One or two HVAC units?

[HankVRSI]

I’m a structural engineer so unfamiliar with most HVAC issues. I am building a two story house and planned to have different upstairs and downstairs HVAC units. However I am using foam insulation in the walls and rafters and have been told I only need one unit for both. Is this correct?

Thanks

Hank

 

[willard3]

You can use only one unit and it is done frequently.

The heating/cooling load is not distributed equally between the first and second floors and, if you are not using some other method for zoning, one unit will do a poor job both heating and cooling on both floors.

Two units, one for 1st and 1 for 2nd floor will do a much better job but will be more expensive initially. Life cycle cost will likely be lower for 2 units.

 

[tombmech]

Two units is the Cadillac option. Better in almost every instance, but technically, not necessary. In fact, it is one of the ways I judge builder quality in house shopping.

With commercial/industrial applications, you can get into significant building airflow problems if a definite segregation between floors and units is not maintained. However, the typical home is small enough that when such mixing occurs, the effects are not that noticeable.

If you can afford it, though, go with separate units per floor in your home.

 

[Ashereng]

A good solution is separate headers for each zone, with flow controllers (actuated vanes) in the inlet of each header. You can have as many headers (zones) as you want to subdivide. The only additional cost is the ducting and the temperature/controller/vanes per header.

Each zone would have a thermostat controlling the vanes in each of the header. In summer, when cooling, the warmer zones would get more flow. In winter, the cooler zones would get more flow.

Aside from upstairs vs downstairs zoning, don't forget the effect of sun exposure. The souther exposure is warmer than the northern. The side with more windows is usuall warmer during daylight than the side without the windows. You may decide that there are actually 4 zones in your 2 story. Up north, up south, down north, down south.

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Albert Einstein
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[Trigout]

I must agree also that two units is preferred.
You don't give square footages of the upstairs and down but
you will be much happier with the management of temperature
and humidity by having two units.
What you were told about one unit being capable of handling the load of both stories is true. I would not suggest doing it however.

 

[HankVRSI]

Looks like there is pretty much agreement that two units will work much better so that’s what I’ll go with. Thanks to everyone who responded.

Hank