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Question DetailsAsked on 6/29/2016

I have a 18W x 60L x 16H room underground. How much larger should I size it for the 16ft. ceiling?

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1 Answer


Since this is in the HVAC category, by "it" I presume you mean the airflow/registers for the HVAC into/out of this room.

Generally speaking, the biggest factor in determining ducting for a room is the volume, not square footage, of the room and the number of air changes per hour the system is being designed for, and of course the ambient wall temperature though that is basically taken care of in the HVAC unit sizing. Therefore, whether the room is 8 or 16 feet high, for a given fixed volume, does not affect the total amount of airflow it should receive per hour. Of course, for a given square footage, a 16' ceiling room takes twice the airflow that an 8' ceiling would. However, long or L-shaped rooms commonly need multiple or deliberately located supply/return registers to ensure that airflow reaches all parts of the room and does not leave part stagnant. Same thing with your high ceiling - you need to ensure airflow mixes in the room before it makes it to the return duct, especially if this room tends to build humidity from damp floor or walls.

This can be done in several ways -

1) locating the registers so the airflow traverses the entire airspace from one end to the other - usually one or the other of the supply/return registers are at ceiling and one at floor,

2) by using "spiral" airflow registers which direct the airflow around the perimeter of the room like a whirlpool leading to a central return (or vice versa),

3) or by using a mixing fan mounted on the ceiling -

4) or sometimes (usually in larger commercial spaces) having an independent mixing system that recirculates air within the room with separate registers and fan unit in addition to the supply/return ducting.

You really need a good HVAC contractor or designer who is familiar with the principles of laying out ducting and of the ACA Manual D for sizing it to match the room - because the different solutions work better in specific situations, and the decision of whether to provide the inlet/outlet high up or down low depends a lot on the heating/cooling requirements for the room/building.

For instance, "normally" the supply is at the floor and the return up high - but in a consistently cool climates the supply is commonly at the floor with sometimes a return at the floor as well as one up higher up so the cold air at the floor gets mixed in with the higher warm air. Likewise, in consistent hot (A/C) environments the supply is usually in the ceiling so the cold air filters down through and mixes with the hot air that has risen to the ceiling as it moves to the return at the floor. The return location in that case might not be exactly at the floor - in larger rooms and foyers and such in real hot areas it is common to place the return partway up the wall in that case so the few feet near the floor stays cooler and compensates somewhat for the loss of cold air through doors when they are opened.

It becomes a tradeoff unless you have multiple supply and return vents in a room, because you want mixing of the air in the room to prevent humidity and staleness buildup, but generally you also want to be removing air that is coolest (in heating case) or warmest (in cooling case) - and of course in probably most cases you are talking a fixed system (except in some commercial systems) doing both heating and cooling at different times of the day or year, so it is impossible to design a system so it handles all conditions in an ideal fashion. You usualy end up designing for the best performance in the most uncomfortable time of the year (coldest or hottest depending on area and duration of the serious heating or cooling season) or sometimes utilize mixing for an acceptable but not ideal temperature situation and design for a primary purpose of ventilation to remove odors or moisture like in bathrooms, kitchens, pool and gym areas, etc.

Rooms with odor or moisture issues commonly place the return vents where the odor or moisture will accumulate - generally with the warm air, so usually in the ceiling like with common kitchen and bathroom and gym and pool exhaust fans/vents. However, in some cases for moisture control, like in damp basements, at times the supply is located at or slightly above the base of the wall or just below the ceiling to provide a vortex flow around the room, with the return located some feet away at the "back" side of the supply to pick up the return flow - so both vents (which are directional in design so the supply puts out a "jet" of airflow) might actually be on the same wall though commonly at different heights - directing the airflow in a three-dimensional whirlpool or spiral around the entire perimeter of the room before it gets back to the return. In that sort of case, depending on whether the issue is just moisture at the floor and walls (like in many basements) or general room humidity, there might be two or more supply and return vents (with downsized ducting of course to maintain proper airflow to that room so as to not steal airflow that should be going to the rest of the conditioned space) at different elevations to ventilate the entire room - or again, soemtimes a ceiling fan or recirculation system to ensure that all the air in the room is involved in the air changes to avoid localized areas of constant high humidity and mold growth. For instance, in rooms with good upper level recirculation from a circulation fan system (common in large shower and bathrooms and pool/spa areas) sometimes both the supply and the return are put down low.

In some cases it works best (especially in basements at times) to have a pass-through flow instead - only a supply or a return in the room but the other being provided by flow from adjacent rooms. For instance, in air conditioning heavy situations, you do not want to put added moisture from the basement into the "living spaces" where it could raise the humidity too high, so moist basement air might be passed directly to the return duct for passage through the A/C evaporator for dehumidification of that air. But in predominately heating environments, sometimes it is best to put a large airflow of heated air into the basement to reduce the humidity and extract the moisture from that area, then pass it into the upstairs living spaces (likevia a larger basement door undercut) to increase the living space humidity in an otherwise too low humidty environment. I have even seen cases with very damp basements where the entire A/C return airflow was ducted and dispersed into the basement to pick up the moisture there on its way to the return side of the central air system, with the basement acting basically as the return plenum of the air handler on the way to the air filter. It depends of course on the importance of the underground room, but generally if it is not a normal part of the "controlled conditioned space" but conditioned air is being passed through it to warm or dehumidify the airflow through it (assuming the air handler is in the basement) would "normally" be done on the return side - making that room vent to the return side of the air handler rather than exhaust to the conditioned space, to prevent adding moisture to the living spaces and to avoid it being a "pre-cooling" influence on the airflow heading to the fully conditioned spaces. At times, humidity control takes precedence over absolute energy efficiency.

And of course, any scenario using the basement as either a pre-cooler or a humidifier outside of but contributing to the "conditioned space" is subject to fire code regulations.

A good HVAC contractor or architect's mechanical designer can evaluate any such situation and determine the proper ducting arrange for the environmental conditions, type of use of the room, humidity control needs, and considering other inflow/outflow sources like openings under doors and such.

For your specific case, especially if this is a pure rectangle like it sounds like and is "inside" the conditioned space, proper amount of supply at one end (probably two supply registers for that width room) and return vents at the other end would be the normal arrangement - supply low and return high if predominately heating environment, supply high and return low if mainly cooling environment.

IF this is not technically a fully conditioned space - more a workspace that is not intended to be strictly temperature controlled and has no thermostatic control, because it will tend to be on the cooler side if it is used as a pass-through plenum to remove humidity then the normal ducting would be to route return register in at one end at the floor level (because incoming air is likely to be warmer than the room's wall temperature) and out the other end, ideally at the floor and ceiling both if a "warm" room, or at the floor if a cold room. Of course, when doing this, the total humidity load has to be calculated, because it is possible to overload the system with moisture and end up with excess moisture in the air coming through the air handler, in which case some removal of humidity in the basement by venting (if outside humidity if low) or dehumidifier may be necessary, and for significant moisture problems is always the preferred first solution to high in-ground room humidity issues.

Answered 4 years ago by LCD

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