Entering Water Temperature for Hot Water Coils – What is Correct?
For many years in the HVAC industry the standard entering water temperature for hot water coils has been either 180 or
200 degrees F. There has been a shift in design strategy over the past few years towards lower entering water
temperature. Engineers are becoming more energy conscience in their designs, realizing that heating water up to 180 or
200 degrees F requires more energy than heating it to 140 degrees F. That difference in water temperature can bring
considerable savings to the end user and ultimately add money back into their bottom line.
With the benefit of increased cost savings to the end user by lowering the water temperature, there are also some
potential pitfalls in system design that should be carefully explored by the design engineer. This article will highlight
some of those pitfalls and allow the engineer to make the best system selection for their design. Below are examples of
terminal unit selections that are selected with 140, 160, 180, and 200 degree F entering water temperatures. In each
example a different parameter is held constant to show the effect that parameter will have on the rest of the system.
An 8 inch inlet and 16 inch inlet terminal unit was selected for the calculations to highlight the differences that occur
between the smaller and larger size terminal units. It’s worth nothing that significant differences in performance are
seen in larger size terminal units.
Calculations for the 8 inch terminal unit:
Calculation
Number
Tag ID
1
200F EWT
180F EWT
160F EWT
140F EWT
2
200F EWT
180F EWT
160F EWT
140F EWT
3
200F EWT
180F EWT
160F EWT
140F EWT
Max
Min
Fluid
Inlet
Reheat
Capacity
Fluid PD
Max
(Primary (Primary
EAT °F LAT °F
EWT °F LWT °F Flow
Rows
Size
(CFM)
(MBH)
(ft.wg)
Air PD
CFM)
CFM)
(GPM)
Solution for Leaing Water Temperature (20 degree F delta T)
8
700
350
350
55.0 92.6
14.3
200.0 180.0
1.5
1.5
1
0.19
8
700
350
350
55.0 86.2
11.9
180.0 160.0
1.2
1.0
1
0.19
8
700
350
350
55.0 79.9
9.5
160.0 140.0
1.0
0.7
1
0.19
8
700
350
350
55.0 73.6
7.1
140.0 120.0
0.7
0.4
1
0.19
Solution for Flow Rate (1.5 GPM at Differing EWT)
8
700
350
350
55.0 92.6
14.3
200.0 180.0
1.5
1.5
1
0.19
8
700
350
350
55.0 87.2
12.2
180.0 162.9
1.5
1.5
1
0.19
8
700
350
350
55.0 81.9
10.2
160.0 145.9
1.5
1.5
1
0.19
8
700
350
350
55.0 76.6
8.2
140.0 128.7
1.5
1.5
1
0.19
Solution for Leaving Air Temperature (minimum 92.6 degrees F)
8
700
350
350
55.0 92.6
14.3
200.0 180.0
1.5
1.5
1
0.19
8
700
350
350
55.0 93.6
14.7
180.0 130.1
0.6
0.1
2
0.41
8
700
350
350
55.0 92.6
14.3
160.0 129.0
0.9
0.2
2
0.41
8
700
350
350
55.0 92.6
14.3
140.0 128.5
2.5
1.0
2
0.41
There are three iterations above for the standard 8” inlet terminal unit. The base iteration for each calculation was
based on 200 degree F entering water temperature. The reheat air flow was set at the minimum primary air flow of the
terminal unit. The base unit had a standard one row hot water coil.
In calculation number 1, the leaving water temperature was locked at a 20 degree F delta T. It’s worth nothing that as
the water temperature decreased and maintaining the 20 degree delta T the capacity of the one row coil decreased
significantly. In order to achieve the same leaving air temperature as the 200 degree entering water temperature
selection each of the remaining three selections would require a two row coil or an increase in water flow rate.
In calculation 2, the water flow rate of 1.5 GPM was held constant across the board on all four selections. It is worth
noting that the capacity is again reduced as the supply water temperature decreases. In order to reach the discharge air
temperature of the 200 degree water coil, the flow rate of the other selections would have to be increased or the coil
changed from 1-row to 2-row.
In calculation 3, the discharge air temperature was held to a minimum of 92.6 degrees F. On the 140, 160 and 180
degree EWT selections the coil was changed to a 2-row configuration. On the 140 degree F EWT selection to maintain
the 92.6 degree F LAT, the water flow rate increased significantly over the 200 degree F EWT selection, however water
pressure drop was reduced. The increase in water flow rate will undoubtedly increase the heating pump sizing.
The same calculations as above were also performed on a larger size terminal unit, a 16 inch inlet single duct terminal
unit. The calculation results are shown below:
Calculation
Number
Tag ID
4
200F EWT
180F EWT
160F EWT
140F EWT
5
200F EWT
180F EWT
160F EWT
140F EWT
6
200F EWT
180F EWT
160F EWT
140F EWT
Max
Min
Fluid
Inlet
Reheat
Capacity
Fluid PD
Max
(Primary (Primary
EAT °F LAT °F
EWT °F LWT °F Flow
Rows
Size
(CFM)
(MBH)
(ft.wg)
Air PD
CFM)
CFM)
(GPM)
Solution for Leaing Water Temperature (20 degree F delta T)
16
3000
1500
1500 55.0 90.2
57.2
200.0 180.0
5.9
6.8
1
0.27
16
3000
1500
1500 55.0 84.5
48.0
180.0 160.0
4.9
4.9
1
0.27
16
3000
1500
1500 55.0 78.8
38.8
160.0 140.0
4.0
3.3
1
0.27
16
3000
1500
1500 55.0 73.2
29.5
140.0 120.0
3.0
2.0
1
0.27
Solution for Flow Rate (5.9 GPM at Differing EWT)
16
3000
1500
1500 55.0 90.2
57.2
200.0 180.0
5.9
6.8
1
0.27
16
3000
1500
1500 55.0 85.2
49.2
180.0 162.9
5.9
6.9
1
0.27
16
3000
1500
1500 55.0 80.3
41.1
160.0 145.8
5.9
7.1
1
0.27
16
3000
1500
1500 55.0 75.4
33.1
140.0 128.6
5.9
7.3
1
0.27
Solution for Leaving Air Temperature (minimum 90.2 degrees F)
16
3000
1500
1500 55.0 90.2
57.2
200.0 180.0
5.9
6.8
1
0.27
16
3000
1500
1500 55.0 90.2
57.3
180.0 114.5
1.8
0.6
2
0.57
16
3000
1500
1500 55.0 90.2
57.3
160.0 119.5
2.9
1.5
2
0.57
16
3000
1500
1500 55.0 90.2
57.3
140.0 124.4
7.4
9.8
2
0.57
In calculation 4, the results are similar to those experienced with the 8 inch inlet terminal unit. In order to achieve the
desired leaving air temperature either the water flow rate would need to be increased or changing the coil from a 1-row
coil to a 2-row coil would be required. The same can be stated for the results in calculation 5. In calculation 6, it’s very
evident that the utilizing 140 degree EWT can have some fairly substantial implications on the overall design of the
system. The water flow rate is much higher than the other selections and the water pressure drop is about 25 percent
higher than at 200 degrees F. The difference in water pressure drop is even greater when compared to the 160 and 180
degree F EWT scenarios.
The differences on the 16 inch terminal units at the various entering water temperatures is more pronounced than on
the smaller size 8 terminal unit. The reheat air flow for the 16 inch terminal units shown above was equal to the
minimum air flow which is 50 percent of the maximum primary air. The following calculations show what happens with
the heating when the terminal unit coils are selected at the maximum terminal unit air flow instead of minimum airflow.
Calculation
Number
Tag ID
7
200F EWT
180F EWT
160F EWT
140F EWT
8
200F EWT
180F EWT
160F EWT
140F EWT
9
200F EWT
180F EWT
160F EWT
140F EWT
10
200F EWT
180F EWT
160F EWT
140F EWT
11
200F EWT
180F EWT
160F EWT
140F EWT
Max
Min
Fluid
Inlet
Reheat
Capacity
Fluid PD
Max
(Primary (Primary
EAT °F LAT °F
EWT °F LWT °F Flow
Rows
Size
(CFM)
(MBH)
(ft.wg)
Air PD
CFM)
CFM)
(GPM)
Solution for Leaing Water Temperature (20 degree F delta T)
16
3000
1500
3000 55.0 79.2
78.8
200.0 180.0
8.2
12.7
1
0.27
16
3000
1500
3000 55.0 75.3
66.1
180.0 160.0
6.8
9.0
1
0.27
16
3000
1500
3000 55.0 71.4
53.3
160.0 140.0
5.4
6.0
1
0.27
16
3000
1500
3000 55.0 67.4
40.5
140.0 120.0
4.1
3.6
1
0.27
Solution for Flow Rate (8.2 GPM at Differing EWT)
16
3000
1500
3000 55.0 79.2
78.8
200.0 180.0
8.2
12.7
1
0.27
16
3000
1500
3000 55.0 75.8
67.7
180.0 163.0
8.2
12.9
1
0.27
16
3000
1500
3000 55.0 72.4
56.6
160.0 145.8
8.2
13.2
1
0.27
16
3000
1500
3000 55.0 69.0
45.6
140.0 128.7
8.2
13.5
1
0.27
Solution for Leaving Air Temperature (minimum 79.2 degrees F)
16
3000
1500
3000 55.0 79.2
78.8
200.0 180.0
8.2
12.7
1
0.27
16
3000
1500
3000 55.0 79.2
78.8
180.0 110.1
2.3
1.0
2
0.57
16
3000
1500
3000 55.0 79.2
78.7
160.0 114.5
3.5
2.2
2
0.57
16
3000
1500
3000 55.0 79.2
78.7
140.0 118.7
7.5
9.9
2
0.57
Solution for Leaing Water Temperature (20 degree F delta T)
16
3000
1500
3000 55.0 101.5 151.4 200.0 178.0 14.2
33.5
2
0.57
16
3000
1500
3000 55.0 94.6 128.9 180.0 160.0 13.2
29.3
2
0.57
16
3000
1500
3000 55.0 99.2 143.8 160.0 140.0 14.7
17.9
3
0.86
16
3000
1500
3000 55.0 95.2 130.9 140.0 120.0 13.3
8.0
4
1.15
Solution for Flow Rate (14.2 GPM at Differing EWT)
16
3000
1500
3000 55.0 101.5 151.4 200.0 178.0 14.2
33.5
2
0.57
16
3000
1500
3000 55.0 107.6 171.2 180.0 155.3 14.2
16.8
3
0.86
16
3000
1500
3000 55.0 99.0 143.1 160.0 139.5 14.2
16.8
3
0.86
16
3000
1500
3000 55.0 95.8 132.8 140.0 121.1 14.2
9.2
4
1.15
In calculation 7 a baseline was established with the 200 degree F entering water temperature system and a 20 degree F
delta T water side. It can be noted that with the other two scenarios above, to achieve the same leaving air temperature
as the 200 degree F system an increase in water flow rate or switching from 1-row coils to 2-row coils would be
required. In calculation 8, the water flow rate was held constant and similar results consistent with the previous
calculations were found. In calculation 9, 2-row coils were required on the 140, 160 and 180 degrees F water systems in
order to meet the desired leaving air temperature. As with the previous 16 inch terminal selections in calculation 6, the
water pressure drop and water flow rate have significantly increased to meet capacity.
Calculation 10 and 11 were additional calculations in order to show what happens when a leaving air temperature of 95
degrees or greater is desired. The base line terminal unit is necessarily a selection an engineer may use on the system
due to the high water flow rate and high water pressure drop, but it allows for a base line of the comparison. In
calculation 10, the water temperature difference of 20 degrees F was maintained and the coil rows were altered to
achieve a leaving air temperature of 95 degrees or higher. It can be noted that on the 140 and 160 degree water
systems that the coil rows have been increased to 4 rows on the 140 degree system and 3 rows on the 160 degree
system to meet the requirements. The air pressure drop on those 3 and 4 row coils is significantly higher than a 2 row as
expected; the higher air pressure drop can increase the fan horsepower of the fan system. In calculation 11, the water
flow rate was held constant through all four selections. It’s noted in order achieve 95 degrees F or greater leaving air
temperature all of the terminal unit selections with the exception of the 200 degree system needed a deeper coil than 2
rows.
So what do all the calculations mean at the end of the day? It means the design engineer needs to review the system as
a whole, even when it comes to something as small as a terminal unit. The engineer needs to review the system and
determine what is going to yield the greatest energy savings for their particular system. Will a lower entering water
temperature save more energy than possible increase in pump energy and fan energy required to handle the larger
coils? The Price Industries All-In-One program is an excellent tool that can be utilized by the design engineer to select
terminal units specifically for their project. The terminal unit schedules can be customized and exported to Microsoft
Excel for easy insertion into project design documents.
Visit http://www.price-hvac.com/software/AIO for information on how to download the selection program, or contact
your local Michigan Air Products sales representative, sales@michiganair.com.
http://www.michiganair.com/newsletters/2014-1/section3.htm