Recommendations for Modeling a 'Split' Heat Pump?

Hi All,

I wonder if someone might have a recommendation for the proper method of modeling ‘split’ ductless heat pump units with IronBug? In particular, I am looking at a multi-family building with the following scenario:

  • Fresh air is delivered to the hallways, with gas heat (no cooling).
  • Heating and Cooling are provided in the residential units with ductless split heat pump systems. This Daikin model in particular: Daikin FXAQ

So I wonder what the recommended modeling strategy for this would be? I wonder if this would fit well with the IronBug ‘Sys16_VRF + DOAS’ template? Or would this be more in line with the ‘Sys4_SZHP’ template? I think maybe I don’t quite understand what systems fall within the ‘VRF’ category?

It seems to me that it’s more a ‘Sys16’ since the ductless units do not have any outdoor air themselves and instead rely on the DOAS for all the fresh-air supply to the zone? But I wonder if that seems right? If anyone knows of any good resources to learn about the differences / boundaries / definitions of those HVAC Systems I’d very much appreciate it.

My thought was to split up the ‘Sys16’ template and apply only the VRF to the resi-zones, then only the DOAS to the hallway zones. So something like:

But I wonder if that seems right? Any thoughts on the appropriate way to approach the above system would be much appreciated!

thanks so much

(btw: IronBug is just incredible - really can’t say that enough!)

Hi @edpmay, I don’t see any issue with your system, but you will need to update ventilation requirements for both residential and hallway zones, because I believe hallway doesn’t have ventilation by default so the DOAS won’t be used at all, and you will need to remove the ventilation from residential zones.

Hope this helps.

Hi @MingboPeng - thanks for taking a look! Appreciate that. Glad to know you don’t think I’m way off base here.

I found some great tutorials here <link1> which describe how to connect / balance DOAS and extract fans. I’m trying to implement that in my solution, and so far so good.

Somone on Unmet Hours recommend looking into ZoneMixing objects for this situation - but it seems that maybe this is overly complex and not strictly needed? It also appears that ZoneMixing is not implemented in OS (?) and so can only be applied as a measure? <link>

Either way - I think I’ll stick with just connecting the Residential Units right to the DOAS using a normal terminal no-reheat, then applying their constant-volume exhaust fans. I think that seems to capture this scenario ok.

regardless - thanks!

Ok, now I know what you are going to do. So you want to supply fresh air from the hallway. Yes, this sounds bit tricky, and I haven’t done anything like this before.

But if you just want to get the energy usage, you could try to set walls between the hallway and room as air boundary which automatically sets the zonemixing for these two zones. You might need to add room’s ventilation load to hallway if the DOSA doesn’t change at all. Since there aren’t many internal loads in the hallway, so it won’t affect the cooling and heating for room’s VRF.

Just some untested thoughts.
Hope this helps.

Thanks @MingboPeng - that’s super helpful and I appreciate the input.

I’ll give that method a whirl and see how it works. I’ll see if there is a substantial difference doing it that way vs. just adding the residential zones to the AirLoop directly.