But there is a lot to think about when replacing an existing unit, or doing a design/build job.
Is quick and easy access important? Maybe we should look at a unit with hinged and toolless access and a slide out blower.
Is comfort critical? Maybe we should look at more cooling stages and adding a VFD.
Is this a mission critical job where no downtime is important? Maybe we should add Fault Detection & diagnostics to give us early failure warning sign alerts.
Do we need to keep utilities on budget? We might want to look at a high efficiency unit, or adding a VFD.
Is ease of scheduling important? Then we might want to look at a control solution, which could require a special MODBUS, Bacnet or N2 board.
What are the existing controls? Maybe there is a proprietary control system that won’t communicate with a standard rooftop board.
Do we need cooling year round? There are lots of ways to do this- economizer, condenser fan cycle control, condenser fan speed control based on liquid pressure, condenser fan speed control based on liquid temperature. Which method is the best for the application? Maybe it’s a combination?
What city is the job in and what are the local energy and mechanical codes? Does code require an economizer? Should that economizer be dry bulb, enthalpy, or dual enthalpy?
How many stages of cooling does code require?
Does code require CO2 sensors or a smoke detector? Maybe two smoke detectors depending on the unit size?
If we have to meet ASHRAE 90.1-2010 we will need to add a VFD if the unit is 10 tons or larger.
Is there a humidity requirement? Might need to consider hot gas reheat. Or we might need to design to a 45 or 50 degF discharge temperature (grab your psychrometric chart first!).
Is it important to have replacement parts available quickly? Might need to make sure we select a unit supported by a distributor with a local stock of parts.
Is building pressure control important? Should we look at on/off power exhaust? Or modulating exhaust with a VFD and building static controller?
And…the list goes on.
Either way, this LONG list is the reason we ask so many questions when we get asked to price a rooftop unit.
We want to make sure we get the right unit on the job the first time.
What questions did we forget to address?
Brad has been in the HVAC industry and at cfm Distributors since 2006, and is actively involved in HARDI as well as the local Kansas City chapter of ASHRAE. Brad has been a semi-professional blogger since late 2015 and is a huge fan of Apple’s new 3D Touch technology.
Latest posts by Brad Telker (see all)
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