Issue of Filter Pressure Drop

[John_187]

This is a common debate, but what do some of you use as common filter (MERV 8 for example) pressure drops. For clean filters, I have seen some around the 0.15" WC range. Dirty upwards of 0.4"WC?

For residential furnaces for example, there is no way you can consider the dirty filter pressure drop. These furnaces only do 0.8" - 0.9" WC max anyways. A dirty filter would take up half of that, which is extremely non-ideal for calculations and not realistic.

This is a pain in the @$$ problem when it comes to pressure drop calcs. Can you please give me your take on typical values you use, as well as whether clean or dirty filter values should be used? As demonstrated above, there are some situations where it is simply not possibly to consider the dirty filter drop.

Thanks for your time

 

[GT-EGR]

You always have to consider it, because it exists regardless. On the residential units if your ductwork has to use up a lot of your external static pressure it just means you have to change the filter more often. And if you don’t then you’ll just see a decrease in airflow and therefore your units ability to heat or cool - and then that will end up prompting you to change the filter.

0.5” of static for your filter loading is great if you can get it, but with a unit that can only do 1” it’s overkill, as you’ll have an oversized fan when the filter is running clean and maybe your fan curve without the pressure shows it unstable. On larger commercial units 0.5” or even 1.0 inch can be common and doable

 

[BronYrAur]

I know I'm throwing a whole segment of the HVAC population under the bus, but I don't think residential installers calculate duct static. From my experience with a contractor that had a residential division, those guys did everything "rule of thumb." Load calculations were based on ranch versus 2-story homes and square footage. That's about it. There were standard duct sizes for various tonnages or CFMs. Nothing was ever calculated on a per-job basis. I was on the commercial side, where we did real engineering, and I always scowled at their crap-shoot approach.

My own home has a unit that is clearly undersized. It I had low ceilings with a "normal" amount of glass, it would have been fine; but, I have cathedral ceilings with tons of East and West glass. To make matters worse, my duct work consists of a short trunk, followed by a spaghetti bowl of flex. I have never taken the time to reverse-engineer it to see how bad it is, but I can only hold 80 deg F on a 90+ sunny day (but at least it is a nice dry 80). I still have R-22, so I will one day have to bite the bullet and upgrade.

So to answer you question, you should consider some level of dirty filter; otherwise, you will always be light on air. As GT-EGR said, you will be heavy on air flow most of the time if you assume a dirty filter. So, give yourself more cushion if the application is critical and less if it is not. More of an art than a science at that point.

 

[miningman]

On a vaguely related matter, my 15 year old residential furnace will soon require replacement. I know that 80,000 BTU will be the required size. I believe that the rule of thumb ( possibly enginneered) is a fan size of 150 cfm per 1,000 BTU or 12,0000 cfm. One specific manufacturer lists three models , all at 80,000 BTU, but with 12,000 or 16,000 cfm. Ducting is essentially fixed and constant, absolutely no desire to replace the system. 16,000 cfm clearly allows for better heat transfer from the heat exchangers but at the expense of a much nosier system.?? SP loss obviously varies between a fresh clean filter and an absolutely filthy one. How do I determine the optimum model and / or cfm rating???

 

[willard3]

miningman: you should hire an HVAC Engineer if you want an accurate answer to your question.

Heat loss/gain would be a good place to start.

 

[BronYrAur]

your CFMs are way too large for a "residential furnace"