Air Conditioner?

[SolaSaint]

I have a HVAC question. I bought a huose last year that is about 1600 sf on the main level which is air conditioned. I'm told I have a 3 ton unit, it struggles to keep our house cooled. We set it a 77 degrees and on days when it gets 90 outside it hardly ever stops running. I get high electric bills (250-300 a month). My neigbbor tells me I should invest in a heat pump that is staged. But he also tells me if I up my unit to a 3.5 ton or higher that the duct work still may not match the bigger unit. My head is spinning. Does anyone have some good advice?

 

[annsni]

I wish I could help! We have window units - some through the walls. That's all we can afford!

 

[Benjamin]

Check the temperature of the air going in the intake and that which comes out. There should either be about a 20 degree split or the air coming out should be between 55-60.

Other than that, if the temperature is correct, it is very likely you have insulation problems. How much insulation is in your attic? I have 18-24 inches in mine. Are your windows dual pane? Are there gaps around your windows? (caulking and weather stripping may help) How about your doors, are they air tight? (again caulking, weatherstripping, and check the threshold) It is hard to do anything about your walls, but they may have poor insulation which can be improved, but $. Trees to shade the walls would also help. Do you have the sun shining directly on the windows? If so, that along with single pane windows can counter all the cold you can get all by itself.

 

[SolaSaint]

Ben,

Our thermostat is set on 77 and the air coming out is at 60. The house is only 7 years old and has double paned windows. The ddors are tight and the previous owning I'm told added insulation in the attic. I'm pretty sure the unit is too small, I was told that by two neighbors. My question is will a new bigger unit really help?

 

[Benjamin]

Ben,

Our thermostat is set on 77 and the air coming out is at 60. The house is only 7 years old and has double paned windows. The ddors are tight and the previous owning I'm told added insulation in the attic. I'm pretty sure the unit is too small, I was told that by two neighbors. My question is will a new bigger unit really help?

I've seen a lot of new houses with poor and even non-existent insulation in the walls. I'd look in the attic and see what's up there and if it covers well. The split doesn't sound too bad so I wonder about the air volume coming out which many units have a separate blower unit. Note: your air conditioner would have to keep up with a higher volume of air and in that case you'd need a better unit. Also direct sun can be a big problem and a well insulated house can keep cool longer even with a a low volume of cold air.

P.S. if you go walking up in the attic don't step on the dry wall. :smilewinkgrin:

 

[Melanie]

:laugh:Ceiling fans are good!!! Can you open your windows?

When it was really hot I would go to the mall with the rest of the city of Mt. Isa. It was a dry blasting heat there and no-one had aircon.


In humid old Brisbane, I had aircon in the bedroom being a shift worker and insulated the ceiling. Walls were not insulated being an old house. Windows were wide open 24 hrs a day (security screens in place) and ceiling fans whirling about. It was just plain uncomfortable except on the veranda sipping bedewed cool drinks watching the sun set and slapping at mosquitoes.:laugh:

I sure don't miss that.

 

[Jim1999]

We don't use air conditioner (icer), but our entire house is covered by 2" foamboard and sealed. We cover windows with good drapes. Windows are the worst source of outside heat and cold. Glass breaks down over time and develops unseen holes (by the naked eye). The attic is usually the same temperature as outdoors, hence, the need for good insulation and sealant (plastic wrap).

A small window type air conditioner should do the whole house with proper insulation and sealant (plastic moistier barrier).

Cheers,

Jim

 

[InTheLight]

There's a lot of other information needed before I can comment. How much of your house is shaded? Do you have a basement? How many floors do you have? Is only the main floor ducted for air conditioning?

Also, the tonnage of the unit refers to the compressor, which is outside of your house. Your duct work would not be an impediment to installing a 3.5 ton unit.

I replaced our A/C and furnace a few years ago. Our house is a two story walkout with 3,000 square feet. There is a southern exposure (all three floors) with no shade. Approx. half the basement level is underground, so it doesn't need to be cooled. In the summer I shut all the air duct registers in the basement and open all the ducts in the upper level.

I set my thermostat at 76 degrees. At this setting the main floor is 76 degrees. The basement is at 72 degrees. The upper level is at 78 degrees. We have ceiling fans in all four bedrooms upstairs. At night the programmable thermostat progressively lowers the setting--74 degrees at 10:00pm and 73 degrees at 1:00am. This lets us sleep in cool comfort.

Even for this size house we only have a 3 ton compressor. When it gets real hot, say 95 degrees, the A/C runs about 45 minutes per hour. I'm with Benjamin on this one, there must be some other factor causing the inefficiency of the cooling of your house.

Another note--the A/C installer said that going to a larger tonnage compressor would certainly cool the house more but would not remove humidity as well.

 

[mandym]

I have a HVAC question. I bought a huose last year that is about 1600 sf on the main level which is air conditioned. I'm told I have a 3 ton unit, it struggles to keep our house cooled. We set it a 77 degrees and on days when it gets 90 outside it hardly ever stops running. I get high electric bills (250-300 a month). My neigbbor tells me I should invest in a heat pump that is staged. But he also tells me if I up my unit to a 3.5 ton or higher that the duct work still may not match the bigger unit. My head is spinning. Does anyone have some good advice?

I used to sell Trane AC. The industry standard says you need one ton for every 500 sq ft. Your neighbor is correct at 3 tons you are pushing it and should be at 3.5 tons. The duct work will be no concern here unless it was done as incorrectly as the unit. You should have an estimate done for a new unit and have the location of the vents checked. Have attic insulation checked as well, it can play a big part of the AC cooling the house. Heat pumps are effective in the south but terrible in the north. Heat pumps basically run in reverse while heating and pull any level of heat in the air into the house from outside. If temps stay at 32 or below then they are useless. They do have axillary heating coils to make up the difference but the coils are smaller than a normal unit since it relies heavily on the heat pump itself. If it has to use the axillary heat coils to much then it will run your heating costs up sky high.

On a 1600 sq ft house it is not likely an estimator will run a heat load on your house. The windows and doors are usually minimal. On larger homes 3000 sq ft or higher with they will run one especially on a house with lots of windows and doors or sky lights. Outside shade such as trees on average size homes also have a minimal effect. American Standard, Trane, or Carrier are great units to go with. Stay away from units made by companies like Lennox, Frigidaire, or Amana, these are very poor manufacturers and you will regret getting them.

Be sure to consider the seer rating on any unit you purchase. Ask questions from the salesman about the available seer ratings. This is an important part of the efficiency and your over all costs of heating and cooling. Much more important than any heat load. I believe the lowest heat seer rating allowed by law currently is 13(its been a while). Stay in the middle to high ranges of available seer ratings.

If you go here http://www.trane.com/Residential/Whats-Right-For-My-Home this will help you get some preliminary understanding of what you might need even if you do not buy a Trane unit.

 

[billwald]

Do you have an exhaust fan in the attic? It will help much!

Our first house had a screened opening either end of the attic under the eaves. I boxed in a plain old square room fan and ran an extension cord to a light switch. Worked fine. A small fan doesn't use much power but it can suck out much hot air.

 

[mandym]

Attic fans can be helpful especially if there is poor insulation in the attic. Unless the gables are vented on both ends attic fans are useless. Most houses have roof venting to allow the heat to escape properly. An R-32 rating and roof vents in the attic does more than anything else to protect the house from attic heat.

 

[menageriekeeper]

Well, I can tell you our experience. We have a two story house with approx. 2800 square ft of floor space. We had two 1 ton units (one downstairs, one up). It cooled okay, but the electric was outrageous. Both were older units. The downstairs went out 4 years ago. We replaced it with a 1 1/2 ton unit with a high energy rating. Our electric bills dropped by a good $75 a month. The second unit died two years ago right at the end of summer. When it turned spring, we couldn't afford a new unit, so we bought the kids (who live in the upstairs bedrooms) each a window unit. Again, our electric bill dropped dramatically.

We do budget billing, so we pay the same amount each month. Our highest budget budget amount before the first unit went out was $480. Now (5 years later), I'm paying $337 a month. We regularly have 6 or 7 people showering, doing laundry etc and we keep the AC set to 73 on cooler days and 71 on days it is really hot. (there is a sharp difference in the two temps that is evidently a pecidillo of the unit) We may never replace the upstairs unit......(an if we do it'll only be ac. We haven't ever used the upstairs heat because of the way the heat rises from the downstairs.)

 

[mandym]

Well, I can tell you our experience. We have a two story house with approx. 2800 square ft of floor space. We had two 1 ton units (one downstairs, one up). It cooled okay, but the electric was outrageous. Both were older units. The downstairs went out 4 years ago. We replaced it with a 1 1/2 ton unit with a high energy rating. Our electric bills dropped by a good $75 a month. The second unit died two years ago right at the end of summer. When it turned spring, we couldn't afford a new unit, so we bought the kids (who live in the upstairs bedrooms) each a window unit. Again, our electric bill dropped dramatically.

We do budget billing, so we pay the same amount each month. Our highest budget budget amount before the first unit went out was $480. Now (5 years later), I'm paying $337 a month. We regularly have 6 or 7 people showering, doing laundry etc and we keep the AC set to 73 on cooler days and 71 on days it is really hot. (there is a sharp difference in the two temps that is evidently a pecidillo of the unit) We may never replace the upstairs unit......(an if we do it'll only be ac. We haven't ever used the upstairs heat because of the way the heat rises from the downstairs.)

A two story home with two floors and 2800 sq ft should have two units. One for the down stairs most likely a 3 ton unit and one upstairs most likely a 2 ton unit. At 2800 sq ft you certainly need at minimum 5 tons depending on the heat load. If you have a basement and that is included in the 2800 sq ft measurement then that would change things. But either way you have been under covered tonage wise.

 

[kyredneck]

HVAC-R is one of my specialties; I've designed and installed/modified many residential & commercial systems. I would be glad to advise if you wish, either through PM or open forum.

From what you've posted I suspect that there COULD be an airflow restriction or distribution problem with the duct inside the house. This could be due to a dirty evaporator (inside) coil), dirty filter, closed dampers, poorly designed/installed ductwork, blower speed too low, etc..

Also, here in KY the rough rule of thumb for sizing a residential system is one ton A/C per 400 sq ft (450 for heat pumps) floor space. A 4 ton system could be justified for your 1600 sq ft main level. However, for the purpose of CREATURE COMFORT (and this is what air conditioning is all about) a three ton system should suffice very nicely simply because of the increased dehumidification that is attained from the long (or continuous) runtime of the equipment. An 82F space with low humidity can be just as comfortable as a 72F space with high humidity.

Are the occupants of the space actually uncomfortable, or are the occupants dissatisfied that the space temp doesn't attain to the setting of the t-stat?

Also, the human body can feel a 2F temperature change immediately. Example, if entering a 79F room from a 77F room it will immediately be sensed and COULD be construed as discomfort. This would also indicate an air distribution (balancing) problem.

Also, A smaller system that runs continuously (IF everything is in good working order, i.e., adequate airflow, evap & condenser temps, good oil return to the compressor) is not bad for the system, nor does it necessarily equate to higher electric bills for the simple reason of the locked rotor amps (LRA) that larger systems will experience that cycle 4-6 times an hour. The LRA can be 5 times the RLA (run loads amps).

Here is my advise:

Obtain the make/model on inside & outside units, let's check see what you actually have.

Check for airflow restrictions within the air distribution system and temp differences between rooms.

Does the system have return air duct or is it free return?

Measure the main duct coming from the air handler (not the plenum) the main duct (or ducts) coming from the plenum. Example, for a 3 ton system you must have ductwork that will move 1200 cfm air @ .08 - .1” static friction loss (don't worry, I've got a ductulator that computes this). A rectangular duct of 20 x 10 carries 1200 cfm @ .08 static.

[edit to add]

One more check to make; AFTER the system has run for 8-10 minutes into a cooling cycle go to the outside unit and grasp/feel the two refrigerant lines connected to the unit (you'll probably/should have to peel back insulation for the big line). The small line should feel WARM, not hot. The large line should feel COOL (and sweaty), not cold.

 

[mandym]

One ton per 400 sq ft would be over kill that would cause significant problems. You end up with condensation on your vents and mold and mildew in your closets and drawers. Even in KY and is not the industry standard. Higher heat loads fro house with large windows and glass doors can vary.

 

[kyredneck]

[sigh]

1 ton AC per 400 sq ft floor space

 

[InTheLight]

A two story home with two floors and 2800 sq ft should have two units. One for the down stairs most likely a 3 ton unit and one upstairs most likely a 2 ton unit. At 2800 sq ft you certainly need at minimum 5 tons depending on the heat load.

Two units? 5 tons total? Really?
My single 3 ton unit cools my 3,000 sq. ft. house just fine.


ACCA (Air Conditioning Contractors of America, developers
of "Manual J") tells us "Efficient single family detached
homes with a normal amount of well distributed glass
typically fall in the 700 to 1,200 range"(para. 2-4, page 9).
That's 700 to 1,200 square feet per ton of cooling;
we're not sure if the includes thicknesses of outside walls,
closets, "air space" beyond balconies on
upper floors, etc.


More from ACCA on the Square Foot Method

"Single family dwellings that are characterized by inefficient
construction and/or inefficient duct system (in an
unconditioned space) may be in the 500 to 700 SqFt/Ton range."

"Single family detached dwellings that have reasonably
efficient construction and a reasonably efficient duct system
may be in the 700 to 1,000 SqFt/Ton range."

"Efficient single family detached dwellings that have a normal
amount of sunlit glass areas may be in the 800 to 1,200 SqFt/Ton
range."

"Efficient single family detached dwellings that have large
sunlit glass areas may be in the 500 to 800 SqFt/Ton range."
Manual J, page 73


http://askweldin.com/Moreisworse.html

 

[kyredneck]

Two units? 5 tons total? Really?
My single 3 ton unit cools my 3,000 sq. ft. house just fine.

In 1984 I installed A/C (condenser unit and 'A' coil) into an existing heating only system for an old two story w/basement house. Load calculation justified 6.5 tons of cooling. Existing ductwork would carry only 2 tons (800 cfm airflow @ .1" static). Owner did not want to replace ductwork or air handler, so I installed 2 ton condenser unit & A coil along with increased HP motor and adjustable sheave (belt driven) to speed the blower up which increased air velocity which increased airflow which also significantly increased NOISE which I explained to the owner beforehand which the owner was OK with. Also on my advice we [strategically] installed a ceiling fan at the top of the main stairwell. It's now 2012, the system has yet to have it's first breakdown and the owner still is satisfied with the COMFORT the system delivers solely due to the excellent dehumidification it delivers.

Once warm weather sets in this system NEVER shuts off, it literally runs nonstop all summer. One of the major bennies from this sort of setup is the LOAD on the evaporator which virtually guarantees a high pressure, high density refrigerant return gas with excellent OIL RETURN to the compressor, which is the life of the system, period.

I would intentionally UNDERSIZE a system for my house for this reason alone.

 

[SaggyWoman]

When I bought my house, it was with an AC unit that was from 1991. The year was 2003. THe AC died the next year. With my old unit, my bills were fairly high, even for me. I purchased a heat pump (mid line--there were some more efficient. I have a 1330 sq ft house and I keep it around 75 in the summer. It doesn't run a whole lot. My bills run from 45 to 120, depending on the need. If you like hot heat, you might go gas or something else. Does it get cold cold in your part of Missouri? I like my heat pump. Fine in summer. Okay in winter, but on cold cold frigid cold nights, I wish I had hotter heat or a man in the house. hahaha.

In the winter I set it on 68.

When I leave the house, I set it on 81 in the summer and 55 in the winter.

 

[InTheLight]

In 1984 I installed A/C (condenser unit and 'A' coil) into an existing heating only system for an old two story w/basement house. Load calculation justified 6.5 tons of cooling. Existing ductwork would carry only 2 tons (800 cfm airflow @ .1" static). Owner did not want to replace ductwork or air handler, so I installed 2 ton condenser unit & A coil along with increased HP motor and adjustable sheave (belt driven) to speed the blower up which increased air velocity which increased airflow which also significantly increased NOISE which I explained to the owner beforehand which the owner was OK with. Also on my advice we [strategically] installed a ceiling fan at the top of the main stairwell. It's now 2012, the system has yet to have it's first breakdown and the owner still is satisfied with the COMFORT the system delivers solely due to the excellent dehumidification it delivers.

Once warm weather sets in this system NEVER shuts off, it literally runs nonstop all summer. One of the major bennies from this sort of setup is the LOAD on the evaporator which virtually guarantees a high pressure, high density refrigerant return gas with excellent OIL RETURN to the compressor, which is the life of the system, period.

I would intentionally UNDERSIZE a system for my house for this reason alone.

Sounds like you know your stuff. Every situation is different and a good installer is critical. In my case the installer had installed A/C in hundreds of houses around our area including dozens of the same model house that we had. This gave me confidence that he knew what he was doing. Both neighbors on either side of me have gone with the same company and are quite pleased.