When an air handling unit cools the air it delivers, it’s doing it in one of two main ways: chilled water or DX (direct expansion). It’s one of the more important decisions in an HVAC design, and it has real consequences for efficiency, cost and maintenance. Here’s how the two compare.
Chilled water cooling
With a chilled water system, a central chiller cools water, and that water is pumped to a cooling coil inside the air handler. As air passes over the coil, it’s cooled before being delivered to the building.
Where it shines: larger buildings, and sites with several air handlers. One central chiller can serve multiple units efficiently, and chilled water systems tend to be very efficient at scale and to offer fine control over temperature. They’re the common choice for big commercial and institutional buildings.
The trade-offs: more infrastructure — a chiller, pumps and pipework — so a bigger up-front installation, and a system that needs its own maintenance regime.
DX (direct expansion) cooling
A DX system cools the air directly using refrigerant. The cooling coil in the air handler is part of a refrigerant circuit, with no water loop in between.
Where it shines: smaller buildings and simpler installations, where a full chilled water plant would be overkill. DX systems are often quicker and cheaper to install and well suited to single units or smaller sites.
The trade-offs: they can be less efficient than chilled water at large scale, and because they use refrigerant they fall under F-Gas regulations, which carry maintenance and compliance obligations worth understanding before you commit.
Which is right for your building?
There’s no universal answer — it comes down to the size of the building, how many units you need, your efficiency goals, and your budget for installation versus running costs. As a rough rule, larger multi-unit sites lean towards chilled water, and smaller or simpler ones towards DX, but the detail matters.
On the mixed-use development at Dalston Cross, the cooling strategy had to balance several different spaces with very different demands, and the right answer there wasn’t the obvious one — which is exactly why it’s worth getting proper advice rather than defaulting to whatever’s most familiar.
We design and install air handling units with both chilled water and DX cooling, and we’ll recommend the approach that genuinely fits your building rather than the one that’s easiest to quote. Get in touch to talk it through.
