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,

Scotty

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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Hmmm. That makes sense. Some loss from accumulator too?

Ok, so help me with the math. 3000 CCDs = x run (or x KWH?) times x1.08?

So if ac costs $1000, HP adds $80?

Any crazy "laundry weighing" examples of experiments you've done?

Bob and Curt, sorry my posts left the cascade I know it makes following the conversational thread frustrating and confusing - ipad does not want to reply in line.  

Bob: " We have to get down into the 30's before the heat runs much."

What does that mean?  Seriously, you aren't suggesting that there is no load on residential structures at 40f, are you?  

If you look at consumption I think you'll find that most of it occurs above 30 for most of the country.  So maybe stepping back and asking: "WHY DO  We have to get down into the 30's before the heat runs much" would open up another area to consider.  

If cycling on and off, overheating air, duct, and house and shutting off are inefficient, what is happening at 30 and above if your equipment "doesn't run much"?  

Furthermore is it comfortable to heat that way?  I think it is not, and so people bump thermostats, use space heaters, overheat parts of their homes (even opening window) in an attempt to get other parts comfortable.  Stupid Pet Tricks - we've done them and seen them.  

So I go back to the idea that having lots of stages is good for efficiency and comfort.  I've sold LOTS of hybrids and I FOLLOW UP.  Reports from happy clients about amazing comfort and amazing energy savings.  

One trick I use is "please send pictures of your meter".  This saves a fair amount of time understanding savings:  http://bit.ly/meterpictures

Curt, have you run the number on SEER savings from HP to straight AC?  I need to factor that into my designs so people understand there is a small sacrifice in efficiency, and how that might translate to $.  

Not saying we don't need any heat in the 40's, but heat loss is relatively low. Once the furnace has recovered from night setback (we setback for comfort reasons more than saving energy) it's done for the day. We've only started hitting the high 30's for morning lows in Oklahoma, it will be interesting to see what happens once winter gets here.

I'm with you on the "stupid pet tricks". People turn their heat to 72-75 in the morning then run the AC in the afternoon of the same day.

I have concerns about ductwork losses when running low BTU outputs. Our ductwork is in the attic and is of decent tightness (not perfect, but better than most). Does running low delta T cause more "loss"? If ductwork looses 5 degrees due to running through unconditioned space wouldn't that hurt the 90F discharge temps more than a 110F discharge temp of the air handler?

I have not; all my residential clients get HP since we have enough of a heating season to justify the expense.

I think you could safely figure the efficiency hit at 5-10%

So in my $1000/$1080 scenario, grid heat would have to cost a couple hundred a year before the reversing valve justified the efficiency loss in cooling..?

Guess climates that don't usually have heating, recommending heat pumps not warranted.

BOB, you may want to talk to the dry climate guys (Healy, MacFarland, Chitwood...), they've done extensive measuring of btu loss of duct at different delivery temps.

Sounds about right. In SoFla, heat might be needed just 10-20 days per year with an energy cost of $50-100

In NoFla, heat is needed intermittently in November and March, consistently Dec-Feb.

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