Corridor Makeup Air Unit 100% Outside Air 1

[cvanoverbeke]

We are designing a midrise condo with a 4 pipe fan coil system for heating and cooling the residential suites. We are introducing ventilation (outside air) through 2 corridor makeup air units located on the roof. I have this Senoior Designer that states "To do it properly you need reheat since there is very little load". I tend to agree with him. The building is located in Waterloo, Ontario where the OA design conditions are 83.5 DB and 73.4 WB. I want to bring the corridor down to 75 DB and 50% RH. I know you need reheat if you look on the psych chart. The main issue is if you cool it meet the DB and not worry about WB conditions then you will have 75 DB and 100 %RH. Feels kind of sticky at those conditions.

Any thoughts would be appreciated. By the way this is a high end condo therefore we can afford to do it right.

Cvanoverbeke

 

[GMcD]

Yes, you'll have to cool that wet outdoor air to wring out the moisture, and then maybe reheat it to avoid overcooling the corridors.

The other thing to think about is how is the make-up air is going to get into the individual suites from the corridor. The accepted "theory" is that there will be enough leakage through the suite door. I've always had trouble with this since the suite door off the corridor is supposed to be fire and smoke rated, so how much real infiltration would there/should there be?

Many "high end" suites out here on the Left Coast are being equipped with their own packaged energy or heat recovery ventilators interlocked to humidistats and the exhaust fans in the suite. The corridor ventilation then becomes a bare minimum, enough to pressurize the corridor slightly to eliminate cooking odours from migrating from suites out into public areas.

 

[RossABQ]

I'd take McD's comments about leakage thru the door gaps a step further. If you did it correctly, by ducting from the corridor into the rooms, you'd need a fire damper.

I do not think this method of providing ventilation meets the intent of ASHRAE standards, certainly not any that address IAQ, and is likely to result in stale suites. You really have no idea where the exfiltrated air is going; a small difference in one door's fitment to its jamb could cause a large portion of the ventilation to go into a single room. A fair amount is likely to go down the stairwells. In short, it does not strike me as an engineered system.

 

[SAK9]

Fully agree wih Ross.It is a crappy system for a high end condo!

 

[Wareagl487]

cvanoverbeke I agree with your approach. The majority of owners/developers are not in tune with ASHRAE recommendations and thus are looking for standard residential split systems or packaged rooftops for corridors.

For residential building’s corridor HVAC systems, the design intent is to provide constant positive pressure, temperature and humidity control utilizing 100% conditioned outside air at a rate of 0.5 cfm/sq.dt. or 3 AC/Hr (whichever is greater) following ASHRAE guidelines. The supply air is constantly tempered (cooled or heated) to maintain acceptable indoor temperature (68°F - 72°) and dehumidified (40% - 60% RH summer) to maintain recommended humidity levels based on ASHRAE Standard 55-2004 – Thermal Environmental Conditions for Human Occupancy. In addition, required outdoor air ventilation rate as prescribed per ASHRAE Standard 62.1-2004 – Ventilation for Acceptable Indoor Air Quality is maintained.

ASHRAE Applications Handbook – 1999 Edition recommends the corridors are maintained under positive pressure to prevent infiltration of warm, moisture laden air through building cracks, gaps, shafts, etc. Assuming a residential split system DX unit is used for pressurization and temperature control (humidity is uncontrolled), the unit would operate as follows. The unit’s thermostat would be set to the “On” position, meaning the fan would run 24 hours a day/7 days a week and outdoor air would be constantly introduced thru the unit’s return air plenum and then to the space. The compressors would cycle on based solely on indoor space temperature requirements. Thus the supply air would only be dehumidified when there is a call for cooling via the local thermostat. ASHRAE states:

“….introducing untreated outside air directly into the return air plenum of the HVAC unit at part load or low load creates a severe high humidity problem, which is one of the causes of mold and mildew. The situation is further aggravated when the HVAC unit operates in on-off cycle during part or low load conditions”

The cooling load profile for the corridors is such that the HVAC equipment must only handle part or low cooling load and peak cooling loads rarely happen. Thus the unit will cycle on and off only to satisfy the space temperature set point. Under the more common part load conditions, satisfying space temperature set point can occur quite rapidly. In “short cycling”, the unit is not in cooling mode for sufficient time period to address the latent load of the moisture in the air – i.e. humidity. ASHRAE further states:

“The quantity of outdoor air introduced into the rooms or corridors is usually slightly higher in excess of the exhaust quantities to pressurize the building. To avoid adding any load to the individual systems, the outdoor air should be treated to conform to indoor air temperature and humidity conditions. In humid climates, special attention must be given to controlling the humidity from outside air. Otherwise, the outdoor air may reach corridor temperature while still containing a significant amount of moisture.”

Changing the operation of the unit (thermostat in “Auto” position) for the fan to only cycle on when there is a call for cooling to prevent untreated outside air from being introduced to the space creates additional issues. Outside air for building pressurization is now only introduced when the unit is in cooling cycle, thus not maintaining ASHRAE recommended building positive pressurization. And again, the humidity levels remain uncontrolled.

Utilizing residential split system DX HVAC units will accomplish space temperature control but is not capable of maintaining constant building positive pressurization, nor control humidity to ASHRAE recommended levels. Residential split system DX HVAC units are not suitable for this application.

Andy W.

 

[RossABQ]

Andy, I have no issue with what you're saying, system-wise, my beef is that using the corridor as the ONLY source of OSA and depending on an architectural feature (door seals) to distribute that air is not what I consider an HVAC design.

 

[Wareagl487]

Ross -

Never said I was using this as make-up air to the residential units - in fact - doing so is against latest codes. The above only relates to corridor conditioning. Maybe I missed the premise of cvanoverbeke's post.

If the residential units do not meet the criteria of natural ventilation, then ducted outside air via wall cap or central ducted conditioned system directly to the space is the only other alternatives.

Andy W.

 

[GMcD]

A summary of my original post and Ross's post is this: yes you'll have to cool, then reheat the fresh air supply to dry it out, and to what extent depends on how you are supplying that fresh air into the suites and corridors.

I spoke from a "standard" residential design approach of just supplying make-up air into the corridors, and the suite exhaust fans are the only air moving device that would draw in a mix of corridor air and whatever leakage there was through the suite's exterior walls/windows for make-up air within the suite. A situation used by 99% of current high rise condo design industry here in the Pacific NW, which has, and will still, lead to building envelope failures. (Uncontrolled leakage through the exterior envelope).

If you plan on ducting a portion of that tempered fresh air from the corridor MUA into each suite, connected to the inlet plenum of the individual suite fan-coil, then you may not need to reheat as much, since the fan-coil heating and cooling coils can be sized to take care of the final mixed air conditions.

And yes, if ducting fresh air from a central MUA into each suite, you'll have to have fire dampers at the suites, and then watch out for your high rise smoke control measures, and you may even need motorized smoke/fire dampers at each suite depending on the configuration of your building, corridor, and local authority's wishes.

 

[RossABQ]

cvanoverbeke, maybe we are misinterpreting your design; please clarify if corridor OSA is the primary source of ventilatio for the suites themselves.

 

[cvanoverbeke]

Yes, the corridor makeup air unit is the primary source of ventilation for the suites. I agree that it is not the best method for introducing ventilation into suites. There must be 10,000 condo units in Greater Toronto area that does it this way. When I discussed the issue with my Senior Designer he indicated that it was the most common method in Ontario. It will pass the local building code since it is such a common method

Cvanoverbeke

 

[RossABQ]

It will pass the local building code since it is such a common method

Not a valid engineering rationale IMO... I'm sure it's a hard sell to do it right in the face of the other firms' designs ($$$).

 

[AbbyNormal]

I would get enginnered air to make you a DX unit to get a supply air dewpoint down below 55F, that turns on some partial hot gas reheat when the corridors get too cold.





Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[cvanoverbeke]

I do not like DX from an operational point of view (cycling of compressors and less efficient). We are proposing an 4 pipe system with reheat from the boilers. Face and bypass may not work. There is a central chiller and central condensing boiler. With proper controls, you can minimize the amount of reheat. I may put a humdity control on the corridor in the summer time to maintain comfortable conditions within the comfort box.

Cvanoverbeke

 

[AbbyNormal]

hot gas is free heat

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

We use a lot of hot water recovery off of chillers down here for domestic hot water, some have tried it for reheat, a pretty expensive approach to install, but the reheat is still free heat.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[cvanoverbeke]

I assume you are from the southern US where the amount of cooling hours is signficantly higher than Waterloo, Ontario. I agree it is free heat but for the small amount of hours that we would probably require it the extra capital cost to use the reheat would be quite expensive. I think a floating setpoint i.e. change DB and WB based on conditions of outside would be cheaper. The goal is to ensure you do not get 75 DB and 100% RH which would feel really crappy.

Further cooling of makeup air units in Canada is considered a luxury in general. I go to many existing highrise condos where the makeup air unit is heating only. Again the whole issue is that it is generarlly pretty cold in Waterloo compared to other places.

I did a energy feasibility study in Hamilton Bermuda. I had to forget everything I knew about office buildings in Canada. The primary mechanical energy consumption was cooling. When I looked at the weather conditions, it was alot warmer (Of course I knew this). It makes you concentrate on different HVAC issues depending where you live and work. For example, ice storage does not make alot of economic sensee in southern Ontario but it probably makes alot of sense in Atlanta Georgia or Miami.

Cvanoverbeke

 

[MoonBeaver]

cvanoverbeke, Introducing OSA into residential units from door cracks is not really the best way. Expect the following: a dirty stripe on the carpet right where it meets the door due to particulate matter being filtered through the nap, cooking odors from neighboring units, excessive spring tension on door closers, and noise introduction from the corridor and neighboring units. It would be much better to have a large air handling unit to supply make up air through ducts to each residential suite. Of course that means lots of extra fire dampers, but the added benefit is being able to control the temperature of that air independently of the corridor air. To satisfy building requirements for air pressure hierarchy means having the fans run 24/7 in the corridors. This is grossly inefficient from an energy savings perspective, especially in climates with extreme ambient air temperature swings. Allowing building owners to set the temperature in the common areas a little cooler or warmer than normal would help them conserve energy costs while not causing residents to complain about a cold or warm draft from under a poorly sealed door.