ACMV: Summer vs Winter

[EnOm]

Hi
I have come across ACMV systems that are designed for Summer time operation but seem to disregard Winter time operation. You have the typical setup. AC units, exhaust fans, and Make-up air fed into the AC units to compensate for the air that is thrown out. I was wondering what the scenario would be like when the AC is turned off come the Winter time. Now the AC and OA are OFF, but the exhaust is still ON. I see two issues with this

1- Is it not a problem that the Exhaust is ultimately pulling in cold air from the outside at a time when the occupants are trying to heat up the space? Or is this considered an equivalent penalty to the AC drawing in hot air in the Summer. Its not freezing here in the Winter (~16 degrees C/61 degrees F) on the coldest days of the year, and those temperatures around 6-8 weeks before the weather starts to warm up again. I'm thinking its not economic to spend money on systems that heat the air if its only cold for a maximum of 8 weeks a year.


2- Maybe cold air is less harmful when it comes to mold formation and similar issues since it carries much less water vapor than the hot air in the Summer, and thus is allowed into the occupancy during the colder months?

PS: I hope I'm not violating any forum policies with all these questions.

Thank you
Best Regards

 

[317069]

keep AC fan running in the winter (fan ON), raise up cooling set point to stop compressors from running, in this case you are bringing fresh air in, and it will get some heat from AC fan motors and duct

 

[Drazen]

your system and climate conditions call for small heat recovery units which will recover cold during summer as well, and which will not need additional heaters given coldest temperatures you mentioned.

 

[willard3]

What is an ACMV?

 

[berkshire]

Willard,
Air Conditioning & Mechanical Ventilation
B.E.

You are judged not by what you know, but by what you can do.

 

[EnOm]

317069: Thank you for the response. That is one way to do it. The budget is available on this one, so I can go with something better. The fresh air to the fans will be supplied close to the face of the fan's filter rack. In the winter when the AC is OFF, the AHU will remain ON and will supply tempered air to the ceiling space which will be slowly drawn into the room through the AC return grills. This air will be tempered somehow, either by a heat recovery wheel, or an electric heater. Any comments are greatly appreciated.

Drazen: Thank you for the response. That makes sense.

willard3: As berkshire said.

Best Regards

 

[willard3]

You will find that hot air rises, ie, if you introduce warm air to the ceiling space, that is where it will stay.

 

[KiwiMace]

At 16 degC, I probably wouldn't bother heating or energy recovering. A few degrees of delta t will help it not do what willard3 correctly said it would.

ACMV is an acronym replacing HVAC in countries that do not normally require heating.

 

[317069]

Enom
Can you describe your system a little more, first you said fresh air come through AC, so this tell AC mean RTU or split system, then you said AC fan off and AHU fan on, what is your system, is it rooftop unit or split system (air conditioner and furnace for example)
what did you mean by AC?

 

[EnOm]

willard3: Thank you for the response. Well I am thinking of introducing it at 75 F. Will it still stay afloat the colder air at this temperature? I'm thinking since the room would also be around 75 then there should be no problem.

KiwiMace: Thank you for the response.

317069: Ok. It is a large palace with a chiller and FCU's. There is an Air Handling Unit that is bringing in Fresh Air and distributing it to the FCUs.
The fresh air is brought in at 110 DB (design day) and cooled down to 75 DB.
There is also a central exhaust fan + ductowrk
What I meant by A.C is Air conditioning System. In this case, Chiller and fan coil units.

Best Regards