Hot Water Loop
Understand how the hot water loop works. Explore component overviews, measurements, and additional resources.
Overview
A hot water heating (HWH) loop system circulates hot water produced by a hydronic boiler to terminal units, such as heating coils in air handling units or radiant equipment before returning it to the boiler. This circulation is managed through a piping network using either primary-only or primary-secondary pumps.
Components
The main components associated with the HWH loop system are primary pumps for primary-flow systems, primary and secondary pumps for primary-secondary flow systems, piping network and terminal units.
Primary HWH Loop Pump and Motor
Circulates water from the boiler to the building in a primary-only system or to the secondary pump and motor in a primary-secondary system through the piping network.
A primary HWH loop pump and motor can operate at:
Secondary HWH Loop Pump and Motor
Circulates water from the primary piping network to the building. This component is found in primary-secondary flow systems.
A secondary HWH loop pump and motor can operate at:
Primary HWH Loop Piping Network
This piping network is connected to the boiler and extends through the building to deliver hot water to that facility in a primary-flow system. In a primary-secondary system, the primary piping network is dedicated to circulating water through the boiler. The piping should be covered with insulation that minimizes heat losses with the environment.
Secondary HWH Loop Piping Network
This piping network is connected to the primary piping network to deliver hot water to the facility. The piping should be covered with insulation that minimizes heat losses with the environment.
Terminal Units
Terminal units provide heating to an end-user, where the heat source can be either steam or hot water. The amount of hot water supplied to the terminal units is generally adjusted with a modulating valve controlled by a temperature sensor, which is typically not part of the HWH loop. Typical terminal units for heating are:
Fan-Coil Units
A fan-coil unit is a smaller, factory-assembled device that is used to circulate and condition air for either heating or cooling. The coil circulates an energy transfer medium, such as hot water, cold water, steam or a refrigerant. The fan blows air over the coil to heat or cool it. Fan coil units can be connected to ducts, which transport outside air or mixed air, or they can be un-ducted in which case they condition the air in the room.
Radiators
Radiators are a common terminal unit to distribute heat to space. By routing hot water through the radiator, the heat is transferred to the piping where it relies on radiation as well as free convection to warm a space.
Convectors
Convectors are similar to radiators in how energy is received but rely almost exclusively on convection to distribute heat to a space. This leads to a cooler temperature to the touch, as well as a smaller terminal unit.
Unit Heaters
A unit heater is a standalone device containing a fan which blows over a small heat exchanger to provide heat to a space.
Typical HW Loop Configurations
Hot water heating loop systems use different pump arrangements to circulate hot water through a facility. Below are the common configurations, how they work, what components they include, and how to quantify the entire system.
Primary-Flow System
A primary-flow HWH loop system consists of pumps that circulate hot water from the designated zone to the boilers.
Primary pumps are usually headered to allow for the minimum flow through the boiler and there are usually multiple pumps to allow for staging and redundancy; however, a boiler-dedicated pumping design can also be found in some facilities. Primary-flow systems can be either constant-primary-flow or variable-primary-flow.
A constant-primary-flow system usually has a staged heating system and constant speed HWH loop pumps.
A variable-primary-flow system has primary HWH loop pumps with variable speed drives that regulate the flow of the system based on the heating load of the building. The heating system may be staged or constant.
How Do I...
Assess System Energy Usage
The primary energy consumption in a HWH loop is the electricity used to run the pump motors. The heat loss across the piping network contributes to the overall energy consumption for the entire HWH plant, in particular the boiler system.
Quantify Electricity Usage (kWh) of a Constant Primary-Flow System
Determine power draw of pump motor and total operating runtime.
Equipment to Measure
What to Measure:
- Total operating time (hours) of the pump.
- Average of spot measurements of true RMS power (kW) of the pump motor.
Quantify Electricity Usage (kWh) of a Variable Primary-Flow System
Determine power draw of pump motor, total operating runtime and average outdoor air temperature.
Equipment to Measure
What to Measure:
- Average hourly true RMS power draw (kW) of the pump motor.
- Total operating time (hours) of the pump.
- Average hourly outdoor air temperature (°F).
Assess System Performance
The performance of the hot water loop system is dependent on the amount of heat delivered by the terminal units.
Quantify Heat Delivered (Btu)
Determine Hot water flow rate and the cooling tower entering and leaving temperatures.
Equipment to Measure
HWH Loop Piping Network
What to Measure:
- Average hourly flow rate (gpm) of hot water in the loop.
- Average hourly temperature (°F) of hot water entering the terminal unit.
- Average hourly temperature (°F) of hot water leaving the terminal unit.
Measurement Locations
Primary-Secondary Flow System
In a system with primary-secondary pumps, the primary pumps circulate the hot water to a low loss header, while the secondary pumps circulate the hot water to the zones in the facility.
The following arrangements can be found in a primary-secondary flow system:
A constant-primary / variable-secondary flow system has constant speed pumps that drive the flow through the primary loop and variable-speed pumps of a separate secondary loop that distribute the heating load to the terminal units. The secondary pumps are controlled by the heat load demand in the terminal units.
A variable-primary / variable-secondary flow system has variable-speed pumps on both the primary and secondary loops. They are controlled by the firing rate in the boiler and the heat load demand in the terminal units.
How Do I...
Assess System Energy Usage
The primary energy consumption of a Hot water loop is the electricity used for the pump motors and the cooling tower fans.
Quantify Electricity Usage (kWh) of a Constant-Primary / Variable-Secondary Flow System
Determine power draw of pump motor, total operating runtime and average outdoor air temperature.
Equipment to Measure
What to Measure:
- Total operating time (hours) of the pump.
- Average of spot measurements of true RMS power (kW) of the pump motor.
What to Measure:
- Average hourly true RMS power draw (kW) of the pump motor.
- Total operating time (hours) of the pump.
- Average hourly outdoor air temperature (°F).
Measurement Locations
Quantify Electricity Usage (kWh) of a Variable-Primary / Variable-Secondary Flow System
Determine power draw of pump motor, total operating runtime and average outdoor air temperature.
Equipment to Measure
What to Measure:
- Average hourly true RMS power draw (kW) of the pump motor.
- Total operating time (hours) of the pump.
- Average hourly outdoor air temperature (°F).
Measurement Locations
Assess System Performance
The performance of the hot water loop system is dependent on the amount of heat delivered by the terminal units.
Quantify Heat Delivered (Btu)
Determine Hot water flow rate and the cooling tower entering and leaving temperatures.
Equipment to Measure
HWH Loop Piping Network
What to Measure:
- Average hourly flow rate (gpm) of hot water in the loop.
- Average hourly temperature (°F) of hot water entering the terminal unit.
- Average hourly temperature (°F) of hot water leaving the terminal unit.
Measurement Locations
Further Reading
ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 13. Hydronic Heating and Cooling. I-P Edition.
ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 44. Centrifugal Pumps. I-P Edition.