I have a 2 story center entrance colonial home. 4 rooms on each floor. Each room has central a/c entering via a ceiling duct. Cold air return and thermostat are in the hallway at the top of the stairs on the second floor. The rooms on the second floor have doors. None of the rooms on the first floor have doors. Three of the four rooms on the second floor are seldom used. Should I close the a/c dampers in these seldom-used rooms and close their doors or leave all dampers and room doors open when running the a/c. Heating system is separate forced hot water base board with separate zones for each floor so not relevant to my question.
Replies
one thermostat for the whole house? if you had a zoned system then you wouldnt gain much by doing this but if only one thermostat it should help some
One Thermostat
The house was being built on spec. The builder was not doing central a/c. We wanted central a/c and the builder agreed to let us have it done but although we were in the process of getting a mortgage the builder would not let us use our on subs and told us we had to use his subs (friends). Instead of cutting the a/c ducts into the walls and refinishing the walls he ran 6" round in the corners of the closets on the second floor....didn't even wrap them. He put the attic air handler over the master bedroom in a plastic pan on the ceiling joists......noise and vibration. Pan was dead flat so it always had water in it and only drained when the water level rose. Emergency overflow was attached to the regular drain line. Eventually had all the attic work redone....suspended the air handler from the roof rafters. Live and Learn!!!!!
Oh, I requested the a/c sub to put the condensor on a concrete pad. He said that if concrete was being poured when he was doing the a/c then I would have a concrete pad......no concrete pad. I'm still kicking myself.
HVAC
Was this house permitted and inspected by AHJ? If so, the A/C must have been installed on a dark moonless night and the inspector was also a friend of the builder.
OOPS
Who/what is AHJ? The inspector was on vacation at the builder's vacation retreat and signed off on everything before he went on vacation. Quick story which I told before on this forum. The builder built 15 homes in this development. There is a handyman guy that has been doing fixit work on a lot of the houses in the development for a lot of years on almost a full-time basis. I was talking to the handyman and mentioned that it seems like he has made his living here 'forever'. He laughed and said that a couple of years ago he had sent a Christmas card to the builder with a note thanking him for the steady supply of fixit work.
FWIW, the a/c install was so bad that I had to have most of it reworked.
AHJ
authorities having jurisdiction
For most efficiency you should do as you suggest. Sometimes it's a good idea to not completely shut off airflow, though, to avoid a "musty" smell, especially keeping in mind that the AC air is dehumidified, and will help keep damaging humidity down. (But this depends a lot on the particular structure. Older, less well sealed structures will be more apt to have a problem.)
Since your home is single zone with a t-state in second floor hallway, I would recommend that you keep the registers and doors open as much as possible on the 2nd floor during the cooling season. If you don't, you're proabably going to end up with a large temperature imbalance between floors of your home and introduce ineffeciencies to your system.
Remember, warm air rises and cool air falls. Most folks (including a few on this forum) seem to forget this basic fact. Your t-state pick up on this fact. Homes that have a single zone heating or cooling system (or both) should be properly sized for the loads of the entire home; not just part of it. Since you have only central air designed for cooling, your A/C technician was designing for cooling loads and had a choice to install the t-stat on the 1st floor or the 2nd. He installed it on the 2nd floor for good reasons. IMO, by closing the dampers of those upstairs rooms and keeping the doors to them pertupually shut you are working against those reasons.
I would close the doors and registers of the unused rooms. I expect there will be no problems with musty odors due to the heat in the rooms during cooling season. This may increase air velocity from the open registers but that's not a problem when cooling, unless the system was not set up properly. I would leave the heat on in those rooms in winter though. Your system could be converted to a dual zone which would be a significant improvement.
And remember hot air rises because Deadnuts just informed us that MOST people, including some on this forum actually do not know this basic fact that is taught in kindergarten to the few children that hadn't already noticed that hot air does in fact rise.
Dual Zone Conversion
How involved is it to convert to a dual zone system. The 2 zone systems I have seen have 2 compressors and 2 air handlers.....2 separate complete systems.
There are a number of ways to zone using baffles that essentially close or open registers as needed. This can get very involved, or be more basic. Thermostats can even be wireless so that eliminates running wires. I would prefer standard thermostats if possible.
There are electrically actuated baffles, and even air inflated baffles that expand to reduce of block airflow.
I can't recommend any brands, but here is a link you can look at and it may give you some ideas. http://zonefirst.com/existing-zone-systems/
Our house originally had two furnaces (with AC only upstairs), and when those needed to be replaced we bought a 2-zone system using a single furnace (with AC). It uses electric baffles to switch the zones on/off. Standard thermostats are fed to a zone controller box which manages the baffles. Works pretty well.
You might want to consider installing a seperate mini-spit ductless system for your downstairs only. You may be more comfortable and save money during the cooling season in the long run. Seems to me that modifying a coil, air handler, and baffeled duct work (presumably) in the attic to operate as a dual zone system which is set up to predominately cool the downstairs is inherently inefficient.
A/C
The a/c system is only a couple of years old. The sytem I replaced was the original and was 25 years old. The house is not that big and here in Massachusetts a/c usage is not all that much....just run it when really hot or humid. At this stage of the game the payback would not be there if I made any changes to the system.