[TRNSYS-users] too-low LWT for Type753 heating coil

Ajit Naik anaik at dbhms.com
Thu Jan 2 14:51:57 PST 2014

Hello All + Happy New Year!

I am modeling a liquid-source heating coil with known liquid inlet temp that conditions outside air entering with a constant flow rate to a constant desired outlet air temperature. My intention is to use the bypass fraction scheme to select a liquid-side pump by recording the coil liquid flow rate (fraction of max liquid flowrate that is NOT bypassed and passed thru coil to condition airstream). This is performed using a Type753e heating coil, a Type2 aquastat (heating), a Type 15 weather data, and an equation block:

1.       The EWT for the liquid loop (60C) and desired outlet air temp are constant (20C) and directly specified as fixed inputs to Type753e.

2.       Type2 observes the outside air temp from Type15 and generates "1" or "0" depending on whether the outside air is above or below a specified fixed setpoint including deadband (20 +/- 3 C).

3.       This control signal is passed to an equation block where it is multiplied by a maximum liquid flow-rate that is passed to Type753e as the liquid inlet flow rate in order to shut off the liquid flow entirely when heating is not required.

4.       The not-bypassed coil liquid outlet rate required to provide the necessary heating is recorded to get an idea of what sort of pump capacity is necessary.

The sim runs fine, but the liquid side temp drop is unreasonably low - something like 40C drop for a corresponding air temp rise of 3-4 C. Correspondingly the not-bypassed coil liquid outlet rate is very low as the temp drop of the liquid is so high.

I have executed the exact same scheme to size a pump for a cooling coil using the equivalent cooling coil component Type 508c... and this works fine (reasonable rise in liquid temp and so reasonable not-bypassed liquid flow whenever the coil is in operation).

All inputs/params are in appropriate units and the max liquid inlet rate is sufficiently high but still on a reasonable order of magnitude.

I am aware that Type753e guesses an initial air outlet temperature (equal to the initial guessed average coil temp) - is it possible that an excessively low guess value is the cause of the problem here?

I appreciate any and all input!

Best of luck in 2014.


Ajit Naik
Energy Engineer
303 W Erie Street, Suite 510
Chicago, IL 60654
p (312) 915-0557 x308
f  (312) 915-0558

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