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How can I model a ductless split HVAC system?

asked 2015-03-29 00:32:43 -0500

eklein12 gravatar image

updated 2015-07-11 10:51:07 -0500

None of the HVAC templates have the option of ductless. Is there a way I can model this system in Open Studio? Thank you in advance.

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answered 2015-03-30 06:24:41 -0500

There are always many ways to skin a cat when it comes to modeling one particular system with templates.

Let's just try and understand what this "ductless split HVAC system" mean.

Split

You have an outdoor unit, also called condenser unit, with a condenser and a compressor.

You have an indoor unit, with the evaporator and a fan (air-handler).

In between, refrigerant is piped.

(The expansion valve's typical location on a split (non multi-split/VRF) would be in the outdoor unit)

Ductless

You have no ducts, so your indoor unit will condition the room it's installed in.

What they mean by "ducted" system is when the indoor air-handler is a proper Air handling unit that will deliver air to multiple rooms.

Modeling

What you need to model this system:

  • An indoor blower fan
  • A DX Cooling Coil
  • Potentially a DX Heating Coil, if it's a heat pump, not just an A/C

So, in EnergyPlus, you could use HVACTemplate:System:UnitarySystem for the greatest flexibility.

In OpenStudio, you could create an empty air loop, and add a DX Cooling coil (and a DX Heating coil if it's a heat pump), and the appropriate controllers.

But, your system has the the same components as a Packaged Terminal Heat Pump (PTHP) (E+: HVACTemplate:Zone:PTHP), and that's built into OpenStudio.

This is a zonal system, you add it on the "Thermal Zones" pane of OpenStudio, and you can't (as of 1.6.0) modify it graphically like the other systems but you'll get a pane to change it its components.

The PTHP template comes with a supplemental (backup) electric heating coil and an outdoor air mixer... Both of which you'll want to modify.

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5

answered 2015-03-31 16:33:12 -0500

updated 2015-03-31 16:53:24 -0500

I'm assuming you're looking for what is often called a Mini-Split Heat Pump which are typically ductless but can also be installed as ducted systems. As Julien mentioned, OpenStudio doesn't model ducts or duct losses so no need to explicitly model this feature of the system. Here are some suggestions based on building type:

Commercial Buildings

These types of systems are commonly found in secondary spaces, for example stairways and electrical rooms. Unless a large part of the building is conditioned by these systems you could use the ZoneHVAC:PackagedTerminalHeatPump as a simplification.

Residential Buildings

For this building type you could build custom air loops for them. For example, BEopt uses the following objects to model MSHPs:

Or as Michael mentioned you could use the VRF objects if that's a better representation of the system. YouTube - OpenStudio 1.3.0 New Features (Exhaust, VRF, and Refrigeration Grid View)

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3

answered 2015-03-30 18:04:20 -0500

If it is a variable-refrigerant flow (VRF) type of ductless system, then you can use HVACTemplate:Zone:VRF and HVACTemplate:System:VRF.

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Asked: 2015-03-29 00:32:43 -0500

Seen: 2,257 times

Last updated: Mar 31 '15