Cooling Performance

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scoward

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I have a question regarding what should be the expected cooling performance of an HVAC unit. And I understand things have been warmer than normal, but things just don’t seem right to me and I’d like to be somewhat knowledgeable when the HVAC folks come back out.

The issue is we had a new unit installed in May and does not seem to keep as well as our old dinosaur it replaced. For instance, once temperatures fall how quickly should an HvAC recover. Yesterday we had a high of 96 at the house. With the unit running all day the house temperature climbed to 77. By 9pm the outside temperature was 81 and the inside temperature was 77 (at two different places in the house). By 11pm it had lowered the temperature by only one degreee (76) and by 5am it was 75 degrees outside it had only lowered the house temp to 73.

This morning I have the the unit set to 72, by the time the outside temperature hit 85 the inside temperature started climbing. . . now 74.

Should a new unit keep up better than that? In addition there seems to me to be excessive condensation on the unit (in the basement). We had this problem with the old unit which was one of the reasons for the replacement (because of mold growth). We were told that the new unit should help solve the condensation issue. Also, some of the vents seem to be blowing well and some you can barely feel cool air coming out.

Thanks!
Steve
 

Sylvan

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Sounds like an undersized unit .

Also R 22 did allow for lower temperatures then the new refringent in some cases
 

WorthFlorida

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There are many variables that can affect the performance.

Generally, old AC units (R22 units) were always oversized by contractors just to eliminate these type of complaints. People expect the same temperature control inside regardless of outside temperature swings. It is possible that the new AC unit is undersized for the heat load but it may fall in the size for the home size. The units being manufactured for about the last ten years is all aluminum coils and the efficiency has gone way up from SEER-10 (early 1990's) to now a minimum of SEER-14. Very little cost to upgrade to SEER-16.

The AC tech will first check that the coils are clear of debris (outside) unit, then put on a set of gauges. There is a chart with every outside compressor unit as the maximum pressure versus ambient temperature. The inside coils should be checked that they are not freezing and the filter is good. Low refrigerant and or a very dirty air filter restricting air flow can cause the inside coils to freeze up. A good tech can tell from the pressure readings if the coils are freezing from the outside reading the pressures. If the pressures are way off and too low that is a sign of a leak on the low pressure side. Since the ambient air temp affects pressure, during the install and especially during cool temperatures tech will keep a low charge since when the weather warms up the pressures can get too high.

Homes that had old units that were oversized usually have a high heat gain during the daytime. Another common issue is old homes the duct work is undersized for the new efficient units. The old units would push the air at a higher rate but the new units are designed for a slower air flow to wringe the air of moisture, therefore larger duct works is better. If you have a 10" main, it now needs to be 14" to get the most efficiency out of the unit. Another is how good the duct work is insulated.

Another old age argument is the temperature of the air after flowing through the coils. Generally, if the temperature drop of about 18-20 degrees is good. Some say it always should be 60 degrees or whatever. If the air intake air is about 80 degrees, right after the coils is should be about 60.

from a google search:
Screen Shot 2021-07-17 at 1.22.19 PM.jpg
 

Fitter30

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Just because the outside temp drops at night the building mass will take hours unless it rains and the evaporation helps cool the mass. Design temp outside for DE 90* for 1% ( hottest day) of the summer 75* indoor. With pocket thermometer measure temp at filter and 18" " from evaporator coil 18*-22* difference. Larger insulator line at condensing unit feel the bare copper pipe should be cold. Have any idea what furnace and tonage of old and new systems?
 

Sylvan

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When I was a stationary engineer the ideal discharge of R 22 running on a medium fan speed was 40 degrees and slightly less on slower speeds and higher on max speed

When I was placed in an engine room and worked on a Central absorption system using utility steam to run the system I quit and become a low pressure boiler and unfired pressure vessel inspector prior to obtaining my master plumbers licenses

I hated having ducts for heating and cooling as it was a compromise

Ideally the hearting would be hydronics with either CI radiators or CI \baseboard and the cooling was separate

Main thing is check the filters / clean the coils, check for possible freeze up due to low charge and ambient indoors for uninsulated ducts
 

Fitter30

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When I was a stationary engineer the ideal discharge of R 22 running on a medium fan speed was 40 degrees and slightly less on slower speeds and higher on max speed

When I was placed in an engine room and worked on a Central absorption system using utility steam to run the system I quit and become a low pressure boiler and unfired pressure vessel inspector prior to obtaining my master plumbers licenses

I hated having ducts for heating and cooling as it was a compromise

Ideally the hearting would be hydronics with either CI radiators or CI \baseboard and the cooling was separate

Main thing is check the filters / clean the coils, check for possible freeze up due to low charge and ambient indoors for uninsulated ducts
What i liked absorption chillers is removing solution into trashcans or when the Lithium bromide would solidified.
Breakout the big torches. Never worked on a gas fire unit just steam.
 
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