The natural response to shut registers in rooms that are “already comfortable” seems like the right thing to do. If the air is not going where it’s not needed than it will go where it’s needed more, right?! Unfortunately, air is not destroyed when you close a register, it has to go somewhere. When you continue to force restrict that air, it increases pressure on your air-handling equipment. When pressure on your air handling equipment gets too high you start blowing motors, capacitors, and evaporator coils. This pressure is called static pressure.
The safe upper limit for closing supply registers is around 20%. Over that, you’re not balancing the system, you’re blocking it. This is important because 90% of room comfort issues are not a result of one room getting too much air. They are a result of the system not effectively distributing air to each room. If you want to fix those issues, it’s time to understand how an HVAC system does that.
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The Return Air Problem Most People Miss
Conditioned air must be able to circulate out of the room for more to enter. If you can’t get that room back to temperature, the return air equation is the reason why. Every supply vent pushes air into a space. That space needs a return path – either through a return air grille, a gap under the door, or a transfer grille built into the wall. When bedroom doors stay closed and there’s no return path, pressure builds up in the room. The incoming conditioned air hits that pressure wall and stops flowing.
The fix isn’t complicated in principle: leave interior doors open, install transfer grilles, or have jump ducts added between the room and the hallway. What won’t fix it is turning up the thermostat and waiting.
Use Your Thermostat Settings to Help Circulation
Most thermostats will have a fan setting that is usually a choice between “Auto” and “On”. When it’s in “auto” mode, the blower will only operate while your system is actively heating or cooling the air. When it’s set to “on,” the fan will run continuously, even if the air isn’t being cooled or heated.
While switching the fan from “auto” to “on” won’t fix an inherently unbalanced duct system, it does assist with the issue of temperature stratification. In two-storey homes, for example, the upstairs can often feel warmer because hot air naturally rises. Likewise, the downstairs can feel cooler, since cold air sinks. When the air is constantly moving, however, temperatures tend to equalize over time.
A ducted air conditioning service can help you determine whether or not your ducts are unbalanced, as well as whether or not the addition of a zoning system would alleviate your particular area deficit. Zoning systems provide independent thermostats for different sections of ducts, allowing you to manually adjust damper settings for different rooms and zones for optimal control.
Start At the Ductwork, Not the Registers
The vents on your walls and floors are actually the final stage in the supply chain. They’re a rather crude adjustment mechanism, at best. If you want to redirect air in a meaningful way, look not to the end of the system, but back further – near the trunk, where the main branch ducts split off from the plenum.
The plenum is the big distribution box that sits right on top of your HVAC unit. Each branch duct that shoots off to a different part of your house should have its own damper – a small, adjustable sheet metal plate with a lever or screw control, on one side of the duct, a foot or so away from the plenum.
By partially closing the dampers of a particular branch, you can reduce the airflow to all the rooms it serves. Unlike closing registers, if you overdo it, the blower motor won’t have to work so hard to push that air through. There won’t be so many idled fans and no dams blocking airflow to the furnace.
Before you start twisting any screws, though, take one operational step: run your heat or AC for about a half an hour, to let the fan reach full speed. Then, take a slow walk around your house to each supply vent – which is a good excuse to put your palm over the air duct for a comfort check.
When the Problem Isn’t Adjustable
Manual balancing can only go so far. Some systems are just too small for the cubic feet they’re supposed to heat or cool. Typical energy losses such as poorly insulated or leaky windows or over-sized equipment can also swamp the smallest of attempts you might make at balancing airflow, or create problems that poor airflow “balance” can mask.
The more likely explanation when one room is constantly too cold or too hot is that it’s getting more or less conditioned air than the space next to it. The simplest reason airflow would be off is that the manual dampers in your ductwork are in the wrong position. Cheaper dampers are notoriously difficult to adjust and even the good ones get knocked around and lose calibration due to pressure changes in your ducts. Other possible reasons are that return vents in that room have been blocked or closed for some reason, or the high and low vents on the air handler itself (if it’s got them) are improperly set.
The Boundary Between Adjusting and Diagnosing
Most people who own a home can accomplish significant airflow balancing by adjusting dampers and managing return air. Those two actions alone solve most of the temperature complaints with the least impact on your equipment.
When that is no longer sufficient, your HVAC is giving you a signal about its real, physical constraints. That’s not a bad thing to find out – it’s useful information. The next step is a professional measurement of what is truly moving through the ductwork, not another seat-of-the-pants adjustment.
