New house build - one or two HVAC systems?

[WolfR1der]

We are looking at new home builds and have noticed some of these large houses have two HVAC systems, one in the basement and one on the second level. Sales people from each builder have given us different stories as to why they do things one way or another. The sales guy for the dual HVAC builder made more sense however they're using Goodman units. I've always been told Goodman HVAC units are crap. The single HVAC builder is using Lennox in basement and their sales guy couldn't look me in the eye while he was explaining their 'revolutionary' way to duct the house with one unit.

House with dual HVAC is 3800 sq/ft with finished basement (basement is calculated into sq/ft). House with single HVAC is 3700 sq/ft with UNfinished basement. I believe total sq/ft with unfinished basement would be 4400sq/ft.

All the info I've found so far only tells me how to calculate how many BTUs you need. Not whether or not you need a second unit for a particular square footage. I'm sure that data is out there, I just haven't found it. Can someone here enlighten me?


EDIT: Naturally shortly after I posted this I changed my search and found some info related to efficiency with a second unit. Other than overall initial cost I can't see a con to having a second unit however, unless the house with the single is zoned it seems like the single HVAC unit is undesirable due to heat differences between upstairs and down. Would this be correct?

 

[HVAC-Novice]

Well, the second floor is more exposed to the exterior than ground level. and warm air rises. So in summer 2nd floor will be warmer. Especially if you have bedrooms in 2nd floor, people like to cool that, but not the grade or basement level that much.
On the other side in winter, you may need more heating in basement. Same with rooms exposed to the West in a summer afternoon and so on.

So you basically create 2 zones with 2 units. Unless you have a furnace system with dampers that also creates 2 zones.

And you also get some redundancy. Lie if one unit fails, they have at least part of the house conditioned.

obviously 2 zones is "better", but also more expensive. So the owner should decide what it is worth to them.

I have a tri-level house with single zone. In summer I block of the lower level vents to prevent over-cooling. That is a crude method to overcome the above problems.

 

[3DDave]

I suspect the main advantage is ducting - the upper floor will get flexible ducts in the attic along with the cooling coil installation, saving trying to push cold air all the way up from the basement.

In the winter, the warm air will be forced to the upper floor and the cool air will naturally return to the lower floor.

Air distribution could be managed from one unit with in-duct boosters and powered baffles to zone the house with a single unit, which makes for great flexibility, but also more items to screw up.

 

[nuuvox000]

If I was building a new house, I would definitely spring for two HVAC units, but only you know if it's a good enough value for you. For instance, my current residence only has one unit. The basement is finished but we're not down there very often so It's okay if it's a little cold. If you have a kitchen or bedrooms or a family room that is used often, it would be great to have a system just for that level. Will you hate your life if you only have one system? No. But being comfortable at all locations in your house is pretty nice.

Keep in mind that when it's time to replace the units, it's also about double the cost.

 

[WolfR1der]

Thanks for all the help. Much appreciated. On a personal note, don't build a house with a home building corporation. They'll screw you at every chance they get.

 

[dbill74]

Bottom line is better control and efficiency. Having more then one unit allows multiple zones which can be different parts of the house. Either east/west, north/south, or by floor.
With more zones and control comes better overall efficiency. When you and your family are in one zone/floor, the unit for that floor will work more than the unit in the unoccupied zone. You can also have different temperature setpoints on your thermostats for different family members or just to allow the one zone/floor get warmer/cooler than the other one.

If building; having 2 or more units will cost more upfront, but you can save money in the long run with smart management of the units. A single unit will try to keep the entire house a single temperature depending where the thermostat is located.

A single unit can be designed with zones, but the trade off is additional ductwork apparatus and a control system similar to what you would find in a commercial application. For a house I would steer away from this as it can get complicated to install and maintain, not to mention the extra cost for such a system.