We are building a new house in Blowing Rock. The builder has recommended a 5 ton unit zoned for upstairs versus downstairs. The unit will be dual fuel to deal with the cold weather here. Any thoughts on this approach versus two smaller units- one for upstairs and one for downstairs? I can't find a definitive answer on the web. My concern is when one zone is off the unit will be oversized for the other zone. Thoughts?
A few thoughts. When you say dual fuel, do I understand that means gas/oil heat and electric A/C or is it a heat pump with gas/oil backup instead of electric?
With today's variable speed compressors and fans, using a single unit for two zones isn't the deal it was when A/C systems were single speed. With single speed compressors that were oversize the issue was that they would technically cool to the set temp, but would not remove enough moisture from the air, so the house would still be uncomfortably humid. With a variable speed compressor and fan, the system should be able to run slowly enough to dehumidify the air even when servicing only one zone. In the heat mode, you need to find out if the burner has multiple burn rates or single. If single, and only one zone is demanding heat, be prepared for pretty heavy air flow, as the air handler has to remove the heat from the firebox and move it to the vents. If only half the house needs heat, it could be a little noisy. Or if the burner has variable heat output, the variable speed fan should be able to run slowly enough to only push the necessary air into one zone.
The important issue in a system like that is whatever is used to separate/switch the zones. That is there needs to be some sort of air flow controllers that allow the HVAC output to be directed to upstairs or downstairs or both as dictated by thermostat settings and readings. This needs to be good enough so that the non-demand area is at near zero air flow.
Another downside is that if you like to run the fan at low speed all the time just to keep stuffiness down it won't happen as much. I run my fan on low all the time because I have an electronic air cleaner and like to keep air moving past it. A single source for heating and cooling will not allow just air circulation in the zone not demanding heating/cooling. If you have two completely separate units, this isn't an issue.
If you have two completely separate systems, it will be extremely rare when you are totally without heating or A/C. If you have only one unit functioning, the heating/cooling may not be as good, but you will have something (half), not nothing, This may or may be important to you, but is something to think about. Say its winter, and it just snowed a foot and your only heating system goes out. Panic time! need a repairman ASAP. Now, if you have two units, and one goes out you simply prepare to be somewhat chilly, as one unit may only keep your house around 60 degrees. But no worries about pipes freezing etc.
With two separate systems, you have twice as much maintenance to do - filter changes, cleanings etc. and almost twice as many things to break, and you have two of everything. Two outside unit and two inside units. How does this work with your house?
With five tons total system need, two systems at 2.5 tons will be near the smallest units made.
An off the wall thought for you: Geothermal sourced heat pump.