Part L2a 2013 - VRF (Split) Providing Heating Only - NCM?

[DesignCO2]

Hello All.

I am currently simulating an office building that derives its heating and cooling by a centralised VRF system. Now, much of the building spaces do require both heating and cooling, therefore the selection of the 'Split or Multi-split' system type is correct (with ventilation defined at room level). Now, the issue I have is that there are several spaces that are not cooled, though these are heated via the same VRF system. What NCM system type should be defined here? Do you:

a) Create a new split/multisplit system and simply change the cooling tab to natural ventilation - invalid setting displayed.

b) Use the 'Other local room heater - fanned' option (assuming the heat is supplied to the space by wall/ceiling mounted fan/cassette units) and define the heat source as an ASHP. If this is the correct approach, how do you account for the fan energy of the room level fan/cassette units? Perhaps this is already accounted for within the Aux calculations for this system type? Alternatively should the air source heat pump seasonal efficiency take these room level fans into consideration - i.e. as you would do normally split/multi-split systems?

Hope that make some sense!

Any thoughts appreciated

It wouldn't make sense to use one of the 'water' heating systems as not water is involved.

[RossThompson87]

Hi,

It's a difficult one to answer. I'm not sure you can enter the system 100% accurately so it's a case of what is the best work around, which is a bit subjective.

What is to say the occupants of the building won't use the VRF system for cooling in a room that 'doesn't have cooling'?

You could also try running a simulation with those rooms on a VRF system with cooling and see if there is much cooling load.

As for the vent, I believe the zone level SFP for 'Split or Multi-split' sytem refers to the general mechanical ventilation rather than the terminal unit recirculating. For some reason this is only an input for the 'fan coil unit' system.

Personally I would probably leave the cooling in the rooms that don't have it. The notional building will mirror this so it should be a fair assessment.

Ross

[DesignCO2]

Thanks Ross. That backs up my thinking that in this case it is a subjective decision. I would be interested to hear IES thoughts on this also, but for the time being I have assumed the pessimistic scenario of central heating using air distribution (using an electric ASHP as the heat source - Seasonal heating efficiency question remains). My justification is that this system does inherently apply a significant auxiliary fan power component which could be argued more than accounts of the room level VRF fans. Fortunately the areas served by this system are relatively small in the scale of the whole building. Clearly the ideal approach in my view would be to be able to define a 'split/multi-split' system that is heating only as well as define cassette fan powers at the room level (as you have stated is possible when FCU systems are used). I think the approach you suggest of assuming these spaces are heated and cooled via a 'split/multi-split' system would also be valid.