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# Zone air supply temperature

Hi, can someone please explain why the zone heating design supply air temperature is 50°C? see link: https://bigladdersoftware.com/epx/doc.... I expected the figure to be near the room set point temperature to be around 21 °C. Thanks

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50°C (122 F) is not unusual (as an initial default value) for zone heating supply air (especially for older designs), although 30°C to 35°C (~90F) is preferable. It may be higher than 50°C in some cases, yet I think 50°C is a code-set maximum temperature for insulated ductwork (foamed plastic insulation). ASHRAE 90.1 requires a maximum supply of 11°C above setpoint (e.g. 32°C if setpoint is 21°C). In any case, a decrease in heating supply air temperature involves a proportionate increase in airflow rate - a balance that would depend namely on peak heating loads IMO.

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Many thanks for your explanation. I will take a look at the Ashrae document you referenced above.

( 2023-11-17 07:36:27 -0500 )edit

Will 50°C not cause occupants thermal discomfort as it seems very warm / hot?

( 2024-03-13 09:12:35 -0500 )edit

Often, yes ... depends on context (e.g. office vs warehouse). Certainly less comfortable than 32°C (re ASHRAE 90.1 required delta-T). Beyond distribution effectiveness, one has essentially 2 variables to play with here: supply m3/s and/or supply T°. If one lowers supply T°, one generally has to increase supply m3/s to meet a given heating load. Increasing the later may not always be possible, or may be quite costly. In my neck of the woods (-26°C winter design T°), older (~uninsulated, leaky) buildings often operate with high supply T° during peak winter conditions.

( 2024-03-13 12:25:55 -0500 )edit

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