Why Does the Office Always Feel Too Cold?

If you’ve ever worked in an office, you’ve probably had this question:

“Why is it so cold in here?”

It seems like every office has one person wearing a sweatshirt in the middle of July. Meanwhile, someone across the room is complaining that it’s too warm and another has a space heater running.

As a mechanical engineer, I see this often. And honestly, getting an office to feel comfortable for everyone is harder than it sounds.

The first issue is that the thermostat doesn't tell the whole story.

Most offices have a thermostat controlling a zone, but that zone can cover a large area. The thermostat might be reading 72°F, but that doesn't mean every desk in the room feels like 72°F. Someone sitting directly underneath a supply diffuser likely has cool air blowing over them all day. Someone sitting next to a large west-facing window might get a lot of sun, but only in the afternoon. They're in the same room, but they're experiencing the space very differently.

This is where air distribution becomes important.

When we're designing an airflow-based HVAC system, we're not just figuring out how much cooling a building needs. We also have to figure out where that cooling is needed, and when. Simply dropping a rush of cold air into a room isn't necessarily going to make the room more comfortable.

Then there is ventilation.

Offices need to bring in a certain amount of outside air to maintain indoor air quality. That outside air needs to be conditioned before it reaches the occupants. In the summer, that can mean taking hot, humid outside air and cooling and dehumidifying it before bringing it into the space.

The interesting part is that most of the time the amount of air we need to bring into a room for ventilation is more than the room

needs for cooling.

That's where reheat can come into play.

A VAV system, for example, may need to provide enough airflow to satisfy the ventilation requirement, even after the room's cooling load has been met. If we continued supplying cold air at that minimum airflow, the room could become too cold.

Instead, the system can reduce the airflow to its minimum ventilation rate and then use a reheat coil to warm that air back up before it enters the room.

 It sounds a little backwards: cool the air, then heat it back up.

So why not just raise the supply air temperature and avoid the reheat?

One reason is dehumidification. In the summer, the air may need to be cooled below the room temperature to remove moisture from the air. Simply raising the supply air temperature could reduce the system's ability to control humidity.

Another reason is that offices often have different zones with very different loads. An exterior zone next to a window might need plenty of cooling in the afternoon because of the sun, while an interior zone with no exterior exposure may have very little cooling load. Both zones can still be served by the same central system.

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The result is that one area may need cold air while another doesn't. Reheat allows the system to provide the airflow needed for ventilation and humidity control without making the interior spaces unnecessarily cold.

So, while cooling the air and then heating it back up may sound inefficient, there is a reason behind it. It's not always about adding more heat to the building. It's about controlling humidity, meeting ventilation requirements, and keeping different spaces comfortable when they have different cooling needs.

There are also a lot of other things going on in an office.

People generate heat. Computers generate heat. Lights generate heat. Even that giant monitor sitting on someone's desk adds a little bit to the cooling load.

A conference room with 15 people in it is going to behave very differently from that same room sitting empty. This is also why some spaces can feel perfectly comfortable in the morning and start feeling warm by the afternoon.

Then you have the human factor.

Some people are comfortable at 68°F. Others think 75°F is freezing. Clothing, activity level, and personal preference all make a difference.

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There are also standards for what is considered comfortable. ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, provides guidance for evaluating thermal comfort in occupied spaces. It considers factors such as air temperature, humidity, air speed, clothing, and activity level when determining whether a space is likely to be comfortable for most occupants.

In other words, there is more to comfort than simply picking a temperature and putting it on the thermostat. HVAC design uses these standards to help establish reasonable comfort conditions, but there will always be some people who are too hot and others who are too cold.

So, what's the perfect office temperature? Unfortunately, there isn't one.

Good HVAC design is about finding a balance between temperature, humidity, airflow, ventilation, and how the space is being used. Sometimes the solution is as simple as adjusting the thermostat. Other times, the real problem is air distribution, balancing, controls, ventilation requirements, or even the need for reheat.

Understanding those factors is what allows engineers to look beyond that thermostat and identify what is really driving comfort issues in a space, and more importantly, how to address them.

Written by:

Alex Wood

Mechanical Project Leader