We are building a 5800 sq foot home with a 2000 ft. basement,2000 ft. main and 1800 ft. upper floor.We are hoping to provide forced air heating and air conditioning to all levels but have been told that because of the volume involved we will need two furnaces ,two air conditioning units,and thus two sets of duct work.We also have a problem with where to place the second furnace.We would be content to use electric baseboards on the upper floor but would still need to address the air conditioning needs of the upper floor which we believe couldn’t be run off of the basement and lower floor system.If it makes a difference,the home is near Vancouver B.C. so the heating and cooling requirements are quite moderate.Any suggestions?
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Make sure you work with a really experienced HVAC person who knows that soggy and moderate climate.
I've read (here and other places) the advantages of double-furnace systems.
- Zoned heating (especially useful if the south side is significantly hotter - not likely in Van).
- will never be completely without heat during repairs (matters more in, say, Winnipeg).
- you might have savings if only one furnace runs in cool weather.
- two smaller furnaces may not be twice the price of one larger furnace. Smaller units are sold in large numbers in price-conscious market. Large serve the upscale market.
For cooling, you won't need much, especially if you design for it by overshooting Vancouver-esque minimums for insulation + shade trees, awnings as applicable, etc.
Consider having it delivered only where needed, like a separately ducted (small vents) top down system that serves only the top floor (some cool air would venture further down), maybe supplemented by fans on lower levels. I've only heard tales about reversing the direction of flow of air through ducts, bottom floor first in winter - top floor first in summer. Don't know if that is myth or reality, but your HVAC people should be able to discuss.
Vannewhome, I have the exact same set up in my house now as you describe. My house was build three years ago and for the same reasons you mentioned I have a two zone heating / air conditioning (forced hot air). Both furnaces are in the basement, one for the second floor one for the first. They are completely independent of each other.
In my first winter in the house the downstairs furnace had a slight problem and would cut off after about 5 minutes. The heating contractor came the next day to fix, however that night we still had the upstairs furnace working fine and therefore we did not freeze to death. The house was slightly colder than normal but having the second independent system was nice.
During the summer, instead of cooling the entire house, we would cool just the floor we were on. At night we were able to turn up the temperature downstairs while keeping the upstairs rooms cooler.
Regarding the duct work, it’s not too bad. The downstairs registers are run in the basement ceiling up to the first floor and the upstairs duct work runs thru one chase up to the attic and then branches off up there to all the rooms.
It is possible to have only one HVAC system.
A two stage variable speed system with a zoned distribution can be designed for your loads. The distribution system is designed with VAV boxes (variable air volume) in the duct work, and the first and second stage heating/cooling plant with a variable a speed air handing unit supplies the appropriate zone requirements based on t-stat settings and programing logic.
It is expensive, but can be as or more efficient than two separate systems. Most residential contractors use the two unit system because it is easier to design. The single system requires extensive Manual-J calculation, and distribution design accounting for every room and area of the house, both supply and return. More work than is reasonable for the competitive market of residential building.
BTW, I hate the idea of planning for system failures. That sales idea has been borrowed from the commercial side of the HVAC business. If a contractor hands me that redundancy line, they are not even going to bid.
Dave