BMC and
UNDERSTANDING OPERATION CONCEPT
The BMC has multiple operating modes that satisfy most hydronic systems. It can change the System Set Point based on outdoor temperature (Outdoor Reset) or it can modulate its stages to achieve an adjustable fixed Set Point.
In Outdoor Reset, the BMC controls a hot water heating system to provide a building with comfortable and even heat levels. The BMC varies the temperature of the circulating heating water in response to changes in the outdoor temperature. The heating water temperature is controlled through the modulation of stages.
The BMC also controls the system circulating pump with an adjustable Outdoor Cutoff. When the outdoor temperature is above Outdoor Cutoff, the pump is off and no heating water is circulated through the system. When the outdoor temperature drops below the Outdoor Cutoff, the system pump relay is activated and the heating water circulates through the system. The temperature of the heating water is controlled by the Reset Ratio, Water Offset, and changes with Outdoor temperature.
| 220 |
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| 1:2 |
| 1:1.5 |
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| 210 |
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| 1:1.25 |
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°F) | 200 |
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190 |
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180 |
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Temperature |
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170 |
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160 |
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| 1.5:1 | |
150 |
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| 2:1 | |
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Water | 140 |
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130 |
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| 120 |
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| 110 |
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| 100 | 60 | 50 | 40 | 30 | 20 | 10 | 0 | ||
| 70 |
Outdoor Temperature (in °F)
Reset Ratio is Presented as
Outdoor Temp. : Water Temp. Ratio
Reset Ratio Curves
RESET RATIO/OUTDOOR RESET
When a building is being heated, heat escapes through the walls, doors, and windows to the colder outside air. The colder the outside temperature, the more heat escapes. If you can input heat into the building at the same rate that it is lost out of the building, then the building temperatures will remain constant. The Reset Ratio is an adjustment that lets you achieve this equilibrium between heat input and heat loss.
The starting point for most systems is the 1.00 (OD):1.00 (SYS) (Outdoor Temperature
:Heating Water Temperature) ratio. This means that for every degree the outdoor temperature drops, the temperature of the heating water will increase one degree. The starting point of the curves is adjustable, but comes factory selected at 70°F Outdoor Temp. and 100°F Water Temp. For example with a 1.00 (OD):1.00 (SYS) ratio, if the outdoor temperature is 50°F, this means the temperature has fallen 20° from the starting point of 70°F. Therefore, the heating water temperature will increase 20° to 120°F.
Each building has different heat loss characteristics. A very well insulated building will not lose much heat to the outside air, and may need a Reset Ratio of 2.00 (OD):1.00 (SYS) (Outdoor:Water). This means the outdoor temperature would have to drop
2 degrees to increase the water temperature 1 degree. On the other hand, a poorly insulated building with insufficient radiation may need a Reset Ratio of 1.00 (OD):2.00 (SYS). This means that for each degree the outdoor temperature dropped the water temperature will increase 2 degrees. The BMC has a full range of Reset Ratios to match any buildings heat loss characteristics.
A heating curve that relies not only on Outdoor temperature but also on type of radiation will improve heat comfort. The following are suggested initial settings for different types of radiation based on average building insulation and heat loss. The contractor can fine tune these adjustments based on the specific building need.
With a 0° Offset, the ratio curves begin at 100° Water Temperature.
With a
With a +20° Offset, the ratio curves begin at 120° Water Temperature.
Temperature | 130 | 1:4 | 1:1 |
120 |
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Water |
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110 |
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100 | 60 | 50 | 40 | |
| 70 | |||
| Outdoor Temperature | |||
Temperature | 110 | 1:4 |
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100 |
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Water | 90 |
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80 |
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| 60 | 50 | 40 | |
| 70 | |||
| Outdoor Temperature | |||
Temperature | 150 | 1:4 |
| 1:1 |
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140 |
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Water | 130 |
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120 |
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| 90 |
| +20 Offset | |
| 100 |
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| 70 | 60 | 50 | 40 |
| Outdoor Temperature | |||
Type of Radiation in Building | Reset Ratio | Offset |
Radiators (Steel & Cast Iron) | 1.00 (OD) : 1.00 (SYS) | 0˚F |
Baseboard (Finned copper tube& Cast Iron) | 1.00 (OD) : 1.00 (SYS) | 0˚F |
Radiant (High Mass/Concrete) | 4.00 (OD) : 1.00 (SYS) | |
Radiant (Low Mass/Joists) | 2.00 (OD) : 1.00 (SYS) | |
Fan Coils & Air Handlers | 1.00 (OD) : 1.00 (SYS) | 20˚F |
WARNING
When controlling a non condensing boiler directly without the use of a mixing valve, minimum boiler water temperature must be set to boiler manufacturer specifications. In that case, system temperature must not go below such temperature.
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