NCARB - Practice Test PDD
The question says hot humid climate, why would you install an active chilled beam system?
Architect's Studio Companion PG 179 -
-
This question tripped me up as well. After looking closer at Architect's Studio Companion, their advantages vs disadvantages of active chilled beams is really misleading. "In comparison to all-air systems, active chilled beams can be quieter, provide more constant airflow and better control humidity." It goes on to say, "Exhaust air is normally not recirculated, reducing the chance of transporting air pollutants or contaminants from one part of the building to another." (both p. 178) Their commentary on humidity control seems to directly contradict their disadvantages, but my best guess was that this is the "best possible solution" scenario and the other two risk contamination, even with DOAS? Maybe someone else has a better explanation.
-
My understanding is active chilled beams can be used in humid climates if the indoor humidity is kept low enough (Heating, Cooling, Lighting, 4th ed p557) or when other systems are checking/monitoring humidity.
Maybe another keyword to consider is "efficient"? Both Active chilled beam and DOAS are energy efficient. MEEB (12th ed p550) mentioned them as a pair very briefly.
Lastly, I don't usually rely on wikipedia for important things but the DOAS page seems to have some good information. Full disclosure -- I haven't heard of DOAS until this question. https://en.wikipedia.org/wiki/Dedicated_outdoor_air_system#cite_ref-ashrae_8-0
-
This question is wrong.
Yes, it is possible to make chilled beams work if you are confident you will be able to control the interior relative humidity, but hospitals specifically (deliberately!) operate at high interior relative humidities.
The question (or rather the answer) would work if NCARB had chosen some other kind of building that doesn’t operate at high interior relative humidities. That said, even with that adjustment, I’d still consider it a bit of an unfair question because of the hot humid designation. Sure the DOAS *reduces* risk in HH climates, but a lot of reasonable practitioners would probably advise against chilled beams anyway. Like suppose the question were reworded to make it a school. If you had operable windows, the DOAS wouldn’t save you and your chilled beams would sweat. Not a great question.
Christine Williamson
www.christine-williamson.com -
As an aside, in my own studying for these exams I've been finding that both the test prep sources and NCARB tend to make mechanical systems sections needlessly complicated by failing to distinguish between *mechanical conditioning* and *mechanical ventilation*.
Conditioning the air means heating or cooling it (or humidifying or dehumidifying, but usually we're just talking about temperature). This is a closed loop: we heat or cool only the air that's already in a space. So the air that comes out of the registers at your house comes through ducts attached to your air handling unit (AHU). The return air grill (large and square, usually), takes air from the space and delivers it through ducts back to the AHU where it is conditioned again (heated or cooled). No new air enters the system.
Providing ventilation means introducing new exterior air into a space. The most popular approach to ventilating houses is called "exhaust-only ventilation" and it works by running a fan (usually a bathroom fan that is set to operate on a timer or whenever we turn on the light). The fan takes interior air and exhausts it to the exterior. This creates a negative pressure and causes exterior air to come into the house somewhere else through defects in the enclosure. (Which is gross for obvious reasons but it "works" and is permitted by code). Here's an image showing the difference between conditioning and ventilation in a house:
A much better way to provide ventilation to buildings is to buy equipment specifically for that purpose and control both the air coming in and the air going out (balanced ventilation). The two most common devices that do this are ERVs (residential and commercial) and Dedicated Outdoor Air Systems (DOAS, commercial and institutional only -- they're very large). These systems do exactly what you'd think: they take air from the building and dump it outside, and they take fresh outside air and bring it inside... but as a bonus, they pre-condition the air first (by warming or cooling it and humidifying or dehumidifying it). It's worth looking up how an ERV works (don't worry about the DOAS) if you haven't already. You won't have to memorize how they work, but it's cool and it will help you to generally understand what's going on.
Now here's where stuff gets all mixed up in NCARB-world:
It is often possible to configure conditioning systems to *also* introduce outdoor air to the space. What we usually do in this case is provide an air intake that uses a damper and a fan to deliver exterior air to the return side of the air handler so that some of the air comes from outside, and some of the air comes from the already-conditioned space. The damper controls how much exterior air we bring in, but it doesn't precondition it like the ERV and DOAS, it just relies on the regular conditioning system to condition the air. This is definitely better (more control) than exhaust-only ventilation, but when we have high ventilation rates or an extreme climate, we can still lose control over humidity. The advantage to using the ERV or DOAS to precondition the air is that it gives us much better control over how much moisture we bring into a space. (But it also matters for heat, too -- if you want fresh breathing air in a cold climate it's nice when you're not bringing in 20F air!). The primary downside to the ERVs and DOAS, btw, is cost. (ERVs cost about $3,000, installed).
Anyway, in this question, the only choice that deals exclusively with ventilation air is the DOAS. The chilled beams, of course, have nothing to do with ventilation and can't be made to handle ventilation. The top three options could be set up to provide ventilation (and often are in commercial settings), but that wasn't made clear.
Finally, one more note for actual professional practice: in addition to (1) controlling temperature and (2) providing ventilation, it's also important to provide (3) humidity control, (4) distribution and mixing and (5) filtration.
The exam (and most study resources) really don't address those last three very well. They assume humidity control will be provided as a side-benefit of controlling temperature and ventilation, but we are increasingly finding that dedicated dehumidification (and sometimes also humidification) is required as our buildings get more airtight, and as a response to our desire for increased ventilation rates (for health reasons). As for distribution and mixing, we handle that with ducts and fans, but we will often have an interest in moving the air around even if there isn't a temperature or humidity load. And filtration is usually added to one of the systems we're already using. I mention it separately because we have flexibility in where we provide it (ie. what system we integrate it into).
Below I included three images that illustrate all of this in a residential application. They are increasingly more expensive, but also increasingly better (superior indoor air quality, comfort, and control).
Christine Williamson
(FYI: this is a cold climate example so the AHU is a furnace. The lines that connect the furnace to the exterior provide the combustion air and the exhaust. This type of system is called "sealed combustion" because neither the air needed to burn the fuel, nor the air being exhausted is connected to the interior space (it's another closed loop). This really has nothing to do with this discussion, I'm just explaining the arrows in case they're confusing).
-
Picking up on this thread, can someone please explain why the option "Central air and water systems" is not a correct answer?
I understand in this system return air cannot be recirculated and therefore good for places like hospitals? Is it because the question is asking to choose two systems that should be used as a combination? Can the Central air and water system be used on its own to satisfied the requirement?
Thanks!!
Please sign in to leave a comment.
Comments
8 comments