VAV Air Handler Unit with Central Heating and no Terminal Reheat

[nuuvox000]

I am currently trying to provide a heating/cooling option to a client that is a little better and not much more expensive than a whole bunch of packaged dx/gas RTU's. My thoughts were to provide an air handler with VAV's and not terminal reheat. All of the heating and cooling would be done at the air handler and the VAVs would just modulate their dampers. I realize that this will not allow for cooling and heating simultaneously to the spaces but are there any major issues I am missing here? I have not ever seen this design implemented in a project and I'm a bit worried that there is a reason for that. Thank you!

 

[BronYrAur]

You would have to reset the temperature at the air handler, so the entire system will be in either heating or cooling. Something would have to decide whether you are in heating or cooling. You may overheat certain spaces in heating mode even with boxes at minimum positions. Plus, some spaces may require cooling all the time (e.g. data rooms).

You are essentially describing a VVT system, except they usually consist of a constant volume packaged rooftop unit. Zone control consists of dampers that operate from temperature sensors. Zones vote whether they want heating or cooling, and the mob rules! This inevitably leaves some zones uncomfortable.

 

[nuuvox000]

Thanks for your input BronYrAur, that's very helpful. I was going to let the maintenance team decide whether it's cooling or heating mode but zone voting would actually work better so thanks for that. We'll have one data room in this building and I'll put a split system in there as well as the conference room.

Due to budget constraints, I think it's either this system or packaged units that serves three to four large classrooms each; what do you think would be the better option? Either scenario would leave some rooms uncomfortable but I'm thinking that the VAV system I'm proposing would do a better job overall. Maybe we could set the cooling supply air temperature a bit higher and the heating supply air temperature a bit lower in the spring/fall so that the minimum airflow to already-satisfied rooms does not affect them as much.

I became aware of this job two days ago so I'm still brainstorming.

 

[GT-EGR]

You will need to have perimeter heating if you want to do it this way with one unit serving a whole floor that includes interior and exterior zones. So in summer they all get cold air, in winter they all get cold air, but perimeter spaces have their VAV boxes at a minimum, and the perimeter heating handles the envelope load plus the small amount of unwanted cooling from the ventilation.

Just imagine the dead of winter - you’ll have every perimeter space needing 90 degree air, and ever interior space will still just need cooling at 55. Whichever one you pick, you’ll have a lot of people upset

The way your setup you described works is if every room you have served by the air handler is either all interior or all exterior spaces.

 

[willard3]

Where will this system be installed?
Do you need perimeter heat? This will certainly complicate your control strategy.

 

[nuuvox000]

Thanks GT-EGR, I'm starting to understand why I don't see these systems. However, almost every room is an exterior space (see image). I think we'll just need a split for the AV closet, an RTU for the kitchen and an RTU for the conference room area but other than that, it may work out quite well. I'll show the supply diffusers on the exterior walls for the main rooms.

willard3, this will be installed in northern Utah.

 

[WilliamTz]

Hello

You central AHU with VAV's is an excellent option but definitely have terminal reheat for final trim in the VAV... either electric or hot water.

Regards

 

[SAK9]

Since most part of the building will require heating in winter,a central VAV AHU with terminal heat does not seem to be appropriate solution. Separate systems for perimeter and internal zones seems to be the way to go.

 

[ACD1]

The option that comes to my mind - Packaged units with terminal reheat, but grouping some of the classrooms with the same exterior exposure directions. If there's a kitchen, you'll probably need to keep that separate - exhaust requirements, varied exhaust? So maybe 3 or 4 rooftop units. I would then provide supply slots at the windows and a single 2'x2' supply diffuser in the rooms. Have about half the required room air going to each slot.

 

[ProcessHVAC]

This can be done on relatively small office areas. As others have said, no issues in the Summer but you will have poor individual zone temperature control during winter. Add some reheat.

 

[nuuvox000]

We decided to go the safe route with packaged units. Thanks everyone for your responses.