Is my HVAC Guy correct? 2 systems instead of 1 system with Dampers

I'll soon be working on converting a second floor of a residential building that was never finished and is being used as an attic for the last 80+ years. Building Size 36'x36', 9 1/2' 1st floor ceiling height, 8' planned ceiling height in 2nd floor with plenty of insulation and air barrier planned.  This is also a DayCare Facility.

I will be adding an open stairway to the floor plan to provide access to the 2nd floor. (Clients request for capitalizing on the natural daylighting.)

The HVAC guy informed me that: having 2 separate HVAC systems is the route he would use because-it was how he learned and is the norm in St Louis:

  • Existing HVAC system for  1st floor is undersized 1 1/2 Ton compressor should be 3 Ton unit. (located in the Basement)
  • 1 system with dampers for climate control could  lead to future mechanical problems if one big system was used.  (I don't believe he had faith in a damper system to work over long periods of time.)
  • Controlling Costs and Not disrupting existing downstairs.  We would have to build a chase for the hvac ducts from the basement to the 2nd floor. (closet space is available to do this)

The RED warning light that is going off in my head and is contrary to all I've learned is that: "What will keep one system from robbing the climatized air from the other system?"  The open stairway will allow air movement up and down.

I agree with him partially on 2 systems if there was not an open stairway planned.

Since this is in design stages to capture the natural lighting I've mentioned a French door with a wall would allow lighting and separate the floors.  Which makes a dual system more appealing to me.

Thanks in advance for any advice.

Build Green,


The following CAD drawings were made using Sketchup and what I've found is an easy way to convert plans into picture files for showing my clients what the finished product will look like.  Yes there will be rails on the stairs, omitted so that it doesn't clutter up the drawing.

Instead of this railing set up a wall with a French door would allow the natural light into the room.  And also alleviate my fears of a child climbing the rails and falling to the 1st floor.  If this design is chosen I plan to build the railing 48" tall.

Tags: Air, Energy, HVAC, Movement, Split, Systems

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I think your HVAC guy is right on this one.

1) that is allot of space being added on. more than 1200 feet

2) Controls for mechanical can be split between the two floors this is a huge advantage over one thermostat in such a large space.

3) Supply and return are on the same floor. 

4) a proper multistage multipoint distribution system is complicated and I would think more expensive at the end of the day. It also has a large amount that can go wrong down the road adding to its cost. I am not sure you are saving money up front or in the long run for such a system

I don't think two system will be competing but rather coexisting if designed and installed properly

Thank you Mr G Gallo for your reply and explanation.

So far I've had 2 people inform me that 2 systems would be ok.  One here and one on Twitter.  I understand but stick by what I wrote.  I'm looking forward to more comments on the issue.  I've never claimed to know everything but I do know where to go to get good advice.  Thanks again and please comment with your suggestions.


2 systems is certainly the way to go for 2 floors. 1.5tons should be plenty for downstairs unless there are some unusual internal loads. The upstairs AC will do most of the work anyway.

Thank you for your reply.  And after a few people have chimed in with their opinion-2 systems here and outside of this forum.  I'll be proposing 2 systems for this project.


Its not the number of systems, it is how it is done & sized

I see plenty of 3 floor buildings running off 1 unit in the 2 to 3 ton range (based on what is needed) & they work just fine. I also see plenty of places with 2 systems that also work just fine. Of course on the flip side I am sure we all have seen both types that don't work well at all and quite simply that is because the needed processes were not done correctly

Being a daycare though, I would opt for two just in case a unit went down - that wont close down the business if the one & only unit did go out say on a hot summer day

Thank you for your reply.  I'll post the final construction drawings and 'action' photos of the project soon.  As well as some "What were they thinking"  pics of the existing hvac system.


Not knowing your climate zone, here are some thoughts.

2500 ft2 of well insulated and sealed building should only need one system.  In our 5000-5500 DD, we go to about 4000+ feet (basements not included) with one system.

The trick is in the distribution.  Remember that we want more cool air to the second floor and more warm air to the first, so this means that controls are very important.  The easy and expensive way out is to put in motorized dampers to do the thinking for you, but these don't usually make people happy.

The less expensive way is to have, say, four separate trunks off the plenum - first floor left and right, and second floor left and right - each with it's own volume damper.  A little fussing in heating season gets it balanced, then you mark that location with a W.  Then a little fussing in coolings season with an S mark.  Now at the change of season, move the dampers.

And return should bea non-issue - one giant return (110% of calculated size) on the first floor and transfer grills out of each room and it works a treat.  I would be sure the thermostat has a "circulate" setting so that on days when the system is not running, the temps are still even.

All this said, there is merit to the 2 systems because it is a day care and if one goes down you're not stuck.  If you do this, NOTHING IN THE ATTIC.

Mr E Minch,

Thank you for your thoughts and ideas-I agree with the whole building approach-with separate trunks and nothing in the attic.  I also like the idea of manual dampers and I will run this option past the Building Owner.


So, 2400 sf square box.  

Existing HVAC system for  1st floor is undersized 1 1/2 Ton compressor should be 3 Ton unit

According to who?  Load calc? 

1 system with dampers for climate control could  lead to future mechanical problems if one big system was used. 

In other words, 2 systems is less complicated and more profitable for ME.  (Are you paying someone for design - or expecting that for free?)

Controls for mechanical can be split between the two floors this is a huge advantage over one thermostat in such a large space.

Why?  Crappy enclosure? Fix building when renovating or pay LOTS to do it later. 

2 systems is certainly the way to go for 2 floors. 

Why?  Crappy enclosure? 

 I would opt for two just in case a unit went down

Why, planning on crappy install?  Probably should opt for three, just to be safe.  Properly designed and installed HVAC systems fail on such a regular basis with catastrophic consequences, loss of life an limb and such.  

If you probably have extra automobiles for when your primary breaks down, definitely get three.  

A lot more information is required for sound advice here. Feels to me like you don't have a designer who is both building scientist and an advocate for YOUR interests.  Hire David Butler.

A lot of AC guys are scared of zoning because they don't know how to do it well (NO you can't put all 1200 CFM from a 3 ton single stage system into the master bedroom and expect a bypass to bail you out...)

We do 3-4 zone systems routinely.

That said, I agree that in this situation, redundancy is important - having a daycare become unlivably hot or shut down completely owing to a single point of failure is a VERY bad thing.

Also, while enclosure is important, internal loads may loom large. In residential load calculation we pretty much ignore lighting and pay limited heed to number of occupants. This is a commercial occupancy - need to figure in lighting energy turned into waste heat, latent and sensible load from a bunch of wild kids and harried teachers.

Depending on code, may need to figure in some forced ventilation, though hopefully they'll let you do that intelligently, such as modulating based on CO2 instead of X CFM per person 24/7, and the persons count being the maximum legal number.

Lastly I shy away from using system blower to circulate during cooling season - blows all the humidity back off the coil when compressor cycles off. Some say stopping blower for a minute allows water to drain away, but if one does the math, a high efficiency unit's evap coil has upwards of 100 square feet of fin and tube area per ton, and that's a lot of wet metal that does not dry in one minute.

Zoning is poorly done so often due to the cost and work of doing it RIGHT. By time you have the ductwork oversized enough to handle all the airflow even when only one zone is calling, the zone control system, damper motors, and possibly a 2 stage system you could buy 2 basic single stage systems for the same price.


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