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Why are the needs of HVAC solutions often oversized?

As an expert in software design for product selection and energy calculations, I believe it is common for solutions for heating, ventilation and cooling (HVAC) to be oversized. I would actually say that it is more common that they are oversized, than the other way around. This is because several premises have overestimated cooling and heating requirements in the initial stages of the design process. This is not only something that has an effect on the investment and possible operation, but also on sustainability. Complicated? Let me, Börje Lehrman, take us through from the beginning.


When HVAC solutions are designed, the focus is to make sure the projected indoor climate meet the set requirements in regards to the building, to its occupancy and to the expected level of activity inside. To overestimate the actual need can to some extent be beneficial as an excess capacity can provide a reserve in the case of changes in occupancy or use pattern. However, the likely consequences of designing an oversized solution is an uncomfortable indoor climate with issues related to draughts, uneven temperatures and/or disturbing noise. Moreover, an over dimensioned solution often turns out to be expensive, and it usually ends up with an increased installation cost, unnecessary use of floor space and/or an impacted ceiling height, the latter parameters can in turn affect the expected rent. Perhaps even more important these days, an oversized solution is probable to have negative effects on sustainability, both from an operational and embodied carbon point of view. Meaning, the energy requirement and the excess use of materials will harmfully affect the environment.

So, why are the needs oversized?


When the needs for heating, ventilation and cooling are calculated, it is common to make rough estimates. Traditionally used values for watts per square meter or airflow per person are for often used rather than to ensure the actual needs. In some cases, somewhat more time is spent on the needs and more precise requirement calculations are conducted. However, far too often time is perceived to be limited to thoroughly study how various internal loads actually contribute or affect the specified need. Generally, the calculations end up to use old and historically relevant values, and normally with good margins.

Internal effects

Computers, lighting and windows are examples of “things” related to the building that affect the need for heating, ventilation and/or cooling. There are values used today which have been used for the past 20 years – can any of those be relevant still?

I would say that we have had a fundamental change in technology during those years. Bulky monitors are completely gone and very few, if anyone, uses a desktop computer anymore. Moreover, computers in the modern world rarely operate at full power, and with a hybrid working model they might not be run in the office for more than a part of the day. Today, a laptop is not more than 80W, about half of the historic estimate that is still used quite often.

In terms of lighting, the traditional energy values are based on bulbs and fluorescent lights. These are no longer sold and they are perhaps not even in the fixtures today. Since quite a few years, energy-efficient lighting is the norm and sensors for presence control and daylight regulation is very common. Lighting nowadays rarely exceeds an energy generation of more than 3W/m2, measured during the dark hours in the darker season but still a third of the casually used value from the past.

Last but not least, windows. The better U-values of today’s windows result in a completely new energy scenario indoors. There is a significantly reduced heating demand during the colder periods of the year compared to when older windows were used. On the other hand, the more modern windows may cause heat to linger inside during the summer. The latter may be avoided by the use of effective sun shading, but inside and outside shading impact differently. None of this is taken into account in the older presumed values for calculation.


Presence is in this case described as the degree of occupancy, and in the design phase of an HVAC solution it is often assumed that all individuals assigned to the building are present all together all day long. Best case, it is decided to calculate with the commonly known figure of 70-80% occupancy rate.

However, a closer examination of a building’s actual presence rate reveals something entirely different. A conference room for instance, is used as little as 10-20% of the office hours, and it is rarely fully occupied. If it happens to be seated to the last chair, is it likely to be during the hottest day of the year? Probably not.

A further highly relevant aspect is that if all conference rooms are used simultaneously and they are fully occupied, then a large share of the other office space is likely to be empty. The common way of calculating the capacity of an indoor climate solution is to think that all indoor space is occupied. Frankly misleading.

A comparison of the past and the present

The difference in cooling and heating demands between yesterday’s estimated need and the present day’s actual requirement is significant. Found below is an example of an office landscape of 40 m2 with 3 windows in a southern direction and 4 people occupying the space.

Traditional estimation

Cooling 50 W/m2 (1.7 times more)

Heating 40 W/m2 (2.7 times more)

Ventilation 1.5 l/s

Today’s reality

Cooling 30 W/m2

Heating 15 W/m2

Ventilation 0.35 - 1.5 l/s

So, when the design of an HVAC solution leans on the old estimations of faulty assumed internal effects and an old-school view of occupancy, the end result ends up to be greatly oversized. It might seem expensive to spend time on careful dimensioning and to challenge the historically known values. But as I said in the beginning, a too large solution will imply a greater investment and a more expensive installation cost. Further, the continuous operation, the excess in materials used as well as the utilisation of floor area and ambient space can be costly if the hardware to the indoor climate solution is oversized.

At Swegon, we have developed a wide range of calculation software beginning either in a product perspective or taking off from an energy calculation angle which will help in the dimensioning phase of the project. Moreover, at Swegon we have many years of experience and we are happy to assist also in person. Visit our website to familiarise with our software and us as experts.