Replaced my A/C this weekend, results of downsizing by 1/3.
Recently my 13 yr old Carrier A/C devolved a leak and I chose to replace the system rather than track down the leak and repair the coil. I had been reading about doing manual J calculations and how contractors are scared to death to undersize. I had been talking to co-workers about this and it's an uphill battle getting them to think different than 1 ton per 500sq ft. So I decided to take confidence in correct sizing and "put my money where my mouth is" and install a correctly sized A/C.
House: 1587sq ft located in Oklahoma City, built 1999.
OLD A/C: Carrier 3 ton 12SEER system original to the house.
New Condenser: Thermalzone (Rebadged Rheem/Ruud 2 ton 14.5 SEER) 2 ton 14.5 SEER.
New coil: 2 ton RFCL 13-16SEER with TXV http://www.rheem.com/documents/rcfl-...ication-sheets
Temperature the old unit cycled until it waswas above 105, clearly oversized for our area. New 2 ton unit can keep 76F inside with a 100 degree outdoor temperature, but runs continuously to do so.
Humidity is HIGHER in the day with the new system even though it runs more. 45% vs. 40% for the old system. I've already reduced the 3 ton blower to it's lowest speed.
As a test I've captured the condensate in a bucket to measure humidity removal. The 3 ton system with piston metering pulled more humidity by day, less by night. The new TXV system pulls more humidity at night than day. Both units keep humidity about 45% at night.
Power Old unit consumed 3,000 to 3,500 watts depending on outside temperature. New unit consumes 1,800 to 2,500watts depending on outdoor temperature. Power use is on my in home wattmeter and includes blower consumption.
Delta T Old unit got about 18 degrees most of the time, day or night. New unit gets 14 degrees in day, 18 degrees at night. Keep in mind the blower speed was lowered when the new A/C was installed.
Cycle time Cycles were running about 6 minutes with the old unit, new unit runs about 8-10 minutes per cycle.
Noise New unit is WAY quieter, both units have 8 pole motors. Scroll compressor can barely be heard compared to the recip compressor of the old unit. With the new unit you can barely hear the fan, very impressed. Neighbor still has his 3 ton builder unit with the 1125 RPM fan and it's twice as loud as my old unit.
The coworker that helped me install it was impressed with how quiet the unit is and how much cooling it put out considering that it's only a 2 ton. He was a bit skeptical that the 2 ton could pull it off, but it's doing so with flying colors IMHO. So, believe the load calculations and don't fudge the input number to get the result you want. Yes they may seem low, but they DO work !!!
new unit 14 degree split. sounds terrible.
Split is measured return grille to supply, not across the coil itself. Don't know how much of a difference it makes.
Originally Posted by drife678
Sounds like a lot of duct gain chewing up a good chunck of your capacity. I'd look into stopping that.
Id like to see a minimum of 18 degree split
Possibly why the humidity rises with the increased run times. Infiltration?
14 degrees delta t could be on target. All acording on indoor dry bulb and wet bulb. There are charts for target delta t.
Maybe you are moving a little to much air? You said you lowered the fan speed, but it sounds like you have more air flow than ideal for 2 tons of cooling. Your rh is dependent on how cold you get the air. Maybe put in a more restrictive filter and see if the rh comes down.
To be fair it was over 100 degrees when I took the daytime delta T reading, it's 5 degrees above design conditions for our area. I would think if I had duct leakage issues it would have affected the 3 ton units day/night offset also. I did notice the suction line felt warm when I took the daytime test on the new unit. I need to do a test with real thermometers across the coil, I question the accuracy of the infrared temperature gun. I'm thinking airflow is probably a little high even on the lowest speed, the 45% humidity is acceptable but I'm curious to see what happens when it's cool/rainy.
Yeah, I wouldn't trust an infrared to do a Delta T. They only measure the surface temp of what you point it at. You need the actual air temp.
76^R, 45%RH equals a 52^F dew point. This equates to 45-47^F coil temp. This is very acceptable. Keep us posted on your %RH when the cooling loads decline with high outdoor dew points. Low/no cooling loads with +50^F outdoor dew points will raise the indoor %RH. How much moisture are you removing during a typical day or night? The amount of infiltrating/ventilation air with its moistue plus the moisture from the occupants equals the moisture removed by the a/c. If we know the number of occpants and the dew point outside/dewpoint inside, we estimate the amount of fresh air. Most homes do not get enough fresh during calm summer weather.
Originally Posted by 54regcab
If you want adequate fresh air throughout the +55^F dew point weather, you will need 25-50 lbs of moisture dehumidified from the house. Without a cooling load, a dehumidifier is ideal. For fresh filtered air, a whole house ventilating dehumidifier a good add-on.
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
The old 3ton A/C filled a 3.5 gallon bucket about every 24hrs. The new 2ton A/C takes about 36hours to fill the bucket. However the weather has been unusually dry lately. Low side pressure varies by outdoor temperature, but was 130 @ 90f outside when we started the system. Looks a lot like the 45-47 degree coil you mentioned.
Calm doesn't exist in Oklahoma, there's almost always some breeze blowing. We have some of the highest average wind speeds in the nation. Outdoor dew points have been high 50's, which is low for this time of year. Very dry summer, dew points are more typically in the upper 60's.