Advice · Commercial
Car park ventilation: fans that run on demand, not all day
Most enclosed car park fans run flat out all day for a peak that happens twice. A variable speed drive with carbon monoxide monitoring runs them to the air instead, and the difference in the running cost is not subtle.
The problem with a fan that only knows on and off
An enclosed or basement car park has to be ventilated, because petrol engines produce carbon monoxide and a poorly ventilated car park will accumulate it. Nobody argues with the requirement.
What does bear arguing with is the way most of them are run. A fan wired straight to the supply has two states, on and off, and it is generally left on. So it runs all day at a speed chosen for the busiest few minutes of the busiest day of the year, when for most of the day the car park is nearly empty and the air is fine.
That is a large motor running continuously to solve a problem that is not currently happening.
What a variable speed drive changes
A variable speed drive sits between the supply and the motor and controls how fast the motor turns, instead of just switching it on.
Pair it with carbon monoxide sensors in the car park and the ventilation becomes demand driven. When the air is clean, the fans idle or drop back to a minimum. When a level fills with cars and the carbon monoxide rises, they speed up to suit, and come back down when it clears. The control has to fail the safe way: if a sensor dies or its signal is lost, the fans go to full speed and stay there, and the sensors need calibrating on a schedule. That is part of the design, not an optional extra, and it is the first thing to ask about any quote.
The energy saving is out of all proportion to the speed reduction, and the reason is arithmetic, not salesmanship. Fan power varies with roughly the cube of the speed, so a fan running at half speed uses in the region of an eighth of the power. Modest reductions in speed produce large reductions in consumption, which is why this is one of the few efficiency measures with a genuinely short payback in a building of any size.
The benefits that are not on the power bill
- Less noise. A fan at part speed is dramatically quieter, which matters when the car park sits under apartments
- Longer plant life. Soft starting and running below full speed reduces mechanical stress on the motor, the bearings and the drive, so the plant lasts longer between replacements
- Gentler starting. Switching a large motor straight on draws a heavy inrush; a drive ramps it up instead, which is easier on the motor and the switchboard. Any saving on a demand tariff comes from running slower for most of the day, not from the start itself, because demand is measured as an average over fifteen or thirty minutes
- Actual monitoring. Sensors that control the fans also tell you what the air in your car park is doing, which is a safety record nobody had before
Who this is for
Body corporates and building managers with enclosed or basement parking, mostly, and resorts and commercial buildings with the same. If your car park has mechanical ventilation running whenever the building is occupied, have it assessed.
The retrofit is generally straightforward. In most cases the existing fans and ductwork stay, and it is the control and the switchboard work that changes, plus the sensors and their cabling.
Before you spend anything, measure
The honest first step is not a quote for a drive, it is finding out what the ventilation currently costs you to run. Power logging on the ventilation circuit for a week or two gives a real consumption figure and a real usage profile, and that is what turns the payback calculation into something you can check.
BH Electrics does the power logging, the switchboard and the control work, and deals with body corporates and building managers on exactly this kind of assessment. We are happy to look at a car park before anybody commits to anything.
Rewritten from an unpublished Word draft, “VSDs for safer Car Parks”. Never published on the old site.
The work behind this
What we do about it
Straight answers
Questions people ask about this
What is a variable speed drive used for in a car park?
It controls the speed of the ventilation fans instead of simply switching them on and off. Combined with carbon monoxide sensors, it lets the fans idle when the air is clean and speed up when cars raise the carbon monoxide level, so the ventilation matches actual demand instead of running flat out all day.
How much energy does a variable speed drive save on ventilation fans?
More than the speed reduction suggests, because fan power varies with roughly the cube of speed. A fan running at half speed draws in the region of an eighth of the power. That is why demand-driven ventilation has a short payback in most buildings, and why the first step is to log the existing consumption so the saving can be calculated instead of estimated.
Do carbon monoxide sensors have to be installed in an enclosed car park?
Ventilation requirements for enclosed car parks come from the building code and the relevant Australian Standard, and what applies to a particular building depends on its classification, size and design. A carbon monoxide monitoring system is the usual way to run ventilation on demand while still meeting them. Have the building assessed. A general answer is no use to a specific car park.
Can variable speed drives be retrofitted to existing car park fans?
Usually, yes. In most cases the existing fans and ductwork stay where they are, and the work is in the switchboard and the control system, plus the carbon monoxide sensors and their cabling. That is what makes it an attractive retrofit for a body corporate: the largest and most disruptive components are already there.
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