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BMC and BMC-6X Installation and Operation Manual
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.
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.
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) -10˚F
Radiant (Low Mass/Joists) 2.00 (OD) : 1.00 (SYS) -10˚F
Fan Coils & Air Handlers 1.00 (OD) : 1.00 (SYS) 20˚F
Outdoor Temperature
Water Temperature
70 405060
130
120
110
100
Outdoor Temperature
Water Temperature
70 405060
110
100
90
80
Outdoor Temperature
Water Temperature
70 405060
150
140
130
120
1:4
With a 0° Offset, the
ratio curves begin at
100° Water Temperature.
With a -20° Offset, the
ratio curves begin at
80° Water Temperature.
With a +20° Offset, the
ratio curves begin at
120° Water Temperature.
1:4
1:4 1:1
1:1
1:1
4:1
4:1
4:1
90
100
+20 Offset
-20 Offset
Outdoor Temperature (in °F)
70 60
50
40 20
30 0 -10
10 -20
100
120
110
130
140
150
160
180
170
190
200
210
220
1:3 1:2 1:1.5
1:1.25
1:1
1.25:1
1.5:1
2:1
3:1
4:1
Water Temperature (in °F)
Reset Ratio Curves
Reset Ratio is Presented as
Outdoor Temp. : Water Temp. Ratio
1:4
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.