Overheating and Generator Sizing in a Kano Secondary School
A 600-student secondary school in Kano is undergoing post-occupancy evaluation. Classrooms are 8 m × 7 m × 3.2 m, with 50 students each, 150 mm sandcrete walls, an uninsulated metal roof, and single-glazed windows at 40% window-to-wall ratio on the long façades. The building is oriented with long axes east–west. During April, measured indoor air temperature peaks at 35°C at 15:00 while outdoor peaks at 39°C; CO₂ reaches 2,800 ppm by the fourth lesson. The client has budget approval to install split air-conditioners in all 18 classrooms and a 150 kVA diesel generator, but the school's electricity supply averages eight hours per day. The architect is asked to sign off the MEP design, which assumes 1.5 TR per classroom and a generator sized at 80% of connected load. The school has no maintenance contract for air-conditioning.
1. Assess whether air-conditioning is the appropriate response, explaining the thermal and ventilation causes.
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Air-conditioning is treating the symptom, not the cause. The 1.5 TR unit provides about 5.3 kW of cooling, but 50 students alone produce roughly 3.5–4 kW of sensible heat, before solar gain through 40% glazing and heat gain through the uninsulated metal roof. The measured 35°C indoor peak against 39°C outdoor shows the building is already moderating temperature, but the roof and windows are driving afternoon gains. The CO₂ level of 2,800 ppm is the more serious indicator: it shows inadequate fresh-air ventilation, not just heat. Closing the windows and adding air-conditioners would make the CO₂ problem worse unless a dedicated fresh-air system is installed. The school also has power for only eight hours a day and no maintenance contract, so air-conditioning would be unreliable and expensive. Passive measures and ventilation should come first, with air-conditioning only for a limited number of critical rooms if justified.
2. Recommend a prioritized passive and low-energy package within the budget, with expected effects.
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Prioritize measures that reduce heat gain and improve air movement. First, treat the roof: add insulation or a ventilated double roof and a reflective finish, because the uninsulated metal roof is the dominant afternoon heat source. Second, shade the windows on the long north and south façades with deep overhangs, fins, or external blinds; internal blinds are much less effective. Third, improve ventilation: high-level operable vents, cross-ventilation, and ceiling fans to raise air velocity; consider night purging to cool the structure. Fourth, reduce internal gains where possible, for example by staggering occupancy or improving lighting efficiency. Fifth, commission and monitor the building, and train staff to operate vents and fans correctly. These measures should reduce peak indoor temperature by several degrees and bring CO₂ down substantially, at far lower capital and running cost than 18 split units and a generator. Retain a small number of air-conditioned rooms only if the client insists.
3. If the client insists on air-conditioning, how should the architect size the system and generator, and what professional risks arise?
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The architect should require a proper room-by-room load calculation to ASHRAE or CIBSE methods, including solar gain, occupancy, equipment, and fresh-air load. The 1.5 TR rule of thumb is not a design calculation and is likely to be inadequate for 50 students in an uninsulated room; undersized units will run continuously and fail to control temperature or humidity. The design must include mechanical fresh air or adequate passive vents, otherwise CO₂ will remain high. The generator should be sized from a load schedule with diversity, starting currents, power factor, and voltage drop, not simply 80% of connected load; 150 kVA may be adequate or marginal depending on the final air-conditioning selection. The architect should not sign off an MEP design she cannot verify. Professional risks include liability for overheating, poor indoor air quality, generator noise and fuel storage, maintenance failure, and approving a design outside her competence without specialist input.