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  1. #1
    Join Date
    Nov 2007
    Posts
    11

    Filter exceeds maximum pressure drop?

    So I've spent the last two months looking at options to improve the IAQ in the house that we just bought. I've started with local room air purifiers and have shifted to upgrading the central air system. I've come to the conclusion that I could only use an external HEPA with it's own fan but I'd love for someone to confirm my logic.

    We have a high efficiency furnace with a variable speed fan (Carrier 58MVP120). Currently we have a Carrier ESP which I have concerns with due to how quickly the the ESP loses it's ability to attract the dust and ozone generation. Our home inspector indicated that he had some concerns with the air flow from our vents but didn't do a full energy balance. The furnace has a nameplate maximum external static pressure of 0.5 in. I was going to measure the static pressure before and after the blower using a Dwyer manometer, but based on some rules of thumb on the web, it appears you don't want to exceed 20% of the furnace rating in the filter pressure drop, which in our case would be 0.1 in. Now based on info on this site the ESP is likely around 0.1 (due mainly to the resistance in the prefilters) and filters like the HC16, Aprilair 5000 etc. have pressure drops that begin in the 0.3-0.4 range and could increase up to 1 in plus when fully loaded.

    So it would seem obvious at this point that I couldn't increase the resistance by including a new filter (or replacing the ESP) and that using a self powered HEPA is the solution, while leaving in the ESP. A HEPA was more money than I was hoping to spend but if that's the only way forward, without replacing the furnace, then we'll do that.

    The question is am I correct or on crack?

    Thanks in advance.

    Nathan

    PS Thanks a lot to the frequent contributors to this site...it's been very helpful.

  2. #2
    Join Date
    May 2005
    Location
    Lubbock Texas
    Posts
    773
    nmaycher your theories and research are quite impressive. But saying that take a look at the external HEPA system you are considering. The CFM that this unit puts out, does it calculate to 4 to 5 air changes an hour in your occupied space? Also because of its central location in your house would you be sampling the whole house? Depending on where you are in the house with a stand alone system you will get a variance in particulate measurements through out the house.
    The best system for filtration would be using your current HVAC system with more frequent use if not continuous uses. I have herd the people in here say that running a fan continues during the summer would raise the humidity in your house. I have tried this for months in my own house and have only seen maybe a 1% in crease in humidity staying in the 40% range on average.
    How manufacture get away with using higher efficient filters is by using a 4 to 6 inch filter to maintain the proper pressure drop. A 6 inch merv 14 is the largest filter I have successful used with out effecting the operation of the a/c in the past. We typically don't recommend use of over a 4 inch merv 13 in a residential system at 300 to 400 fpm. The lower the velocity across the filter the better it functions.

  3. #3
    Join Date
    Nov 2007
    Posts
    11
    Thanks for the compliment Genesis...not sure how many hours it has taken to get here but it's too many.

    Unfortunately I'm a little confused by your response. My understanding is that even the Lennox HC16, which is a deep 4" high surface area filter, still has pressure drops that start in the 0.3 in range and may rise to 1 in, which would be multiples of what my HVAC system could handle (without system significantly reducing air flow and efficiency). What am I missing?

    I am looking at the Amaircare 10,000 system which in my home would be about 2 air changes per hour. I am also going to run my furnace fan continuously and install a nERV on the outside air line. Although this doesn't reach the ideal 4 ACH it is pretty good and given the 0.5 MESP limitation of my furnace, I don't see another option.

    Nathan

  4. #4
    Join Date
    Aug 2003
    Posts
    906
    Quote Originally Posted by nmaycher View Post
    (...)
    My understanding is that even the Lennox HC16, which is a deep 4" high surface area filter, still has pressure drops that start in the 0.3 in range and may rise to 1 in, which would be multiples of what my HVAC system could handle (without system significantly reducing air flow and efficiency). What am I missing?
    Residential HVAC system manufacturers are S L O W and cautious, and HOs are cheapskates. I've been complaining of this very issue for years: weak blowers in residential furnaces. The 0.5" spec dates from the days of "hog hair" filters that don't even rate a MERV 6. What doesn't help is that HVAC contractors are often confused and measure the static pressure after the A/C coil, whereas the 0.5" spec is between the return and the furnace exhaust -- before the A/C coil. The static pressure drop of a wet coil is often around 0.3"; the return and supply ducts each take about 0.1" typically, so that leaves 0 (zero) for the furnace filter. That means that only a hog hair filter can be installed while respecting the 0.5" spec.

    Now that the manufacturers provide brand-name high MERV filters themselves, they'll have to adjust their blower design eventually because it doesn't make sense anymore. Either that, or face ridicule and warranty claims -- how could they not honor the warranty when all the equipment is their own brand name, installed in a way that is consistent with their instructions and marketing materials? I know marketing material isn't what HVAC contractors should use, but either it's false or misleading advertisement, or the design is dodgy. To compound the problem, some HVAC contractors design their systems with filters operating at 500 fpm instead of 300 fpm, which can cause twice as much filter pressure drop (up to 25/9, depending). In practice, I bet that a systematic survey of static pressures in installed furnaces would commonly find 1" pressure drops when the A/C is operated.

    So, you're correct, currently to respect that 0.5" spec you have to install an independently ducted or bypass type system. Using grossly oversized filters (or multiple filters) to lower the fpm as much as possible can help but you'll still exceed the 0.5" spec (to get 300 fpm you need a 24x24 filter for 1200 cfm or a 3 ton A/C, not the 16x20 you'll often find).

    Or, you can just say "s**** it" and deal with the repair bills as they come, knowing this industry is hopelessly messed up.

  5. #5
    Join Date
    May 2005
    Location
    Lubbock Texas
    Posts
    773
    Nice post pmeunier, I am so busy right now I can't get my head screwed on straight. I know what I need to say but it doesn't always come out correctly.

  6. #6
    Join Date
    Nov 2007
    Location
    Central Ga.
    Posts
    44
    Quote Originally Posted by pmeunier View Post
    Residential HVAC system manufacturers are S L O W and cautious, and HOs are cheapskates. I've been complaining of this very issue for years: weak blowers in residential furnaces.... That means that only a hog hair filter can be installed while respecting the 0.5" spec.
    This statement is DEAD on!! I have seen it too many times. The design engineer will spec a residential furnace with a max. operating static of 0.5" then slap on an "A" type D/X coil on top then wonder why the system does not perform with the 300' of return and supply ducting. And almost always the manufacturer will blame the installer, the installer will blame the engineer... and nothing ever gets resolved. I cringe when I walk onto a commercial job with this type residential equipment installed.

  7. #7
    Join Date
    Nov 2007
    Posts
    11
    Thanks alot of the info pmeunier and genesis. Just wanted to make sure I was on the right track. Since the furnance is only 2 years old and high efficiency, I struggled to think that it was not properly sized but alas this seems to be a problem with the industry.

    Nathan

  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    The detailed fan data charts for that furnace sho it can operate at higher then .5 ESP.

    Often return duct modifications are needed to add "better" filters,plus using two instead of one filters will reduce the static.

    You need to test the static/ESP to see where you are today,and what mdifications may be needed.

    If the mfr. provided higher static blowers ,it would kill the SEER ratings,so ducts must be modified instead.

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