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Thread: Health Care Facilities - 100% OA or Recirculation

  1. #1

    Health Care Facilities - 100% OA or Recirculation

    I've been involved in energy simulations of some healthcare facilities recently. It seems that the norm in my area is to supply 100% OA to critical areas such as Emergency and Surgery. As a result, energy use is significantly higher unless high efficiency heat recovery is installed and even so, energy use is still increased due to additional fan energy. There is also the additional maintenance related to the heat recovery device.

    As the relevant codes and standards (CSA, ASHRAE) don't require 100% OA (at least in most areas), I've been interested in the history behind 100% OA and recirculation in the health care setting. I did a little searching into the old handbooks at our office and was able to go back as far as the 1971 Applications Handbook which states that:

    "Until recent years, available air filter media could remove only a relatively small percentage of the pathogenic organisms in an air stream as compared with the high-efficiency air filter media available today. Because of this low efficiency, past authorities believed it was better to use all outdoor air which contained very few pathogenic organisms than to reuse by recirculation the more contaminated air from within the hospital for the ventilation of operating rooms and similar sensitive areas. Today, however, it has been demonstrated that properly designed air systems using high-efficiency air filter media can deliver air equally free of bacterial contamination, whether it be outdoor air or air recirculated from within the area."

    So it looks like ASHRAE has had the belief that recirculation systems with proper filtration can deliver air equally free of bacteria since at least the early 1970's.

    Do the energy savings benefits of recirculation systems outweigh the potential health benefits of 100% OA systems? What are your thoughts?

  2. #2
    I've also worked with several large hospitals that were designed from the beginning with 100% OA for the entire hospital's patient areas. From what I remember, they all were built before the 70's, and I had always thought that the design philosophy was that energy was nearly free (fan and OA conditioning), and filtration was less than totally reliable. So the 100% OA was done. From what I remember, the main constraint against designing 100 OA systems was actually the real estate used and construction costs of the huge ducts.

    In today's world, there is another consideration, and that is heat recovery may NOT be an option. I work in a state (NC) where the local Authority Having Jurisdiction has ruled that the building code saying "no leakage" across heat recover coils for critical spaces like patient areas, means 0.000% leakage. None. Even the best HRU wheel has a tiny bit of leakage clinging to the wheel. About the only system that could be used are run-around coils, and I do use them, but their energy transfer is pretty small in comparison. Most users compare that small benefit vs the confidence of clean air that comes from a 100% OA system, and choose the latter.

  3. #3
    We design large hospitals in Phoenix. 100% OSA would be a mistake for us as it would require cooling 125F outside air down!

    We follow the worst case of the guidelines of ASHRAE Design Manual for Hospitals, Joint Commission, or, FGI, etc. I'd say that our large central airhandlers end up at about 40% OSA, typically. That includes proper filtration at the units, proper exhaust where required (emergency lobbies, etc), proper use of HEPA filters where required, etc.

    Our clients are also very aware of energy use and they are willing to change their interal workflows to allow energy to be saved. For example they may require a few additional PE (protective enviroment rooms) that are dedicated exhaust. They understand that is benefical rather than having every room be a pseudo-PE room becuase it has 100% exhaust.

    We use run around coils sometimes but not regularly.

    I have been involved with a hospital that is of '70's era that is operated at 100% OSA, but that is becuase it's located in coastal California where the outside temperature is never higher than 75F!!! So even if they didn't design to 100% OSA, the system would have inherently operated in economizer mode 99% of the year.

    Gabe

  4. #4
    Yes in effect high efficiency filter media can remove almost just about anything you need to remove in a hopsital but it is important for HVAC and building contractor to involve the media developper at an early stage in order to ensure a fit for purpose design, achieving this way both the clean air environment free of contaminants AND energy savings.
    Anouk

  5. #5
    Recirculating air from the space is the most efficient form of energy recovery - it is 100% efficient! It is my belief that the 100% outdoor air craze started in the part of the country where the climate allows running on economizer most of the year anyway. Therefore, there was no energy penalty. If you live in the north part of the country where I live, and you have airside economizers like I do, then much of the year you are bringing in more than the minimum outdoor air. So, those who believe that more outside air is better can have an efficient design and still have plenty of outside air most of the year with no energy penalty. This is especially true now that ASHRAE has lowered the humification requirements.

    As far as filtration - it's all about the seals. You might consider specifying HEPA filter housings instead of MERV filter housings for final filters. Even if your client chooses to use sub-hepa filters, the filters that will work in the HEPA housing will have renewable seals on them and so will have minimal bypass.

  6. #6
    I was also brought up with 100% OA systems however the reality of modern day filtration is that 100% OA is a wasteful solution to Theatre air conditioning systems particularly with the large volumes of air that are required in laminar flow applications. Air recirculation with appropriate filtration and correct air handling unit design is not a recommendation but in my view a must in this day and age of energy scarcity. We use plug fans with direct coupled motors and VSDs to provide a constant air flow. The rate of recirculation is app 85% of the supply air which saves a huge amount of energy. Personally I would rather use recirculation than 100% OA and energy recovery on the exhaust system as there is never a 100% guarantee of air tightness between the different energy recovery wheel chambers.

  7. #7
    Using 100% OA or Recirculation on in Healthcare Facility can be decided on perticular zone climate and type of activity. All the areas in the Healthcare facility do not require 100% OA except few critical area and that also can be recirculated with min OA requirement by using High Grade filter with Low pressure losses (NON FIBRE HEPA FILTERS) and UV lights. Many Critical area require cont. exhaust (Waste of energy) a cross flow or heat pipe can be used for heat recovery (Heat Wheel are not recommended).

  8. #8
    Be very wary of using in duct UVGI systems there is no guarantee that they will work. HEPA filters are supplied with a performance certificate so you are guaranteed of a given performance. At this stage as far as I know there is no supplier of UVGI systems that can provide you with any certification regarding particle size removal or extermination of bacteria, all they can supply you with are test results but no certification. I am not sure which critical areas in a Hospital you are referring to that require continuous exhaust, as far as I know there in actual fact very few if any at all in which air after correct treatment cannot be recirculated with a modicum of fresh make up air.

  9. #9
    Occasional User Bill Bahnfleth's Avatar
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    Quote Originally Posted by mikeriv@carifro.com View Post
    Be very wary of using in duct UVGI systems there is no guarantee that they will work. HEPA filters are supplied with a performance certificate so you are guaranteed of a given performance. At this stage as far as I know there is no supplier of UVGI systems that can provide you with any certification regarding particle size removal or extermination of bacteria, all they can supply you with are test results but no certification. I am not sure which critical areas in a Hospital you are referring to that require continuous exhaust, as far as I know there in actual fact very few if any at all in which air after correct treatment cannot be recirculated with a modicum of fresh make up air.
    UVGI doesn't remove particles. It inactivates bioaerosols and the particles may remain airborne. It's a technology that works and is approved by CDC (upper room). As an industry, it does lag behind others in the standardization. ASHRAE is developing consensus standards for determining the performance of equipment (185.1 and 185.2) that would help resolve your concern. On the other hand, knowing that a component was tested or comes from a product line that is certified is not an absolute guarantee of installed performance for a variety of reasons.
    Bill Bahnfleth
    Penn State
    Dept. of Architectural Engineering
    ASHRAE President 2013-14
    eMail: ashraepresident@ashrae.org
    Web: www.facebook.com/ASHRAEPresident

  10. #10
    You are correct UVGI does inactivate aerosols however at this stage there is no certified performance of any system. I live in south Africa and here for example the Department of Health has placed a moratorium on the use of UVGI systems simply because there is no available standard for measuring their performance once installed. I agree that with filtration there is no guarantee that it will perform as intended/designed/certified but at least the systems can be verified during commissioning with simple tests, the same cannot at this stage be done with UVGI systems. I will be the first to include UVGI systems in my design work once they are supplied with a verifiable performance certificate. Until then HEPA filters remain my choice.

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