Heating with Lower-Temperature Hot Water

Engineers Newsletter | Volume 51-4 | August 2022

Introduction

This Engineers Newsletter examines how to select coils for lower hot-water supply temperatures, in order to maximize the performance of newer heating systems driven by decarbonization and electrification trends.

Decarbonization and Electrification in HVAC

Sustainability is a key focus, leading to increased interest in reducing the carbon footprint of buildings. In HVAC, decarbonization typically involves:

Improving overall system energy efficiency to reduce emissions.
Using refrigerants with a low Global Warming Potential (GWP) and minimizing leakage.
Reducing fossil fuel use by installing electrified HVAC equipment powered by carbon-free energy sources (solar, wind, renewables).

Electrification can present a challenge for building heating.

Electrified Heating Solutions

Electrified heating solutions include resistance-based electric heat, heat pumps (air-to-air or air-to-water), and heat recovery systems. For hydronic systems, the hot-water supply (HWS) temperature is a critical variable affecting performance.

Figure 1 & 2 Description: Graphs illustrating the impact of Hot-Water Supply (HWS) and outdoor temperatures on the heating Coefficient of Performance (COP) and capacity of an Air-to-Water Heat Pump (AWHP). Higher COP indicates better efficiency. Lower HWS temperatures generally lead to higher COP and capacity, especially in colder outdoor conditions.

Impact of Hot-Water Supply Temperature on Heating Coil Selection

While the choice of hot-water supply temperature also impacts the sizing and selection of piping, pumps, and valves, this newsletter focuses on how this choice impacts the selection of hot-water coils in various HVAC equipment types: VAV terminal units, multiple-zone VAV air-handling units, fan-coil units, dedicated (100-percent) outdoor air-handling units, and single-zone VAV air-handling units.

General Impact: Designing for lower HWS temperatures typically requires coils with more rows, leading to higher fluid flow rates. This affects the sizing and cost of piping, pumps, and valves, and can increase pumping energy use. However, it also increases the capacity and efficiency of heating equipment, reducing overall heating energy consumption. The goal is to balance these factors for optimal installed cost and energy use.

VAV Terminal Units

When a zone requires heating, the hot-water coil in a single-duct VAV terminal unit heats the supply air.

Figure 3 Description: An example VAV terminal unit operating at a design heating condition. It receives 325 cfm of 60°F primary air and heats it to 90°F using a hot-water coil with a capacity of 10,600 Btu/h.

Table 1: Example VAV Terminal Unit Selections
Hot-water supply temperature (°F)	Coil rows	Inlet (VAV damper) diameter, in.	Coil heating capacity, Btu/h	Entering fluid temperature, °F	Leaving fluid temperature, °F	Fluid flow rate, gpm	Fluid pressure drop, ft. H₂O	Airside pressure drop at design cooling airflow, in. H₂O	Airside pressure drop at maximum heating airflow, in. H₂O
180	1	8	10,600	180	132	0.44	0.66	0.23	0.06
140	2	8	10,600	140	118	0.95	0.10	0.43	0.11
105	3	8	10,600	105	98	3.27	1.13	0.63	0.16
105	4	8	10,600	105	93	1.71	0.45	0.83	0.21
105	3	10	10,600	105	91	1.52	0.70	0.31	0.08
105	4	10	10,600	105	85	1.05	0.51	0.41	0.10

Note: The table assumes design cooling airflow = 650 cfm, minimum airflow = 130 cfm (165 cfm for 10-in. inlet diameter), and maximum heating airflow = 325 cfm, using the "dual maximums" control sequence.

For lower HWS temperatures, VAV terminal units often require multiple-row coils. Upsizing the VAV terminal unit can help minimize impacts on pumping and fan energy use.

Multiple-Zone VAV Air-Handling Units

A hot-water coil in a multiple-zone VAV system's central air-handling unit warms supply air during cold weather to prevent excessively cold air from reaching zones.

Figure 4 Description: An example multiple-zone VAV air-handling unit operating at a design heating condition. It mixes 10°F outdoor air (OA) with 70°F recirculated air (RA) to create 40°F mixed air (MA), which is then heated by a hot-water coil to 60°F before distribution.

Table 2: Example Multiple-Zone VAV Air-Handling Hot-Water Coil Selections
Hot-water supply temperature (°F)	Coil rows	Fin density, fins/ft	Coil heating capacity, Btu/h	Entering fluid temperature, °F	Leaving fluid temperature, °F	Fluid flow rate, gpm	Fluid pressure drop, ft. H₂O	Airside pressure drop of coil at design supply airflow, in. H₂O	Airside pressure drop of coil at design heating condition, in. H₂O
180	1	80	86,800	180	75	1.65	0.15	0.091	0.017
140	1	80	86,800	140	81	2.92	0.41	0.091	0.017
105	1	80	86,800	105	91	12.8	5.51	0.091	0.024
105	1	120	86,800	105	70	5.00	1.10	0.121	0.024

Note: Design supply airflow = 10,000 cfm, supply airflow at design heating condition = 4000 cfm.

While a one-row coil can still provide capacity, it requires a higher fluid flow rate. Increasing fin density can help reduce fluid flow rate and pressure drop, though it slightly increases airside pressure drop.

Consider incorporating exhaust-air energy recovery to preheat incoming outdoor air, reducing the load on the heating coil.

Fan-Coil (or Blower-Coil) Units

Fan-coil units are often the most challenging for lower HWS temperatures due to limited coil options and space constraints. Using a "changeover coil" for both cooling and heating is a common strategy.

Figure 5 Description: Illustrates a four-pipe distribution system for fan-coil units, featuring a shared (changeover) coil used for both cooling and heating. It shows the flow paths for chilled and hot water, with diverting valves on the return and supply sides to manage the changeover.

Table 3: Example Fan-Coil Unit Selections
Hot-water supply temperature (°F)	Coil rows	Coil heating capacity, Btu/h	Entering fluid temperature, °F	Leaving fluid temperature, °F	Fluid flow rate, gpm	Fluid pressure drop, ft. H₂O	Airside pressure drop of unit, in. H₂O
180	1 (HW) 2 (CHW)	32,600	180	103	0.85	0.05	0.30
140	2 (HW) 2 (CHW)	32,600	140	103	1.39	0.29	0.32
110	2 (HW) 2 (CHW)	32,600	110	82	9.06	4.32	0.32
105	4 (changeover)	32,600	105	82	2.83	0.84	0.39

Note: The example involves heating 1200 cfm of 65°F recirculated air to 90°F discharge air.

Using a four-row changeover coil offers advantages in fluid flow rate, pressure drop, and airside pressure drop compared to multiple standard coils, while also allowing more rows for cooling.

Upsizing the fan-coil unit can also facilitate the use of lower HWS temperatures. Blower-coil units, designed for ducted applications, often offer more coil rows and are well-suited for lower HWS temperatures.

Dedicated (100-percent) Outdoor Air-Handling Units (DOAS)

In a DOAS, a hot-water coil heats cold outdoor air to a suitable discharge-air temperature for zones, typically no warmer than 70°F.

Table 4: Example DOAS Hot-Water Coil Selections
Hot-water supply temperature (°F)	Coil rows	Coil heating capacity, Btu/h	Entering fluid temperature, °F	Leaving fluid temperature, °F	Fluid flow rate, gpm	Fluid pressure drop, ft. H₂O	Airside pressure drop of coil, in. H₂O
180	2 (HW) 8 (CHW)	325,000	180	122	11.3	0.39	0.12 (HW) 1.21 (CHW)
140	2 (HW) 8 (CHW)	325,000	140	100	16.3	0.78	0.15 (HW) 1.21 (CHW)
105	4 (HW) 8 (CHW)	325,000	105	75	21.7	0.41	0.34 (HW) 1.21 (CHW)
105	8 (changeover)	325,000	105	57	13.5	0.30	1.21

Note: The example involves heating 5000 cfm of 10°F outdoor air to 70°F discharge air.

An eight-row changeover coil provides benefits in flow rate, pressure drop, and airside pressure drop, as only a single coil is in the airstream. Many DOAS units include exhaust-air energy recovery, which reduces the heating coil's capacity requirement.

Single-Zone VAV Air-Handling Units

In a single-zone VAV AHU, the hot-water coil heats supply air to a temperature warmer than the zone.

Table 5: Example Single-Zone VAV Air-Handling Unit Hot-Water Coil Selections
Hot-water supply temperature (°F)	Coil rows	Coil heating capacity, Btu/h	Entering fluid temperature, °F	Leaving fluid temperature, °F	Fluid flow rate, gpm	Fluid pressure drop, ft. H₂O	Airside pressure drop of coil, in. H₂O
180	2 (HW) 6 (CHW)	86,800	180	150	5.78	0.05	0.13 (HW) 0.53 (CHW)
140	2 (HW) 6 (CHW)	86,800	140	120	8.69	0.29	0.17 (HW) 0.53 (CHW)
105	4 (HW) 6 (CHW)	86,800	105	85	8.70	1.00	0.39 (HW) 0.53 (CHW)
105	6 (changeover)	86,800	105	81	7.24	0.98	0.53

Note: The example involves heating 2000 cfm from 50°F to 90°F discharge air.

A six-row changeover coil offers advantages in fluid flow rate, pressure drop, and airside pressure drop. Incorporating exhaust-air energy recovery can also reduce the heating coil's capacity requirement.

Sidebar: "Dual Maximums" VAV Control Sequence

Section 6.5.2.1 of ASHRAE® Standard 90.1 permits reheat in VAV terminal units when using a "dual maximums" control sequence. This sequence involves modulating the hot-water valve to maintain zone temperature while adjusting the air damper (between minimum and maximum heating airflow setpoints) to maintain a design discharge-air temperature limit (e.g., 90°F). This higher airflow during heating helps prevent overly hot discharge air, reducing stratification and improving zone air distribution, which in turn facilitates the selection of coils for lower HWS temperatures.

Figure 6 Description: A control mode chart illustrating the "dual maximums" VAV control sequence. It shows how the system maintains zone temperature by modulating the hot-water valve and, if necessary, the air damper to manage discharge-air temperature and airflow.

This sequence requires a modulating hot-water valve (or modulating electric heater) and a discharge-air temperature sensor.

Conclusion

Providing the necessary heating capacity with a lower hot-water supply (HWS) temperature requires a higher fluid flow rate, which increases the size and cost of pipes, pumps, and valves, and likely increases pumping energy use. However, a lower HWS temperature increases both the capacity and efficiency of the heating equipment, which reduces the size of this equipment and reduces heating energy use. Finding the right balance optimizes both installed cost and overall system energy use.

Table 6: General Recommendations for Designing a Heating System for a Lower Hot-Water Supply Temperature
Equipment type	Typical minimum HWS temperature (°F)	Expected fluid ΔT at minimum HWS temperature (°F)	General recommendations
VAV terminal units	100 to 105	8 to 20	Implement the "dual maximums" control sequence. Select hot-water coils with multiple rows. Consider upsizing the VAV terminal unit.
Multiple-zone VAV air-handling units	80 to 105	18 to 30	Include a centralized heating coil. Implement supply-air temperature reset. Incorporate exhaust-air energy recovery.
Fan-coil or blower-coil units	95 to 115	8 to 30	Select hot-water coils with multiple rows. Use a changeover coil. Possibly consider upsizing the fan-coil unit.
Dedicated outdoor air-handling units	80	20 to 40	Consider using a changeover coil. Incorporate exhaust-air energy recovery.
Single-zone VAV air-handling units	100 to 105	12 to 26	Consider using a changeover coil. Incorporate exhaust-air energy recovery.

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