Residential AC capacity/replacement

[ChuckstrPT]

So,
We've got slightly less than 1000 sqft upstairs, and 2 ton unit that can't cool it down below 80 degrees when it hits 95degrees here (Alabama). Hotest room is our nursery, has the worst run of ducting. Have to nearly close off all the rest of the ducts to get any air in there. Plus attic access is in that closet, and does get hot. The system pretty much runs 24/7 during hot summer months.
So we have the system looked at....seems to be working fine, coolant checks out. Rec replace the whole system with a 2.5 ton unit for $5700. I asked about a 3 ton unit, was told Too Big. Funny, I didn't think you could ever have TOO MUCH AC? Apparently it's a humidity thing.
Does all that sound correct? and is that a fair price? We need resolution before July shows up again.
Thanks for any input!
Andy

 

[8Mud]

Ducting cold air never has been very efficient IMO. Getting the system balanced is often pretty rough.
Many of the systems draw warm or hot air and then cool it and push it to where it's needed. The household systems rarley have any ductwork for recirculation. The over pressure forces it out of the room. Works OK in restuarants, where you need a large air turnover, but really isn't very efficient.
*Window shakers* use a lot of electricity, for the BTU rating, but at least you can recirculate the same air and not have to continiously cool the whole load of air from scratch.
Chillers work well, each room has it's own heat exchanger.
A couple of easy things to increase efficiency, is to pull the intake air from the north side of the building, which is also a good spot for the unit (or the cellar). Make sure your exhaust heat isn't being drawn in for fresh air. You'd be surprised how many units I've seen with the fresh air intake within feet of the condensor outlet.
They make some stuff here, that is a quarter of an inch of styrofoam with a cardboard covering, that is used like wall paper. It does help. As does reflective blinds on the windows and other stuff that adds up.
I use ceiling fans in large office areas, to help homogonize the air layers, cold and warm air don't mix and have a tendancy to form micro climates. One corner is hot and the other cool.
I measure my wall thickness, outside wall (roof) and window area, the number of bends in the duct work (which really kills you) the CFM of the fan and a couple of other factors and take it down to an engineer at the local wholesaler. It's all in the book, in tables and graphs, saves a lot of guess work. You can look it up yourself, but the books are what an engineer does for a living. Avoid salesman.

 

[ChuckstrPT]

Thanks for the info! Any more AC specialists out there in XJ land?:sunshine:

 

[8Mud]

To clarify a bit, the object of the whole exercise is to move the heat from the inside of the house to the outside. If you force air (cool air), in effect some of the heat is displaced by the cooler air (forced outside or into the next room along with some of the cooler air, by overpressure), some of the air is somewhat homoginized into little pockets of warm and cooler air. And actually most times the poorest methode, the warm air gives up some heat to the cooler air ( in effect homogonizing it in a finer way).
There are two air intakes and exhausts, one for the condensor which spits out heat and one for the evaporator which spits out cooler air (or air with less heat). (Or sometimes the other way around for a freon pump heating unit).
I'm guessing you don't have a swamp cooler in a high humidity area? And your using a refer unit and forced air.
The heat a forced air unit is venting outside, is the heat it removes from the outside air (not the inside air) without a recirculating ductwork. Unless you have a recirculating air duct, which takes air from a room and returns it to the cool air intake, you are wasting a lot of effiency. You may have a recirculating forced air setup, which requires double the ducting. And often requires an air mixing setup so a percentage of the cooled air is always outside (fresh) air. Not real common in residential units. They usually cool nearly double the volumn of air they actually need (at often near double the temperature of the room to be cooled at the intake), pump it into a room and let the whole works kind of average it's way out. Windows that open at the top and can be opened a crack, can help the process some. If your good, they can be set up to vent warm air and not draw hot air from outside (in the absence of a breeze).
A window shaker (window unit) is designed for one room and can be adjusted to recirculate room air, draw outside air to be cooled and often even mix some intake air for the evaporator (cooler). It spits the heat from the room outdoors. It has no duct work to absorb heat and reduce the velocity of the cool air.
Chillers use a liguid coolant that is pumped to what is in effect a radiator in each room. But instead of radiating heat, it actually absorbes heat, returns it to a tank to be cooled by the unit. The heat is in effect spit outdoors. Fairly efficient, the coolant pipes are alot smaller than duct work and are easy to insulate, the evaporators (cold) are often relativley small and use small fans to circulate the air (sometimes they work by convection/the air circulates by itself as the temperature changes). They are designed to recirculate the air in the room and to use windows or the natural air exchange (in a room or building) for fresh air.

Clear as Mud and way more than you wanted to know.

My air conditioner is a giant Walnut tree on the south side of the house. 80 feet of hose that runs from the well (cold water) to the crown and into a mister (fine sprinkler). Uses about as much electricity as a couple of light bulbs, the water evaporates mostly before it hits the ground, it cools about 6-7000 square feet. And a few circulating fans inside the house. Germany rarley gets over 90, three days in a row with 80 degree temperatures is concidered a heat wave. If global warming keeps on keeping on, I'm gonna set up a chiller type system, using well water and then returning it to the water table (the earth as a big heat sink). I might even use my chiller water to flush the toilets. I also have a half cellar and can open the ground level North side windows and open two large attic vents. And move cool air from the cellar throughout the house. I can even water the garden in front of the north side cellar windows (well water pump on a timer). There is more than one way to stay comfortable.

 

[KP60]

Some advice I can give is make sure the condensor coils are clean (the outside part of the system)...ie; no leaves, dirt, or junk in it. You would be suprised how much stuff you can wash out of there. Be sure to turn off ALL power (inside at the fuse box and outside at the disconnect.....I figured this out the fun way one time:shocked: ) and flush with lots of water from the garden hose with no nozzle until the water runs clear, this will greatly reduce the cooling capicity of the unit if its dirty. Make sure the condensor fan is running properly (not running, or slow, or not at all) How old is the unit? on some older units you needed to adjust the fan pullys on the house fan motor to increase the fan speed for cooling and slow the fan down for heating if you didnt have a muli speed blower motor. Another thing we found out on older houses is have the duct work cleaned out, I couldnt belive the amopunt of stuff that came out of there (like a small trashbag of lint, crayons, small toys from privious owners kids ect.)I have a 2200sf home in Kansas and I have a 2 ton unit cooling the whole house with no problem on +100deg days in the summer wth 80% humidity. Same things apply to window units except adjusting the fan speeds. Hope this helps.

 

[Ed A. Stevens]

So,
We've got slightly less than 1000 sqft upstairs, and 2 ton unit that can't cool it down below 80 degrees when it hits 95degrees here (Alabama). Hotest room is our nursery, has the worst run of ducting. Have to nearly close off all the rest of the ducts to get any air in there. Plus attic access is in that closet, and does get hot. The system pretty much runs 24/7 during hot summer months.
So we have the system looked at....seems to be working fine, coolant checks out. Rec replace the whole system with a 2.5 ton unit for $5700. I asked about a 3 ton unit, was told Too Big. Funny, I didn't think you could ever have TOO MUCH AC? Apparently it's a humidity thing.
Does all that sound correct? and is that a fair price? We need resolution before July shows up again.
Thanks for any input!
Andy

I underlined a few clues to the problem.



Problem #1:

Is the attic/ceiling insulated? Wall insulation? (R-19 at ceiling and R-13 at walls -- minimum, more is better).

The old rule of thumb was 1-ton of cooling for every 400 square foot of insulated conditioned space.

Rule of thumb: 1000 sq-ft => 2.5-tons (maybe a little short). Yes, a higher tonnage will condition an uninsulated space, for a greater utility cost (but only if you can move the greater airflow).

Problem #2:

No airflow to the nursery?

Is the air handler fan direct drive or belt driven? What is the duct pressure and airflow restriction compared to the blower rating?

Get the most airflow and pressure from your system.

Direct drive fan motors are typically multitap/multispeed motors, is yours on the high speed (high static pressure & airflow) motor wiring taps?

Belt driven fans are typically more robust that direct drive fans (more commercial grade). If you have a belt driven fan is it sheaved to run at enough speed and static pressure to overcome the long duct run restriction (the same question as the direct drive system)?

You need blower pressure and airflow capacity to get the refrigeration capacity delivered (and your system is not delivering the air, even if it's cooling fine).

Problem #3:

Is the duct to the nursery sized correctly?

What size (of the nursery & duct), what length duct, how many duct bends and twists?

The duct to the nursery is likely to be too small, and the register is also likely to be too small (your description of the air balance effort merits this diagnosis). Air is compressible and will not flow if the duct is too small or too restrictive (even with all other outlets closed, unless you increase the power by a cube factor).

Has anyone mentioned serving the nursery with larger ducting?

Problem #4:

Is return air ducted from the upper floor to remove the heat where it naturally travels (convection heat rises)?

If you donot have an upper floor returnair grille and duct, adding one will help extract the heat transferring through the walls and ceiling.

Pricing is specific to your area. $120/hr labor in San Franscisco will drive the cost there up more than the typical $45/hr in Littlerock. Equipment cost is similar nationwide but labor cost varies.

Get multiple estimates and ask questions. Ask them what the problem they see, and how they plan to fix it (besides isolating the solution to only a larger unit). If the current system cannot deliver airflow (develop sufficient pressure) to cool the nursery a unit change out is not expected to improve the performance unless it has high pressure features (commercial belt driven blower) or unless they reduce the duct restriction.

Insulate, match the system blower performance to the constructed duct restriction, reduce the duct destriction to better match (balance by design) the airflow restriction to each outlet, and extract (return) the hot air close to the source or where it collects.

Good Luck.

 

[8Mud]

Ditto, I'm not a very good communicator, what they both said.
30 BTU per square foot, 1 ton is 12,000 BTU. But a lot of variables, windows, south side, direct sun etc. How well the house is insulated etc.
More capacity isn't going to help much if it can't flow. Some way to vent the room, preferably some type of return duct or recirculation system.
A window shaker for the nursurey, would be a likely solution. Panasonic makes some fairly quiet units.
Saleman usually just try to sell you more tons. I really dislike throwing good money after bad.

 

[azdesertrhino]

Some good points made here prior to me but it equates to a minumum on 2.5 ton if a well insulated space. Windows, the direction the house faces, construction material all figure into the calculations. don't have time to go into all of it.

A very large unit will cool the space down too fast and not remove the humidity but I don't think a 3 ton would be excessive but calculations would have to be done.

Find someone with a "J" manual and you should be able to figure it out pretty close.

 

[Ironmen77]

I underlined a few clues to the problem.



Problem #1:

Is the attic/ceiling insulated? Wall insulation? (R-19 at ceiling and R-13 at walls -- minimum, more is better).

The old rule of thumb was 1-ton of cooling for every 400 square foot of insulated conditioned space.

Rule of thumb: 1000 sq-ft => 2.5-tons (maybe a little short). Yes, a higher tonnage will condition an uninsulated space, for a greater utility cost (but only if you can move the greater airflow).

Problem #2:

No airflow to the nursery?

Is the air handler fan direct drive or belt driven? What is the duct pressure and airflow restriction compared to the blower rating?

Get the most airflow and pressure from your system.

Direct drive fan motors are typically multitap/multispeed motors, is yours on the high speed (high static pressure & airflow) motor wiring taps?

Belt driven fans are typically more robust that direct drive fans (more commercial grade). If you have a belt driven fan is it sheaved to run at enough speed and static pressure to overcome the long duct run restriction (the same question as the direct drive system)?

You need blower pressure and airflow capacity to get the refrigeration capacity delivered (and your system is not delivering the air, even if it's cooling fine).

Problem #3:

Is the duct to the nursery sized correctly?

What size (of the nursery & duct), what length duct, how many duct bends and twists?

The duct to the nursery is likely to be too small, and the register is also likely to be too small (your description of the air balance effort merits this diagnosis). Air is compressible and will not flow if the duct is too small or too restrictive (even with all other outlets closed, unless you increase the power by a cube factor).

Has anyone mentioned serving the nursery with larger ducting?

Problem #4:

Is return air ducted from the upper floor to remove the heat where it naturally travels (convection heat rises)?

If you donot have an upper floor returnair grille and duct, adding one will help extract the heat transferring through the walls and ceiling.

Pricing is specific to your area. $120/hr labor in San Franscisco will drive the cost there up more than the typical $45/hr in Littlerock. Equipment cost is similar nationwide but labor cost varies.

Get multiple estimates and ask questions. Ask them what the problem they see, and how they plan to fix it (besides isolating the solution to only a larger unit). If the current system cannot deliver airflow (develop sufficient pressure) to cool the nursery a unit change out is not expected to improve the performance unless it has high pressure features (commercial belt driven blower) or unless they reduce the duct restriction.

Insulate, match the system blower performance to the constructed duct restriction, reduce the duct destriction to better match (balance by design) the airflow restriction to each outlet, and extract (return) the hot air close to the source or where it collects.

Good Luck.

X2

Also, don't close off all your other vents upstairs, it might cause the coil to freeze in your indoor unit.
Is your downstairs unit cooling allright?

 

[ChuckstrPT]

X2

Also, don't close off all your other vents upstairs, it might cause the coil to freeze in your indoor unit.
Is your downstairs unit cooling allright?

If we don't close down the other vents, in addition to not cooling the nursery, we freeze in the master! The master is still consistently cooler than the other two upstairs rooms. The nursery is by far the warmest.
THank you for all the input!

 

[Ironmen77]

If we don't close down the other vents, in addition to not cooling the nursery, we freeze in the master! The master is still consistently cooler than the other two upstairs rooms. The nursery is by far the warmest.
THank you for all the input!

Is the nursery at the end of the duct run? Do the other rooms closer to the indoor blower get good airflow? Many times with a dirty evap. coil the futher away from the coil the worse the air flow and the colder the air to the closer rooms.
Is this a package system (everything in one unit outside) or a split system(indoor furnace/air handler with a/c or heatpump outside)? Are your filters clean? Do you change them before they get real dirty? Flex duct or metal duct?