Air Conditioning - Manual J calculation
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geshields
05-10-08, 09:21 AM
I purchased HVAC-CALC to find the proper size A/C for my house. Original unit on the lower floor is a 5 ton unit and the Manual J calculation states that it only need a 3 ton(which I concur with). Our current downstairs unit cools the house very fast but does nothing for humidity removal.
However, I completed the upstairs calculations and it is saying I only need a 1 1/2 ton. We currently have a 2 1/2 ton for the upstairs and I believe that is properly sized. Upstairs had good humidity removal and during the summer months it runs fairly often(1-2 times/hr) Why would I go with a lower tonnage when the current one almost seems too small now??
Greg
However, I completed the upstairs calculations and it is saying I only need a 1 1/2 ton. We currently have a 2 1/2 ton for the upstairs and I believe that is properly sized. Upstairs had good humidity removal and during the summer months it runs fairly often(1-2 times/hr) Why would I go with a lower tonnage when the current one almost seems too small now??
Greg
airman.1994
05-10-08, 12:00 PM
If your unit is cycleling 1 to 2 times an hour on the hottest days it is over sized. A properly sized system on the hottest days will run none stop from afternoon well into the evening.
geshields
05-10-08, 02:32 PM
Actually I take that back with regards to the upstairs unit. Today we had summer temps (88 feels like 97) and it has been going constantly since around 12pm and has shut off only once.
HVAC RETIRED
05-26-08, 06:02 AM
Actually I take that back with regards to the upstairs unit. Today we had summer temps (88 feels like 97) and it has been going constantly since around 12pm and has shut off only once.
The 2.5-ton may be delivering only around 1.5-ton of heat removal! There are many reasons why an AC is not delivering anywhere near its tonnage rating.
It also sounds like you have a high humidity day which puts a much bigger load on the system, especially if there is high air infiltration upstairs.
Are the ducts well insulate & sealed tight? Could the return air be drawing hot air from the attic?
http://www.udarrell.com/airconditioning-sizing.html
- HVAC RETIRED
The 2.5-ton may be delivering only around 1.5-ton of heat removal! There are many reasons why an AC is not delivering anywhere near its tonnage rating.
It also sounds like you have a high humidity day which puts a much bigger load on the system, especially if there is high air infiltration upstairs.
Are the ducts well insulate & sealed tight? Could the return air be drawing hot air from the attic?
http://www.udarrell.com/airconditioning-sizing.html
- HVAC RETIRED
geshields
05-26-08, 07:13 AM
Lately the temps have been in the low 90's and very muggy outside. From about noon or so until 10 at night the a/c almost runs constantly. The upstairs is saying it is 78 when I have the thermostat set to 75. RH upstairs is about 43-45%. About the only way to cool it down to the thermostat setting during the day is to have the downstairs thermostat set to 74 or so...