Now that the Building Regulations TGD L 2017 has been published, which includes the Nearly Zero Energy Building (NZEB) Requirements for Commercial Buildings, a few interesting scenarios arise.

We all know it was the “Letting Agent” that required air-conditioning in offices in Dublin to ensure they achieved the maximum rent available! The current AC Solution in favour in Central Dublin is the good old 4-pipe Fan Coil System – which in fairness is the most flexible and possibly most suitable AC System for the Dublin Office Market.  The VRV / VRF Solution was always banished to Industrial Parks and beyond the M50 where rent was lower.

However, with NZEB and the requirements for a Renewable Energy Ratio (RER) of between 10 and 20% in Commercial Buildings, and in the absence of the actual calculation method which has yet to be released by SEAI, us M&E Consulting Engineers are looking at cost effective options and to generate discussion.

I note the new TDG L does classify Heat Pumps as Renewable Energy, therefore if you take the following scenarios based on a 70,000 sq. ft. new office building, taking all the individual component parts being compliant with the NZEB Reference building as set out in the new TGDL 2017 with the only variable being the Air-conditioning System.

Scenario 1       4-pipe Fan Coil System

Scenario 2       VRV System

Scenario 3       4-pipe Fan Coil System with min number of PV Panels to achieve compliance


  Target for Compliance FCU

Scenario 1


Scenario 2

FCU with 350m² PV

Scenario 3

EPC <1.0 0.91 0.66 0.70
CPC <1.15 0.91 0.67 0.71
RER >0.10 0 0.15 0.1
Compliant with NZEB?   No Yes Yes


The 4-pipe Fan Coil System by itself does not comply with the Renewable Energy Ratio (RER) and requires additional technology to comply such as CHP, PV, etc. We have used PV to demonstrate minimum compliance in Scenario 3.

The VRV System achieves a RER of 15% and is therefore compliant with no requirement for extra goodies!!!

So, the VRV with all its technical inadequacies as an AC Solution compared to a 4-pipe Fan Coil such as control during defrost cycle, low supply air temperature control, especially at change from heat to cooling mode, the toilet flushing sound of the BC Control box etc. and as much as I hate to admit it the VRV Solution does allow a building achieve compliance with TGD L 2017 NZEB at a cost much lower than a 4-pipe Fan Coil Solution!

What’s the world coming too!?


Simon O’Brien, CEng

Joint Managing Director of Homan O’Brien Consulting Engineers