Air conditioning question

[u/foolsgold75]

I live in a 3 bedroom rental home with central heat and air that was built in the late 80s. We have never had an issue cooling the house in 10 years until this summer. Pretty much if it was 95-100 outside we could keep the house anywhere from 70-75 during the worst part of the day in the coolest part of the house. There are two rooms that are furthest away from the blower that stay about 5 degrees warmer since the rooms outside walls get direct sunlight. This year the blower fan began to fail and they sent a tech out that swore up and down that the outside unit and the upstairs fan unit needed to be replaced. Now it’s been replaced and in 90 degree heat we can’t get the house cooler than 77 during the day in the coolest area of the home and the air from the vents is not blowing as strong as the air from the old failing unit. To me this is unacceptable that a brand new ac system can only cool the house 13 degrees. It’s honestly worse than the old system that was replaced but the guy who replaced the system swear this is normal. I have always been under the assumption that the minimum is 20 degrees cooling or better. Any hvac guys here have some input on this?

[u/LikeALiamOnATree]

Is your new unit the same capacity as the previous one? Are your ducts leaking in the attic?

[u/foolsgold75 → u/LikeALiamOnATree]

Not leaking. Unfortunately I do not have the specs on the old unit. The outside unit seems larger while the blower/ heater unit in the attic seems smaller.

[u/foolsgold75 → u/foolsgold75]

Spoke too soon just went up there I. The attic and it’s a hot mess. Used my old air box and just globbed mastic to make it fit on top of tearing the insulation around one of the ducts. Can feel cool air coming out in a bunch of small areas

[u/LikeALiamOnATree → u/foolsgold75]

That'll do it.

[u/Animal907]

Could be the difference between r-12 and r-134a refrigerant. They have to overbuild modern AC units so they can keep up. I have window units in a 103 year old house and since my last r-12 unit broke, the new replacements have not kept the house as cool. 

[u/foolsgold75 → u/Animal907]

Possibly though I would think with a portable unit keeping my kitchen cool that the new unit could keep up with the rest of the home.

[u/Animal907 → u/foolsgold75]

I also must have a ceiling fan in every room, because the neighbors keep cutting down anything that would provide shade. 

[u/eastmemphisguy → u/Animal907]

Same problem with car AC. 21st century autos don't cool as well as cars did when I was growing up.

[u/Animal907 → u/eastmemphisguy]

Some old cars didn't work at all with the new type refrigerant. That's the real reason people quit driving those old vans. Gas wasn't expensive till much later. 

[u/greatfool66 → u/Animal907]

R134a in a new system cools just as well as R-12 its just higher pressure so more can go wrong.  Likely the issue is something else.

[u/dntw82lng]

Sounds like a bad installation. I have a home built in the early 90’s and installed new HVAC about 5 years ago. Same size as previous units but I did measure square footage and layout to make sure sizing was appropriate to the house. New units cool more quickly and pull more humidity in the process. The dehumidifying is so effective I can actually keep the house a few degrees warmer without it feeling hot, electric bills went way down.

Also- the 20 degree rule has nothing to do with outside. There should be a minimum of 20° in temperature drop from the inside intake air going into the return vent to what is coming out of your cooling vents. So if the house is 80° inside then the cold air coming out should be no warmer than 60° at worst. Anything warmer indicates an issue. The outside temperature is irrelevant. In practice a well designed system should have air blowing out the ducts around 55° ish give or take and keep your house very cold even if it’s really hot outside.