Return Air Grill

[tlhiv]

I have a 3 ton evaporator unit that is rated at 1200cfm airflow, but my return air grill is only 14" x 24". Am I correct in thinking that this is too small? I have read many places that I should expect to get about 2cfm/in² from my return air grill, and following this, I expect only to get about

2cfm/in² x 14in x 24in = 672cfm

which is barely over ½ the rated 1200cfm of my evaporator unit. What other considerations should I be making? I have a house with about 1400-1500 ft² living space, and this fairly new unit does WILL NO cool my house in 90° heat (it can only maintain the inside temperature and sometimes not even).

I am wondering if my return air is a bottleneck for my system.

Thanks in advance!

[second opinion]

I have a 3 ton evaporator unit that is rated at 1200cfm airflow, but my return air grill is only 14" x 24". Am I correct in thinking that this is too small? I have read many places that I should expect to get about 2cfm/in² from my return air grill, and following this, I expect only to get about

2cfm/in² x 14in x 24in = 672cfm

which is barely over ½ the rated 1200cfm of my evaporator unit. What other considerations should I be making? I have a house with about 1400-1500 ft² living space, and this fairly new unit does WILL NO cool my house in 90° heat (it can only maintain the inside temperature and sometimes not even).

I am wondering if my return air is a bottleneck for my system.

Thanks in advance!

You would be correct

[genduct]

second opinion is right, the face velocity is high BUT the real issue is what the duct connection looks like between the grill and the filter.
If the connection is short and sweet the extra pressure drop with the grill won't be noticed, the extra grill noise might be another story
So IMO it,s the pressure drop on the return/ intake side that really makes the difference

[tlhiv]

You would be correct

Should I add the square area of the supply registers and try to match that with the return air grill size? I assume that I am not going to be able to get 1200cfm of airflow total out of my supply registers even if I add another 14x24 return grill. Is this correct?

[tlhiv]

second opinion is right, the face velocity is high BUT the real issue is what the duct connection looks like between the grill and the filter.


There is no duct work connecting the evaporator unit to the return air grill and filter. The evaporator unit is in a mini-closet in my hall which is elevated about 2ft above the floor. A portion of the bottom of this elevated closet is open to a cavity in the wall below it which is (of course) about 2ft high and is probably 3ft x3ft square. The existing return air grill is on one of the walls of this cavity, namely the hall wall. Thus, the sheet rock itself is serving as the "return duct" (if you will). It would be relatively easy to add an additional 14" x 24" return grate on the adjacent wall. Is this something that I should consider?

[hcong]

1600 CFM / 500 FPM = 3.2 SF

24*14 = 2.3 SF * 50% free area = 1.2 SF. Yep... bigger return grille... atleast 3.2 SF of free area.

[tlhiv]

24X14 = 1800 CFM @ 0.05 w.c. /100 ft.

http://www.hydro-temp.com/help/drawings/duct%20chart.pdf

Have someone take some static pressure measurements to see if it is air flow.

Am I misunderstanding something? I cannot see how there should be any noticable pressure drop between the unit and the return air grill/filter since there is no duct and the unit sits directly above the cavity in which the return air grill is on one wall of this cavity.

[hcong]

Am I misunderstanding something? I cannot see how there should be any noticable pressure drop between the unit and the return air grill/filter since there is no duct and the unit sits directly above the cavity in which the return air grill is on one wall of this cavity.

Sorry.. I misread... I edited my post.. see above :)

There is static pressure drop over the grille face... if you go to the manufacturer specs... it will have how much for a your grille. I would say about 0.016 for Return.. Yours is higher since you don't have are much free area.

[tlhiv]

1600 CFM / 500 FPM = 3.2 SF

24*14 = 2.3 SF * 50% free area = 1.2 SF. Yep... bigger return grille... atleast 3.2 SF of free area.

Am I correct in assuming that my evaporator unit is "choking"? That is, does it seem like the return air is a bottleneck for the system?

[hcong]

Am I correct in assuming that my evaporator unit is "choking"? That is, does it seem like the return air is a bottleneck for the system?

How is your airflow in your supply registers? Are you getting around 55 F coming out of your supply registers? You are probably correct in that there is too much static pressure drop for your blower fan to handle, but there could be system issues too. Have you checked that your coils aren't dirty? Wall cavity returns are never as good as ducted.

[tlhiv]

How is your airflow in your supply registers? Are you getting around 55 F coming out of your supply registers? You are probably correct in that there is too much static pressure drop for your blower fan to handle, but there could be system issues too. Have you checked that your coils aren't dirty? Wall cavity returns are never as good as ducted.

I am getting anywhere between 52F to 57F from my supply registers depending on the current temperature in the house (typically there is about a 18F difference between the return air and supply registers). My condensor coils are not dirty, and I cannot easily access the evaporator coils to check them, but I change my filters pretty regularly, so I assume (perhaps incorrectly) that they are not dirty.

[genduct]

with no real return duct so the grill size seems to be not an issue since the air is getting back to the unit through all the nooks and crannies.
You can add another grill but I doubt if you'll notice any more air flow

[tlhiv]

I guess I would like to know if adding another return grill and filter (same size) on an adjacent wall if I could expect more cool air to enter through the supply registers and thus cool the rooms faster. As it stands now, my unit does not power cycle at all throughout the day if the temperature is more than about 85F - 90F. I have been told by more than one professional that a 3 ton unit should be sufficient for my house, but my friend (who is also a professional) suggested that my return air may be too small. I'm wondering if I increased it whether I could supply a larger volume of the same temperature (cold) air into the house (thus cooling the house better than it currently does).

[hcong]

Yeah, your supply temperature is good and 3 tons should be pretty close to what your need... so dirty coils doesn't seem as likely...

The 18 F difference between return and supply means your house is around 75 F.... That is pretty normal design temperature too. How cold do you set your thermostat? How old is your house?

[hvacmike85]

24x14 with no duct ya might be noisy but should be enough area, a 4" filter would help too, some of those 3m menards filters are tooooo restrictive for a system that isnt already perfect (which not to many are) the side of a furnace cut out is 24-14 and the only time I go bigger is when its a 5ton use bottom or two sides. Adding another can never hurt, helps it breath if you will but cost of install and more filters is the only downfall...

[George2]

Check the airflow with the door to the AH closed and then check it with the door open.

That is an easy way to determine if the unit is getting enough return air.

I agree with Genduct that even though the grille is too small, your AH is getting air (maybe enough or not) through the nooks and crannies.

Try the above and let us know. P.S. How many suppies are in the home?

[tlhiv]

Yeah, your supply temperature is good and 3 tons should be pretty close to what your need... so dirty coils doesn't seem as likely...

The 18 F difference between return and supply means your house is around 75 F.... That is pretty normal design temperature too. How cold do you set your thermostat? How old is your house?

Now don't laugh, but we like to keep it around 70F throughout the day, and on many days our system will "maintain" this temperature but if we decide to let it get to 72F for some reason, then the unit will not typically bring the temperature in the house back down to 70F until the outside temperature has significantly dropped.

Our house (and unit) is 5 years old. We bought both completely new. We are the only homeowners of this house, and the unit was installed the day before we moved in.

[hcong]

I guess I would like to know if adding another return grill and filter (same size) on an adjacent wall if I could expect more cool air to enter through the supply registers and thus cool the rooms faster. As it stands now, my unit does not power cycle at all throughout the day if the temperature is more than about 85F - 90F. I have been told by more than one professional that a 3 ton unit should be sufficient for my house, but my friend (who is also a professional) suggested that my return air may be too small. I'm wondering if I increased it whether I could supply a larger volume of the same temperature (cold) air into the house (thus cooling the house better than it currently does).

The pressure drop over the return grille is probably around 0.06 w.c. (http://www.krueger-hvac.com/ecatalog/getfile2.aspx?fileid=3400) which means at most you can get is 5-10 more CFM with a typical Carrier 80% furnace blower... so adding another return won't make much of a difference in my opinion.

[hvacmike85]

Check the airflow with the door to the AH closed and then check it with the door open.

That is an easy way to determine if the unit is getting enough return air.

I agree with Genduct that even though the grille is too small, your AH is getting air (maybe enough or not) through the nooks and crannies.

Try the above and let us know. P.S. How many suppies are in the home?

I'm not understanding your description. If you take the coil door off of course the static is going to change the fan is bypassing the coil right in through the door, path of least resistance, just ckecking r/a alone and total esp will tell you alot, maybe like I said, not understading you method.

[hcong]

Now don't laugh, but we like to keep it around 70F throughout the day, and on many days our system will "maintain" this temperature but if we decide to let it get to 72F for some reason, then the unit will not typically bring the temperature in the house back down to 70F until the outside temperature has significantly dropped.

Our house (and unit) is 5 years old. We bought both completely new. We are the only homeowners of this house, and the unit was installed the day before we moved in.

70 F is okay, I like it 70 F on a hot summer day :). Unfortunately your unit is probably sized for a 74-75 F design temperature. House and unit are both fairly new... so it doesn't seem like a leaky house... No leaky supply ducts or long supply duct runs? Do you have flexible supply ducts?

You should see if your supply are giving you 1200 cfm... if not you might need a bigger blower.

[tlhiv]

70 F is okay, I like it 70 F on a hot summer day :). Unfortunately your unit is probably sized for a 74-75 F design temperature. House and unit are both fairly new... so it doesn't seem like a leaky house... No leaky supply ducts or long supply duct runs? Do you have flexible supply ducts?

You should see if your supply are giving you 1200 cfm... if not you might need a bigger blower.

When you say it is "sized" for a 74-75 design temperature, do you mean the tonage (3 ton) of the compressor/condensor unit and the (3 ton) evaporator unit?

I don't know of any leaks as, like you said, it is a new house. To my knowledge there are no leaky ducts. We do not have very long duct runs, and most (perhaps all) of the runs from the trunk to each supply register is flex.

[hcong]

When you say it is "sized" for a 74-75 design temperature, do you mean the tonage (3 ton) of the compressor/condensor unit and the (3 ton) evaporator unit?

I don't know of any leaks as, like you said, it is a new house. To my knowledge there are no leaky ducts. We do not have very long duct runs, and most (perhaps all) of the runs from the trunk to each supply register is flex.

Yeah, if you assume everything else is in perfect condition... then your 3 ton might not be able to hold 70 F. Usually when engineers or contractor do the load calculations to choose your condenser and coils, they assume the residents will run the AC to hold at least 74-75 F. The goal these days is to undersize units... I would still maybe sure you are getting the correct amount of air through your supply registers... adding the return grille from my quick calculations won't give you much.

If you bought one of those houses from a national builder... they will put the same system in all their homes no matter which way the windows face or house is oriented.

[tlhiv]

I would still maybe sure you are getting the correct amount of air through your supply registers... adding the return grille from my quick calculations won't give you much.

How can I measure this? Do I have to have a special meter to measure the amount of air flow?

[hcong]

http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Dtools&field-keywords=CFM+meter

Might be cheaper to get someone who has the device and just get a full inspections so you know nothing else is wrong.

[tlhiv]

http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Dtools&field-keywords=CFM+meter

Might be cheaper to get someone who has the device and just get a full inspections so you know nothing else is wrong.

Indeed :gah: