Air flow with central air (furnace, heating, insulated, ceiling)

[MT Man]

I've been keeping three extra bedrooms shut (doors and vents) while we are not using them. Today, I was told that it is better for my energy bill to open the rooms and the vents so that the air flow throughout the home is even. Would anyone agree with this concept or should I continue to keep my rooms blocked off from the rest of the house.

[ShaneSA]

Is it 2 story, one story, are the rooms at the end of the air distributiuon. I would think if the rooms are at the end of the air distribution and the return is not near, then leave it closed. if it is 2 story, where is the air return. There's alot of variables, maybe someone who knows air systmes has a better answer.

[MT Man]

It's a 2 story house with an open air balcony. The air return is at one end of the upstairs hallway and 2 of the closed rooms are at the other end (also where the balcony is). The third closed bedroom is next to the ceiling return in the upstairs.

The downstairs return is at the bottom of the stairs, which lead down from an open balcony and 2 closed bedrooms.

[Cosmic]

If this is not just air flow but is some sort of cooling, then closing off three rooms should lower your energy bill.

If it is not cooling but just air flow then a lot will depend on your equipment and how you have it set up to run. Is it very fancy high tech with variable speed capabilities or just fixed speed fan supplying the air?

If variable speed, the equipment can probably sense the volume it has to pump and adjust according. To supply less of the house, takes less power, got to move less air. You get less pressure drop overall. All depends on how smart your equipment is.

If it is just fixed speed, in theory you should also save. The fan again has to move less air total as a system. It will try to pump more to the rooms on service but will probably ride back on its curve a bit. In effect again doing less work.

You have to look at it as a system and how various combinations of vents / flow paths present demand to the system. As a general rule with the same main trunk duct work and less possible flow paths off it, system should have to do less work, maybe by wee amounts but still less. There is far more savings in being able to change fan speeds or have a very smart variable speed system that reacts to a entire range of inputs / conditions / variables. There the system is not just looking at flow in a dumb way but trying to react to demand as defined by something like temperature from a thermostat or some other variable in addition like outside temps. Total flow and power used will vary as demand varies.

Another way of saying you have not defined the problem well enough for us and we don't know enough about your system. Or how you have it set up to run. Does it run all the time or does it start / stop based on demand? Less demand should give a lower bill in most all cases, if your actions accomplished that, should be less.

[sailordave]

I'd keep the vents open. Depending on the weather conditions, keeping the vents closed could potentially develop mold growth around the closed air vents and other places in the room. If possible, I would either fully open or partially open the vents in the rooms. Unplug anything electrical in the rooms not being used.

[NCN]

Originally Posted by MT Man

I've been keeping three extra bedrooms shut (doors and vents) while we are not using them. Today, I was told that it is better for my energy bill to open the rooms and the vents so that the air flow throughout the home is even. Would anyone agree with this concept or should I continue to keep my rooms blocked off from the rest of the house.

The answer is in who told you that. We have a heat pump and there was a note that came with the heat pump when we first installed it that said for heating and cooling efficiency we should never block off more than 1/4 of our home. We just decided to leave it all open. I don't want to start calculating how big the rooms are and the effect they will cause on the unit.

BTW The electric bill for a heat pump is much lower than we had for an electric furnace.

[BigJon3475]

Originally Posted by MT Man

I've been keeping three extra bedrooms shut (doors and vents) while we are not using them. Today, I was told that it is better for my energy bill to open the rooms and the vents so that the air flow throughout the home is even. Would anyone agree with this concept or should I continue to keep my rooms blocked off from the rest of the house.

I agree. Undercuts and transfer grilles should be used when airflow is a problem between rooms and returns. If you do not want to cut your door to 1.5" or 2" undercuts then transfer grilles are the way to go.

http://www.hartandcooley.com/tech%20...ech%20Talk.pdf

More good tech questions:
http://www.hartandcooley.com/tech_corner.htm

[MT Man]

Thanks everyone. It comes down to whether heating and cooling the rooms that are not used is more efficient than leaving them closed off with the central system vents closed. I guess the answer is "it depends on the house."

[BigJon3475]

Interior walls are usually not insulated so letting one get hotter or cooler allows the heat to transfer from the ones that are faster. The greater the temp. difference the faster heat travels. Also by closing the system off, though not linear on a performance curve because the way HVAC systems work, the larger the effective system becomes. Over-sizing short cycles your system and doesn't allow it to equalize as it should. The very first bit of your system running is the point where it's using the most power. If you'll notice the higher efficiency systems run longer, although mostly on the lower stage, keeping air flowing and the system constantly removing heat. This tends to remove more moisture from the home allowing you to keep the set temp, dry bulb setting, higher. The wetbulb and reltive humidity readings will be lower.