Our remodeling projects include screen porch to sunroom conversions. We probably average one a year (we currently have one in design.) Will you need a HVAC return in your sunroom? Maybe. To answer the question, first you need a strategy for heating, ventilating and air conditioning the converted space. If your remodeler and HVAC contractor are at odds over whether to install a return, it's a safe bet that there isn't a strategy.

Typically, sunrooms heat up faster in summer and cool down faster in winter than other living areas. While your existing HVAC system may have the capacity to condition the new sunroom, I suspect it will be difficult to maintain comfortable conditions in the sunroom without adding zone controls and a separate thermostat in the sunroom. If you go that route, you'll need either a return in the sunroom or a transfer grill in the wall so that a return in an adjacent space can serve the purpose. Another (better, in my opinion) option is to provide a dedicated system for the sunroom such as a ductless mini-split heat pump. That gives you the option to control the conditions in the sunroom independent of other spaces, avoids the need to integrate new ductwork into your existing ductwork system (which can cause problems conditioning the existing living areas) and avoids the cost and complexity of a zoning system. I suggest you call a "time out" and review options with your remodeler and HVAC contractor.

Charles brings up good points to consider.

If you provide supply vent(s) to the new room and keep the door closed, the room will pressurize which is not good! For starters, in the winter, pressurization will force moisture laden indoor air through all the leakage paths to the outside. The moisture will likely condense in the walls as the temperature drops towards the outside, potentially leading to mold and rot issues within the wall assembly.

Do what Charles suggests and get things figured out now.

Thank you for all the information—very helpful. But how come the same principles do not apply to bedrooms or other rooms in a house which don’t have returns? We have three returns—one towards the front of the house near the garage entry, one in the back of the house in the LR going into the new sunroom, and one upstairs but the upstairs is on a separate thermostat.

Also, forgot to add, if it makes a difference, there will be a half wall of 36” all around and windows on just two sides. Thanks again!

"But how come the same principles do not apply to bedrooms or other rooms in a house which don’t have returns?" Best practice is to install returns in each bedroom of a home to ensure comfort. Installation of returns in some rooms such as kitchens, bathrooms, and closets are prohibited by code.

If you have separate thermostats on each level of your home you either have separate systems dedicated to each floor or a single system with zone controls.

Bedroom doors typically have a 1" gap at the bottom of the door which functions as a "return". The sunroom will have weather stripping all around and at the bottom. As Charles says, the heating/cooling load of the sunroom will typically be greater than a comparably sized bedroom, so you'll need some kind of air return functionality if you want warm air delivered to the sunroom. Without a return, the sunroom room will present a much higher resistance than the rest of the house so warm/cold air from the HVAC system will just go where the resistance is lowest (ie the rest of the house).

Are the two supply vents connected and operational? If yes then turn on the furnace so it is running at the highest speed and partially open the door leading into the sun room. If the air pushes the door open of closed (depending on how it swings) then that is evidence you need a return in the room. Air flow for cooling is typically higher than than that of heating. The problem may become worse in the summer.

Thanks again. The door going into the sunroom (from the LR) will be an interior door with a gap at the bottom. The HVAC system is almost 5 years old as is the house itself.

I don't advocate the use of door undercuts to provide a path for return air flow. The Building America Solution Center says this about them:

"Door undercuts are a commonly used method for pressure balancing between rooms (Figure 1) but they are not a recommended approach as they do always provide an adequate return air pathway for HVAC systems. Seasonal changes in wood can result in variations in the height of the door undercut opening. Changes to the flooring material, such as adding carpet, can obstruct the return pathway. The ENERGY STAR pressure-balancing thresholds are difficult to achieve with just a door undercut for most applications, except for rooms with minimal supply airflow (<20 cfm). In most applications, the door undercut would need to be 1.5 inches or greater to allow enough air to pass. There are some through-the-door alternatives that can be effective. Consider installing a transfer grille in the door or employing another return air pathway method, such as transfer grilles or jump ducts."

I don't disagree but door undercuts are what you find in 99% of homes!

There's a difference between best practices and common practices. Yes, you'll often find door undercuts substituted for proper returns in homes. They're a cost-cutting shortcut that compromises occupant comfort.