Office Air Conditioning: zoning, heat recovery and who pays for it

The simultaneity problem, and why it drives the system choice

An office building faces in several directions at once. On a bright spring morning the east and south elevations take substantial solar gain while the north side takes none. The rooms are separated by a few metres of carpet and need opposite things from the air conditioning at the same moment.

That is the single most important fact about office cooling, and it explains why heat-recovery VRF — a three-pipe system that can run some indoor units in cooling and others in heating simultaneously — is the mainstream answer on multi-zone office floors. It is not just that it can do both; it is that the heat extracted from the warm side is moved to the cool side rather than rejected outside. On a shoulder-season day with mixed demand, a meaningful share of the heating is effectively free. Our VRF and VRV page covers the technology.

The corollary is that zoning should follow aspect. Grouping the whole east side on one controller and the whole west side on another reflects how the building actually behaves. Zoning by department or by partition line — which is what tends to happen when the fit-out designer sets the zones — produces the classic complaint that half the floor is uncomfortable whatever the setpoint.

Where an office's heat actually comes from

Worth being concrete, because it changes decisions. Occupants contribute roughly 100–140 W of sensible heat each at typical office activity — for a 60-person floor that alone is several kilowatts. Lighting matters much less than it used to; an LED relighting scheme can remove a surprising slice of the cooling load as a side effect, which is worth knowing if both projects are on the table. IT equipment should be assessed on measured draw rather than nameplate ratings, which are routinely around double reality. Solar gain depends on glazing area, orientation, glazing specification and shading — and on a heavily-glazed elevation it can dominate everything else.

Two zones inside an office behave unlike the rest and need separate thought. Meeting rooms carry a high transient load: an empty room and the same room with twelve people in it are very different problems, and occupancy or CO₂-based control avoids both overcooling an empty room and being caught out when it fills. Comms rooms run continuously all year and should never be served from the general office system, which will be off overnight and at weekends precisely when the comms room still needs cooling — see server and comms room cooling.

Open plan versus cellular

Open-plan floors are easier to condition — larger air volume, more mixing, fewer boundaries — but harder to keep everyone happy in, because occupants at a fixed desk experience a fixed position in the airflow pattern. Draught complaints in open plan are usually about terminal selection and diffuser choice rather than temperature, and they are far cheaper to avoid at design stage than to fix afterwards.

Cellular offices give better individual control and worse economics, since more zones means more indoor units, more controls and more commissioning. They also raise the A2L refrigerant charge limit question: BS EN 378 caps charge relative to the volume of the smallest room served, and a system serving many small offices can hit that ceiling. This is a common reason a straightforward-looking replacement turns into a redesign — see air conditioning replacement.

Most real offices are a mix, and get reconfigured every few years. Designing with some flexibility — spare capacity on branch controllers, containment routes with room in them — costs little at installation and saves a great deal at the first churn.

Noise: the specification people forget

An office is a place where people concentrate and take calls, so the acoustic performance of the air conditioning is a genuine requirement rather than a nicety. It should be specified as a target noise rating for the space, and checked at commissioning.

Two separate concerns. Indoors: indoor unit selection, fan speed and diffuser design determine whether the system is unobtrusive or a constant presence. A unit running at high fan speed because it is undersized will always be noisier than a correctly-sized one running at medium. Outdoors: condensers on a roof or in a courtyard need a BS 4142 assessment against the background at the nearest noise-sensitive receptor, which in a mixed-use area often means residential flats. That assessment can dictate plant location, attenuation or night-time limits, and it is far cheaper to design around than to retrofit.

Landlord, tenant, and who actually pays

In multi-let offices the commercial arrangement shapes the technical answer, and it is worth resolving early.

Who owns the plant — landlord-provided base building system, or tenant-installed within the demise — determines who specifies it, who maintains it and who replaces it. After-hours running needs a mechanism: a tenant working late in one zone should not require the whole floor's plant to run, and the ability to bill it accurately depends on both zoning and metering being designed in. Service charge treatment of maintenance and eventual replacement is a lease question with real technical consequences — a landlord who cannot recover replacement cost tends to keep repairing ageing plant well past the point where replacement would be better for everyone. And dilapidations: a tenant installing air conditioning should know at the outset whether it is to be removed at lease end, because that changes how it should be installed.

EPC, MEES and the letting risk

For any office that is let or is going to be, the air conditioning contributes to the building's EPC, and the EPC determines whether the space can legally be let. Under MEES, EPC E has been required to continue letting commercial space since 1 April 2023, with a proposed EPC B by 2031 for privately rented non-domestic buildings over 1,000 m², subject to secondary legislation — the previously-proposed 2027 interim EPC C requirement was dropped. Confirm the current position before making an investment decision; our MEES and EPC guide covers where it stands.

The practical implication for a landlord is that replacing old, inefficient cooling is not purely an operating-cost decision — it can be what keeps a floor lettable. Efficient plant, good controls and a properly commissioned system all help the assessment, and a TM44 inspection report (required above 12 kW anyway) is a useful piece of evidence about how the system is actually performing. See servicing and TM44.

Running cost and the controls that matter

The cheapest efficiency measures in an office are almost always in the controls rather than the plant. Time schedules matched to real occupancy, with a proper optimum-start rather than plant running from 6am for an office that fills at 9. A sensible deadband between heating and cooling setpoints, because a narrow band means the system heats and cools against itself continuously — this single fault wastes more energy in UK offices than almost anything else. Setpoint limits so a local user cannot demand 17°C in August. And integration with the BMS so the air conditioning is not fighting the heating system.

Where an office has no sub-metering on its cooling, a commercial energy audit is the sensible starting point — you cannot verify a saving without a baseline. Because office cooling peaks on bright summer afternoons, its demand profile also aligns unusually well with on-site generation; commercial solar covers that side.

Frequently asked questions

Why is one side of our office always too hot and the other too cold?

Almost always a zoning problem rather than a plant fault. If the zones follow the floor plan or the org chart instead of the building aspect, the south and east sides will be taking solar gain while the north side is not, and a single setpoint cannot satisfy both. Re-zoning by aspect, and using a heat-recovery system that can heat and cool simultaneously, is the real fix.

What is the best air conditioning system for an office?

For a multi-zone floor, heat-recovery VRF is usually the strongest fit, because it can cool the sunny side while heating the shaded side and moves the heat between them rather than rejecting it. For a small suite of a few rooms, splits or a multi-split are simpler and cheaper. The deciding factor is how many independently-controlled zones the building genuinely needs.

Should our comms room be on the main office system?

No. A comms room produces heat continuously, including overnight and at weekends when the office system will be scheduled off. It needs its own cooling, specified for continuous duty and for low outside ambient temperatures so it can still cool in winter, with resilience matched to how costly an outage would be.

Who is responsible for air conditioning in a leased office?

It depends on the lease and on whether the plant is a landlord base-building system or tenant-installed within the demise. That determines who specifies, maintains and replaces it, how after-hours running is charged, how maintenance and replacement are treated in the service charge, and whether the tenant must remove it at dilapidations. Worth settling before installation, not after.

Does air conditioning affect our EPC rating?

Yes. The cooling system contributes to the building's EPC, and the EPC governs whether the space can be let under MEES — currently EPC E to continue letting commercial space, with a proposed EPC B by 2031 for privately rented non-domestic buildings over 1,000 m², subject to secondary legislation. For a landlord, replacing inefficient plant can be what keeps a floor lettable.

We design and install office air conditioning across the UK. See VRF and VRV systems, comms room cooling, installation and costs. Other building types are on the sectors index, or request a quote.