Terminal Units

Terminal units deliver heating and cooling to individual zones from the central plant and air handling system. They are numerous: a single office floor can have dozens, and a tower hundreds. Their condition is assessed as a population, by sample and by controls data, rather than one by one.

Terminal units are where the tenant meets the building's systems, so they are often relocated, added and altered in fit-outs. Lease make-good obligations frequently cover them, and their ownership and maintenance responsibility can be split between owner and tenant.

What shortens life:

• Blocked condensate drains on fan coils.
• Lack of filter maintenance in fan coils.
• Poor water treatment, which blocks small coils and valves.
• Fit-out changes that relocate units, cut ductwork and lose commissioning.
• Obsolete controllers that cannot be replaced like for like.

Fan coil condensate failures

Fan coil drain pans block with dust and slime, overflow, and drip into ceilings and onto fit-outs; pans corrode; drain pipework sags and holds water. Water damage to ceilings below fan coils is one of the commonest maintenance calls in office buildings, and the pans are also a microbial growth site.

Fan coil fan and coil faults

Fan motors fail and bearings get noisy; filters clog and dust fouls coils; small coils block with sludge from poorly treated water; valves stick.

VAV box faults

Damper actuators fail, leaving boxes stuck open or closed; airflow sensors foul and drift so the controller delivers the wrong airflow; reheat valves stick open so the box heats and cools at once, wasting energy; controllers lose communication or become unsupported. Many problems hide behind controls overrides.

Chilled beam condensation

Chilled beams have no drain. If the chilled water supply is colder than the room dew point, they condense and drip. They depend on dew point control and on dry primary air; failures of either cause dripping.

Induction unit faults

Induction units at the perimeter suffer blocked nozzles, fouled coils, noisy high-pressure primary air and leaking valves. Many are fifty years old, and some contain asbestos in linings and insulation.

Controls and commissioning drift

Over years of fit-outs and maintenance, setpoints drift, schedules are overridden, zones are put in manual, and units fight each other (one heating while its neighbour cools). The equipment works but the building performs badly.

In the building

• Stained ceiling tiles below fan coils; water marks on fit-outs.
• Hot and cold complaints concentrated in particular zones.
• Noise from fan coils; draughts from diffusers.
• Musty smells from fan coils.

At the units (sample)

• Drain pans: standing water, slime, corrosion.
• Filters: overloaded, missing.
• Coils: dust, corrosion, leaks.
• Actuators: position against command.
• Labels and controller types; asbestos in induction units.

In the BMS

• Zones in manual override.
• VAV boxes at full open (starved) or minimum (oversupplied).
• Reheat valves open while cooling.
• Controllers offline.
• Temperature trends showing persistent deviations.

In the records

• Maintenance logs and reactive call-outs for leaks and comfort.
• Test and balance reports after fit-outs.
• Controller types and support status.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A mechanical services engineer leads, with the controls contractor operating the BMS during the inspection.

• Sample inspection of a representative share of units of each type on each floor, weighted to areas with complaints and leaks: drain pans and drains, filters, coils, fans and motors, valves and actuators, controllers, casing and insulation.
• Functional testing through the BMS: command a sample of VAV dampers, valves and fans and confirm the response at the unit; read airflow and temperature against setpoints.
• Controls audit: overrides, alarms, offline controllers, trend data for zone temperatures and VAV damper positions, controller models and vendor support.
• Airflow measurement at a sample of VAV boxes and diffusers; comparison with test and balance records.
• Comfort survey and complaint analysis to locate problem zones.
• Asbestos check of induction units and older fan coils.
• Fit-out review: changes to layouts, ceilings and ductwork since commissioning.

The report should give fault rates by type and floor, controller support status, the controls faults found in the trends, safety items, and a strategy: repair and recommission, retrofit controls, or replace floor by floor.

• Drain programme: clean and treat fan coil drain pans and drains on a schedule; fix falls and traps; condensate overflow switches on units above sensitive areas.
• Fan motor upgrade to electronically commutated motors, cutting energy and noise.
• Coil cleaning and filter regime.
• Controls retrofit: replace unsupported unitary controllers and actuators with open-protocol controllers, often in the same programme as a BMS upgrade.
• Airflow sensor recalibration and test and balance.
• Recommissioning: setpoints, deadbands, schedules, and sequences to stop simultaneous heating and cooling.
• Chilled beam protection: dew point control and condensation sensors.
• Replacement floor by floor, timed with lease expiries and fit-outs, sometimes moving from fan coils to VAV or chilled beams.
• Induction unit replacement with asbestos removal and new perimeter systems.

Sample inspection and controls audits are lump sums. Repairs and motor replacements are per unit. Controls retrofits are per unit or per floor. Replacement is per unit or per floor including ceiling works, ductwork and pipework connections, controls and commissioning, and is usually timed with tenancy changes. Lease make-good obligations can carry part of the cost of units altered by tenants.

Replacement is lifecycle; repairs, recommissioning and drain maintenance are repairs and maintenance.

• Australia. AS 1668.2 sets ventilation and AS/NZS 3666 microbial control for air-handling systems including terminal units; NABERS Energy and Indoor Environment ratings reflect terminal unit and controls performance; lease make-good clauses commonly address tenant alterations to terminal units.
• United Kingdom. Approved Documents F and L apply; the British Council for Offices guide to specification sets market expectations for office conditions; BESA TR/19 covers hygiene; air-conditioning energy inspections apply to systems over 12 kW.
• United States. ASHRAE 55 sets thermal comfort, 62.1 ventilation and 90.1 energy, and Guideline 36 sets VAV and other terminal sequences; state energy codes such as California's Title 24 add requirements.

Asbestos rules in all three jurisdictions govern work on older induction units and fan coils with asbestos-containing linings or insulation.

• Maintenance logs and reactive call-out records, especially for leaks and comfort.
• Test and balance reports, original and after fit-outs.
• BMS point lists and trend data for zones and terminal units.
• Controller types, firmware and vendor support status.
• Asbestos register for older units.
• Fit-out records and lease make-good obligations.
• Comfort survey or complaint records.