Specifying a Replacement AHU: What to Include in Your Tender

A vague specification produces quotes you can’t compare. One supplier prices EC fans and a proper controls package, another prices the cheapest unit that meets the airflow, and on paper the second looks better value until it’s installed.

Here’s what a specification needs to cover if you want comparable prices and a unit that works.

Duties and performance

Start with what the unit has to do:

• Supply and extract air volumes in m³/s
• External static pressure
• On and off coil conditions for heating and cooling
• Required supply air temperature and any humidity requirements
• Filtration grades on supply and extract
• Sound limits, both breakout in the plant room and in the ducted path. See AHU noise reduction techniques

Be honest about whether these are the original design figures or what the building needs now. Buildings change. We regularly replace units that were correctly sized for a layout that hasn’t existed for fifteen years.

Coils and heat sources

State what’s available on site: LTHW, chilled water, DX, electric, gas. Include flow and return temperatures, because coil selections stand or fall on them. On a poultry facility we supplied units with LTHW, chilled water and reheat coils in the same casing, which is straightforward if it’s specified up front and awkward if it appears later. Details are on the poultry farm project page.

If heat pumps are part of the plan, flow temperatures will be lower and coils have to be selected accordingly.

Heat recovery

Say what you want, and why. Thermal wheel, plate exchanger and run-around coil all behave differently on cross-contamination, efficiency and space. Our guide to HVAC energy recovery systems compares them.

Fans and motors

Specify EC plug fans unless there’s a reason not to, and state whether duty and standby arrangements are required. See EC vs AC fan motors.

Casing construction

This is where cheap units are cheap:

• Panel construction, insulation type and thickness
• Thermal transmittance and thermal bridging classification
• Casing leakage class
• Internal and external finishes, and whether stainless is needed for wet sections
• Drain pan material and drainage arrangement
• Access door positions and which side they open

Specify access properly. A unit that meets every duty but can’t have its filters changed without dismantling something is a problem for the next ten years.

Controls and integration

State whether controls are supplied with the unit or by others, what protocol they need to speak to the BMS, and what points are required. Our guide to AHU controls and building management systems covers the common arrangements. Frost protection should be specified too, as covered in AHU frost protection.

Site constraints and delivery

This is the section most often left out, and it changes the price more than anything else:

• Plant room dimensions, door and hatch sizes, lift limitations
• Craneage, roof access, out of hours restrictions
• Whether the unit needs to arrive in sections and be built in place. See flat-pack AHUs
• Whether the building stays live during the works, as at the B&Q distribution units and the work we’ve done in live airport facilities

Commissioning, documentation and support

Say who commissions, what records you expect, and what happens afterwards:

• Commissioning and witnessing requirements. See AHU commissioning
• O&M documentation and as-built drawings
• Warranty terms
• Spares availability and lead times, covered in AHU spare parts sourcing
• Whether a maintenance contract is required from handover

Comparing the returns

Once quotes are in, compare like for like on casing class, fan type, coil selections and controls scope before comparing price. Our guide to AHU lifecycle costs is a useful frame, and choosing AHU suppliers covers what to look for beyond the numbers.

How we could help

We manufacture standard and bespoke air handling units, and we’re happy to help develop a specification before it goes out. Visit our air handling units page or get in touch.

Frequently asked questions