Help with HVAC Design

[mrtone]

My wife and I are looking at homes in SC and came across one that appeals to us but I question the HVAC system layout. I am going to have a licensed contractor inspect it if we resolve other matters, but thought I'd ask the experts on the board.
The home is approx. 1600 sf with 2 BR's upstairs, crawl space foundation. While looking at it this weekend, I noticed that there were no return air ducts in the upstairs at all. In fact, there was only one (about 18"X24") in the entire home, placed under the stairway and facing the great room (vaulted). The house was built in 1996 with this HVAC system installed.
I don't see how this could possibly provided circulation in the upstairs BR's, let alone in the downstairs rooms - just think about how this works when the doors to rooms are closed?
Any have any comments or suggestions?

[CraigFL]

In Florida, this doesn't seem uncommon either. I've seen many homes the way you described. My two story home happens to have seperate A/C-Heat units for each floor. There is only one cold air return for each unit located in a hallway on each floor.
It's interesting you bring this up because I was just a my sister-in-laws new home yesterday(a "manufactured" type) and I noticed they had placed vents above each bedroom/bath door to allow air circulation.

[mrtone]

CraigFL: I might be able to understand one return for each floor, but this house has one return - period! No way is the air upstairs going to move. I'm ok with putting another system in IF I could get the price of the house adjusted enough.

[chuckspeed]

A central return system is not uncommon in a house of this size. Usually there is an air gap between the bottom of the door and the floor in each bedroom - this is the 'return' for the system - as odd as that may seem.

Depending on whether or not you're trying to solve for heating or A/C as your design condition - the central return system can work well (heating) or be somewhat compromised (cooling).

the 'fix' is really quite simple - install return grating above each bedroom door - a standard sized diffuser will be enough. A high-level return in each bedroom will allow the warm air to vent and be displaced by cooler air. over time, the air in the great room will mix enough to destratify - assuming the hvac system has enough flow to do so.

NetNet - while it's not a perfect system, it will work. My 1600 sq ft house in California was built the same way - but with the vents over the bedroom doors. The A/C system kept the house cool, no prob; it was built in 1992.

[Jumpin Johnnie]

Poor design...you need to return as much air to the system as you deliver. A diffuser above each door is a bandade approch. Proper duct design is key to a comfortable house. I'm sure the upstairs will never be comfortable. One thermostat can't satisfy both floors. It may work, but that doesn't mean it's right.
Johnnie

[mleichtle]

I have hydronic base board heat in the house, the only way I get air circulation is to get up and walk to a different room.

[mrtone]

Each upstairs bedroom does have a ~12"X14" grate about 4 inches from the ceiling on the wall that faces the great room (vaulted). Seems to me that during the heating season, heat builds up at the ceiling - what's going to pull this hot air down and remix it? The cold air is going to drop down stairs.

In cooling season, again the upstairs rooms are going to get the rising hot air.

I made the mistate of buying a home with insufficient return air ducting in Chicago. With the AC on full blast, the upstairs BR's were stifling.

But I guess if I knew the answers I wouldn't be asking!!

[BoostAddiction]

This isn't much help with your design question, but most homes in our area with two-story houses have 2 completely separate zones, each with their own HVAC unit and ducting. I think the reason is that whether cooling or heating, the heating and cooling load on one floor will never be in balance with the other, and trying to heat or cool them with one unit will always result in a temp imbalance.

In other words, if you really want the house, discount it by the amount it would take to add a second, properly-engineered zone. Otherwise, I predict you will never be happy.

-Will

[mrtone]

Boostaddiction: I think you're right on that, and that's how it will probably go if we proceed with this house. I'd like it to work out since the house has a lot of possibilities and its bound to be cheaper that building one from start.
Its just the two of us, kids are grown and gone, so we can put more time and money into making changes to it.

[mrtone]

Update: A HVAC company did an inspection today:
- system functions well
- would operate even better with a larger return duct
- $500. to install larger duct (not too bad)
- Gas pack system is expandable so addition is possible
- Insulation in crawl space is good

Tony