Wood floors are said to add warmth to a room. Sometimes that can be literal, as the registers can can make a room colder (or warmer) than desired. To understand this, let’s take a look at HVAC basics.
Understanding HVAC
The objective of a forced-air heating, ventilation and cooling (HVAC) system is to put enough heated or cooled air into a room to maintain the desired temperature. (In this article, we’ll just refer to “heating”.) Each room in a house requires a certain amount of heat (in BTUs/hour) to maintain that temperature. Heat is added to air moving through a furnace or heat pump. Then a certain amount of air (in cubic feet per minute, CFM) is sent through ducts to rooms.
A fan in the HVAC system is used to circulate air through the ducts. Each fan has pressure from friction of the air moving through ducts, filters, registers, grills, and anything else the moving air contacts, such as UV lights and humidifiers.
An HVAC system designer calculates the heating requirement, or load, for the entire building and individual rooms. Then the system is designed to get the necessary heat to and from each room; this includes ducts that terminate in a grill or register.
A quick clarification on vocabulary: Vent is a generic term for the open end of a duct. A grill is a cover over the open end of a duct. A register is a grill with an adjustable damper, flap or other means to adjust air flow.
Selecting a supply register
Supply registers affect the whole system. They have the following characteristics:
Air flow and pressure loss in a register are based on the size and shape of the register’s openings. The area of the openings is referred to as the free area (FA) and ranges from less than 9 in2 to almost 30 in2 in a 4-by-10-inch register. A larger FA typically means more air flow and less pressure loss.
Area coefficient (Ak) is similar to FA but takes into consideration the size and shape of individual openings. A larger Ak is generally better than a smaller Ak.
Spread and throw are indicators of the pattern of the plume of air coming out of the register. Spread and throw affect distribution of the air in the room, but are not as important now due to more energy-efficient construction.
RELATED: How Inside Air Affects Wood Floors
Noise is determined by the velocity of air coming out of a register. Standards say that face velocity at registers should be between 500 and 750 FPM (feet per minute) in residential situations. Many designers aim for no more than 600 FPM.
While spread, throw and noise may affect overall occupant happiness, getting the right amount of heat into a room is a building code requirement. The wrong register can reduce the air flow, resulting in uncomfortable temperatures and building code violations.
A typical challenge
Say you install wood flooring in a bedroom and choose wood registers for the existing two vents. Come winter, the room isn’t as warm as expected. The chosen wood registers created more friction that the metal ones planned by the HVAC installer, so the air flow into the room is less than required. Some wood registers will only provide two-thirds the air as a metal register at the same pressure. Some plastic registers will only provide one-fifth the air.
Tweaking an existing system
You can sometimes tweak a setting in the HVAC fan to get more air flow, but the face velocity will be higher, potentially making more noise. Also, if you tweak a fan setting, you may get more air out of registers in other rooms, causing them to be hotter than designed. Adjusting dampers in the other rooms might help. But if you have installed wood flooring and registers in multiple rooms, adjusting the fan and dampers may not be enough. Then major changes to the HVAC system, like changing ducts and adding vents, may be necessary.
Choosing a wood floor register
I am absolutely not saying that wood floor registers are bad. What I am saying is that there are bad uses of wood floor registers—and bad uses of metal and plastic registers, as well. Blindly sticking them in an existing HVAC system can cause problems. Buy your registers from reputable companies who can provide the data that allow HVAC professionals to design appropriate systems and help select registers for existing systems.
