperature). So, if the water temperature in the upper portion of the thermal storage tank is below this target temperature, it is too cool to supply the heating load. In this case, a relay contact within the outdoor reset controller closes to signal the auxiliary heat source to operate. If the water temperature is at or above the target temperature it can supply the load.
Figure 4 shows a typical configuration in which a thermal storage tank is heated by some renewable energy heat source.
Sensor (T1) monitors the water temperature in the upper portion of the thermal storage tank and sends information to the outdoor reset controller (ORC). When a call for heat arrives from the distribution system, the ORC is turned on and quickly measures the temperature at sensor (T1). It then determines if this temperature is suitable to supply the current heating load. If it is, diverting valve (DV1) remains off and flow is directed through the thermal storage tank (e.g., from port AB to port B of the diverting valve). The auxiliary boiler remains off.
If the measured temperature at (T1) is a few degrees below the target temperature, the relay contact in the ORC closes. This powers up the diverting valve (DV1), which then directs flow returning from the distribution system around the tank (e.g., out port A of the diverting valve). This contact closure also enables the auxiliary boiler to operate. In this condition the load solely is supplied by the auxiliary boiler. The thermal storage tank awaits further heat input from the renewable energy heat source.
To keep the system stable, an “on/off” type outdoor reset controller must have a differential between the temperatures at which it opens and closes its relay contacts. This differential should be specified by the system designer and entered into the controller during installation. Some outdoor reset controllers allow for differentials ranging 4 to 40°. The wider the differential, the greater the allowed variation in thermal storage tank temperature between conditions where the auxiliary heat source is turned on and off.
For low temperature distribution systems I suggest a 10° differential as a starting point. Once the system is operational, this differential can be decreased if there are perceived variations in thermal comfort. It could also be increased if no such variations in comfort are detected. The latter adjustment provides the benefit of fewer but longer operating cycles of the auxiliary heat source, which generally improves performance and longevity.
Figure 5 illustrates a situation where the outdoor reset controller has been configured for a heating distribution system that requires 110° supply water when the outdoor temperature is -10°. The differential of the outdoor reset controller has also been set to 5°.
Assume the outdoor reset controller is powered on when the outdoor temperature is 20°. It immediately calculates a target supply water temperature of 95° as shown by the blue dot on the reset line.
If the water temperature in the thermal storage tank is less than or equal to 92.5° (e.g., target temperature minus half the differential) the outdoor reset controller turns on DV1 and the auxiliary boiler shown in Figure 4. The auxiliary boiler would be on for any combination of tank temperature and outdoor temperature that falls within the green shaded area of the graph.
If the measured water temperature is between 92.5° and 97.5° and the auxiliary boiler is not already operating, the contacts in the outdoor reset controller remain open and the thermal storage tank supplies the load until the temperature at the top of the tank drops to 92.5°.
If the measured water temperature is between 92.5° and 97.5°, and the auxiliary boiler is operating, it continues to operate until either the demand for heat stops or the temperature in the upper portion of the tank reaches 97.5°. The latter would be the result of heat input to the thermal storage tank from the renewable energy heat source.
The auxiliary boiler would be off for any combination of tank temperature and outdoor temperature that falls within the red shaded area of the graph.
This control action allows the tank to supply the load whenever possible, but also quickly invokes auxiliary heating when necessary to ensure comfort is maintained in the building.
Outdoor reset control also can be used to operate mixing valves between the thermal storage tank and a low temperature heating distribution system. The objective is to blend hot water and cooler water returning from the distribution system such that the water temperature leaving the mixing valve is at the calculated target temperature. This potentially allows hot water in the thermal storage tank to be “metered” out to the distribution system as needed to maintain ideal comfort conditions.
Both “on/off” and mixing reset controllers are readily available and inexpensive. They can be creatively applied in many systems supplied by renewable energy heat sources.
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