How many BMS points does an AHU need?

A typical supply and extract air handling unit, controlled directly by the BMS, lands between 20 and 30 points. A packaged unit with its own controls needs a handful. The gap between those two answers is worth thousands of pounds per unit, so before counting anything, settle which kind of AHU the tender is actually asking for.

The hardwired shape

Take an ordinary supply and extract AHU with an LTHW heating coil and inverter-driven fans. The BMS controls it directly, so every sensor, switch and actuator wires back to a controller. A defensible starting count looks like this:

• Supply fan: enable (DO), run status (DI), trip or fault (DI), speed (AO).
• Extract fan: the same four again.
• Filters: a differential pressure switch on each side (2 DI).
• Temperatures: supply air, extract air and outside air sensors (3 AI).
• Frost protection: a frost stat (DI) and often a coil return temperature (AI).
• Heating coil: valve actuator (AO).
• Dampers: motorised isolation dampers, an open command and an end switch each (2 DO, 2 DI).

That is 24 points before anything unusual appears. It is why an experienced estimator, asked to price an AHU blind, will say "call it 25" and be roughly right.

What moves the number

Heat recovery adds points: a thermal wheel brings a speed signal, a status and a fault; a run-around coil brings a pump and a valve. A cooling coil adds another valve. Duct pressure control adds pressure sensors. Humidifiers, where they still appear, add several points and several arguments. Smoke and fire interfaces add inputs that must be there and must be shown to be there. A large hospital AHU can pass 40 points without any of them being padding.

The packaged fork

The other answer is much smaller. A packaged AHU arrives with the manufacturer's controller fitted and commissioned, and the BMS talks to it over BACnet or Modbus rather than wiring to it. The hardwired count collapses to an enable and perhaps a common fault, and the rest becomes a single high-level integration carrying dozens of software points: temperatures, speeds, alarms and setpoints read over the network.

The two routes need different hardware, different labour and different commissioning, and they produce very different prices. Pricing the hardwired shape for a packaged unit means quoting for IO modules and cabling nobody will install. Pricing the packaged shape for a hardwired unit means winning a job with the controls missing from the number.

The question to settle first

Here is the catch: tenders regularly say both. The specification promises a packaged unit with integrated controls, and the schematic shows a full set of hardwired IO. Neither document is wrong; they were drawn at different stages by different people. The estimator has to notice the disagreement and choose a route, and the tender pack will not announce that the disagreement exists.

This is exactly the kind of conflict Tenderfly surfaces rather than resolves silently. When the documents disagree about an AHU's control route, it puts the choice in front of the estimator with both routes priced, and cites the page each version came from. The counting is automatic; the decision stays with the person who carries the risk.