I bought a house last summer that's a bit smaller than 2100 sq.ft. It had the original HVAC system which, like the house, was 18 years old. I replaced it last fall with an American Standard system.

Both units were/are 4 tons, despite the fact that my HVAC's company salesman supposedly did a heat load calculation and said I could get by with 3.5 tons.

Once it got hot this summer, I found that the new unit couldn't keep up with temps over 95. Additionally, my layman's measurements of the temperature drop of the supply vent closest to the attic unit and the return was only 11-13 degrees. So the HVAC company sent some guys out yesterday to check on it. They found out that the fan was set for a 5 ton unit so they changed the switches to a 4 ton and added additional refrigerant.

Their changes have helped a lot but not as much as I was hoping. They measured the delta-T right at the coils and got only an 18 degree difference. Is that about right or should it be more? The delta T between the return and the supply vent inside my home is now about 15-17, which seems to be the bare minimum. The system can now very slowly cool my house until outside temps get to ~99 or so.

I was expecting better numbers than that given it's a new system and slightly oversized (supposedly) for my house. Are my expectations unrealistic? If not, what else can I do; what are my options?

Thanks, guys.

So you delta T seems sane, and hopefully they determined your state of charge using superheat/subcooling as appropriate depending on if your unit has a txv valve. But I wonder if your load calculation is a real Manual J or a swag that a 2100 sq foot house needs 4 tons. Did they give you a copy? Oversized units are not a good thing, insufficient run time keeps your humidity out of control. There are online calculators where you can compute a real Man J to verify. Also things like duct leaks, insufficient return air and other defects can affect operation. Is it a Single/multiple/variable speed unit& compressor? Heat Pump or conventional?

You need to know what his design temp was. I can almost guarantee that it was not 100. It probably wasn't even 95. Either way the numbers you posted after they adjusted things sound more than reasonable.

NM

Delta T on a properly running/sized system should be 15-20 difference measured from the shortest supply duct and return air

How deep is your attic insulation ?

Thanks guys. I did have my house, Including the ductwork, pressure tested about a year ago and everything came out OK. I did not see the HVAC guy's heat load calculations and he no longer works for the HVAC company. I also had a bunch more insulation blown into my attic. If the numbers sound good to ya'll, then I guess it is what it is.

Do your houses just basically stay at temperature with no reduction with the AC running constantly when it hits 100 or thereabouts outside?

Not sure what you have the thermostat set to, but load calcs are normally run using about a 25 degree temperature difference. So 100 degrees outside, around 75 inside.

What temperature are you setting your thermostat at, and at what time of day are you adjusting it? That can make a huge difference. I keep my house pretty cold, (72/day, 65 upstairs at night), and my AC still doesn't run constantly, even in the dead of summer in central TX. However, I don't adjust it much. For example, when we are not at home, we keep the house at 74, so it doesn't have much cooling to do when we get home. The thermostat at night gets turned down when the sun is going down. And since the house is already at 72, it doesn't have too far to go.

I have my thermostat set at 75 and just leave it there.

NM

IMO a properly running system should be AT LEAST 20* delta T between ambient room temp and supply air temp measured at a supply air grille. I have two Trane systems at my house, they give me 25* and 29* delta Ts.

drummer0415 said:

---

IMO a properly running system should be AT LEAST 20* delta T between ambient room temp and supply air temp measured at a supply air grille. I have two Trane systems at my house, they give me 25* and 29* delta Ts.

---

So what do you do if they're not at that level, but the company that installed them said it's good enough?

Also, I have never heard of delta Ts at that level, not even right at the coil. Everything I have read says it should be between 16-22. Is your attic sealed and foamed by any chance?

I know nothing about A/C's, but I have a similar sized house and also replaced the original 19 year old 5 ton system with another 5 ton. Cost exactly $8,500.

If you replaced the furnace too, that is a heck of a price. I got 4-5 quotes for replacing my 4 ton unit and none were remotely close to that price. Pat yourself on the back.

First company I talked too was quoted 12-15k. A friend recommended me to an honest sole proprietor and he made it happen quickly without trying to upsell me. Ruud system.

All of my quotes were in that range as well. Wish I'd been fortunate enough to stumble into an honest sole prop like you did.