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PTHP Template: How to fix this error: "Gross Rated Total Cooling Capacity [W] will be limited by the maximum rated volume flow per rated total capacity ratio

asked 2020-01-13 11:53:18 -0600

Cristiano gravatar image

updated 2020-01-13 14:59:23 -0600

I'm using the HVACTemplate:Zone:PTHP template and I'm getting the following warnings:

Warning ** SizeDXCoil Coil:Cooling:DX:SingleSpeed DORM2 PTHP COOLING COIL
**   ~~~   ** ...Gross Rated Total Cooling Capacity [W] will be limited by the maximum rated volume flow per rated total capacity ratio.
**   ~~~   ** ...DX coil volume flow rate ( m3/s ) = 0.170368
**   ~~~   ** ...Requested capacity ( W ) = 1554.202
**   ~~~   ** ...Requested flow/capacity ratio ( m3/s/W ) = 1.096E-004
**   ~~~   ** ...Maximum flow/capacity ratio ( m3/s/W ) = 6.041E-005
**   ~~~   ** ...Adjusted capacity ( W ) = 2820.210
** Warning ** SizeDXCoil Coil:Heating:DX:SingleSpeed DORM2 PTHP HEATING COIL
**   ~~~   ** ...Gross Rated Heating Capacity [W] will be limited by the maximum rated volume flow per rated total capacity ratio.
**   ~~~   ** ...DX coil volume flow rate ( m3/s ) = 0.170368
**   ~~~   ** ...Requested capacity ( W ) = 2820.210
**   ~~~   ** ...Requested flow/capacity ratio ( m3/s/W ) = 6.041E-005
**   ~~~   ** ...Maximum flow/capacity ratio ( m3/s/W ) = 6.041E-005
**   ~~~   ** ...Adjusted capacity ( W ) = 2820.210

I'm estimating the cooling and heating loads of a generig AC split unit with COP of 3.00 during one year. As the size of that room may vary, I am using the cooling and heating capacity in autosize. Both cooling and heating supply air flow rate are set to autosize too. Does anyone know how to fix this problem? are these warnings something to worry about?

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answered 2020-01-13 14:44:45 -0600

updated 2020-01-14 10:58:21 -0600

This is a result of zone air flow sizing. The zone air flow is 0.170368 m3/s and the coil load at this time is 1554.202 W which gives a flow to capacity ratio of 1.093E-4. This ratio is too high for DX coils at about 800 cfm/ton or twice the normal flow rate. Nominal flow to capacity ratio is 5.0E-5. To correct this, adjust the supply air temperatures in the Sizing:Zone object so that zone air flow decreases. For cooling, a lower supply air temperature will lower zone cooling air flow and a higher heating supply air temperature will lower zone heating air flow. The PTHP uses the greater of these 2 zone flow rates so only one or both may be causing this sizing issue. In your case it looks like the zone cooling air flow rate is causing this warning.

Look in the eio file for zone load and air flow sizing information and use this information to guide the adjustments. Note in this example the 0.28446 Calculated Des Air Flow Rate and 0.34135 User Des Air Flow Rate (includes sizing factors from Sizing:Parameters object) for cooling and Calc = 0.0380418 and User = 0.10384 for heating (higher than a factor of 1.2 because of the zone OA limit, see 2nd to last value in the eio output for heating as Calc Outdoor Air Flow Rate). The higher of the User Des flow rates will be used for PTHP air flow sizing. Notice the zone loads also have Calculated and User values where the User values are 1.2 times the Calc values.

Sizing:Parameters,
  1.2,                     !- Heating Sizing Factor
  1.2;                     !- Cooling Sizing Factor

 ! <Zone Sizing Information>, Zone Name, Load Type, Calc Des Load {W}, User Des Load {W}, Calc Des Air Flow Rate {m3/s}, User Des Air Flow Rate {m3/s}, Design Day Name, Date/Time of Peak, Temperature at Peak {C}, Humidity Ratio at Peak {kgWater/kgDryAir}, Floor Area {m2}, # Occupants, Calc Outdoor Air Flow Rate {m3/s}, Calc DOAS Heat Addition Rate {W}
 Zone Sizing Information, SPACE1-1, Cooling, 3487.37160, 4184.84592, 0.28446, 0.34135, MIAMI INTL AP ANN CLG .4% CONDNS DB=>MWB, 7/21 15:45:00, 32.89850, 1.71418E-002, 99.16000, 11.00000, 0.10384, 0.00000
 Zone Sizing Information, SPACE1-1, Heating, 1259.72572, 1511.67086, 3.80418E-002, 0.10384, MIAMI INTL AP ANN HTG 99.6% CONDNS DB, 1/21 23:15:00, 8.70000, 6.99118E-003, 99.16000, 11.00000, 0.10384, 0.00000
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Asked: 2020-01-13 11:53:18 -0600

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Last updated: Jan 14