My basement is often warmer then the main floor on a cold windy winter day then the main floor - I attribute the amount of doors/windows to that, and it does have open stairs between floors and no thermal insulation between floors.
so on cold windy days do you hang out in the basement because its warmer than the main floor? i suspect not. the heat (energy) in the basement is being lost out to many things other than the floor above it, which means still some wasted heat (energy). this doesnt mean you are putting heat (energy) in the basement, just means that energy is still being wasted whether you are paying for it or not.
convection will be primary mode for heat to travel up the stairs into the main floor, where it will then mix and be lost via windows & doors. almost every 1btu you put it, is wasted at some point, because at any given time t the space around you is not the space you are in. take for example you sitting at dinner table on main floor, all that energy you paid for to cool the 2nd & 3rd floors is basically wasted because you are not there on those floors.
another example is a big 1story home, 5 bedrooms, multiple play rooms, 6k sq.ft. it would be extremely energy saving if AC & heat was activated only in the space you were in as you moved around, like occupancy sensors for lights.
but to generalize, all the energy we consume is wasted. all that heat we suck out of a home and exhausted via the outside condenser coil is lost energy (you pay to move it and then lose it, you paid for more than what the system moved, etc). all that heat could be used to pre-heat water to the water heater, which eventually gets lost anyways, but we can control the rate of loss via using efficient systems. the net sum of all energy consumed is exothermic and then lost forever into space, entropy of the universe is always increasing, yet the entropy density of universe is decreasing. unfortunately for Earth, the human processes used cause atmospheric conditions that trap more heat, thus elevating avg temps, which is not good. maybe one day instead of giant wind generators we will have giant radiators up in the air, because it will be so hot you can simply just heat water from the moving air and then use that hot water to turn generators and to heat public water.
the energy we use in HVAC in homes is extremely wasteful, or in other terms, very low efficiency. these systems are made to keep a giant volume of air at comfortable temp, but 99.99% of that volume you at not in at any given time. if you could wear clothes that kept you at a comfortable temp, would you need HVAC at all? we do not need HVAC when working outside in 30F temp, our clothes have enough R to keep us warm, no need to have a big IR heater while working.
as for water heater in home, sure, if its in a location that can use the lost heat. but that only works for when the home needs heat, certainly does no good when you need AC. my water heater is in the garage, lost heat there serves me no good year-round. how about cooking that turkey in the oven, all that nice heat coming out when its 20F outside, but what about when its middle of summer 90F and humid, how often do you use the oven in summer time?
ok, went went off the rails here, but you get the point.
