Renovate or Replace an Ageing HVAC System?

Short answer: decide on measured evidence, not on the age of the machines. Log what the plant delivers and what it draws, recheck the load the building has now, price the staged options between repairing and replacing everything, and compare lifecycle costs with the uncertainty written down. Replacement wins when refrigerant, condition, capacity and efficiency all point the same way; a retrofit wins when most of the loss sits in controls, distribution or maintenance that can be fixed for less.

An ageing system's running cost is the largest number in the decision and the least measured. Quotations for new plant usually compare a new machine's rated efficiency with an old machine's nameplate, which flatters replacement; a cheap overhaul quotation assumes the old plant can be restored, which flatters a retrofit. A measured baseline is the only way to tell which assumption is wrong.

1. Build a measured baseline and read the maintenance history

BEE's manual on energy-efficient retrofits of existing commercial buildings (2025) lists what to collect before choosing measures — chiller design specifications and daily log books, plant schematics and operating hours, AHU design data, at least a year of electricity bills and LT log books, and any metered energy data — followed by field measurement and validation. For a retrofit-or-replace decision, the baseline comes down to four numbers for each load band: cooling delivered, power drawn, the resulting plant kW per TR, and the hours a year spent at that load.

Cooling delivered by a chilled-water circuit is roughly TR = flow in m³/h × temperature difference in °C ÷ 3.02, where 1 TR is 3.517 kW of cooling. Compare old and new plant on the same boundary — the chiller alone, or the chiller with its pumps and tower fans — or the comparison means nothing. BEE's star-labelling schedule for chillers (Schedule 21, revised 20 September 2021) rates chillers on ISEER, a weighted figure built from efficiency at 100, 75, 50 and 25% load, with full-load COP as a prequalification. Chiller labelling is in BEE's voluntary regime, so ask whether a quoted machine carries a label, and ask for its part-load data either way: most plants spend most of their hours below peak.

2. Recheck capacity against the building as it is now

Buildings change under their HVAC. Floors are densified or emptied, server rooms and kitchens appear, shifts change. A like-for-like replacement repeats the original sizing decision, including any oversizing, and an oversized plant spends its life at part load. Before any capacity is quoted:

• compare the measured peak cooling delivered with the installed capacity;
• recalculate the load for the current layout and occupancy, including fresh air — NBC 2016 Part 8, Section 3 includes minimum ventilation rates in the breathing zone, and a denser floor needs more outdoor air;
• separate loads that run on a different schedule, such as an IT room running all night, which may be better served by their own system;
• keep the redundancy you actually need, and no more.

The AC cooling-load screening calculator and the chiller capacity calculator give first numbers to hold against a quoted capacity.

3. The options are a ladder, not a choice of two

1. Recommission. Correct set points and sequences, clean condensers and tubes, fix leaks and rebalance. Measure the effect before spending more.
2. Upgrade the system around the chiller. Variable-speed drives on pumps, tower fans and AHU fans, cooling-tower refurbishment and better controls (see chiller plant VFD retrofits).
3. Replace the worst machine. One new, efficient chiller sized to the base load, with the remaining old units kept for peak and standby.
4. Replace the plant. When refrigerant, condition, capacity and efficiency all point the same way — possibly with a different type of system (see VRF vs chiller vs ductable at North Indian ambients).

Refrigerant often decides the timing. The Ozone Cell's handbook on HCFC phase-out in buildings describes India's plan to phase out HCFCs by 2030, except for a small quantity for servicing existing equipment, and the end of HCFC-based air-conditioning manufacture from 1 January 2025. It also warns that converting a unit to another refrigerant usually changes its capacity and efficiency, should be done only under the original manufacturer's guidance, and should not be done to a normally working unit merely to phase out HCFCs. For some HCFC-123 centrifugal chillers, it puts the cost of a retrofit that restores capacity and efficiency as high as 80% of a new chiller.

Where ECSBC 2024 applies — its scope is commercial and office buildings with a connected load of 100 kW or more or a contract demand of 120 kVA or more, and whether it is mandatory depends on your state's notification — section 3.2.5 lets existing space-conditioning systems continue, but new equipment installed must meet the code's requirements for that equipment. Check its status in your state before specifying.

4. Lifecycle cost, with the uncertainty written down

The energy arithmetic is the same one the chiller retrofit savings calculator uses: annual kWh saved = the sum, over each load band, of TR × hours × (old kW/TR − new kW/TR), and net annual saving = kWh saved × tariff − any added maintenance cost. Simple payback means something only if the net saving is positive. For a larger investment, BEE's retrofit manual points to life-cycle cost, net present value or internal rate of return rather than payback alone — over the same horizon for every option, with major overhauls, refrigerant costs and residual value included.

A worked example with constructed figures: a plant spends 1,200 hours a year delivering 180 TR, 2,400 hours at 120 TR and 1,400 hours at 70 TR — 6,02,000 TR-hours in all. Measured plant efficiency is 1.10 kW/TR, and a replacement's data at the same loads and on the same boundary shows 0.80. The saving is 6,02,000 × 0.30 = 1,80,600 kWh a year, worth about ₹16.25 lakh at an illustrative ₹9 per kWh, with maintenance cost assumed unchanged. On a six-year simple payback, the replacement would need to cost under about ₹97.5 lakh installed.

Now the uncertainty. If the new plant achieves 0.90 kW/TR at these loads rather than 0.80, the saving falls by a third, to about ₹10.84 lakh a year, and the six-year break-even cost falls to about ₹65 lakh. In this example, a 0.1 kW/TR error in the baseline or in a vendor's part-load figure is worth about ₹32.5 lakh of justified capital. That is why the baseline comes first, why both plants must be compared on the same boundary, and why a proposal should show its savings as a range across tariff, run hours and efficiency rather than as one number. If your tariff bills in kVAh, value the saving at your measured power factor.

Under a turnkey EPC scope, staged HVAC works are planned together with the electrical and controls changes they depend on; the HVAC and mechanical service covers the plant itself, and maintenance contracts keep the records the next decision will need. More planning guides are on Insights, or you can share the existing layout, landlord provisions and renovation scope for a review.

Sources and how to verify

• BEE retrofit and energy-efficiency publications, Bureau of Energy Efficiency — including Energy Efficiency Solutions for Existing Buildings: Commercial (first printed February 2025) and ECSBC 2024 (scope, section 2; additions and alterations, section 3.2.5).
• Schedule 21 — Chillers, Bureau of Energy Efficiency (14 September 2018, Revision 2 of 20 September 2021), with the Standards and Labelling programme page listing chillers in the voluntary regime.
• HCFC Phase-out and Energy Efficiency in Buildings, Ozone Cell, Ministry of Environment, Forest and Climate Change (2017).
• Guide for Using NBC 2016, Bureau of Indian Standards — Part 8, Section 3 (Air Conditioning, Heating and Mechanical Ventilation).

Confirm locally: whether ECSBC 2024 has been notified in your state, the manufacturer's retrofit options and refrigerant availability for your machines, and whether your tariff bills in kWh or kVAh.