1

We just bought/built a new home. We were not given any options regarding the central ac/heat system. We were told simply the system would be sized to fit the house. Soon after moving in it already seems the system has a hard time keeping up with the Texas heat. By 10am the temp has already risen 4-5 degrees from a 68 or 70 setpoint. I know that is a lower than typical 75 setpoint they are designed for, but at 10am it is barely 80 outside. The home is 2700sqft, two story with a single condenser 3.5ton heat pump system with dampers. I called the company that installed the unit and they said I may only get 20 degrees. So in a 105 degrees, the best I may get in my house is 85, does that sound right? I feel like this system is way undersize. I am afraid my electric bills will be outrageous and my system is going to have a shortened lifespan because it is running constantly. Should I hire a different hvac company to do calculations and see if it is correct? Should I push this issue to the builder and/or installer, or am I being unrealistic?

  • 1
    As a homebuyer, you are absolutely well within your right to question the design decisions being made with your money, and challenge something you think is wrong. But realize that the contractor expects to get paid and certainly wont tolerate not getting paid, you'll need pro or legal backup. If you have already paid them, then they got what they came for. It might be a fight, and one that will be hard to win if the contract you signed called out the equipment they put in. They will claim that's the contract & you'll pay more otherwise. Unf.I cant size an AC so I cant mark an answer. – noybman Aug 20 '17 at 5:49
  • 5
    You're concentrating only on one side of the equation, the heating/cooling system itself. Which probably is sized correctly. The other part of the equation is insulation and air sealing. Keeping the heat out and cold in. (I think you misunderstood the "20 degrees of cooling" statement, commonly repair techs measure air temp in the return duct and the supply duct close to the furnace they are looking for 20 degrees difference in the two temps.) – Tyson Aug 20 '17 at 11:55
  • Where is your house located – Jim Stewart Aug 20 '17 at 12:30
  • Belton TX, 1hr north of Austin – Willis Aug 20 '17 at 13:19
  • 3
    I'd be careful not to oversize the unit -- running continuously is actually OK for a compressor. – ThreePhaseEel Aug 20 '17 at 13:26
6

Your concern about the unit wearing out from running constantly is misplaced. Most of the wear is on start-up so it is easier on the compressor to just stay on continuously.

Your electric bills will not be higher with an undersized a/c. The electric power consumed would be higher with a larger unit. The amount of power consumed is determined by the outside temperature, the insulation in your house, the amount of heat you are generating within the house, and the set point you choose. We keep ours at 75 F and some neighbors keep it at 78 F.

We have had a somewhat undersized unit in service for 26 years and it's still cooling. The compressor is original; the condensing unit fan motor was replaced 2 years ago. Our system is a Carrier 12 SEER 3.5 ton condensing unit (with I think a 4 ton evaporator) with a "scroll" compressor. The evaporator has a TXV not a fixed orifice. Heating is with a natural gas fired furnace so our a/c is not a heat pump.

The house is in Dallas TX 2050 sq ft on a 10,000 sq ft lot with some tall trees around it, but very poorly insulated. On a >100 deg F day (peak temp) the condensing unit might stay on from say 11 am to 5 pm. Our thermostat is set at 75 deg F. If I turned it down to 72 deg F, the a/c would probably be on 24 hrs.

A lot of the heat that you are removing from the house is heat generated within the house--body heat of humans and dogs, TVs, computers, refrigerators/freezers washing machine, dryer, hot showers, and cooking. The exhaust fans over the range and in the bathrooms force out cool air which is replaced by hot air infiltrating.

Put you thermostat on 75 F and see how it performs. It should be able to keep the house at 75 deg F when the outside temp is 100 F.

When we replace the unit I will probably get a 16 SEER single stage scroll compressor, and I might increase to a 4 ton. But I think the increment in cost (4 ton vs 3.5) might be better spent on increasing the insulation in the attic.

  • 2
    excellent point on the heat generated in the home. We have a 55" Samsung TV, and a home theater system, my wife turns it on and often leaves it on all day. I set it to auto off to save cost, but it draws AMPS (multiple) even idle, and is its own little electric heater! – noybman Aug 20 '17 at 14:39
4

Square footage is mostly meaningless because there are so many factors involved. The only way to determine the equipment size is to do heat load calculations on the home. ACCA Manual J is one of the most recognized methods. The total square footage of the floors, ceiling, windows doors and walls must be measured individually. manual J then gives multipliers for each construction material which is then multiplied by the total square footage of each material. Plug in the mean maximum daily temperature for your area and subtract the temperature you want the house to be. This number gets plugged in and gives you the total BTU the house is gaining. Add about 30% for latent heat and that will give you the total BTU needed. Bottom line, if they can’t produce some calculations they are just guessing and so would I based upon the information you gave. If they didn’t use some recognized calculation method it is a crap shoot wether it is sized properly or not. For illustration purposes I once did a heat load calculation on two fictitious houses, both 2500 square feet. One house needed twice as much cooling as the other based on construction and how many stories.

1

Since I live about 30 miles north of Pittsburgh, Pa. I can not tell you if the A/C unit installed is the correct size for your area. Some things that effect A/C sizing are, orientation of your home, type of construction, including wall thickness and insulation, size and type and construction of windows, color of roofing, do you have a basement or are you on a slab. You could get a thermometer and check the temperature drop across the cooling coil. (return at air handler VS. supply at air handler). This would just be a starting point. An 18-20F. degree drop is normal. Higher than 20 degrees would usually mean not enough air flow. Then I would call 1 or 2 HVAC companies and explain your concerns and ask if they could appraise your whole system including the size of the duct work, placement of supply and return registers, and their recommended size of an A/C system. They should also check the A/C unit for the correct refrigerant charge and overall system performance. A few things you can check/do; cover the sun facing windows to reduce the solar load during the day, Install thermostatically controlled fans or fans that run on solar power to lower the extreme temperatures in the attic, make sure that there is enough attic insulation (more is better than less), Try to reduce the use of kitchen appliances that produce heat (oven) and laundry equipment on the hottest days. And yes, A/C is expensive and so is home ownership. Hope this helps.

  • 1
    100% on the quality & engineering of the rest of the home! Not only did the contractors make decisions with the size of the unit, (and hopefully the right one), but adequate home venting and insulating is crucial too, and decisions were made in that aspect of your homes design as well. – noybman Aug 20 '17 at 14:36
  • A comment; You wrote that you have a 2 story home somewhere in Texas that has 2700 sq. ft. of conditioned space using a 3.5 ton heat pump.That equates to 771 sq. ft. per ton of A/C. Jim Stewart wrote that he has a 2050 sq. ft. house in Dallas, Texas using a 3.5 ton A/C unit. That equates to 585 sq.ft. per ton of A/C. He thinks his unit may be a little to small. but it manages to keep his house at about 75 degrees. . However his A/C seems sized better for the extreme heat you guys get. As far as good insulation goes it only slows down the transfer of heat into the home so A/C runs at night. – d.george Aug 21 '17 at 11:48
  • One last idea; buy a large dehumidifier, 70 pints/day capacity minimum and set it to run until the humidity is around 40%. This will remove a lot of the cooling load from the A/C system and may provide enough extra cooling capacity to keep your house at a decent temperature. I run my dehumidifier on automatic set at 45% which keeps the humidity at or below that setting. and makes my house very comfortable. One thing to remember is that most A/C systems are not capable of removing enough of the homes humidity to provide for the best comfort. – d.george Aug 27 '17 at 11:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.