> For the complete documentation index, see [llms.txt](https://docs.betterbuilding.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.betterbuilding.io/user-guide/2d-building/building/hvac/plant-hvac/hot-water-loop.md).
# Hot Water Loop
The Hot Water Loop comprises supply-side equipment like (gas) boilers or heat pumps, pumps, pipes, and control devices. The demand side includes heating coils and heat exchangers connecting to other loops (e.g., auxiliary condenser water loop), forming a comprehensive system for efficient heat distribution.
## System
### Min. Loop Temp.
Specify the Min. Loop Temp defines the maximum allowable temperature in Celsius for this loop.
### Max Loop Temp.
Specify the Max Loop Temp setting the minimum allowable temperature in Celsius for this loop.
### Load Distribution Scheme
Specify the Load Distribution Scheme from five options: Optimal, Sequential Load, Uniform Load, Sequential Uniform PLR, and Uniform PLR. This scheme determines the algorithm used to sequence equipment operation to meet the plant loop demand.
### Design Loop Exit Temp.
Specify the Design Loop Exit Temp., setting the water temperature in degrees Celsius at the exit of the supply side of the plant loop. This represents the temperature of the water supplied to the demand side of the loop.
### Design Loop Temp. Difference
Specify the Design Loop Temp. Difference, defining the design temperature rise (for cooling or condenser loops) or fall (for heating loops) in degrees Celsius across the demand side of a plant loop. This temperature difference is crucial for component models to determine the required flow rates to meet design capacities efficiently.
## Setpoint Manager
### Setpoint Manager Type
Specify the Setpoint Manager Type, choosing between Constant and Outdoor Air Reset. In Constant mode, a constant hot water supply temperature is maintained. In Outdoor Air Reset mode, the hot water supply temperature from the heating devices varies based on the outdoor air temperature.
### Supply Temp.
Specify the Supply Temp, applicable when the Setpoint Manager Type is Constant. Set the constant supply temperature for the output of the heating devices on the hot water loop.
### Setpoint at Outdoor Low Temperature
Specify the Setpoint at Outdoor Low Temperature, applicable when the Setpoint Manager Type is Outdoor Air Reset. This sets the supply air temperature setpoint in degrees Celsius at the outdoor low temperature for the first reset rule.
### Outdoor Low Value
Specify the Outdoor Low Value, applicable when the Setpoint Manager Type is Outdoor Air Reset. Set the outdoor air low temperature in degrees Celsius for the first supply air temperature reset rule.
### Setpoint at Outdoor High Temperature
Specify the Setpoint at Outdoor High Temperature, applicable when the Setpoint Manager Type is Outdoor Air Reset. This represents the outdoor air low temperature in degrees Celsius for the first supply air temperature reset rule, typically indicating the maximum supply temperature at this outdoor air temperature.
### Outdoor High Value
Specify the Outdoor High Value, applicable when the Setpoint Manager Type is Outdoor Air Reset. This field represents the outdoor air high temperature in degrees Celsius for the first supply air temperature reset rule.
## Sizing
### Sizing Option
Specify the Sizing Option, choosing between Coincident or Non-coincident. Coincident sizing triggers additional sizing simulations iterations, using coincident flow rates of each plant loop component to size the total loop flow rate. Non-coincident sizing means that plant loop flow rates are the sum of the component flow rates.
### Sizing Method
Specify the Sizing Method, choosing from Autosize or Manual. For Manual sizing, ensure all numerical fields are completed with valid entries.
### Min. Loop Flow Rate
Specify the Min. Loop Flow Rate is a numeric field containing the minimum loop flow rate in cubic meters per second.
### Max. Loop Flow Rate
{% content-ref url="/spaces/fKJnzF2TwHA3APPbiCpF/pages/wvtKXTVJoRkkoTeQ47s6" %}
[Heating Device - Gas Boiler](/user-guide/2d-building/building/hvac/plant-hvac/hot-water-loop/heating-device-gas-boiler.md)
{% endcontent-ref %}
Specify the Max. Loop Flow Rate, a numeric field containing the maximum loop flow rate in cubic meters per second. This parameter is also used when autocalculating the loop volume.
### Plant Loop Volume
Volume of the plant loop in m3. This numeric field contains the loop volume for the entire loop, i.e. both the demand side and the supply side. This is used for the loop capacitance calculation. Loop volume (m3) could be calculated from pipe size data but this is not usually known. If zero volume is specified the loop has no fluid heat capacity. If a very large capacitance is specified unrealistic time delay may result and there may be poor response to changes in loop setpoint temperature. The autocalculate option sets the loop volume to the product of the Maximum Loop Flow Rate and the user input for Loop Circulation Time which defaults to 2 minutes.
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