There is no graceful way to explain our thoughts on HVAC, given the variables, so without further ado, a random list of thoughts/facts on High-Performance HVAC Critical Details:
In 2011, the average residential customer in the U.S. spent 11.7 cents/KWH via U.S. Energy Information Administration. The EIA reports that each KWH via Electricity = 3,412 BTUs of heat. According to the U.S. DOE electric heaters are 100% AFUE*(the wasted heat of the fan motor is helpful so “no waste”) & homeowners should expect to pay roughly $35 per 1 million BTUs of heat via electricity.
Natural gas is sold in “therms”, where 1 therm = 100,000 BTUs. The EIA reports that one therm costs about $1.01 as of November 2012 [and we believe the price will steadily decline over the next 20 years] Gas furnaces in the U.S. must have a minimum AFUE of 78% thus NG consumers should expect to pay roughly $12.96 per 1 million BTUs of heat. While prices for gas and electricity vary over time and by region, gas furnaces typically cost much less to operate than electric furnaces or heaters. The crappiest natural gas furnace allowable in NC is a Package Unit (outdoor air-handler) and will last about 9-10 years prior to failure.
If you happen to want the most efficient furnace available, maybe you’ll find a 95% efficient variable speed split-system, you can be spending just ~$11.00/1 million BTUs.
PROPANE costs ~$22.00+ per million BTUs, a 40% savings over Electricity, but you can’t completely cool a home with propane, thus the popularity of electric heat pumps.
Even the worst NG equipment makes the ~$10.00 monthly service charge ($120/year) for NG WELL WORTH IT, not to mention the environmental savings which are gargantuan if you factor in water heating and space heating and cooking and the ease of a gas fireplace.
Now we can assume that a person has chosen a fuel and equipment type for heating/AC. Now you need to design the specifics as far as locating all the parts and pieces and deciding how they are shaped and go together. Duct-work may affect the framing/foundation plan just like plumbing.
Ideally the entire system and the duct-work would be housed in a friendly space such as a sealed crawlspace, a sealed attic, or inside the building itself, routed through the ceilings in the hallways or in the floor system between floors. Actually, for tighter and better insulated homes, or homes with tricky/modern architecture, a ductless system, or a mini-split system is ideal, and wastes virtually zero space.
If you are planing very long-term, then you should think about which areas your city/county taxes. In Raleigh, the city/county taxes the heated square footage and partially finished areas at maybe $0.80 to $2.00 per SF per year so it might not make sense to devote an interior closet or finished basement to mechanical equipment. Imagine a 4′ x 4′ closet or 16SF taxed at $32.00/year for 30 years = ~$1,000 which would probably pay for a new water heater every 20 years, including interest. Even when your home is paid for you get tax bills and utility bills.
We have consulted on dozens of new homes with varying timelines/priorities in view. One client, we’ll call him Darren, wanted to build the ultimate service friendly home. He was going to build a nearly on-grade ranch on a vented crawlspace with a sealed attic (spray-foam) with a heat-pump air-handler in the attic and propane for a tankless-water-heater, stove, and fireplace.
I asked, where are you planning on storing your crap, and he said nothing. He had not thought about storage or the fact that having HVAC equipment and ducts in an attic drastically reduces its user-friendliness, storage-friendliness, and opens the home up to the possibility of HVAC condensation over-flowing or leaking into the ceiling/living-space.
The benefit of ceiling vents is pretty obvious, and having a high-return in every room is ideal, so there is a pretty good case for the attic-air-handler if the drainage is awesome, there is a wet-sensor, and the air-flow is really stellar. Also, you then have a completely wide-open crawlspace which is great. A sealed crawlspace only needs a single 4″ duct into it to keep it perfectly dry if the moisture management is working, and the cost of insulating the walls of the crawlspace should be about 1/2 the cost of insulating the floors of the home.
After you decide on attic or crawlspace or both for the locations of the equipment/ducts, you can decide or design the shape of them.
CRAWSPACES: In crawlspaces, make the ductwork and equipment as high and tight as possible, We don’t care if your crawlspace is 14′ tall you should still hang the air-handler cabinet from the floor system above with just barely enough clearance enough to get your venting in or access to the top-screws. Make the plenum as squatty as possible to allow for maximum crawlspace head-room. If you need to connect ten 6″ flexible ducts to a supply plenum make it long and shallow:
If you are lucky enough to be building or working on a house that is less than 50′ across the diagonal of the footprint then you have a pretty good chance to locate the supply distribution plenum or box centrally and to have really even air-flow across the home. this means, that regardless of the location of the thermostat, the home will heat up and cool down evenly and be evenly comfortable.
In the attic I think it is safe to say that having your ducts running over-head, where is gets scorching hot in most attics is a bad idea. You can lay the ducts directly on the ceiling joists so they are in the lower attic strata and potentially cover them with ceiling insulation to make them super insulated. Anytime you put equipment or duct-work in an attic you make it more inaccessible.
Zoning systems are popular but inneficiently designed in most cases. Restricting the air-flow by closing a zone, or short-circuiting a zone by supplying that zone’s air directly back to the return, aka bypass zoning, are energy wasting strategies. If you really think you need to be able to favor an area, do just that, design the duct-work so that you can supply 60% or 65% or 70% of the air to the zone in need and continue to provide the remainder to the other zone. What we have found is that the systems behaves, there is no restriction, and the other side gets enough to be comfortable as well.