The Benefits Of Displacement Ventilation

The world has been shaken to its core by a respiratory virus pandemic. Humanity has been raiding the toolbox for every possible weapon in the fight, whether that be masks, vaccinations, or advanced antiviral treatments.

As far as medicine has come in tackling COVID-19 in the past two years, the ultimate solution would be to cut the number of people exposed to the pathogen in the first place. Improving our ventilation methods may just be a great way to cut down on the spread. After all, it’s what they did in the wake of the Spanish Flu.

Up, Up and Away

The trick to displacement ventilation is supplying air at ground level at low velocity. Buoyancy forces it to the top of the room where it can be extracted. This creates a one-way flow in the space that carries nasty stuff up and away. “Floor Vent” by Daniel Morrison

When most of us think of Heating, Ventilation, and Air Conditioning, or HVAC, our mind probably wanders to ducted heating and cooling systems and those typical vents in the ceiling. Whether it’s at home, in the office, or in retail settings, this is one of the most typical kinds of ventilation used in modern buildings.

Some systems run in a recirculating manner, while others bring in additional fresh air from outside. Either way, they essentially spray conditioned air into the top of a room. They rely on the velocity of the air to mix the conditioned air throughout the entire space.

This is referred to as “mixing ventilation” for the way it ends with a relatively homogenous mixture of air throughout a given space. When dealing with a virulent airborne pathogen, the problems with this method are obvious. It’s a great way to ensure viral particles get evenly distributed in a room, where they can infect many people.

Displacement ventilation works differently, by taking advantage of the buoyancy of warmer air. When ventilating in this manner, low velocity air is fed to the lower levels of a space. This air then picks up warmth from occupants of the space, lighting, and such, and then gradually rises to where it is extracted at the ceiling.

The air can be cooled or simply fresh air of ambient temperature, or the system can be used to deliver warm air for heating a space. In the latter case, it may not be the most efficient heating method, as the warm air quickly rises upon entering the space. However, it still works, and maintains the fundamental principle of ventilating a space from bottom to top.

This method has powerful benefits for human-occupied spaces. The flow generated from the ground to the ceiling helps carry away undesirable elements in the atmosphere, immediately removing them from the occupied zone. In contrast, mixing air systems tend to churn all these up in a room, distributing them homogenously. All the bad stuff, from viral particles to carbon dioxide, gets sucked up and out. All one need do is supply fresh (and ideally filtered air) at the lower level and the room is nicely ventilated with a healthy atmosphere. Studies have shown that displacement ventilation can cut CO2 levels in classrooms by 17-27%, with potentially beneficial knock on effects to health and cognition.

It’s not actually a new idea; displacement ventilation has been around for a long time. Such a system was famously implemented in the Orpheum Theatre in Vancouver, BC, when it was built in 1927 in the wake of the Spanish Flu epidemic. Fresh air was brought in from outside, and dust and contaminants were removed by passing it through a water spray system. It was then heated as desired, and delivered to the theatre via mushroom-like vents placed under the seats. The air delivered would then rise through the space, exiting through various openings in the porous ceiling. This was notably a great way to remove cigarette smoke from the theatre during a performance, and came with benefits to carbon dioxide levels and pathogen spread, too.

Displacement ventilation isn’t the norm, particularly in residential applications. Those that don’t work in the world of HVAC may not have even heard of it. However, buildings from Carnegie Hall to lecture theatres at the Norweigian Institute of Technology all rely on this method of ventilation.

However, the fundamental equipment and principles involved are simple. Any properly qualified HVAC technician could apply these principles to a given space. It’s straightforward to implement, and it’s not cost prohibitive. For spaces in which it’s desired to cut down on pathogen transmission, the benefits are there for the reaping!

[Banner photo: “Orpheum Theatre Vancouver View Of Stage” by MichaelThoeny]

27 thoughts on “The Benefits Of Displacement Ventilation

  1. Displacement ventilation is pretty common in residential construction when a house is built on a crawl space. In areas where slab construction is prevalent, I doubt displacement ventilation is used.

    1. Yeah, that part threw me as well. In the United States, I suspect that we do more crawl space foundations than not, so I’m more familiar with seeing displacement-style ventilation in residential, and the mixing ventilation in commercial buildings.

      And even if the house doesn’t have a crawl space, the builders ultimately do what’s easiest, even if that means a mixture of both ventilation styles. For example, my brother’s 2-story house in Missouri appears to be built on a slab, so all the ductwork is ran through the first story ceiling. If you’re on the first floor, the vents are on the ceiling (hence mixing ventilation), and if you’re on the second floor, the vents are on the floor (displacement ventilation).

  2. In my experience, that “floor vent” photo would indicate a gravity feed furnace, not passive ventilation. Though I guess it would definitely still qualify as displacement ventilation.

    In early 20th century homes, such a furnace had huge grates like that on hot outlets and cold inlets. Hot air from the furnace would rise up from the basement, into the living spaces, and push cold air down the inlets.

    In college in the 80s one of my housemates figured out he could suck almost all the hot air into his room with a box fan. It took the rest of us a while to figure out why we had huge heating bills but a freezing cold house.

    1. Yeah, it is also called laminar flow ventilation. I don’t think it is practically ever actually used in residential. Certainly that vent in the pic is actually a floor intake vent, not a low speed output vent. Poetic license I guess. However it’s inaccurate because you need a larger orifice to get the low air velocity required.

  3. The college housemate comment points out the problem with displacement ventilation when used in a heated space. Normal forced air systems recycle the heated air after it passes through the living spaces, being filtered before passing back into the heater (or heat pump heat exchanger) cabinet. While the air cools a bit in this cycle from heater outlet to heater inlet, the air re-entering the heater/heat pump is much warmer than outside air (assuming you are in a substantially cold place, like the northern US in winter).

    Throwing away your heated air and importing new, cold air for every heating cycle is much more expensive than the recirculating systems in use today.

    On the plus side, most of us forced-air users can put HEPA filters in ‘the loop’, mitigating the spread of air-borne disease. UV lights in our heater/ac cabinets help this way too. Of course, CO2 concentration remains an issue and is one reason modern forced air homes are designed to be ‘leaky’ enough to mitigate CO2 accumulation. I wonder how much a CO2 ‘scrubber’ would cost to add to a house?

    1. That’s why they have Heat Recovery Ventilators (HRVs) and Energy Recovery Ventilators (ERVs). They recover some of the heat or cooling from the fresh air coming in while exhausting the excess quantity of air returning the the air handler unit.

      1. Quality HRV units are expensive and always a pain to place in the system. Of course not all the heat is recovered so energy use to condition the fresh air is high. This is the reason “100% fresh air” (or much over whatever minimum healthy amount of fresh air) is not typically done in residenses or most commercial buildings unless required for special purpases. Not only do you have to heat the air in cold climates you have to remove water vapor in cooling mode and cool the air.

        1. DOAS (Dedicated Outdoor Air Source) units are used for commercial kitchens to make up the exhaust air over cooking appliances or in hospital rooms or labs with up to 100% fresh air requirements. They are 2-3 times bigger than recirculating type HVAC units because outside air can be that much hotter or colder than your usual 75-80 degree return air. And they are staged or modulating to make up for the outside temperature variation from day to night and season to season. Super expensive to buy the unit upfront, and expensive to run, they aren’t really a good solution to stopping the spread of disease in the face of climate change.

          For most places increased filtering and sanitization is a much more cost effective and energy efficient strategy. Most times you can’t just slap HEPA filters in a standard air handler, the airflow restrictions have to be engineered for. UV lights were really only meant to keep the coils from growing mold. Ozone generators are the cheapest solution, but as ozone can be toxic in high concentrations it has to be designed right as well. And most of all be careful, there’s a lot of “snake oil” feel good solutions being sold out there that don’t do anything.

      2. Yes, roughly 20 percent of all global warming emissions are caused by the need to heat all this fresh air, about half of all building heating requirements.

        An erv can save 85 percent of that. Theoretically, 17 percent of all global warming emissions. Pretty good for a single bit of tech!

        I have designed an open source printable single room erv,

        Unfortunately as some people point of no they are not free, however because they have energy they save money, so they can pay for themselves. An. Openerv at $500 purchase price pays for itself in only two years displacing electric heat. That’s very good as any green tech goes. Solar panels are more like seven years payback time.

    2. Once you’re using forced-air, you mas as well dump the displacement concept and switch to downdraft flow – as used in clean-rooms – with the added benefit of scrubbing particles down to floor level and out rather than trying to pull them up (which may result in suspended particles at varying heights dependant on particle size and mass, and that major particle generates like carpets and footwear being at the start of airflow rather than the end).

  4. While in grade school in the late 50s and early 60s, I lived in a house built in the late 30s. It had a central furnace that was converted from coal to natural gas (they just kept the coal door locked). That furnace had a large air intake (about 20 by 40 inches), which was the only “cold air” intake into the system (of course, the vents were covered in asbestos). The furnace itself was about 4 feet in diameter (maybe because of the original coal firing).

    Now I live in a house built around 1915 (per an electrical inspection sticker found on a 2×4 during some remodeling that listed the rating as “15 LAMPS” — not Amps!). This house also used to use coal (same Bunting HW store door) but now has recirculating hot water and radiators — NO VENTILATION !!! Talk about dry air in the winter!

    1. The reason the air inside is so dry in your structure in winter is precisely because the house has so many air leaks. The very dry and cold air from outside is getting inside. Heating systems do NOT dry out the air.

  5. In the 90s I was familiar with this big old 1930s Cinema that had been repurposed and their ventilation system was pretty powerful… really only got used in the summer when the place would heat up quick with a lot of people in there… then it would start up with a boom, whirrrrrrr… and you’d feel it in minutes, musta changed the air several times an hour… guess it had a motor with low efficiency and they didn’t like the cost of running it the entire time.

    1. I used to work at a beautiful old Music Hall up the Hudson River. Before they changed out the AC 10 – 15 years ago, they had the original unit from the.. 30s? It could cool the place down fast with brute force – it’s coolant lines were 3″ diameter.
      When they rented out the space, they’d charge an extra $40 / hour to run the AC, and I suspect between maintenance and electricity costs, they weren’t turning much of a profit on that.

  6. The description of the Orpheum Theater took me back to my college job in building maintenance. Our main building and theater was built in 1929 had exactly that setup. The water sprayed out from fan-spray nozzles in a 10′ square (3m) duct. It was amazing how much silt had to be shoveled out of the catch tray after running the system. Seats had mushroom outlets under the seats and air intakes near the top of the theater. The blower motor was the size of a 55 gallon drum and the starter capacitor was the size of a loaf of bread.. We started the motor by moving a 4′ (1.3m) long handle with exposed contacts from the capacitor position to the normal running position. Oh and the pulley and belt was fully exposed. It’s hard to believe some of the things I used to do for $6/hour

  7. I suspect that most residential homes in the US that don’t have a basement are using high-mounted air vents, along with air returns in the ceilings, because that way all the ugly-looking ducting is in the ceiling. Same for commercial properties that also don’t have a basement or a raised floor.

    My house was built in 1961; and IIRC, the original equipment for it was a gas-fired heater in winter, and for summer cooling an evaporative cooler was connected to the same ductwork. It was retrofitted (probably in the late 80’s early 90’s) with a first gen heat pump that was woefully undersized for the task, and the swamp cooler was just left on the roof to rust away. I ended up replacing that in 2014/2015 with a more modern and properly sized heat pump, along with adding an extra ceiling vent and air return, but the old return vent from the 1961 era heater still remains, because that would require me to make some moderate modifications to the closet that used to house the heater. (the prior owner never re-framed the opening for a proper door, nor did they actually do anything with that space as far as closing it up. :( )

  8. We just use open windows and have a house surrounded by vegetation. Nobody in our large household has bothered with masks, jabs, or got sick with anything bad enough to get tested. Viral particles are just tasty treats for all of the other tiny living things that are normally found in a healthy and natural environment. It must suck to be trapped in an urban desert.

  9. What about energy efficiency? Here in Europe you aren’t allowed to build anything new without balanced ventilation. Natural ventilation is common in older homes, but it wastes a lot of energy in winter.

    1. >Natural ventilation is common in older homes, but it wastes a lot of energy in winter.

      Not really universally true, it certainly can be poor efficiency wise but it doesn’t have to be, and as always you design the system for the climate its built in, which changes how efficient any one type of system can be what is the right choice for x…

      Don’t get me wrong the active balanced systems are good, have extra potential like air filtering and are pretty adaptable to the climate. However they won’t blow just having the right airflow design to function passively (or just more passively) away efficiency wise in all situations, though they may well have better air quality (with how filthy city air in particular can be) and temperature control for that loss in efficiency.

  10. Many older houses in the Netherlands (“old” being pre 1990’s) lack any form of mechanical ventilation other than maybe some switch-operated extraction fans in bathroom, toilet and above the kitchen range.
    My parents house (brick farm barn, reconverted to living space in 1983) only had one extraction fan over the range. This place was originally heated with gas stoves in every room (renovated 5 years ago to floor heating + radiators, though in practice they use their wood stove most of the time). There were ventilation holes in all walls (which we taped shut in summer because birds nested in them and the fleas jumped in) to get some fresh air.

    My house (1967 appartment block, 6th floor, renovated in 2004) has permanent extraction in kitchen and bathroom, heating is via hot water radiators under the windows. Fresh air has to come from opening a window (which is cold in winter) or having ventilation openings I love the air quality, except it’s very, very dry in winter.

  11. Floor vents are terrible. You can’t have wall-to-wall carpeting without cutting an extra hole in the carpet. Small things like eyeglass screws fall into the vent, as does household dirt that builds up over time. Vents limit furniture placement in a room, and they’re something else to trip over. Bad as they are, they’re not as ugly as wall or ceiling vents.

    1. Complaints about the poor aesthetics of vents misses the point. And worse, their interference with “wall-to-wall” carpets, when carpets are one of the worst problems for indoor air quality (e.g. dirt, mold, bugs, formaldehyde, etc.), plus chemicals used their fireproofing treatments that are known to cause health problems including cancer. But, you’d think that with hundreds of millions to billions of people that this affects, that someone would come up with a more optimal solution.

    2. Why someone would want a wall to wall carpet is beyond my understanding. Carpets are nothing but pains in the backside to keep clean, especially if you house semi domesticated carnivores. Bad enough with 2/3 of the floor covered in carpet. Only reason i use carpets is to cut down on the echo and to keep my feet from freezing in the winter.

  12. I object to the focus on pathogen transmission. There are hundreds of reasons for good ventilation, and pathogen transmission is only one. It is the sum of them that matters. It is foolish to focus on the latest fad. This pandemic will pass. There are many other transmission vectors.

    Ventilation is general purpose and a quality of life issue. It should be obtained and provided efficiently for all buildings, all people , at all times, all seasons. It is worth it pandemic or not, the details are not really going to change that. So the way forward is clear. That’s a more sensible way to think of it imo.

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