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DOAS Chilled Beam no way to dehumidify

DOAS Chilled Beam no way to dehumidify

DOAS Chilled Beam no way to dehumidify

Doing a design review of a 90,000ft^2 two story building with DOAS serving chilled beams in classrooms. Currently there is an ERW that has a configuration of: first a dissicant wheel then the heating coil and finally the chilled water coil. The rooms typically have one wall exposed with 70% glass, 30 students and about 8 2'x2' active chilled beams. The outer bank ACBs have a dual coil that can always use hot water. The inner ACBs have only a single coil that is either receiving chilled water or hot water (depending on what the plant is doing).

My concern with the design: There is no way to dehumidify the air going to the chilled beams, cooling coil is after heating coil and the only reheat available is the 3 outer bank ACB in each classroom. Is this a valid concern or am I over thinking this?

My concerns:

-I fear that there is no way to dry out the building during a Monday morning cool down after a long muggy weekend.
-Return air humidity will slowly increase, which in turn will increase the return air dew point and cause the high temp chilled water set point to climb in the mid to lower 60s causing over heating of rooms.
-Rooms will slowly sub cool over time, "cold and clammy feel"

Oh and they have a new Gym and this unit is configured the same way, heating coil and then cooling coil. The dehumidification sequence I got from the engineer is to slow the fan down to 50%. My concern here is with the new wood floor and maintaining a consistent 55% humidity in the gym to prevent wood floor problems.

RE: DOAS Chilled Beam no way to dehumidify

Depending on your climate zone, which you didn't mention, I assume it's a moderate climate with humid spring, summer and fall conditions?

Then yes, the DOA isn't configured properly at all. Typically I'd suggest the dessicant energy wheel as air on first, then the cooling coil, then the reheat coil as air on last. That way you have controlled temperature and humidity of the fresh air supply and can dry it out as much as you can before it hits the chilled beams. I've seen some configurations where the reheat coil is actually a heat pipe coil, or other non-enthalpy type heat wheel as the last thing the air out of the DOAS sees.

With the cooling coil as the last coil the air sees going out of the DOAS, it will certainly dehumidify the fresh air supply, but may be over-cooling the spaces during shoulder seasons - again this is location and climate dependant - maybe that's not an issue, considering there is terminal reheat at the perimeter chilled beams. The problem may occur in the interior zones where there is no potential ability to reheat in shoulder seasons unless you can change over the heating/cooling water to those zones.

RE: DOAS Chilled Beam no way to dehumidify

What is the sensible heat ratio of the room? Is this a good application for ACB given the high latent load?The ventilation air qty is going to be determined by the latent load rather than the number of occupants.So what will be the pecentage of sensible load handled by the ACB?What happens to temperature control when the sensible load goes down?Sorry more questions than answers!

RE: DOAS Chilled Beam no way to dehumidify

My experience is only based on research since a consultant wanted to talk us into Cb for an office building. i had googled a lot, talked to multiple CB designers (hospital etc.) and met with and called multiple operators in the mid-west. the systems they operated had all kind ove version of CB, with all kind of operating conditions and different levels of sophistication. some had humidity sensors in each room, and very complex DOAs setups, some were simpler. The applications were university buildings, offices etc. An energy simulation also was performed for our project. For all the reasons below we decided to use VAV. So take my advice as you may. i really spent a lot of time on this topic and read every article because my employer fell for the architect who claimed CB to be the solution to every problem:
- unless you have an application that has 24/7 ventilation and high return requirement (i.e. hospital patient room), there is no significant energy benefit, quite the contrary.
- You definitely need a DOAS with de-humidification coild and a second drying wheel (unless you are in a really arid climate). Some systems i saw also have complex DX de-humidification/reheat/scavenge heat setup.
- if you don't have very complex humidity sensor control etc and everything works perfectly, you may end up the AHU 24/7, which obviously is not efficient. google "rain in building"
- with very complex de-humidification you need frequent maintenance, sensor calibration and your client should have a controls person on staff. CB is not a system you get from contractor, commission it, and then it works.
- you need a building with very tight envelope, if you have infiltration, this will rain.

Out of all the people i interviewed no one could claim to be happy with the system the way it was intended, they all had to modify operation (i.e. run 24/7 instead of schedule) to stop the rain. among the interviewees was one university with 3 CB buildings (all large buildings), one university with 2-3 CB buildings, a commercial large office. They also often had a weird tolerance for system failure ("it fails sometimes, but only when it is hot and humid"..... like this is when your client wants AC to fail).
With our local university that has 3 CB systems we met with 4-5 maintenance people. they basically set us down and told us they finally convinced the State to not use CB anymore, and they would strongly advise to do anything to prevent a CB system for office application.

the energy simulation didn't really show CB benefits vs. VAV. Yes you can manipulate the simulation to show what you want, but the simulation doesn't even take into account just a few extra hours of run time to prevent raining at startup. Yes. literally rain in the building.

I really recommend your client decides what the actual benefit is, for a school it is not energy, upfront cost is higher, and you have very high risk of failure.

It is great for the right application, hospital patient room is one such example, but not offices and classroom. Maybe if you have a 24/7 classroom, and they use incandescent light or have some other sensible load... and you really need the DOAS to dehumidify beyond regular cooling coil cooling.

RE: DOAS Chilled Beam no way to dehumidify

Are you sure the desiccant isn't setup to perform all of the dehumidification? It may not just be recovering energy from the exhaust stream. Is the exhaust by any chance heated prior to hitting the desiccant wheel? If so, then I would suspect the wheel is dehumidifying and the downstream cooling coil is sensible only.

RE: DOAS Chilled Beam no way to dehumidify

climate is southwest MN and no heat source prior to the wheel in the exhaust stream.

I really think this is going to be a problem and have raised my concerns but no one seems to care as it is a new school.

But I guess I will get the last say because I will be doing the Cx.

RE: DOAS Chilled Beam no way to dehumidify

Since this unit is only delivering the outside air requirement, is the volume large enough to overcool the space? Perhaps it is designed for 55 deg (or whatever) all the time with no chance of overcooling due to the low volume of air compared to the spaces it serves.

RE: DOAS Chilled Beam no way to dehumidify

each room has 6 ACBs with a design of 60 cfm of primary air.

RE: DOAS Chilled Beam no way to dehumidify

Oh and that reminds me, they don't have space humidity sensors. So the only way they are controlling the high temp chilled water is based on the return air back at the DOAS. I have been asking for a final sequence of operations from the engineer because the initial sequence that went out to bid clearly showed the heating coil after the cooling coil and when the return air humidity rose above set point they were to flood the chilled water coil while modulating the hot water to maintain set DAT. Well since that is not the case they have not given the team direction on how to control this cold clam machine they are installing.

just a sad thing to see, all this money they are getting paid to do such a poor lack of thought job. The real sad thing is the owner will be the one fighting with it for the next 50 years to make it work.

RE: DOAS Chilled Beam no way to dehumidify


Design Space Conditions: 75°F DB / 50% RH
Primary Air Conditions: 50°F DB / 95% RH
Primary Air flow: 360 cfm

Latent load: 30 students * (115 BTU/hr/student) = 3450 BTU/hr

Humidity Ratio in Space = 0.0092 lbw/lb
Humidity Ratio of Primary Air = 0.0072 lbw/lb

[Qlat (BTU/hr)] = [Airflow (CFM)] * 4840 * [deltaW (lbw/lb)]
Qlat = 360 * 4840 * (0.0072 - 0.0092)
Qlat = -3484 BTU/hr

I would assume DOAS is set to supply 50°F air.

If so:

[Qsens (BTU/hr)] = 1.08 * [Airflow (CFM)] * [delaT (°F)]
Qsens = 1.08 * 360 * (50 - 75)
Qsens = -9389 BTU/hr

Qsens (ESTIMATE!) = people + glass + wall + lights (also floor + ceiling but I am lazy)
Qsens (ESTIMATE!) = [30 students * (230 BTU/hr/student)] * [0.55 sq.ft*hr*°F/BTU * (20ft * 10ft) * 0.7 * (90°F - 75°F) + 0.064 sq.ft*hr*°F/BTU * (20ft * 10ft) * (90°F - 75°F)] + [0.8 W/sq.ft * 500 sq.ft * 3.412 BTU/Whr]
Qsens (ESTIMATE!) = 6900 BTU/hr + [1155 BTU/hr + 58 BTU/hr] + 1365 BTU/hr
Qsens = 9478 BTU/hr

Doesn't seem like the ACBs will run very often. During part load as you have said rooms might overcool. Assuming space thermostats only modulate ACB water valve, then it does look like the DOAS should have some way of reheating air.

EDIT: Basically it seems the room load is ventilation-driven, not sensible-driven. This is not a useful application of ACB in my opinion as the DOAS primary air can supply almost all the required sensible cooling while delivering the constant CFM required for latent cooling.

Anybody care to check my high level numbers?

RE: DOAS Chilled Beam no way to dehumidify

Just as a reference, the VA does not allow chilled beam for hospital HVAC.

RE: DOAS Chilled Beam no way to dehumidify

RandomUserName - you need to send a bill for your engineering services now...

But yes, I agree - a school is primarily ventilation driven and the ventilation air supply will probably be able to do all the heating and cooling of the classrooms with just the 100% ventilation air supply alone, if the building has a pretty good envelope and reasonable window to wall ratio, with some really good glazing (triple glazing hopefully in MN) If it's a typical "to Code" envelope there may be some room by room perimeter trim heating and cooling required and that could be passive ceiling radiant panels and keep it simple. If the ventilation ait was supplied as low level displacement style, then a lot of the space cooling loads can be discounted (lights = 0, stratified heat off equipment and people = 50%, etc.) and the actual room cooling loads could be pretty well met by the DV ventilation air supply.

EnergyProfessional- your experience has been similar to mine, and is a sad commentary on the state of HVAC engineering these days - it's not getting any better. Contractors and equipment suppliers (and even architects?!) all trying to get their systems of the month thrown in there without any due diligence. Keep up the good fight.

RE: DOAS Chilled Beam no way to dehumidify

60 cfm for classrooms? that sounds little.
and you think as CxA you have the final say? That is contrary to anything I've seen. The owner will take your advice, dismiss it, and then probably listen to the architect and later on will blame you because the system you commissioned doesn't work....

RE: DOAS Chilled Beam no way to dehumidify

Umm EnergyProfessional- I interpreted his post to mean he had 6 chilled beams at 60cfm each of primary air which makes sense.

However, I still view ASHRAE 62.1 as a "minimum standard", and up here in my world we commonly use a minimum of 400 cfm per classroom in elementary schools, and up to 600 cfm per High School classroom, based on a minimum class size of 30-35 people and an instructor, so in the Pacific NW we can pretty well do all the heating and cooling from just the required outdoor air ventilation as long as the window to wall ratio and glazing is high performance and has some exterior shading on south and west sides.

RE: DOAS Chilled Beam no way to dehumidify

its 60 cfm per ACB, so each room has roughly 480 cfm of primary air.

RE: DOAS Chilled Beam no way to dehumidify

GMcD - Took all of 15 minutes ... at my rate it would probably only be $20! lol

RE: DOAS Chilled Beam no way to dehumidify

Random -

The design supply air is 55F in lieu of 50F and the space design of 75F 50% RH, how can that 50%RH be maintained without sending dryer air to the space. When the cooling is on and condensing water out of the air the supply air will be more like 54F and 95% RH.

RE: DOAS Chilled Beam no way to dehumidify

If design numbers are as you said (DOAS SAT 55F), then the air will not remove the entire latent load; humidity will rise.

The reason I had come to the conclusion that DOAS SAT was 50F was because the latent load (3450 BTU/hr) closely matched what 360 CFM could remove between 50F DB / 95% RH and 75 F DB / 50% RH (3484 BTU/hr)

Bump up DOAS SAT to 55F and you have to move a lot more primary air to maintain 75 F DB / 50 % RH. About 1425 CFM if my quick numbers are good and I am using the calculator on this Android tablet correctly!

But then again, I am doing these calculations because I think they are fun while sitting at home watching YouTube videos. Maybe you should just call the mechanical engineer and set up a meeting to sit down and go through his thoughts on system design? Surely it would be benefit the both of you as things progress onward.

RE: DOAS Chilled Beam no way to dehumidify

you'll need some enhanced de-humidification. Like an energywheel, cooling coil and then another desiccant wheel that transfers more water into exhaust stream (and also slightly reheats supply air. Like the Semco Pinacle unit etc. You also could use some heatpipes, or some sort of DX. The wheel-coil-wheel is the most common method. the energy recovery of such unit is so good, you may even skip the heating coil.

you always want the heating coil in an AHU upstream of cooling coil. This heating coil is not for reheat, it is for heating. in summer it can freeze when your cooling coil freezes (i.e. in DX systems.... ask me how I know...). either way, cooling and re-heating is inherently wasteful anyway.

RE: DOAS Chilled Beam no way to dehumidify

I agree that they need some sort of reheat after the cooling coil. Since the unit is chilled water it would have to be a sensible wheel or a run around loop. Another option that they could have done was used a desuperheater on the air cooled chiller for reheat of the two DOAS units.

RE: DOAS Chilled Beam no way to dehumidify

Further movement on this. The design firm clearly wants this sequence to happen.

When return air humidity rises above set point (55% RH), the chilled water valve modulates open as a function of deviation from set point.

I my mind, won't this make the situation even worse because now you could be sending 46F air into the spaces, lol.

Testing is going to start in the next month so I'll let everyone know how it goes.

RE: DOAS Chilled Beam no way to dehumidify

Thanks for following up! This has been a fun one.

"When return air humidity [...]"

What return air? I thought this was a DOAS + ACB system? If so there is no return air, only supply air from the DOAS and exhaust air.

Even assuming they meant exhaust air humidity, they way that is worded sounds like the sensor will be in the exhaust air duct? I hope all rooms are similarly loaded. You are taking a common average humidity of combined rooms at this point; could be poor control.

I would hope the RH sensor is in each room. Even then, if the RH sensor is modulating the ACB water valve, the only way we will take moisture out of the air is by condensing it out and draining it via the ACB. That is not a typical design for an ACB; they usually provide sensible heat rejection only. Sounds like it's gonna rain baby!

The ONLY way to control humidity is via the DOAS. And with no reheat downstream of the final CC on the DOAS, you will over cool your rooms as we have discussed before.


Have you ever called the mechanical engineer just to discuss what you believe will be an issue with him? That is usually the best way to approach these things.

Here is what I see happening in the very near future with your system:

The addition of RH sensors in each space and temperature sensors in the supply air duct work controlling a reheat coil on each branch supply air duct serving said space. If the RH sensor calls for dehumidification, a signal is sent back to the DOAS to run the CC valve wide open. All reheat coils will be tasked with maintaining a supply air temp setpoint via the duct mounted supply air temp sensor and a hot water valve on the reheat coil.

RE: DOAS Chilled Beam no way to dehumidify

The heating coil in the DOAS is a preheat coil to protect the downstream chilled water coil from freezing if there is not enough heat from the ERW or if the ERW is down. The space heating is by the reheat coil in the perimeter ACB. The indoor ACBs will have hot water in their coil when heating is required. For morning cool down or warm up, turn on units earlier.
Adequate dehumidified air must be supplied to the ACBs to do sensible and latent cooling without condensation. Ask engineer for list of schools with this type system and check them out.

RE: DOAS Chilled Beam no way to dehumidify

Randomusername -

you are correct that the humidity sensor is in the exhaust air stream at the DOAS unit.

you are not correct, no space humidity sensors are being installed.

Yes I have contacted. THe response I received is exactly what lilluput1 typed, lol. Maybe this is a common design now, maybe in northern climates this will work. Although it seems lately that we are seeing higher than normal wet bulb temperatures for longer periods of time.

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