VRF heating coil sizing improvement

[rongpengzhang]

Issue overview

In the VRF sizing, the heating coil capacity is determined using the cooling load. Although the cooling coil and heating coil are actually one coil in the real VRF system, the current approach should be improved given that 1) heating capacity may be slightly larger than cooling capacity, and 2) heating capacity will be undersized for heating-dominated climates. Solution can be similar to the sizing approach in the new UnitarySystem HP model, where the larger of the heating and cooling capacity is used.

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

• [x] Defect file added ( VariableRefrigerantFlow_5Zone.idf)
• [ ] Ticket added to Pivotal for defect (development team task)
• [ ] Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)

[ejhw]

From a residential equipment perspective, it would be nice to have some flexibility in the sizing approach. I agree that being able to size VRFs based on heating load would be a good option to have. However, it would be nice to retain the current approach because ACCA Manual S specifies that heat pumps are sized based on cooling load (with up to 30% oversizing allowed for variable speed heat pumps in certain climates when the heating load exceeds the cooling load).

Residential variable speed heat pumps often utilize an auxiliary heating source which makes sizing for 99% of heating design unnecessary. Undersizing the heating capacity (e.g., sizing for 95% of design conditions instead of 99%) is often more cost-optimal than sizing for 99%.

Happy to discuss further. I'd be interested to hear what solution you end up proposing.

[rraustad]

@ejhw, your reasoning is based on a real need. My thought is to allow a new sizing method to be generally used for all systems. Since there is both zone and air loop equipment, and that VRF is modeled as zone equipment, I think the best place to consider is the DesignSpecification:ZoneHVAC:Sizing object. It's not set up to allow the described sizing however this might be a simple add. For example, adding \key GreaterOfCoolingOrHeating or maybe \key AllowIncreasedCoolingIfHeatingLoadisGreater might provide the flexibility requested. Another input may be needed to limit the increased cooling capacity, for example, LimitOnIncreasedCoolingCapacity with a default of 1.0 and max of 1.5 or 2.0. I could consider using the Sizing:System object, however, that object is not currently required for VRF. But using just one object (instead of multiple zone sizing objects that could cause conflicting inputs) might be more logical. Although thinking of VRF, maybe the zone sizing method is better for that model. Just my thoughts so far.

DesignSpecification:ZoneHVAC:Sizing,
  A2, \field Cooling Supply Air Flow Rate Method
     \type choice
     \key None
     \key SupplyAirFlowRate
     \key FlowPerFloorArea
     \key FractionOfAutosizedCoolingAirflow
     \key FlowPerCoolingCapacity
  A3, \field No Load Supply Air Flow Rate Method
     \type choice
     \key None
     \key SupplyAirFlowRate
     \key FlowPerFloorArea
     \key FractionOfAutosizedCoolingAirflow
     \key FractionOfAutosizedHeatingAirflow
  A4, \field Heating Supply Air Flow Rate Method
     \type choice
     \key None
     \key SupplyAirFlowRate
     \key FlowPerFloorArea
     \key FractionOfAutosizedHeatingAirflow
     \key FlowPerHeatingCapacity

Sizing:System,
  A6, \field Cooling Supply Air Flow Rate Method
      \type choice
      \key Flow/System
      \key DesignDay
      \key FlowPerFloorArea
      \key FractionOfAutosizedCoolingAirflow
      \key FlowPerCoolingCapacity
  A7, \field Heating Supply Air Flow Rate Method
      \type choice
      \key Flow/System
      \key DesignDay
      \key FlowPerFloorArea
      \key FractionOfAutosizedHeatingAirflow
      \key FractionOfAutosizedCoolingAirflow
      \key FlowPerHeatingCapacity
  A9, \field Cooling Design Capacity Method
      \type choice
      \key None
      \key CoolingDesignCapacity
      \key CapacityPerFloorArea
      \key FractionOfAutosizedCoolingCapacity
 A10, \field Heating Design Capacity Method
      \type choice
      \key None
      \key HeatingDesignCapacity
      \key CapacityPerFloorArea
      \key FractionOfAutosizedHeatingCapacity

[ejhw]

Thanks @rraustad. I'll let @shorowit and Jon Winkler comment.

[rraustad]

There is some new discussion on coil sizing and I would like some input from others. @shorowit @ejhw please add others to this discussion if needed.

We currently use:

CoilCapacity_cooling = cooling_load / CapFTemp_cooling CoilCapacity_heating = CoilCapacity_cooling

For heating dominated climates would you use: CoilCapacity_heating = heating_load / CapFTemp_heating CoilCapacity_cooling = CoilCapacity_heating

This could lead to coil sizes that are 3-4 times larger where a CapFTemp_heating value on a heating design day might be 0.25 - 0.35. Would it be more economical to size the HP the way we do now and rely on a supplemental heating coil to pick of the remaining heating load on very cold days? If so, then how would you size the HP?

The question is, how would you want to size the HP in heating dominated climates?

[ejhw]

@rraustad I think my comment above still stands—there are multiple methods in practice for sizing HPs in residential applications. If we're not allowing a choice, then I guess I'd choose what you describe above (max(htg_load, clg_load)) over cooling load priority since it is more intuitive and not based on the specific ACCA industry organization manual which may not apply everywhere. Users can always choose manual sizing.

Not sure which method is typical for commercial VRF sizing; perhaps it is max(htg_load, clg_load) based on this ASHRAE article.

Note that there is often an offset between the heating and cooling capacities under rated conditions; e.g., CoilCapacity_heating_rated = CoilCapacity_cooling_rated + 2300 Btu/h so I don't know if you can you the equations written above as is.

cc @shorowit

[PeaseP]

Bumping this requests to add new methods to size VRF heating capacity. The current method initially appears as an error to engineers who do not expect cooling and heating capacity to be equal in any case. While the current approach may work for energy modeling purposes ensuring capacity is there) for sizing the actual VRF system a method to see the actual required capacity in heating operation. Would be great if this can be picked up soon.