Improper engineering

I have a client that is in a 3900 sq.ft. 2 story house. 19ft entry way and 10 foot ceilings downstairs and 8 ft upstairs. He mentions that during the summers in Oklahoma when he has parties he can not regulate the temperature in his house to cool down. He sets the thermostat at 67 degrees and it remains warm in the house. There is 2 zones with a combined 7 1/2 tons. What can be done to help fix this. He has had a HVAC tech. evaluate the systems and he has told them they need more. Is this true or what else can be done. I am going to meet with him next week. Can anyone help. What should i tell him.

Tell him to quit having parties. No, seriously, without seeing or knowing what kind of system he has installed it would be pretty difficult to determine what the problem(s) could be. One thing to remember is for every person that is present at one of his parties that raises the room temperature, humdity (warm bodies), food being served, etc. , people going in and out, poor circulation, lots of big windows, etc. Too many variables. Sounds like the AC can not keep up with the load being placed on it. Think “tipping point”.

I think 7.5 tons is not enough for that size house. Do you know the builder? All house should have a heat load calculation done prior to installing A/C. Can you find out if that was done?

A good rule of thumb is to divide 400 into the total square feet to get a educated guess of the required tonnage. Many other factors go into the calculation but that is a good place to start.

I would be careful qouting the 400 square feet per ton rule of thumb. Most HVAC contractors think that this is a bogus rule of thumb and would not hesitate to thrash any home inspector that qouted it. Most HVAC contractors think home inspectors are idiots. Tell your client to hire a HVAC contractor that can perform a Manual J calculation. This is the only reliable method to determine how many tons are required.

Agree, I use to own a heating and cooling company. A bogus rule indeed. The tonnage sounds right. I have not seen a zoned system work right yet.
I have seen silly things like blowers running backwards on furnaces running backwards causing this problem too. Yes, one of my guys did it once. Or he could just stop having parties. Heating and cooling systems are designed for normal use. It should not be tailored for extreme uses because it will just cause problems for the next home owners.

Turn the air down 1 day before the party.

You are asking the impossible here not a standard build home to begin with. You stated 2 zones combined tonnage. Combined tonnage means more than one system which would not be a zoned system, zoned means two or more zones on one system. No way to determine without being on site not even for the experienced HVAC contractor.

As for the myth of sq footage per ton of A/C no it is not a myth on a standard built home in the state of Ok you can take a standard built 1500 square foot home and do all of the heat load calculation you want and it will come out 500 square feet per ton a 3 ton unit on every one every time.

Contractors do not do heat loads on every home they install on if they are installing from a blueprint with a builder that they install for on a regular basis they are familiar with how he builds and knows from experience what the tonnage will be. They will use heat loads when they run into a situation that they have not installed on before.

The 3900 sq foot home you are describing is not a standard built home and would require a heat load calculation to even be in the ball park.

BTW I don’t have HVAC contractors call me an idiot when I make a call I have the facts to back me up, at least to my face.


Like most, I am reluctant to provide diagnosis on something that I cannot see and feel. However, the “V” in HVAC stands for ventilation. When you don’t get enough cool, the first response is to make the equipment bigger. There is a substantial number of issues associated with “comfort” that have nothing to do with the size of the HVAC equipment. On a home this size and larger, I find a substantial number of deficiencies due to improper air flow, inadequate insulation in the complex framing structure and above all excessive heat in the attic (often where the HVAC equipment is located). If nothing, else the duct system is installed in the attic in an environment that is so hot it is impossible to receive cool enough air to affect the temperature in the living space of the house.

The system design is obviously pushing the limit. Adding a few partying fools that are adding 12 or 1300 BTUs each is just pushing things over the top. More door opening and closing substantially increases the load during the party time etc…

Adding more tonnage to the HVAC system may improve the situations during these adverse conditions, however the increased capacity may cause difficulty during low load conditions the rest of the year (this can include mold growth and premature compressor failure). Large equipment also consumes more power. Larger equipment also cycles more under low load conditions often leading to premature failure.

Reducing the thermal transmittance of window glass, reducing air infiltration, increasing ventilation to reduce the heat transfer rate between the living space and the exterior and the excessively hot areas like the attic can dramatically reduce the load requirements on HVAC equipment and get things back in line.

As posted, using a square foot per ton is a crapshoot. When you’re talking about 1500 square-foot homes (built like a square box), those that are built alike will require the same BTUs. However, when you start looking at the complex building structures of 4000 square-foot homes, a lot more variance occurs due to the complex roofing, angles and elevations of the structure. Just the buildings orientation to North will have a major load change requirement on the HVAC equipment between two buildings which are identical. Heating and cooling load calculations is the only way to go. Evaluation of the existing structure and its capacity to reduce thermal conductance must be addressed first.

Troubleshooting over the internet is difficult. Just can’t see exactly how things are installed. First step would be determine if the airflow is correct for the tonnage of cooling. Then measure return air and supply air wet bulb temperature. Then using an enthalpy chart and a formula they can calculate if the system is indeed delivering 7 1/2 tons of cooling. If the systems are not delivering 7 1/2 tons of cooling then that needs to be addressed. If it indeed is delivering 7 1/2 tons of cooling the next step would be to determine if there is any return air leakage or thermal temperature gain if any of the ducting is in the attic. This can be done by measuring the dry bulb temperature at a return air grille and at the air handler return. If there is a temperature rise there is possible return air leak sucking hot attic air or the heat in the attic is being radiated into the ductwork. Duct leakage can be determined by using a Ductblaster reference the Energy Conservatory

In addition to what Charlie said you can expect an additional 1 Ton of cooling for every 10 people (on average) in a confined area. So if this guy is having huge parties and his HVAC contractor was not informed of this then his AC system probably was not designed to control the additional heat load.