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Plant sizing: how to choose the design loop exit temperature and temperature difference ?

asked 2016-05-03 06:19:07 -0600

Urado gravatar image

I have a hot water loop with everything set to 40 °C:

  • The design water outlet temperature of my Boiler.
  • The setpoint manager (Hot water temperature schedule, including design day profiles).
  • The rated Inlet water temperature of my Coils.

The flow rate and capacity of systems are autosized.

My question is about these two fields:

What is the right way to choose these parameters ? I have observed they have a significant impact on simulation results. Depending on theses values, I also have some sizing errors (it seems that I need a high value for Design Loop Exit Temperature).

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answered 2016-05-03 07:20:55 -0600

updated 2016-05-03 07:24:18 -0600

Design Loop Exit Temperature should be set to 40°C in your case.

For the Loop Design Temperature Difference, it should be set in concordance with your coils so that:

  • Design Loop Exit Temperature =$LWT_{plant}$
  • Design Loop Temperature Difference =$\Delta T_{plant}$
  • Heating coil's Rated Outlet Water Temperature = $LWT_{Heating Coil}$

$$LWT_{plant} - \Delta T_{plant} = LWT_{Heating Coil}$$

If your boiler and coils are designed for 40°C supply and 30°C return temperature for example, you would use Heating Coil's Rated Outlet Water Temperature = 30°C and Loop Design Temperature Difference = 10°C.

See Heating Coil Water: Rated Outlet Water Temperature

All in all, you're - maybe unfortunately - responsible for ensuring consistency between your plant and the coils' sizing objects.

Likewise, the temperatures you put for the heating coil air inlet and outlet temperature should match the Sizing:System object of the airloop on which it's located

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This is exactly what I though but the 40°C / 10°C for these two fields brings a severe error for coil sizing. To get the simulation working I have to set for example 50°C / 10°C which doesn't seem logical to me...

My Heating Coil's Rated Outlet Water Temperature was already set to 30°C.

Urado gravatar imageUrado ( 2016-05-03 07:37:43 -0600 )edit

What severe error do you get?

Julien Marrec gravatar imageJulien Marrec ( 2016-05-03 07:40:37 -0600 )edit
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The hot water coil sizing routines break down when the Design Loop Exit Temperature goes below about 45C. It is unfortunate but the sizing method seems to have been developed for warmer hot water. It should be okay to use one set of values for sizing and then still operate the loop at the 40C.

Archmage gravatar imageArchmage ( 2016-05-03 07:50:48 -0600 )edit

@Julien Marrec "Autosizing of heating coil UA failed for Coil:Heating:Water. Bad starting values for UA."

@Archmage Thanks, it explains why this is working for 50°C and not for 40°C !

Urado gravatar imageUrado ( 2016-05-03 08:11:27 -0600 )edit
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@Archmage: are you aware of an issue on github for this? (I searched but only found 5445 and 5442 which I believe is only concerning the ZoneHVAC:UnitHeater).

Or anywhere in the documentation where it can be found? This is typically the kind of "voodoo-like" information that us mere mortals don't have and will keep us banging our heads against for days at a time until it's addressed or at least documented :)

Julien Marrec gravatar imageJulien Marrec ( 2016-05-03 08:23:20 -0600 )edit

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Asked: 2016-05-03 06:19:07 -0600

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Last updated: May 03 '16