Interstitial Space

[urgross]

I am starting on design for replacement of interstitial HVAC support space for a hospital, and have run into a problem at the onset: for U.S., State, Local and Federal complaince, I can find no basis or criteria for design. nearest thing located, which does not appear to be in ASHRAE or NFPA, is standard given by IMC for unoccupied space. Anyone availab;e that has expereince or knows criteria relating to HVAC support of interstitial? I have found lighting requirements and fire/smoke control guidance (going back to 1986 NBS criteria), nothing on HVAC.

 

[dbill74]

By Interstitial space, are you referring to the space above ceilings? If this is correct, then there are no special requirements for the HVAC system to provide any kind of conditioning. This isn't to say that you can design as if the space doesn't exist and must be accounted for in your load calculations.

If on the other hand your interstitial spaces are more like intermediate floors of the hospital containing HVAC or other support equipment, then such a space would be considered a 'mechanical' space and fall under the requirements of your regular mechanical rooms.

Perhaps a little more information about the spaces in question would help. I.E. Where are the spaces located relative to occupied spaces? What is in the interstitial spaces?

 

[urgross]

The interstital floors contain duct and piping. No equipment is allowed. The interstitial floors are unoccupied space, unlike mechanical or equipment rooms, and have separate designation for lighting requirements by IESNA, are their own smoke control zones separate from mechanical/equipemt rooms, do not require sprinkler coverage as would typcially be required fo mechanical/equipment rooms, and are recognized as different space than MER's by the 1998 ASHRAE HVAC Design Manual for Hospitals and Clinics. Having worked in multiple locations, hospital and laboratory, the current location is the first time I have seen ventilation air included. The interstitials have a 2-hour separation from occupied floors above and below, and 1-hour separation from mechanical rooms.

IMC 406.1 specifically includes ventilation air requirements for unoccupied space. ASHRAE 62.1 does not have a standard for unoccupied space, though the 0.06*SF would probably be closest category. Generally, they are interior zones, and need cooiling/dehumidication only, as on a cool day like today (36*F) you need to take off your coat to chase out duct or pipe runs.

I'm in favor of ventilatio air even if not mandated, as purge will be needed whenever renovation is performed to change occupancies in above or below occiped areas, and the interstitial floors include large amounts of asbestos containing material. Construction was done before ASHRAE 15 publication and includes refrigerant lines. That would probably dictate purge air, but not ventilation air.

 

[ChasBean1]

ASHRAE 62.1
NFPA
IMC
NBS
IESNA
ASHRAE 15
Isn’t it amazing how many standards there are that prevent you from exercising your own brain power as an engineer? Codes and standards should prevent idiocracy, not dictate our every decision.

 

[dbill74]

From your description of the spaces, treat it like an attic space. If the spaces have natural ventilation, nothing needs to be done. On the other hand, if there is no natural ventilation it wouldn't be hard to add a tap off the air handling units to meet the IMC minimum amount of ventilation air (IMC 2012 406.1 is 0.02 CFM/SF) to keep relative humidity below 60%.

The IESNA lighting requirements are for the very rare occasion people are actually in the space (once a year maybe). The lack of sprinkler requirements is because the space is not intended to be occupied on a regular basis such as a storage room.

Next time you are on the site, you may want to take a temperature measurement of these spaces and use that information in your load calculations. In Carrier's HAP there is a tab on the room input page to include partitions with unconditioned spaces.

Bill

 

[urgross]

I've trended the interstitial mixing boxes long enough to see no cooling load exists, and all heating elements have been isolated.

Just attaching to the existing AHU's is the problem. The customer wants to eliminate all NFPA 92 and move solely to NFPA 90A. Right now I'm thinking DOAS based on IMC 406.1 and VRV with purge to match refrigerant.

 

[chicopee]

Per your explanation the interstitial space is a sealed shaft(horizontal, vertical or whatever) so as to prevent the spread of smoke and fire thru the hospital. In all likely hood that interstitial space is of fire resistive construction( 1hr, 2hr or whatever). Being a sealed space, I doubt that there will be much of an A/C load loss thru that space. If purging is required due to asbestos, you will need HEPA filters before exhausting the air to the outside, however, since it would be a sealed space, purging will need an air inlet in addition to the outlet where presumably the air handler would be placed. I would try to get the architect's construction prints and figure out from them the information that you are soliciting.

 

[chicopee]

By the way, the hospital engineering department would also have a set of construction prints.

 

[dbill74]

Urgross, it sounds like you are trying to make this more complicated than it needs to be. Remember to KISS (Keep It Simple Silly).

IMC 406.1 requires an "exhaust" rate, and does not require "outside air". In other words, air leaving the unoccupied space is required to go to the outside (not returned to an AHU); however air entering the unoccupied space, the supply air (different from outside air), can be from anywhere. Think of it as you would a restroom (except EA <= SA), you don't provide a DOAS just for restrooms do you? That would be overkill.
You already know there is no cooling/heating load, so what do you think a VRV system is going to do? Other than add maintenance to the building and make your interstitial space a "mechanical room".
You can't get away from NFPA 92, so don't try. Any new duct work penetrating fire and smoke barriers will need to be appropriately protected.

You already have existing interstitial mixing boxes? This tells me you already have a means of getting air into the interstitial spaces. Then all you need is to provide some exhaust (2 CFM per 100 SF) and balance the system.

 

[urgross]

dbill74,

Not making it tougher than is needed, and yes, you can get away from NFPA 92. That is the specific customer request. Current arrangement has 19 different hospital central AHU's providing ventilation and conditioning for three interstitial floors. If interstitial floors were defined under ASHRAE 62.1, then it might be easier to see the infectious control issues with supplying air to the interstitial and returning that air to patient support areas as is the existing. Not having a FSCS, biannual witness testing of smoke control dampers, pneumatics and controllers not having UL rating, and age of all the above (over 30-years) requires replacement in-kind or with a preferred system, and dedicated units appears to be the only way around NFPA 92.

I've read through the IMC several times, until it struck me that there is no ventilation rate per se; a mechanical system controlled to keep RH below 60% is required; no high humidity, no ventilation air. Cooling is still a separate issue, even during winter the temperature stays in high 80's/low 90's. Mold formation or worse are a concern if humidity cannot be controlled and temperature starts getting above 90*F. If mold starts growing on interstitial ductwork because of failed humidity control, much costlier issues can develop.

Yes, for exterior wall patient bathrooms, I do specify and install mechanical cooling and heating beyond transfer air.

 

[chicopee]

So it seems that you have answered your own question by rereading IMC a few more times and realizing there is no requirement to ventilate interstitial spaces.