New AC Unit Questions

Cherokeekid88

Cherokeekid88

Well-Known Member
Joined
Jun 30, 2007
Location
High Point, NC
So my house was built in 1988, and it still has the original Air Handler/Furnace..so they are going on 30 years old. The outside AC unit was replaced around 6-7 years ago, we bought the house in 2013 and have just had issues just about every year with either the heat or the AC. We have had the motor replaced, had some things here and there replaced, but our house just won't stay cool. Right now, we are leaving it on 73 degrees 24/7. We used to be able to knock it down to 70 at night to sleep more comfortably, but it seems if we knock it down to anything lower than 73 degrees, it will just run and run and run. With the T Stat set to 73 degrees, it will run for about 10 min, cut off for 5 min, and then start up again for another 10 min or so...and does this constantly.
But our issue is that our upstairs never seems to cool down, we have to run a ceiling fan and a tower fan and its still hot as all get out. Now I know it has been really hot lately, but even when its cooler at night, we have always had issues with cooling our house...Now, I am guessing that the main reason our house isn't cooling is the unit being as old as it is, but I also wonder about the insulation in our house and if its letting too much cool air escape. Now we did have a new front door installed two years ago as well as a New bar door ad door to garage and made sure everything was insulated and well sealed. Our windows are original as well and probably another variable to the problem.

So with all that said, my house is roughly 1500sqft and my wife and I have been thinking of looking to see about getting all new stuff.
Roughly, how much is it to replace the outside unit and indoor unit with a new Air Handler/Furnace? Nothing fancy, just something that will efficiently cool my 1500sqft house and allow us to turn our air down below 70 and actually have a cool nights sleep.
 
I replaced 2 inside units and 2 outside heat pumps and added a zone 2 years ago with Goodman equipment which at the time had the best warranty I could find for a little under 8k
 
1500 sq ft, you should be able to get away with a 2.5 ton, maybe 3 ton if your load calcs call for it.

Most units that size that don't require any duct rework other than at the air handler (expected) should cost in $6k to $8k range depending on who does the work and brands of units, etc. You may be able to keep your supply and return lines which would save you some $. If you add a heat strip, that may add to the cost as well. Some installers want to install all new drip pans and condensate lines which may add costs as well. If yours are in good shape, then reuse them maybe. They are cheap but it all adds up.
 
rabb11d said:
If your upper floor doesn't have a door i think you will always struggle against heat rising.
Click to expand...
what do you mean by door?
Sorry for my newbie-ness, but I am just not that educated with AC stuff.

But the 6-8k is basically what I am looking for. We plan on staying in this house for a good while I think, and with a 30 year old furnace/Air Handler, I just stress everyday that something is going to happen at any moment.
 
Personally, if it were my house, Id want a split system.
With a separate system for upstairs and down.

Its possible to make a single unit work appropriately for multiple levels. Not Easy but possible.
 
Ron said:
Personally, if it were my house, Id want a split system.
With a separate system for upstairs and down.

Its possible to make a single unit work appropriately for multiple levels. Not Easy but possible.
Click to expand...

How do you do that; motorized dampers and multiple thermostats? I think that's how the house we rented was setup, but I never took a close look to see how the split zones were done with one air handler.

I've been considering looking into this for our house with the same upstairs/downstairs problem. We have cathedral ceilings in half of the house so it funnels all the heat upstairs very effectively, especially from the kitchen. The duct split (upstairs/downstairs) is done in the crawlspace before the duct runs up a wall chase, so controlling the air may not be very hard. And our gas pack is ooold.
 
Ron said:
Personally, if it were my house, Id want a split system.
With a separate system for upstairs and down.

Its possible to make a single unit work appropriately for multiple levels. Not Easy but possible.
Click to expand...
1540 sq ft
Split system 2 2 tons works great
 
Fabrik8 said:
How do you do that; motorized dampers and multiple thermostats? I think that's how the house we rented was setup, but I never took a close look to see how the split zones were done with one air handler.

I've been considering looking into this for our house with the same upstairs/downstairs problem. We have cathedral ceilings in half of the house so it funnels all the heat upstairs very effectively, especially from the kitchen. The duct split (upstairs/downstairs) is done in the crawlspace before the duct runs up a wall chase, so controlling the air may not be very hard. And our gas pack is ooold.
Click to expand...


We have an upstairs that is covered with one unit. Well, it's a bonus room but the same idea....

We have multiple thermostats to a computer that controls dampers and the unit. We can theoretically have the upstairs on heat and downstairs be on AC. When one conditioning cycle is done, the other thermostat kicks the unit on (or adjusts the dampers) as needed for whatever is needed upstairs. If I can keep dampers working properly without breaking, the system works like a charm.
 
Fabrik8 said:
I'm a HVAC newb, but isn't that a recipe for creating super high relative humidity with that much oversizing?
Click to expand...
I'm not sure, about the humidity problem but since 92 the system has worked flawlessly.
 
That's a ton per 400sf. I'm no mechanical engineer, but that doesn't sound too bad.

And if they're both heat pumps with electric elements, the upstairs one might be a little big in the winter, but the downstairs one is a little big in the summer.
 
I'd say you need 2 -3 estimates from companies that can give you ideas on what problems your having. My house is a single a story, 1200 sq.ft. I replaced a 32 yr old heat pump that worked fine & did the job. I Knew it wasn't efficient, costing me money, & didn't want to wait til it Quit! My new unit runs at 1/2 the cost, & I'm more comfortable at a higher thermostat [73] in the summer, & about 70/71 winter. Btw, Estimates were from $6k to $10k. I choose the $7k unit. The 6 was a basic unit, & the others were overpriced. The 10K was a lifetime warranty, but Their Trick was I would be paying a hidden $2k to pay for a third party warranty.
1200 Sq Ft. Fixed it, thanks!
 
Last edited:
rodney eppes said:
m more comfortable at a higher thermostat [73] in the summer, & about 70/71 winter
Click to expand...

Unless the temps (and RH) were confirmed by a separate device, they don't hold any water. I've seen 3-4F differences in the stated temps on residential thermostats. It's most likely a calibration issue.
 
rodney eppes said:
I'd say you need 2 -3 estimates from companies that can give you ideas on what problems your having. My house is a single a story, 12K sq.ft. I replaced a 32 yr old heat pump that worked fine & did the job. I Knew it wasn't efficient, costing me money, & didn't want to wait til it Quit! My new unit runs at 1/2 the cost, & I'm more comfortable at a higher thermostat [73] in the summer, & about 70/71 winter. Btw, Estimates were from $6k to $10k. I choose the $7k unit. The 6 was a basic unit, & the others were overpriced. The 10K was a lifetime warranty, but Their Trick was I would be paying a hidden $2k to pay for a third party warranty.
Click to expand...

You have a 12,000 sq ft single story house? ;)
 
shawn said:
That's a ton per 400sf. I'm no mechanical engineer, but that doesn't sound too bad.
Click to expand...

That's a normal "out of your ass rule of thumb" for commercial ballpark estimates, where you can assume leakage, minimal insulation and typical heat transfer, as long as there's not some ridiculous array of storefront (you'd be shocked how many BIG named firm architects simply CAN'T get low-E :rolleyes: ) but (new) homes are substantially tighter so it's usually better to run loads to get a more accurate number. I SAY that.......but usually I pull a number out of my ass, then spend 2 days modelling a space and come within a 1/4 ton of what I ballparked it at o_O

I'll be honest, homes really throw us a curveball. MOST good HVAC contractors are better at getting the tonnage right on a house than we are, because of their shear experience. We tend to number it to death. GREAT for 50yr cost analysis and getting a precise setpoint with conditioned outside air, but not so much for a house

Having said that.....While a single unit may seem to work flawlessly for 2 floors, you're not trending your numbers to see what efficiency you have or know how much money you've really thrown out the window in wasted energy. (Not that it's important to many, but a $300+ electric bill DOES freak some people out) and you KNOW there are hot spots and cold spots. Obviously you've tuned the vents for airflow, or you'd have a laundry room or bathroom on the first floor (some isolated interior space that's small with a grille) that was 44°F when the door is closed :lol:
I'm in no way saying you're lying...what I'm saying is...."Comfort" is relative as hell! What MAY "feel" wonderful to Susie (22 yr old athletic build) may be sweltering to Sally (Post-menopausal heavyweight former East-German wrestler) hence why we have AHSRAE standards and target information to shoot for in design. But for 2 floors you really NEED 2 zones (units) and for larger houses, 2 units per floor is really ideal (smaller zones to separate S/W exposure from N/E)
While the traditional boxes and ceiling diffusers are ugly as hell, the MOST ideal unit is the VRF....or, Multi-Zone. When I build, this will be the route I take, but with bulkheads and concealed ductless splits (i.e. living room ceiling at 10'-0" and adjacent room 8'-0" with the ductless split unit blowing sidewall into the living area over the 8'-0" ceiling room) But they are coming out with some creative solutions

This way you can control every room individually with one exterior unit


multi-zone.jpg



rodney eppes said:
If you Oversize the AC, it will cool Fast, but not run long enough to remove the humidity. So you still feel hot & sticky!
Click to expand...

This! The key to cooling (especially in the South) is to strip the moisture from the air. Also, oversizing an AC unit causes it to cycle on and off at way too great an intervals. A motor, compressor, mechanical device is always more happy when it can either run....or stop.....not do both non-stop. A properly sized AC unit will run continually on a design day (95°F) so it won't keep the house 68°F on a day like today (expected 97°F here) but usually we only see a couple of weeks of those "design days" This means 90% of the rest of cooling season the unit is slightly over-sized and will cycle.
 
Last edited:
CasterTroy said:
That's a normal "out of your ass rule of thumb" for commercial ballpark estimates, where you can assume leakage, minimal insulation and typical heat transfer, as long as there's not some ridiculous array of storefront (you'd be shocked how many BIG named firm architects simply CAN'T get low-E :rolleyes: ) but (new) homes are substantially tighter so it's usually better to run loads to get a more accurate number. I SAY that.......but usually I pull a number out of my ass, then spend 2 days modelling a space and come within a 1/4 ton of what I ballparked it at
Click to expand...

True, but in a 2-story house, the upper unit is essentially cooling at 2 tons per.... let's call it 1200SF, while the downstairs unit is heating at 2 tons per 1200SF. I'm guessing that each unit is doing half of the work for the other (half the hot air is drifting upstairs) because of the open stair. It seems like a reasonable number. We have 2 tons on a 880SF single-story, and it works great. Meanwhile, we have 4 tons on a 2800SF ranch, and it cools and heats fine from a capability standpoint, but needs 2-3 zones in order to work "well". There are more variables than just tons/SF.
 
CasterTroy said:
While the traditional boxes and ceiling diffusers are ugly as hell, the MOST ideal unit is the VRF....or, Multi-Zone. When I build, this will be the route I take, but with bulkheads and concealed ductless splits (i.e. living room ceiling at 10'-0" and adjacent room 8'-0" with the ductless split unit blowing sidewall into the living area over the 8'-0" ceiling room) But they are coming out with some creative solutions
Click to expand...

Slot diffusers for ductless units? I'm starting to totally change my plan for our (eventual) gaspack replacement. I looooove slot diffusers.

How are those installed in a wall by the way; do you need a full length (length of the slot diffuser) header for a standard wall to accommodate the duct boot unless it's a non load bearing interior wall? They're usually a couple of feet long, so you're spanning at least a few stud bays.
 
shawn said:
I'm guessing that each unit is doing half of the work for the other (half the hot air is drifting upstairs) because of the open stair. It seems like a reasonable number.
Click to expand...

That's not at all correct. Loads are calculated on cooling. The heating phase (heat pump) is grossly oversized if the cooling load is designed correctly
 
CasterTroy said:
That's not at all correct. Loads are calculated on cooling. The heating phase (heat pump) is grossly oversized if the cooling load is designed correctly
Click to expand...

Then how else do you account for the heat loss/gain via the stair?
 
Back
Top