Russell King, M.E.

Disclaimer:  The vast majority of my HVAC design experience is for new construction, production homes, in hot-dry climates (CA and NV).  I’ve estimated that I designed at least a couple thousand plans, each plan built many times in multiple orientations.  I never had any significant comfort complaints result from my designs.  We always used Manual J/S/D.  We designed to high airflow (>400 cfm/ton) low-ish external static pressure (<0.6 IWC) and we were careful not to oversize the air conditioners.  We also field tested most of these designs.  This article is based on that experience.  I don’t claim to be an expert on the topic of room pressurization, nor have I done a whole lot of research on the topic.  I’m just sharing my experience and opinions so that, good or bad, others may add it to their knowledge base and make informed decisions. Please feel free to comment and give your opinion and experience.

To start, let’s define the problem.  First, imagine a house with no interior doors and one central return grill in a hallway.  The return path from a supply register in a bedroom is out of the room through the door and down the hallway.  The rooms and hallways are essentially return ducts – nice, big, open ducts with little to no resistance to the air.  I think (I hope) we can all agree that in this situation, there would be no need for ducted return grilles in every bedroom.

Now add the interior doors.  These doors are essentially dampers in our nice big return ducts.  When these door/dampers are closed it creates significant blockage in our nice big return “ducts”.  I think (I hope) we can all agree that it’s the doors that are causing the problem.  Theoretically, they stop the air from leaving the room, which prevents supply air from entering the room, potentially causing under-conditioning of the room.

1. My first question is:  How often and when are these doors closed?

• My second question is: When a door is closed how big is the impact on the supply air flow to that room?

• My third question is: How should we address the problem caused by closed doors?

My experience designing and testing HVAC systems in homes leads me, personally, to the following answers:

#1. Not often enough during peak load conditions to be a problem.

#2. Not big enough during peak load conditions to be a problem.

#3. What problem?

Keep in mind that the homes I design are very energy efficient (compared to other homes built at that time), the systems are properly sized, and the ducts are well designed for good airflow. This means that during peak loads (very hot days) the system is running almost continuously, the air is mixing well, and the supply air is not coming out of the registers super cold or super hot.  I never really made a big effort to check, but I recall that most of the bedroom doors in these homes had decent undercuts, say, 1” above the flooring.  I should also mention that I always insisted on a dedicated return in the master suite. If all these boxes are checked, I contend that pressure relief in secondary bedrooms is not necessary for comfort reasons.

I always try to make a point to say this in every class I teach: “A well-designed system will forgive a lot of sins.”  What I mean by that is a house with a properly sized system (not oversized) and good airflow will work fine despite:

• Less than optimal register locations
• Less than optimal room by room air balancing
• Less than optimal thermostat location
• Less than optimal owner behavior, and
• Some room pressurization

The reason I emphasize comfort a couple paragraphs earlier is because there may also be energy efficiency reasons for pressure relief.  The Florida Solar Energy Center (FSEC) did a study on how room pressurization increases infiltration (exfiltration) and therefore energy consumption.  I have not read this study yet and do not know how tight these homes were, how often the doors were closed, how effective the door blocked airflow, how well the ducts were sized, etc.  Assuming that there is increased energy consumption, a cost effectiveness evaluation is warranted to justify pressure relief strategies.  As we will see, some strategies are more expensive than others.

I have tested houses designed by other people where there were severe comfort issues and room pressurization seemed to be part of the problem.  They installed pressure relief strategies and the problems mostly went away, but unfortunately, they also did other fixes, such as increasing return air capacity, sealing ducts, etc., so it is impossible to know how much impact the pressure relief strategies had or how much of a problem they were to begin with.

I guess you could say that my most used pressure relief strategy is to have big supply air ducts.  By “big” I mean a lot bigger than those installed by people who do not used Manual D.  I might put an 8” duct in a room where someone else might think a 6” duct is fine.  In very simplistic terms, when your duct system is big, the air is moving slower, the pressure drop into the room is lower and the overall system can compensate more easily to parts of the system being blocked off.  For example, if you were to close off a register in a room in a house with “big” ducts, that back pressure is easily absorbed by the rest of the system.  If the house had small ducts, the new back pressure is felt all the way back at the fan.  In other words, the pressure behind the closed off damper at the register is greater in a system with small ducts than in a system with large ducts.  I have not personally tested this, but if that damper is now the bedroom door, rather than the register, the pressure in the room should also be less when the ducts are big, all else being equal.  On the other side, one could argue that a system with big ducts is more likely to reduce airflow to a room when a door is closed because there is “room” for it to go elsewhere.  True, but the room is less pressurized, so the energy impact is reduced.

So, other than big ducts, what are the more common pressure relief strategies?  I categorize them as follows:

1. Louvered bypass grilles
2. Jumper ducts
3. Dedicated ducted returns

I will evaluate them based on the following qualities (or lack thereof):

• Balance/Comfort –Does it reduce the impact of opening and closing doors and thereby increase comfort?
• Noise/Privacy – We have doors on rooms for a reason.  Does the pressure relief strategy allow outside noise to disturb the room occupants? Does it allow private conversations inside the room to be eavesdropped on from outside the room?  Note that this is very hard to quantify and varies widely from family to family.  I personally think this issue gets more attention that it deserves, but I never lived in a large family.
• Aesthetics – Is it ugly?  Is it visually conspicuous?
• Cost – Is it expensive to install relative to the other options?
• Energy – Does it increase energy consumption?

Here are the three strategies, in detail:

1. Louvered Bypass Grilles

These basically just provide additional pressure relief much like the undercut of the door.  They are usually the simplest and easiest to install.  They can be as simple as a louvered door, or back-to-back grilles in an interior wall (one side to room, one side to hallway), or ducted high/low grilles on either side of an interior wall.

There are some very nice-looking louvered doors available.  Personally, if I could design my own house from scratch, it would have fully louvered doors everywhere but on the bathrooms.  I just like the look and the good air circulation.  Doors can be fully louvered or partially louvered.  Even off-the-shelf panel doors from the big box stores have room at the bottom to install a 6” tall louvered panel.  These louvered door panels are common in commercial applications

Back-to-back louvers are similar but, in a wall, rather than a door.  See drawing, right side.

High/low louvers are similar but to mitigate sound transmission one side is up high and one side is down low.  See drawing, left side.  I have never seen these installed.  I heard of someone who wanted to, but a sharp building inspector pointed out that the section of the code that prevents us from using unducted building cavities as return ducts could apply here and that the stud bay should probably be lined with an approved ducting material.   In a hot-dry climate, I would not have any issues with passing room air through a well-sealed, unlined, stud bay, but in a humid climate it could be a mold issue. 

Sizing these louvers is tricky.  It depends on a lot of things.  I’m sure there are fancy equations for calculating the pressure drop of the various options, but I prefer empirical data and experimentation.  A target pressure differential across the door that I have seen referenced many times is 3 Pa.  I’m not sure how this number was arrived at, but it seems reasonable.  If you don’t like it, pick a target – the lower, the better.  It would be quite easy to build a test chamber using a calibrated fan, such as a duct tester and test different size louvered configurations at different airflows.  If the grilles you are using have good performance data, such as for supply registers, you could use that to predict pressure drop too. Be sure to account for the door undercut, or just use that as “safety margin”.

Here is my “scoring” of louvered bypass grilles as a pressure relief strategy.

• Balance/Comfort – Very good.  Assuming proper sizing these should perform fine.
• Noise/Privacy – Probably not good, especially fully louvered doors.  With louvered grilles I can imagine a younger brother being caught with his ear to a louvered register spying on his big sister’s phone conversations.  (It’s amazing what you can hear with your ear to a floor register when the system is not running, but no one complains about that.)
• Aesthetics – I love the look of louvered doors, but then, I also like jalousie windows because they remind me of living in Hawaii as a child and the windows on our old VW camper.  On the room side, louvered grilles can be hidden behind furniture, but they might not look great on the hallway side.
• Cost – Louvered doors can be pricey.  Back-to-back louvers installed in walls (or doors) are probably the cheapest of all options mentioned here.  High/low louvered would be expensive if you had to duct the wall cavity, otherwise pretty cheap.
• Energy – Great.  No negative impact on energy use.

• Jumper Ducts

These are very common in some parts of the country and in some energy efficiency programs.  They are probably the most common pressure relief strategy.  They are similar in function as louvered bypass grilles.  They provide an alternative return path past the door in addition to the door undercut.  The main difference is that they are in the ceiling rather than a wall and they are ducted.  There is one register in the room and then there can be a shared register or individual registers in the hall or common area.  When the system is running and the door is closed the air goes up into the register in the ceiling of the room, through the duct and out the register in the hall and back to the return.  See diagram.

Again, sizing of the ducts and registers can be tricky, but I have seen some sizing charts that people have put together specifically for this application.

Here’s my evaluation of jumper ducts:

• Balance/Comfort – I think these are very good too.  Assuming proper sizing, they should work very well.
  • Noise/Privacy – Definitely better than the louvered bypass grilles.  Being up in the ceiling and ducted limits most sound transfer.
  • Aesthetics – Probably better to have the grilles in the ceiling than in the walls.  The ability to share the hallway grille helps too.
  • Cost – Substantially more than louvered bypass registers.  You have a ceiling boot at both ends, the duct material and the labor to install it all.
  • Energy – If the bypass duct is in conditioned space, there should be no energy impact, but this is probably rare.  In the more common scenario of jumper ducts in a vented attic, I think the energy impacts are their biggest disadvantage. They increase surface area for conduction, and unless perfectly sealed, they increase building infiltration.

• Dedicated Ducted Return

This is basically putting a return duct in every bedroom.  Each bedroom will have a supply and a matching return, presumably sized to handle the same amount of air.  As I mentioned earlier, I insisted that all my designs had a dedicated return in the master bedroom.  This was because in new homes the master “suite” was quite large.  They usually included the master bedroom, master bath, toilet room walk in closets and sometimes a separate retreat area. The amount of supply air going to that side of the main door was quite large and the pressure across the closed door could be substantial.  I had good success with this strategy, but occasionally I found that if the owners kept the master bedroom door closed a lot, like all day, that room was not well monitored by the thermostat out in the hall and sometimes the temperature drifted away from the thermostat setpoint.  In larger homes that had more than one system, we often put the thermostat in the master bedroom for that reason.

• Balance/Comfort – Poor.  Now, I’m admitting a bias here.  I only designed one project where the builder insisted on returns in every bedroom, and it was a nightmare.  This was a subdivision of large (3500-5000 sf), one-story production homes.  There were three or four models.  The builder was new to Las Vegas and had previously built homes on the east coast.  Despite my objections, he insisted that a return in every bedroom is what made a house a “quality” house.  The problem was that the City of Las Vegas required balance testing on the sales models of all subdivision projects.  This meant you had to measure and report all supply and return airflows at every register and they had to be within 10% of design.  This was a very hard criteria to meet for regular systems.  I had no idea how hard it was going to be for multiple-return systems.  At that time they did not specify if the test was to be performed with bedroom doors open or closed, so we tested both ways in hopes that one would pass more easily. 

I won’t go into the specifics, but even though we very carefully sized the ducts according to strict Manual J/S/D protocols, it was a nightmare to balance these systems.  Here’s just a small example:  We would measure the supply and return airflows to a bedroom with the door open.  They would be different despite being the same size ducts, registers, duct length, everything.  It is impossible to intentionally make the total equivalent length (TEL) of a supply and return duct be the same.  We could adjust the registers and maybe some balancing dampers until they were close, but then we would close the door and measure airflows.  They would both change by different amounts!  Without making any further changes, we would open the door and measure again.  Would they go back to the original measurements? NO! They would be totally different than before.  No other changes were made.  It was infuriating.  It has to do with the fact that there are two pressures acting on the room instead of just one and that there is more than just static pressure forcing air down certain pathways in the ducts.  There is also velocity pressure.  Think of it as the momentum of the air.  When you change velocity pressure it changes the way the air “wants” to go.  When you change the static pressure back, the air might say, “Nah, I kind of liked going this way.” 

I suspect that few contractors who install systems with returns in every bedroom ever had to balance them like we did.  If they had to, I’m sure they would find an easier way to achieve pressure relief.  Once we finally got them dialed in I told the builder that because he insisted on dedicated returns against my advice, we would not be responsible for any additional “fine tuning” requested by homeowners.  There weren’t many requests, fortunately, but the systems weren’t as trouble free as one would have expected for all the extra expense and work that went into them.

Despite that bad memory, I also think there are other problems caused by this design strategy.  I alluded to it earlier where I mentioned putting the thermostat inside the master suite. Everything, of course, depends on the system and layout, but for the “typical” system I have found that comfort is best achieved when the thermostat is measuring a good representative sample of all the air in the house.  This depends greatly on the thermostat location and that is a whole other topic to discuss later.

When a system is running, you want the air passing by the thermostat to be a good representation of air from all the rooms.  When you give a room its own return you effectively take that room out of them mix.  You take away its “vote”, so to speak.  If the thermostat is in the hallway where the bedrooms are and all bedrooms have their own returns, that hallway can actually become stagnant with no moving air.  Thermostat location is very important in these types of systems.

• Noise/Privacy – Good. No sound transmission
• Aesthetics – Good. Fewer registers than the other options.
• Cost – Poor.  Much more expensive than the other options.
• Energy – Poor.  Unless ducts are located within conditioned space, this strategy will greatly increase duct leakage and convection.  Also, note that longer return ducts add significant equivalent lengths to the entire system, which changes the friction rate for every run.  This potentially could require larger ducts on both the supply and return sides to compensate for this additional resistance.

Conclusion

I can’t emphasize enough that a good design (not oversized equipment, good airflow, low design ESP) greatly reduces the need for these types of “enhancements” to a ducted HVAC system. 

Secondly, I don’t want to discount the induced infiltration and energy problem.  It is worth evaluating the energy savings vs the cost of these strategies on a case-by-case basis.  More research is needed here.  I suspect that in tighter homes with better designed duct systems, the impact is greatly reduced.

Lastly, until I have seen some detailed temperature vs time data logger analysis of systems with and without different pressure relief strategies, I will be skeptical of their overall benefit to comfort.  Here’s why: If closing a door supposedly creates pressure, which reduces supply air to a room, then the result would be an underconditioned room (hot in the summer, cold in the winter).  What I have actually heard from as many homeowners as not, is that the rooms where the doors are often closed get over conditioned.  In fact, in my own house, in heating mode, if we close our master bedroom door, it gets too hot at night.  If we leave the door open, it’s just fine.  We do not have a return in that room, btw.  This makes no sense to me.  Clearly there are other factors at play besides just a pressure difference across the door and it will take more to remedy than just pressure relief.