Closing vents in unused rooms
user-1120876
| Posted in Mechanicals on
My local utility had the following info on it’s web site regarding closing vents in unused rooms. Any contrary thoughts on this?
“It’s true that closing a vent or two may reduce the furnace’s efficiency slightly. But if you’re closing a vent to avoid heating an area that doesn’t need to be heated, the energy you save will be greater than the energy lost due to the slight increase in inefficiency.
Bottom line: If you don’t need to heat a room, close the vents in that room.”
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Replies
Stephen,
Your local utility is giving you bad advice.
Iain Walker, a staff scientist at Lawrence Berkeley National Laboratory, researched the issue. In his paper ("Register Closing Effects on Forced Air Heating System Performance"), Walker reported, “The results of this study showed that register closing led to increased energy use for a typical California house over a wide combination of climate, duct leakage, and number of closed registers. The reduction in building thermal loads due to conditioning only part of the house was offset by increased duct system losses, mostly due to increased duct leakage.”
For more information, see More Energy Myths.
Your utility is not alone. Many other utilities also provide bad advice. For more examples, see Stupid Energy-Saving Tips.
I'm glad that Martin publish the link to the study that he basis his response. I read the study, and from what I could understand, the study was done for a typical California house, which has it ducts located outside
conditioned space. The study basically found that the losses via duct leakage was higher enough that closing registers simply caused any saving to be leak out into the non conditioned space. The study also seem to say that closing only a couple of vents (out of ten), might not reduce the efficiency below acceptable minimum efficiency of the furnace. The study also said that closing registers farthest from the furnace has the smallest effect on duct pressures. The study might be more interesting if it had been done with a system who duct where inside the conditioned space, instead of outside it. It that case, the energy lost via leakage might be a great deal smaller.
The study was done in 2003, so changes in technology might make a difference. I replace my heat pump six years ago with one that has a ECM fan, and it seems to me that at least in heat mode, the system tries to kept the duct pressure the same, so closing several registers should result in the furnace slowing down the fan and saving some energy. I now have a smart electrical meter on my town house, so if I can figure out how to pick two weeks with the same HDDs, I will try closing off one of my rooms that I don't use.
I'm seeing a lot of ads for thermostat-controlled registers that can be set remotely from a smartphone to create an easily retrofit zoning system. People love gadgets so these will probably be popular. It may be a good time to re-evaluate and either confirm or update the results of the California tests with an up-to-date sealed modulating installation as Carl suggests.
"The study basically found that the losses via duct leakage was higher enough that closing registers simply caused any saving to be leak out into the non conditioned space. "
It's not just that the duct leakage may be into non-conditioned space, but that the pressure imbalance between rooms created when the return ducts still being open when the supply ducts closed drives the whole HOUSE ENVELOPE air leakage rates higher. A constant-pressure ECM drive air handler it would mitigate the duct leakage issue, but not the duct imbalance & room depressurization problem that arises when supply registers close without a corresponding reduction in return duct impedance.
The thermostatic controlled registers may be good for comfort, but they are not an energy saving device. Since they are often advertised as a energy saving device it's incumbent upon them to actually make their case with real measurements rather than not fully-baked sorta-plausible arm waving theory.
Carl, Yes those concerns do come to mind. Seems like a bit of a weak study to use as a "myth buster"...
As with most studies and HVAC based conservation measures location and system details matter greatly. The depressurizing of rooms is often touted as an issue as well - however last I checked you can block or restrict return as well...
If I recall Martin's blog regarding the myths had some good comments/discussion.
However with most residential based energy measures - the savings is often so small thhat it can easily be lost on bill data as almost error. (or things like behavior, occupancy, plug loads,weather, sun, etc...skew results)
Catch 22? Even leaving aside Dana's point about whole-house pressure balance issues, a well-insulated relatively airtight home with modern HVAC system has the least to gain from room-by-room setbacks, while a leaky poorly insulated old house with crappy HVAC running through unconditioned space has the most to lose from ductwork losses. Lose/lose for register-controlled setback savings I guess, unless as Dana suggests the industry can demonstrate otherwise.