Entering Water Temperature for Hot Water Coil w

7/25/2019 Entering Water Temperature for Hot Water Coil w

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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. here has been a shift in design strategy o!er the

past few years towards lower entering water temperature. "ngineers are becoming more

energy conscience in their designs# reali$ing that heating water up to 180 or 200 degrees Fre%uires more energy than heating it to 1&0 degrees F. hat difference in water temperature

can bring considerable sa!ings to the end user and ultimately add money bac' into their

bottom line.

(ith the benefit of increased cost sa!ings to the end user by lowering the water temperature#

there are also some potential pitfalls in system design that should be carefully e)plored by the

design engineer. his article will highlight some of those pitfalls and allow the engineer to

ma'e the best system selection for their design. *elow are e)amples of terminal unit

selections that are selected with 1&0# 1+0# 180# and 200 degree F entering water

temperatures. ,n each e)ample a different parameter is held constant to show the effect that

parameter will ha!e on the rest of the system. An 8 inch inlet and 1+ inch inlet terminal unitwas selected for the calculations to highlight the differences that occur between the smaller

and larger si$e terminal units. ,t-s worth nothing that significant differences in performance

are seen in larger si$e terminal units.

Calculations for the 8 inch terminal unit

here are three iterations abo!e for the standard 8/ inlet terminal unit. he base iteration for

each calculation was based on 200 degree F entering water temperature. he reheat air flow

was set at the minimum primary air flow of the terminal unit. he base unit had a standard

one row hot water coil.

,n calculation number 1# the lea!ing water temperature was loc'ed at a 20 degree F delta .

,t-s worth nothing that as the water temperature decreased and maintaining the 20 degree

delta the capacity of the one row coil decreased significantly. ,n order to achie!e the same

lea!ing air temperature as the 200 degree entering water temperature selection each of the

remaining three selections would re%uire a two row coil or an increase in water flow rate.



,n calculation a baseline was established with the 200 degree F entering water temperature

system and a 20 degree F delta water side. ,t can be noted that with the other two scenarios

abo!e# to achie!e the same lea!ing air temperature as the 200 degree F system an increase in

water flow rate or switching from 1row coils to 2row coils would be re%uired. ,n

calculation 8# the water flow rate was held constant and similar results consistent with the

pre!ious calculations were found. ,n calculation # 2row coils were re%uired on the 1&0# 1+0

and 180 degrees F water systems in order to meet the desired lea!ing air temperature. As

with the pre!ious 1+ inch terminal selections in calculation +# the water pressure drop and

water flow rate ha!e significantly increased to meet capacity.

Calculation 10 and 11 were additional calculations in order to show what happens when alea!ing air temperature of 3 degrees or greater is desired. he 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. ,n calculation 10#

the water temperature difference of 20 degrees F was maintained and the coil rows were

altered to achie!e a lea!ing air temperature of 3 degrees or higher. ,t can be noted that on

the 1&0 and 1+0 degree water systems that the coil rows ha!e been increased to & rows on the

1&0 degree system and 4 rows on the 1+0 degree system to meet the re%uirements. he air

pressure drop on those 4 and & row coils is significantly higher than a 2 row as e)pected5 the

higher air pressure drop can increase the fan horsepower of the fan system. ,n calculation 11#

the water flow rate was held constant through all four selections. ,t-s noted in order achie!e

3 degrees F or greater lea!ing air temperature all of the terminal unit selections with the

e)ception of the 200 degree system needed a deeper coil than 2 rows.



6o what do all the calculations mean at the end of the day7 ,t means the design engineer

needs to re!iew the system as a whole# e!en when it comes to something as small as a

terminal unit. he engineer needs to re!iew the system and determine what is going to yield

the greatest energy sa!ings for their particular system. (ill a lower entering water

temperature sa!e more energy than possible increase in pump energy and fan energy re%uired

to handle the larger coils7 he rice ,ndustries All,n9ne program is an e)cellent tool thatcan be utili$ed by the design engineer to select terminal units specifically for their pro:ect.

he terminal unit schedules can be customi$ed and e)ported to ;icrosoft ")cel for easy

insertion into pro:ect design documents.

Visit http<<www.priceh!ac.com<software<A,9 for information on how to download the

selection program# or contact your local ;ichigan Air roducts sales representati!e#

sales=michiganair.com.

http://www.price-hvac.com/software/AIO


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Entering Water Temperature for Hot Water Coil w