HVAC & Filters?

[mpire]

I have a relatively new A/C units. (9 months old & 2 years old)

The condos are both roughly 800 square feet. 14x8 ducts in 2 bedrooms, plus two in the main room, and then 4x8 ducts in each bathroom.

I replaced the original A/C units (1975) which were 2 Ton GE units with a flat evaporator, vs the "A" style that the newer units come with. They worked good, but the evaporator drain pans were rusted beyond repair and had eventually rotted out the floors underneath them.

The New units are 2.5 ton units to hopefully deal with the increased sun loads on the side of the units, there were a lot more trees around when they were built.

The first one I installed was a rush job, so there is no return plenum, I just use the slide in filter track on the bottom of the unit. It is a 15x20x1 filter.

The second one I installed has a 24" square return plenum with rails that the AC sits on. I have a side cut out and it goes to a 16x20 return grate.

Considering they are new-ish systems, can they handle a high merv rated filter element, or should I stick to the lower rated ones?

The AC with no plenum feels like it blows a lot stronger than the other one, but the only other difference is flex hoses in the attic instead of the 45 year old rigid fiberglass board ducting.

My primary concern is keeping the evaporator clean long term.

What would you use?

 

[nadogail]

How about a medium merv filter?

 

[Jim greengo]

2.5 tons for 800 sq feet?

 

[tonyciambrone]

I use Merv 8 filters from discountfilters or Amazon, whoever's cheaper. sometimes Menards blows out a random brand and I'll buy those. I change them every 30 days max. filters are cheap cheap cheap compared to HVAC components, and I like good indoor air quality.

I have seen some videos from hvac guys saying the really high Merv filters just don't flow enough for the way most air handlers are installed. you would need to use two filters i.e 16x50 total filter area in most cases they said.

 

[mpire]

2.5 tons for 800 sq feet?

Too much or too little?

Its half a ton more than it had before, and this is Florida with a full wall of windows.

 

[bonneyman]

Too much or too little?

Its half a ton more than it had before, and this is Florida with a full wall of windows.

Gone are the days when the ole "rule of thumb estimating" worked. I don't have the heat load calc book in front of me but with the new systems you gotta perform that calculation and size the unit accordingly.

I do know that alot of systems have way too little filtering available for the proper airflow. I have an older (prolly 90's) informational document that computes inlet area of filter for different tonnages - if the OP would like I can try and find and post it. FWIW

 

[justinjoyal]

Honestly, the whole thing seems like one hell of a mess....

Why not do it right?

 

[SGKent]

Each ton need 400 CFM. (cubic feet per minute). The manufacturer lists what the maximum vacuum in inches if water column (WC) that the system is designed for. A typical system is designed for .50" WC but not all are, you have to read the manual. On most filters today, but not all yet (HD ones lack it as of last week at least here) there is a small grid chart that spells out what the filter handles at different inches WC. My 2.5 ton Carrier is designed for .50" WC plus .10" WC for a filter. I have a 25"x20"x1" and a 12"x 24"x1" combined (788 sq in) into that unit. It barely makes it under the design spec. 2.5 tons needs 1000 CFM per minute of air flow. I am using the lowest resistance 1" thick pleated filter made. The 3M Filterete 1900 13 merv filter has lower resistance than any other filter available (including the medium MERV filters, that I could find greater than a MERV 1 basic filter because that Filterete filter has so many pleats.

Here is a chart that shows how many cubic feet of air a given size of duct can handle although your ducts appear large enough. If you don't provide enough air thru the filter and thru the return duct the motor labors which can quickly burn it and the circuit board that controls it up. https://www.buildingincalifornia.com/wp-content/uploads/2014/03/duct-sizing-chart.pdf

This is what the chart on a filter will look like. California will require it soon from my understanding because so many of the filter ads are misleading. I believe it will be shown nationally on all consumer filters. This is for a 25x20x1 Filtrete 1900 filter. All the middle MERV filters I could find are actually worse air flow. Many HVAC units have about .40" WC design resistance as built - meaning fins, air handler size, squirrel cage inlet etc.. No matter what you do that number is in the design of the actual heater. If your limit in the manual is .50 " WC plus .10" WC for the filter then .50" + .10" = .60" limit minus the .40" design equals .10 " to .20" in the duct and air filter. So ... you have between .10" WC and .20" WC for the filter and duct period. This 20x25x1 filter flows 770 CFM at .13" WC so that would be good for a max of about 1.5 to 1.75 ton AC unit. You might get by with this on 2 tons if everything else in the ducting was perfect and you could use the .18" at 1025 CFM with zero duct resistance.

View media item 100719
Good luck. You have a good question but answering it is not a simple answer unless someone does the math on your specific units, and anything they say is a general statement only and should be taken as such. I spent close to a year researching that question and many HVAC people can't answer it, that is why they install the MERV 1 filters - it prevents callbacks because the HVAC motor or circuit board burned up.

Your 15x20 is 60% as large as the 25x20 on mine. That means yours at the max resistance a HVAC unit is designed for is about 470 to 600 CFM or 1 ton to 1 1/2 ton AC. You need to double the filter size or use a MERV 1 filter. Sorry.

 

[Jeepster04]

Each ton need 400 CFM. (cubic feet per minute). The manufacturer lists what the maximum vacuum in inches if water column (WC) that the system is designed for. A typical system is designed for .50" WC but not all are, you have to read the manual. On most filters today, but not all yet (HD ones lack it as of last week at least here) there is a small grid chart that spells out what the filter handles at different inches WC. My 2.5 ton Carrier is designed for .50" WC plus .10" WC for a filter. I have a 25"x20"x1" and a 12"x 24"x1" combined (788 sq in) into that unit. It barely makes it under the design spec. 2.5 tons needs 1000 CFM per minute of air flow. I am using the lowest resistance 1" thick pleated filter made. The 3M Filterete 1900 13 merv filter has lower resistance than any other filter available (including the medium MERV filters, that I could find greater than a MERV 1 basic filter because that Filterete filter has so many pleats.

Here is a chart that shows how many cubic feet of air a given size of duct can handle although your ducts appear large enough. If you don't provide enough air thru the filter and thru the return duct the motor labors which can quickly burn it and the circuit board that controls it up. https://www.buildingincalifornia.com/wp-content/uploads/2014/03/duct-sizing-chart.pdf

This is what the chart on a filter will look like. California will require it soon from my understanding because so many of the filter ads are misleading. I believe it will be shown nationally on all consumer filters. This is for a 25x20x1 Filtrete 1900 filter. All the middle MERV filters I could find are actually worse air flow. Many HVAC units have about .40" WC design resistance as built - meaning fins, air handler size, squirrel cage inlet etc.. No matter what you do that number is in the design of the actual heater. If your limit in the manual is .50 " WC plus .10" WC for the filter then .50" + .10" = .60" limit minus the .40" design equals .10 " to .20" in the duct and air filter. So ... you have between .10" WC and .20" WC for the filter and duct period. This 20x25x1 filter flows 770 CFM at .13" WC so that would be good for a max of about 1.5 to 1.75 ton AC unit. You might get by with this on 2 tons if everything else in the ducting was perfect and you could use the .18" at 1025 CFM with zero duct resistance.

View media item 100719
Good luck. You have a good question but answering it is not a simple answer unless someone does the math on your specific units, and anything they say is a general statement only and should be taken as such. I spent close to a year researching that question and many HVAC people can't answer it, that is why they install the MERV 1 filters - it prevents callbacks because the HVAC motor or circuit board burned up.

Your 15x20 is 60% as large as the 25x20 on mine. That means yours at the max resistance a HVAC unit is designed for is about 470 to 600 CFM or 1 ton to 1 1/2 ton AC. You need to double the filter size or use a MERV 1 filter. Sorry.

This is very interesting to me. So you believe the filter in the air handler sufficiently filters the air in the house? I've always read that the unit doesn't run enough to actually filter the air in the house, the filter is just to protect the unit.

I've always used the filtrete basic filters but may go up to the 600 MPR (MERV7) filters. My unit seems to be perfectly sized for my house and may run for 15min-20min/hour depending on the outdoor temp.

 

[jbwilkins]

Gone are the days when the ole "rule of thumb estimating" worked. I don't have the heat load calc book in front of me but with the new systems you gotta perform that calculation and size the unit accordingly.

And we're talking about a condo in Fl that sounds to be built in the 70's......I'm guessing single pane aluminum windows and not sealed well......Sizing isn't quite as critical as on new construction or a remodel that was brought up to current code/standards......

 

[LS6 Tommy]

I've always read that the unit doesn't run enough to actually filter the air in the house, the filter is just to protect the unit.

For residential and light commercial equipment, that's pretty much correct. Systems that actually filter the air, like the ones at the pharma company I work for, are designed much differently to allow the proper airflow for the required filtration.

Tommy

 

[SGKent]

I've always used the filtrete basic filters but may go up to the 600 MPR (MERV7) filters. My unit seems to be perfectly sized for my house and may run for 15min-20min/hour depending on the outdoor temp.

you want low resistance filters. The MERV7 filters are higher resistance than the Filtrete 1900 MERV13. State of California did extensive testing for energy reasons and discovered that. That is why filters will have a chart printed on them to be sold in California. It will be nationwide rather than make different filters just for California. It's a good thing because it turns out many of those pretty labels you see on things are misleading. The chart is what really happens. Multiply the number of tons you have X 400 CFM per ton. That will be close to the air flow in CFM your unit needs. Then look on the chart to see what the resistance is at that air flow. Most conventional air handlers have about .40" WC resistance. Add the number on the chart to .40" WC and you should not exceed .50" WC to in some cases .60" WC unless your manual specifies otherwise. That assumes your ducting is clean and properly sized. The chart varies by filter size. This one is for a Filtrete 1900 20"x 25"x 1".

View media item 100719

 

[99LeCouch]

Stick with the lower rated ones. The most basic pleated filter will work just fine. It's there to protect the blower and A coil, not to act as an air purifier.

If you want to filter the air, buy a standalone air purifier. Or one that ties into the ductwork.

For one unit blowing stronger, check that the flex duct is pulled tight, no sagging, and routed straight. Any 90* bends should be hard piped.