HVAC problem (undersized unit)

[TJay74]

You got some really good advice here.....
And then some you should ignore
It's not always out of refrigant, pipe is Not ductwork, and under sizing the unit to keep it running is just..Crazy
Your unit is undersized or not installed correctly or your blower speed is set too low
Call a reputable H\A company and they will fix it......buy guns here......
Unless your house is a custom, the smallest unit that could legally be put in it.......was put in it
These days they're using 500 sq ft per ton, rule of thumb is 400 or 1 cfm per sq foot, I've never seen 600, which would seem what yours is getting close too
Anyway, it seems a window unit or maybe even a mini (there Still expensive) would work....Good Luck and hug your HVAC guy.....he prolly needs it this time of year

Hate to tell you this, under-sizing is not crazy. It serves a purpose and works very well if it has been done correctly. Both of our houses have been under sized by about a .5 ton, both of them ran lower bills than the surrounding neighbors.

People can complain about the OG&E smart hours all they want, the program works and can save you a boat load of money provided you can cut and or eliminate usage between the 2p-7p time frame. I turn the AC on at 7am to 70° and turn it up to 80° at 2pm, house never gets higher than 75-76° during that time frame. Then it stays at 74° until around 10pm when it drops to 73° for the rest of the night. When stuff is not in use it is not plugged in other than TV and appliances.

It is pretty easy to lower the bill, just have to do some research and change the usage habits in the home.

 

[ProBusiness]

Really do not know a lot about this.

Did you mention that amount of insulation you have in the attic?

I have 2175 sq ft and a 3 ton unit.

when I replaced my unit several years ago i got all types of quotes and prices. some said 3 was not enough, some professionals wanted 4 and 5. the 3 ton unit will keep the house at 76 -74 during day and night which is fine with us.

it will have problems keeping it cool when very, very hot like 110 or a house full of people on a very hot day.

you might put your hand around the locks on your doors and feel the air flow and check your fireplace.

we had all windows replaced, some top of the line dual pane windows which reflect heat. they had small pieces of metal specs in the outside pane which would reflect heat. this lowered our bill.

i work at home so i keep it cool all day. around 200-250 month.

 

[twoguns?]

Sorry , think what you want, I'm not selling anything
There Can be some savings by changing, inside and outside coils, fan speed
Too much info to give here, that's why I said ....call a pro
The rest, I don't know, I believe it to be sales mumbo jumbo, that's why I'm not trying to sell it...... :-$

 

[NikatKimber]

You got some really good advice here.....
And then some you should ignore
It's not always out of refrigant, pipe is Not ductwork, and under sizing the unit to keep it running is just..Crazy
Your unit is undersized or not installed correctly or your blower speed is set too low
Call a reputable H\A company and they will fix it......buy guns here......
Unless your house is a custom, the smallest unit that could legally be put in it.......was put in it
These days they're using 500 sq ft per ton, rule of thumb is 400 or 1 cfm per sq foot, I've never seen 600, which would seem what yours is getting close too
Anyway, it seems a window unit or maybe even a mini (there Still expensive) would work....Good Luck and hug your HVAC guy.....he prolly needs it this time of year

Sorry , think what you want, I'm not selling anything
There Can be some savings by changing, inside and outside coils, fan speed
Too much info to give here, that's why I said ....call a pro
The rest, I don't know, I believe it to be sales mumbo jumbo, that's why I'm not trying to sell it...... :-$

Neither TJay nor I are selling anything either.

I know myself, although I can't speak for TJay, that I am NOT saying his unit is fine, but that there could be other causes to the high bill.

"Rules of thumb" are just that: not hard and fast. Accurately sizing an A/C unit depends on multiple things: 1) heat load (external surface area, appliances, window area and quality, how many people/animals, how frequently doors will be opened) 2) thermal efficiency of structure (insulation, window quality) 3) efficiency of unit

Not being familiar with the HVAC industry, I would imagine an accurate "sizing" of the A/C system is not regularly done on residential units, as this is expensive, and can change from the original owner.

Simply put, if the 3ton vs 3.5ton units are the only (approximate) difference from your house to your neighbors', then that .5ton undersize should not cause your bill to be double theirs.

 

[ConstitutionCowboy]

Another consideration is whether your inside unit (the evaporator) and the outside unit (the condenser) are from the same manufacturer and rated the same.

Have your HVAC guy measure the temperature differential between a return air grill and an outlet grill while your A/C has been running for several minuets. This will indicate how efficient your unit is. (I don't recall what the differential range should be when mine was measured, but your HVAC guy will know.)

It might be a good idea to have your electric meter checked as well. It is also possible you have an electrical drain elsewhere in the house causing your high bills.

You could also have all your cooled air seeping out between your foundation stem wall and the soleplate due to a lack of proper sealing when the house was framed.

Regardless, find the problem before you spend money on a new unit!

Woody

 

[Viper16]

A 3-Ton could do it, the problem is your builder was getting extra credit back from the rebates because of the energy star rating he gave the house. I bet you have no trouble heating the home?

I would have a contractor that is willing to run a heat load on the building. A block load is good enough to correctly select a tonnage for cooling.

Alot of the problem is the systems are only designed to run at their capacity when it is 95F Ambient outside temp. If the sun is radiant on the condenser and we are above 95F then you get a reduction in capacity...the house has trouble cooling.

I have a 3 ton in my 1500 sq ft home, and have trouble cooling it with my bad insulation/windows...although I am asking my system to run outside its design conditions. I want a 95F ambient outdoor condenser to run my house below 75F-78F indoor temp...and I complain because I want it to be 70-72F in the house. If you look at the AHRI ratings they are for cooling "Nomnical Cooling Capacities are based on: Indoor Temp of 80F DB / 67F WB. Outdoor Temp of 95F DB, 75F WB.

What temperature are you trying to achieve in your home?

 

[twoguns?]

All units installed in new houses must be accompanied with an ACCA approved load calc and layout.
Same as with complete change our of equipment and ductwork
I know what rule of thumb is , but I don't beat my wife.....anymore...=-O
The type of equipment available today, variable speed fans, multi speed compressors, sencing t stats, the old way of mixing sizes. Isn't economic.
Nor is it in line with the energy conservation codes being enforced.....
My apologies for thinking it was a sale, I read the OP as it wasn't cooling properly, and when someone starts out with credentials, I assume that also
Anyways, it seems he has a builder grade model, it can either be changed out, add units or torque the unit for max output
Just my experience......call someone in the bizz you trust

 

[_CY_]

some clarification about sizing ... purposely undersizing is not good .. neither is oversizing. what you really want is to correctly size .. a pro will do a Manual J calculation. 500sf per ton can be used as a rule of thumb but going by rules of thumb can sometimes get you in trouble.

let's forget about labeling for a minute and examine what type problems under/over sizing causes. below only covers main points

1. if HVAC unit is truly undersized unit will struggle to keep up and run almost all the time. naturally this drives up electric bills. but note several other factors can cause HVAC unit not to run at peak efficiency and cause unit to run all the time too.

2. if HVAC unit is oversized .. unit will not run long enough to properly dehumidify the house. for air to feel comfortable besides lowering temperature, air has to be at proper humidity levels. your evaporator condenses an amazing amount of water .. if the inside drain line should plug one finds out quickly how much water is generated.

a properly size HVAC should be able to pull temps down to a comfortable range on the hottest days. when unit cycles on should run long enough to properly dehumidify house to feel comfortable. then unit should not cycle again for awhile .. how long depends on outside temps, time of day, etc. etc.

 

[twoguns?]

I have one 5 tons and a 3 tons at my 3500sf house, last month's electric bill was $256 with OG&E.

5+3=8tons....8x500 (the average sq ft per ton on load calc)=4000
Your unit meets common load calc, and I doubt its a builder grade units
The window , doors ,wall and ceiling insulation all add up to the efficiency of the house, heck even the way the house faces is calculated
Air flow is very important, that could be why your units are rated ....larger
Do you know what the adjusted SEER, EER ratings for your eq is?
LOL it can get confusing

 

[twoguns?]

Thanks cy