[EnergyPlus_Support] autosizing components in hybrid DV/radiant heating system

["josh_miarch" <jeemang@xxxxxxxxx>]

I'm attempting to model a cold climate system in which heating is
provided by a combination of centrally conditioned air (delivered via
displacement ventilation) and a radiant panel. The zone in question
has a constant setpoint of 21 C. Air is delivered from the central AHU
at 19 C, with the balance of the heating load being made up by the
radiant panel.

I've modeled this with a DIRECT AIR object given heating priority 1
and a LOW TEMP RADIANT SYSTEM:HYDRONIC object (heating control
temperature = 18 C, throttling range = 3 C) given heating priority 2. 

The problem I'm having is that the autosizable fields of several
objects are being filled incorrectly. For example, the maximum hot
water flow of the radiant panel is being autosized to 0. Strangely, I
noticed that when I increase the "zone heating design supply air
temperature" field in the ZONE SIZING object from its original value
of 19 C, at a certain point the maximum hot water flow of the radiant
panel starts getting autosized to numbers greater than 0. 

Investigating this further, I noticed that the "DesLoad" outputted in
the "Zone Sizing Information" section of the .eio output file changed
when I changed the zone heating design supply air temperature in the
ZONE SIZING object. For example, when the design supply air temp was
set to 19 C, the "DesLoad" was 0.20703 W, and when it was set to 80 C,
the "DesLoad" was 13194 W. 

I'm guessing that this is the source of my problems. It seems strange
to me that the design load would change based on the supply air
temperature -- shouldn't these variables be independent of one another? 

I know that EnergyPlus can sometimes struggle with hybrid systems like
this; I'm wondering if there is an error on my part, or if this an
example of EnergyPlus' limitations in this area. Any advice that
anyone could give me that would get me closer to an accurate
simulation would be greatly appreciated.

Thanks,

Josh 

--
Josh Kjenner, EIT, LEED AP
Manasc Isaac Architects Ltd.
10225 100 Avenue :: Edmonton, AB T5J 0A1
780.429.3977





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