Dim Question About Air Con (Office)

A stupid question - apologies!

Does the typical air con systems that you get in offices bring in air from outside or does it just cool and re-circulate existing air?

We’re trying to think about COVID ventilation for our offices.

It depends on what you already have or intend to buy
Fresh air exchange will normally be included in whole building integral systems & include combined heat/cool/fresh air climate control.
Add on systems that bolt on the wall after the building is built (with individual outside condensers) can be specified with fresh air exchange, you need to look at the spec.
Mobile wheel around units are not fresh air exchange.

Typical aircon units simply cool the air, transferring the heat, not air, to a box outside to dissipate it. Think of it as the office being the inside of a fridge. That is the same in a variety of countries I have visited, and is distinct from an integrated ventilation system that may incorporate cooling (and/or heating) capability, and are much more substantial constructions with ducting around the building.

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or does it just cool and re-circulate existing air?


What @Innocent_Bystander said is correct. A heat exchange air conditioner doesn’t go to all that effort to cool (or heat) air only to pump it all outside.

Not at all stupid, this question is already affecting many organisations world wide. The short and slightly unhelpful answer is, “It depends on the type of environmental control systems which are installed in your work place.”

By definition, real ‘Air Conditioning’ should include, at least some fresh air, some ‘comfort cooling’ systems (not strictly full air con) are 100% fresh air, humidity control (tends to be expensive and require even more maintenance), rudimentary air filtering and of course cooling.

In the UK it is quite common for office managers to call ‘comfort cooling’ air conditioning, it isn’t. Alright, yes I am being a bit pedantic.

In the UK I would expect most competent office ‘comfort cooling’ systems to include a percentage of fresh air, if this is introduced through opening windows, then you have to be careful not to try to cool the world!

The issue of ventilation/air con and Covid 19 is a hot topic in the world of building services design companies and their associated institutions. There is a lot of talk about treating air with UV light, filtering and increasing the percentage of fresh air rates plus more robust maintenance routines. Including this in new or introducing these into existing buildings is likely to be expensive, time consuming and disruptive.

It has been a long held belief within the industry that in town centres especially, properly considered, designed and operated mechanically ventilated spaces with energy recovery can be efficient and good for staff. So called fresh air is not always as fresh as people would like to think.

If you want or need to address your concerns about how your existing office ‘air con’ deals with Covid, if at all, you should seek professional help. Given that most companies lease their buildings that would mean tackling your Landlord as well.

Many papers are emerging within the industry and company policies, such as ours have and still are changing in acknowledgement of this challenge.


Thanks for all the input. I will, of course, check the spec for what we have to be certain, but I’m pretty sure it’s a simple cooling system that simply transfers the heat to the outside.

We’re pondering a return to the office and thinking about how we might provide additional ventilation (as Roog discusses above). We have three floors - two for desks and one for meetings. There are only 18 of us in total, and we’re unlikely to be all in at once - perhaps 12-15 people. One of the ‘desk floors’ has some air con, and whilst it’s fairly recent, I doubt it’s a Rolls Royce version. For all floors we’re probably going to be relying on opening the windows for extra ventilation. That’ll work okay for now, but I’m not sure how that will happen when we’re deep into winter.

This a layout of a project I was working on yesterday. The small offices rely on windows for ventilation and VRF system for cooling. The large open plan offices have VRF for cooling and a heat recovery unit providing supply and extract ventilation.

The VRF system is Mitsubishi and the HRU is a Lossnay unit, controlled by the VRF system. If you have an office with just VRF, it’s quite simple to install a HRU and simply switch on and off when office occupied.

Can you see any supply and extract grills on the ceilings?

In the UK education system there is now a focus on using CO2 monitors to ensure adequate ventilation (although not really funded fully…but thats a political area I’m staying away from here) and fresh air

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For quite a few years now, (before Covid) new schools have been built with Co2 sensors in the classrooms. These are very often specified and not very expensive.

They are sometimes connected to the ventilation system, (making their use expensive), or sometimes simply a warning to the teacher to open the windows

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Mrs Roog works in the local Primary school and they worked with all of the windows open last Winter, they got cold. :0(

A typical large office block will comprise of the following
AHU probably located on the roof which supplies conditioned fresh air into the ceiling void, this is typically at 10 l/s per person, although this can be up to 90% recirc
In the office above the ceiling will be Fan Coil Units these are 80-90% recirc but has the fresh air from the AHU feeding them, typically these will deliver around 300-400 l/s into a common area through the ceiling grilles, these units have local temperature sensors and have chilled water and heating coils
There will be a common extract, a bell mouth within the ceiling near the riser, this will pull air from the dummy return air grilles or through the light fittings
The cassette type units on the wall are purely recirc

For the record I’m a building services commissioning manager, it’s my responsibility to deliver all the MEP systems in a building fully functioning, our MEP contracts are typically about £25M


I’ll have to check next time I’m in - but not sure when that’ll be.

On a slightly larger scale than our £10-£15k ‘air con’ units!

My wife does too - the guidance was use thermals and gloves if needed!!

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All very well as long as teachers can still write on the board / operate keyboard etc, and students can take notes / operate keyboard etc. A bit difficult with thick gloves, or if fingers are numb!


Mitsubishi DX or VRF systems are the most common in the UK in commercial buildings. If ceiling void HRU’s are required (usually in small buildings). Mitsubishi Lossnay units are usually used. Looking at link below it suggests cool air in the extract is recovered.


For stand alone HRU’s, Nuaire are usually specified. Smaller and less expensive.

For classroom ventilation in a new buildings. Gilberts or Breathing Building systems are usually specified.

DX and VRF are uncommon in commercial buildings for the main climate control, this is achieved by fan coil units with piped LTHW & CHW

The DX / VRF / CRAC units are generally used in small comms rooms, equipment rooms ie LV/HV switch rooms and a few areas to provide comfort cooling where running pipework would be impractical


Regarding DX & VRF
The biggest issue for me with the commissioning and testing is the pressure test,
Ok it’s done with NFO but it’s still in excess of 30 Bar and for 12 hours so the area under test has to be an exclusion zone, we would normally test from 6pm



As you’re dealing with commissioning, that’s going on just before handover (hopefully :grinning:).

I’m involved at the design and tender stage, so 1 or 2 years ahead of you. I’m looking at a couple of mechanical specifications a week, VRF systems far out number LTHW/CHW FCU’s.

Gas boiler generated LTHW seems to be a thing of the past, it’s mostly ASHP for heating/DHW in smaller building and VRF in larger buildings. I’d say approx 80% of building don’t even have any gas on site.

However, somebody decided a few years ago to build a number of massive energy centres containing massive gas fire boilers around London to provide LTHW to the nearby developments. So they’ll still be using LTHW/CHW FCU’s. :scream_cat:

PS. I actually meant Mitsubishi where the most common VRF manufacturer. As opposed to Daiken or Toshiba.

I’ve seen a few projects recently were Mitsubishi Hybid systems were specified. This is a system were there is no refrigerant in the occupied areas. (office or hotel bedroom).
The refrigerant is pumped to a heat exchanger which has water on the secondary side which is then pumped to the occupied areas.

Would this overcome the problem?

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