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  1. #1

    Rooftop filters versus interior filters

    I own a commercial building (5000 sq ft) and have 5 rooftop package units ranging from 2 tons to 3 tons. My tenant (a bank) has just completed a buildout of the interior and their HVAC engineer specified 13 returns (24" x 24" inlays) that do not accommodate filters. Historically, the filters were always located in the returns inside the space. Hence we have no filters in place now.

    The engineer's specification seems rather odd to me since every retail property I have seen has filters on the inside. My primary concern (the tenant is responsible for HVAC maintenance) is the foot traffic on the roof and the potential for water leaks. I sent my HVAC on the roof to install filters until this issue is resolved and we have already had one leak caused by the improper replacement of a panel on one unit.

    Other than reducing traffic on the rood is there any other compelling reason to have five filters on the roof or thirteen filters in the interior. The first thing that comes to mind is the frequency of filter changes, which would obviously be more frequent with 5 filters versus 13.

    Any other thoughts?

  2. #2
    Join Date
    Nov 2007
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    First thing that comes to mind is you should definitely have filters in your rooftop unit if you want to add them to your return air grilles that will help you even further as long as it isn't too restrictive in the airflow department. Here is why it is imperative you keep filters in your rooftop unit. Your unit does not only pull its air from your floorspace it also has to introduce a certain amount of fresh air to your space as this is code. Without a filter in the unit your evapporator has no protection from any debris in the outside air and will foul very quickly. If your unit utilises an economizer the amount of air you will be bringing in to the space will be huge. So the long and the short of it is you absolutely must have filters inside your unit and the ones in the space ie return grilles are optional. Hope this helps

  3. #3
    Join Date
    Nov 2006
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    Quote Originally Posted by pforst View Post
    I own a commercial building (5000 sq ft) and have 5 rooftop package units ranging from 2 tons to 3 tons. My tenant (a bank) has just completed a buildout of the interior and their HVAC engineer specified 13 returns (24" x 24" inlays) that do not accommodate filters. Historically, the filters were always located in the returns inside the space. Hence we have no filters in place now.

    The engineer's specification seems rather odd to me since every retail property I have seen has filters on the inside. My primary concern (the tenant is responsible for HVAC maintenance) is the foot traffic on the roof and the potential for water leaks. I sent my HVAC on the roof to install filters until this issue is resolved and we have already had one leak caused by the improper replacement of a panel on one unit.

    Other than reducing traffic on the rood is there any other compelling reason to have five filters on the roof or thirteen filters in the interior. The first thing that comes to mind is the frequency of filter changes, which would obviously be more frequent with 5 filters versus 13.

    Any other thoughts?
    Yes, I do.

    99%+ of my work is commercial retail spaces: banks, jewelry, clothing, shoes, hair salons, etc.

    Interior filters in return ducts are rare, which is different from your experience. They are most often found in hair salons with "split" systems (where there is an air handler hidden in the ceiling that requires removal of tiles for simple maintenance) and they are used to allow frequent filter changes associated with the airborne chemicals that rapidly clog them. Additionally, having five small units with one filter each is also rare, and suggests to me that this space is made up of several former small spaces. Economics suggest that you will be stuck with these five small, inefficient units for some time to come.

    While you could put additional filters in the return ducts to supplement the filters in the RTU's, I would not recommend that. Every time I find that scenario, the filters in the return have been overlooked and are passing maybe 10% of their intended volume. You and you tenant will want to avoid that situation. If you eliminate the RTU filters, you will have a different bad outcome with dirty evaporators, as mentioned above. Fresh outside air from an economizer is necessary, and filters must protect that evaporator from dust and debris.

    I see many types of roofing material, and many repairs. What I do not see is many repairs directly due to foot traffic or mishandling of equipment. Perhaps your roofer is in the habit of attributing leaks to the HVAC trade.

    Usually, roof repairs are due to age and poor installation of the roofing materials. A good contemporary roof will have additional squares of rubber to protect the rubber or PVC roof membrane from abrasion or piercing. The squares look like a pathway of paving stones would from the street to your front door, and lead from the hatch or exterior metal ladder to the units. That's a best-case scenario, and you can still have a positive outcome with regular visits to the roof without the extra squares of rubber. Most commercial contractors are careful and mindful of the importance of the surface they walk upon.

    Perhaps the best reason for RTU filters is that their placement leads to regular inspections of the units on the roof, a vital requirement for all rooftop equipment. Finding problems sooner saves you the added costs of more involved repairs as a situation degrades and takes other components along for the ride. For example, a defective contactor, which is not expensive, can ruin a compressor. It's better to find that contactor and replace it before you have the added expense and equipment on the roof needed for the repair.

    So:

    Have a ready and safe roof access, a roof that starts out in good condition, filters in the rooftop units, and enjoy less maintenance costs and a happier tenant.
    [Avatar photo from a Florida training accident. Everyone walked away.]
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  4. #4

    Thanks for your input!!

    I appreciate you taking the time to offer your advice.

  5. #5
    Join Date
    Sep 2006
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    Ohio
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    I only do commercial and industrial HVAC and find very few units with filter grills inside the building. It would be best to only put filters in the units. When filter grills are used, it can be very disruptive for the people in the building when the filters need changed. You have to set up a ladder and work above people. It will also be faster for the HVAC tech to change the filters in the unit while doing the PM's and not have to also go inside to change filters. It will also cost less to have fewer filters.

  6. #6
    Join Date
    Jun 2004
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    Clayton,NC
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    I too did mostly commercial work and wether or not the filters were at the units I still had to go on the rooftop to PM the HVAC unit. There are a lot of things that should be looked at as part of the PM and since when the tennet leave you still own the unit I would make sure the HVAC guys look at the units each time they are there no matter where the filters are located.

  7. #7
    Join Date
    Mar 2004
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    Austin, Texas
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    This is an age old argument with good points on both sides. I agree with the comment about filters are needed at the unit even with filters in the return grill. One good point about return grill filters is they protect the return duct from dirt build up.

    What I recommend is put a good filter at the unit. If you want a filter at the return grill, make sure it is a cheap fiberglass filter. And make sure both filters are checked and replaced regularly.
    If "I have always done it this way" is a good reason to do it again, how many times do I have to do something wrong - before it becomes right?

  8. #8
    Join Date
    Dec 2007
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    Soudern Kal-e-phone-e-ya
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    sometimes my employer will enter into a multi-year lease of a particular property, and they usually negotiate a lower "per sq ft" price if we agree to do our own maintenance.

    One of the first things I do, (assuming its a RTU), is ADD A FILTER BOX to the ductwork on the roof, unless I can convert the unit.

    Return grille filters = PITA

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