Manual J calculation and CFM

mkreb · Oct 9, 2020

Hi,

I have a question about the results I have from a manual J calculation. The info about the house was input (room sizes, type of construction, insulation or lack there of, window and door sizes and types, etc.). The report says the following:

Heating BTUH: 39,701
Cooling BTUH: 38,364
Sensible cooling: 33,902
Latent cooling: 4,461

CFM: 1,712

My house is in Houston, TX. My understanding is that these results indicate a 4 ton A/C system. Is the CFM calculated (1,712 cfm) representative of a 4 ton system? I thought there was 400 cfm/ton. Does the CFM on a manual J calculation influence the size of the A/C unit? Thanks.

kdean1 · Oct 9, 2020

How do you figure that to be a 4 ton cooling load? I see it as 3.5 ton or even 3 tons if you tighen the house some more.

pecmsg · Oct 9, 2020

Mow you need the Manual "S" for the equipment selection and "D" for what the ducts can handle.

dan sw fl · Oct 9, 2020

mkreb said:
Hi,

I have a question about the results I have from a manual J calculation. The info about the house was input (room sizes, type of construction, insulation or lack there of, window and door sizes and types, etc.). The report says the following:

Heating BTUH: 39,701
Cooling BTUH: 38,364
Sensible cooling: 33,902
Latent cooling: 4,461

CFM: 1,712

My house is in Houston, TX. My understanding is that these results indicate a 4 ton A/C system.

Is the CFM calculated (1,712 cfm) representative of a 4 ton system?
I thought there was 400 cfm/ton.

Does the CFM on a manual J calculation influence the size of the A/C unit?
Thanks.

One does NOT know the LDB temperature until a system is selected.
_________________ Leaving Dry Bulb

beenthere · Oct 10, 2020

mkreb said:
Hi,

I have a question about the results I have from a manual J calculation. The info about the house was input (room sizes, type of construction, insulation or lack there of, window and door sizes and types, etc.). The report says the following:

Heating BTUH: 39,701
Cooling BTUH: 38,364
Sensible cooling: 33,902
Latent cooling: 4,461

CFM: 1,712

My house is in Houston, TX. My understanding is that these results indicate a 4 ton A/C system. Is the CFM calculated (1,712 cfm) representative of a 4 ton system? I thought there was 400 cfm/ton. Does the CFM on a manual J calculation influence the size of the A/C unit? Thanks.

First, what size system do you currently have?
Does it maintain thermostat set point during the hottest/hotter parts of the day?
During the hottest parts of the day, does it run constantly, or does it cycle on and off?

mkreb · Oct 13, 2020

beenthere said:
First, what size system do you currently have?
Does it maintain thermostat set point during the hottest/hotter parts of the day?
During the hottest parts of the day, does it run constantly, or does it cycle on and off?

Thank you for asking. We bought the house a little while ago but haven't lived in it yet so I don't know the answers to most of your questions. I know of couple of the rooms are off by about 3-4 degrees but I don't know why (not enough air to it or what). Unfortunately the condensing unit which is currently a 4 ton system has failed and I need to replace it. There is no insulation in the walls but I would like to have some put in. I have the option of having cellulose or foam put in from the outside. The numbers I posted are with R-13 insulation in the walls.

beenthere · Oct 13, 2020

Might want to get the insulation done first,and then rerun the numbers.

But as toy our original question.Yes,it would take a 4 ton unit to meet the sensible load you posted.

teddy bear · Oct 14, 2020

mkreb said:
Hi,

I have a question about the results I have from a manual J calculation. The info about the house was input (room sizes, type of construction, insulation or lack there of, window and door sizes and types, etc.). The report says the following:

Heating BTUH: 39,701
Cooling BTUH: 38,364
Sensible cooling: 33,902
Latent cooling: 4,461

CFM: 1,712

My house is in Houston, TX. My understanding is that these results indicate a 4 ton A/C system. Is the CFM calculated (1,712 cfm) representative of a 4 ton system? I thought there was 400 cfm/ton. Does the CFM on a manual J calculation influence the size of the A/C unit? Thanks.

In reality, you expect this a/c to maintain maintain <50%RH with 75^F t-stat setting??

if yes on the 50%RH at a minimum, the air flow through the cooling coil must be slowed to <320 cfm per ton of capacity. The coil will be <45^F with a 75^F, 50%RH return air. The a/c will have less sensible capacity when the air flow is slowed to get adequate latent capacity. With 4,461 btus of latent load calculation, you will need a setup that removes 10,000 btus of latent cooling. This is about 25% latent cooling S/R ratio. 30,000 sensible cooling would be normal. Keep in mind that latent loads are much more steady throughout 24 hours of a day and rainy days. This a minimum latent ratio to maintain 50%RH during moderate cooling loads.

Evenings and rainy days, that latent load is much higher than the sensible cooling load on a home. You are opening up a real can of worms. But it is the worth a discussion to get an understanding cooling a properly insulated and ventilated home in a green grass climate.

If you want a healthy properly ventilated home with normal moisture generating occupants in a green grass climate, you will need 4,000-6000 btus of latent cooling most hours of spring/summer/fall in Houston to maintain 50%RH. This will require an a/c setup as I described and a small whole house dehumidifier. Or remove even more moisture during peak sensible cooling to over-dry the home, eliminate as much fresh air infiltration as possible, and learn to live with many hours +60%RH during low/no sensible cooling, humane occupancy, and higher humidity levels.

The seasoned a/c contractor does all this with much fuss and most occupants learn to live with the results.

This was a good post with the possibility of further discussion.
Keep us posted.
Regards Teddy Bear

Manual J calculation and CFM

mkreb

kdean1

pecmsg

dan sw fl

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