My previous post on Multi-Zone HVACs brought a few questions from agents, who basically asked: "Why not just add another zone (i.e. a second furnace and/or A/C system)?" But these houses already have two completely separate systems...so I digressed into a history of regulating heating and cooling zones. See the reply below.
The short answer to your question "would a separate (second/third...) system be more efficient? is: It Depends.
So let's get some history of forced air systems that arrived in great measure after WWII. The air from one furnace was pushed from the furnace (usually in the basement) up and into the rooms in the home of one, two, or three floors. Heat rises, so the top floors, for the most part, would get more heat rising up the open staircases. So the first solution was to install adjustable supply registers in each room where they could reduce the flow of air to reduce the amount of hot air coming out. Problem solved - as long as the owners understood how to monitor and regulate flow to control the heat in each room. "I like it cooler, so I'll close my bedroom down, and you like yours warmer so you keep yours open."
Then along came the addition of cental air conditioning to add to the complexity. Since hot air rises, now we have the problem of pushing more cold air upstairs to cool the rooms closest to the attic space. So we could re-adjust the registers...or, the idea was then to install mechanical dampers (movable doors) into the ductwork in accessible spaces (i.e the basement). The dampers could then be closed slightly for the vents in the lower floors in the summer (to allow for more cold air flow in the upper floor), and reversed to allow less hot air upstairs in the winter. This worked - again, as long as someone living in the home understood the principle and how to adjust the damper doors and registers.
Then along came energy efficiency in the 1970's, and the manufacturers joined in by recommending two A/C systems for a home of any significant size, shape, or number of floors. This in reality was a good thing, albeit the initial cost of installing the dual systems - as long as it was engineered and installed correctly. One system cools the upper floor and one system cools the lower floors. If the upper floor system was installed in the attic, then it was even more efficient (if properly insulated), as it took less energy to push the air, as cold air drops down into the floor below, and the hot air rises and gets sucked into the return vent. Two systems - two thermostats. And you are using energy to cool the upstairs where more of the heat resides, while the system for the lower floors takes a rest.
But we still had the problem of mismatched systems, bad designs and installations, and people just not getting the knowledge and idea that they could adjust the air flow in one or more ways to maintain the level of cooling and heating they desired. And there was the increase we have seen in the size of homes from 1,500 sq. ft. to 4,000 sq. ft. Now through the entrance of the age of electronics, we have the ability to control everything in our life through the use of electronic sensors, motors, valves, and systems with pre-programed memories.
So the newest energy efficient idea is to control the temperature in centally located areas of a home through an electonically zoned approach. This takes on two main concepts that one must understand to properly design, install, and program a home. First is the concept of re-use - if there is more hot air upstairs than is needed and more is needed downstairs, then let's move that air to where it is desired. So here is where the electronic dampers come in. The hot air upstairs is pulled through the return ducts and at the central point of the air handler, it is pushed back out, but not before the dampers in the ducts are told to close down for the upstairs and open up more for the downstairs. The heat is then redistributed without having to add as much new heat to the downstairs, and thus the entire home. Of course this concept is more theory.
The second concept (more implementation) is to determine how, what, and where to set up the components of the system. In the original cases I posted, the main failure was the lack of proper installation of thermostats in locations that could properly provide regulation for the centrally located heating and cooling systems in the home. In theory, we are looking for points where heat is generated (kitchen, bathrooms, family room, and upstairs in the summer), heat is lost (upstairs in the winter) , and heat is retained (interior areas, south side and where masonry exists, and basement). Placing the thermostats in the areas where they will catch the highest and most long-term differences in heating and cooling variation is where they will be the most efficient. And here-in the problem lies. How do we train the specialists, and design the systems to effectively use all this new technology to its maximum capacity? For the most part, no two homes are the same, or will have the same energy loss and energy gain.
Adding another A/C unit or furnace would solve the lack of proper cooling or heating, but that solution is unable to recapture the energy lost from the other systems working in the home, and would only use more energy. We are finding more, and hearing more, ways for energy recapture, seemingly on a weekly basis. California and it's water crisis is effectively recapturing more water than ever before seen in this country.
So let's get some history of forced air systems that arrived in great measure after WWII. The air from one furnace was pushed from the furnace (usually in the basement) up and into the rooms in the home of one, two, or three floors. Heat rises, so the top floors, for the most part, would get more heat rising up the open staircases. So the first solution was to install adjustable supply registers in each room where they could reduce the flow of air to reduce the amount of hot air coming out. Problem solved - as long as the owners understood how to monitor and regulate flow to control the heat in each room. "I like it cooler, so I'll close my bedroom down, and you like yours warmer so you keep yours open."
Then along came the addition of cental air conditioning to add to the complexity. Since hot air rises, now we have the problem of pushing more cold air upstairs to cool the rooms closest to the attic space. So we could re-adjust the registers...or, the idea was then to install mechanical dampers (movable doors) into the ductwork in accessible spaces (i.e the basement). The dampers could then be closed slightly for the vents in the lower floors in the summer (to allow for more cold air flow in the upper floor), and reversed to allow less hot air upstairs in the winter. This worked - again, as long as someone living in the home understood the principle and how to adjust the damper doors and registers.
Then along came energy efficiency in the 1970's, and the manufacturers joined in by recommending two A/C systems for a home of any significant size, shape, or number of floors. This in reality was a good thing, albeit the initial cost of installing the dual systems - as long as it was engineered and installed correctly. One system cools the upper floor and one system cools the lower floors. If the upper floor system was installed in the attic, then it was even more efficient (if properly insulated), as it took less energy to push the air, as cold air drops down into the floor below, and the hot air rises and gets sucked into the return vent. Two systems - two thermostats. And you are using energy to cool the upstairs where more of the heat resides, while the system for the lower floors takes a rest.
But we still had the problem of mismatched systems, bad designs and installations, and people just not getting the knowledge and idea that they could adjust the air flow in one or more ways to maintain the level of cooling and heating they desired. And there was the increase we have seen in the size of homes from 1,500 sq. ft. to 4,000 sq. ft. Now through the entrance of the age of electronics, we have the ability to control everything in our life through the use of electronic sensors, motors, valves, and systems with pre-programed memories.
So the newest energy efficient idea is to control the temperature in centally located areas of a home through an electonically zoned approach. This takes on two main concepts that one must understand to properly design, install, and program a home. First is the concept of re-use - if there is more hot air upstairs than is needed and more is needed downstairs, then let's move that air to where it is desired. So here is where the electronic dampers come in. The hot air upstairs is pulled through the return ducts and at the central point of the air handler, it is pushed back out, but not before the dampers in the ducts are told to close down for the upstairs and open up more for the downstairs. The heat is then redistributed without having to add as much new heat to the downstairs, and thus the entire home. Of course this concept is more theory.
The second concept (more implementation) is to determine how, what, and where to set up the components of the system. In the original cases I posted, the main failure was the lack of proper installation of thermostats in locations that could properly provide regulation for the centrally located heating and cooling systems in the home. In theory, we are looking for points where heat is generated (kitchen, bathrooms, family room, and upstairs in the summer), heat is lost (upstairs in the winter) , and heat is retained (interior areas, south side and where masonry exists, and basement). Placing the thermostats in the areas where they will catch the highest and most long-term differences in heating and cooling variation is where they will be the most efficient. And here-in the problem lies. How do we train the specialists, and design the systems to effectively use all this new technology to its maximum capacity? For the most part, no two homes are the same, or will have the same energy loss and energy gain.
Adding another A/C unit or furnace would solve the lack of proper cooling or heating, but that solution is unable to recapture the energy lost from the other systems working in the home, and would only use more energy. We are finding more, and hearing more, ways for energy recapture, seemingly on a weekly basis. California and it's water crisis is effectively recapturing more water than ever before seen in this country.