The Air Conditioning Contractors of America (ACCA) publishes ANSI Standards for HVAC Contractors to follow. These professional standards are well known in the Home Performance Industry and the HVAC Contractors. I found a link on their website this morning to a study showing the problems caused with problems that can easily occur if the Industry Professional Standards are not followed.
A quick review of the reports shows many technical details and mathematical formulae. That forms the back ground for comparisons with other studies. However, they did write in some plain English for the rest of us. Here is one of their findings, about the performance of heat pumps.
A Heat Pump provides both heat and air conditioning. It is efficient because it moves heat. In the summer, it moves heat from inside to outside, just like an ac unit. The same technology and principals. In the winter they move heat from outside to inside. Yes, there is heat outside at cold temperatures.
Think about your refrigerator. When it gets up to around 40° F inside, the unit turns on and finds the heat in that 40° temperature and moves it out of the refrigerator to keep your food cool and fresh. If you watch your temperatures outside, you know more heating is done at 40° and above than at 20°.
This findings for a heat pump cooling your home apply to geothermal heat pumps, regular heat pumps and to air conditioners. They are all working on the same principals.
The first fault they report on is having the wrong size unit.
Changing the size of the heat pump for a given house – either undersizing or oversizing – impacts the heat pump performance in several ways:
- Cycling losses increase as the unit gets larger; the unit runs for shorter periods and the degraded performance at startup has more impact (parameters used in simulations are: time constant = 45 seconds, or CD ~ 0.15).
- In the cooling mode, the shorter run periods impact the moisture removal capability (i.e., ability to control indoor humidity levels) because operational steady-state conditions are an even smaller portion of the runtime fraction.
- In the cooling mode, continuous fan operation with compressor cycling greatly increases moisture evaporation from the cooling coil. However, this impact is minimal with auto fan control (indoor fan time ‘on’ and ‘off’ the same as that of the compressor), since only a small amount of evaporation occurs with the assumed 4 % airflow during the off-cycle with the indoor fan off. If the air conditioner controls include an off-cycle fan delay – that keeps the fan on for 30-90 seconds after the compressor stops – then the impact of off-cycle evaporation is in between these two extremes (Shirey et al., 2006). The results in this study assumed auto fan operation with no fan delay.
- Heat pump sizing also affects the level of duct losses.
A improperly sized unit will run for shorter periods, and will turn on and off more often. That is just like driving your car around downtown Wichita. Lots of stop lights. Compare you gas mileage to highway driving at a steady speed of 55 mph or higher.
OK, I know efficiency isn’t as important as being comfortable. So the next item is the comfort side. Running you AC like your car in Down Town, doesn’t remove the humidity as well. Your house temperature is cool, but the humidity is high and you turn down the thermostat again to get rid of the humidity. Pretty soon you are cold, and yet you are not comfortable because you feel the high humidity. Then you go out and buy a de-humidifier.
The third item is also a comfort issue. That is the advice of many HVAC technicians to set your AC fan to run continuously. When the AC shuts off and the fan keeps running, the humidity the AC just removed from your house is put back in. Oops!
Finally the report mentions duct losses. They are important in a home that has no basement. In those homes ducts can be located in the attic or the crawl space, or both. If you have one of those houses, I would be glad to address that one in the comments. So leave a comment if your are interested.
How do you get a right sized AC unit? Your HVAC contractor can run a series of calculation found in the professional standard from ACCA Manual J. It is a Load Calculation. For cooling it considers the insulation in your home, the windows and size of the home and other details. It can be done on a paper worksheet or a spreadsheet. Both are free from ACCA. Most contractors have a software program to run these Load Calculations.
Load calculations for cooling, are a balancing act. The homeowner wants a home that is cool in the summer and in Wichita, they want the humidity controlled. Without a lot of formulae or detail, how do, the rest of us, understand a cooling Load Calculation?
My advice is to look at the out door design conditions that must be factored into the calculation. For cooling, ACCA Manual J has 6 outdoor conditions.
- Two relate to location. Cooling in Atlanta, is different from Wichita and is different from Denver.
- One is the outside temperature. The choice is a temperature that covers all but 1% of the hours in a year that require cooling for that location.
- There are four design conditions that relate to Relative Humidity.
This obviously requires a balance. If your contractor varies from these conditions, it still must balance. You may or may not like the balance, if one item is changed without consideration of the others.
What should you as a consumer do?
Insist that professional standards be used.
Require that all the items of the standard be measured and reported.
For Air Conditioning the standards require measurements of:
- Air Flow Actual compared to manufacturer’s requirements. (+/- 15%)
- Air Flow Static Pressure (ESP) Actual within the manufacturer’s acceptable range
- No more than 25% or 0.10 IWC over the design pressure for the duct system.
- Refrigerant Charge Verification
- Superheat method: Within +/- 5% of manufacturers superheat value.
- Sub cooling method: Within =/- 3% of the manufacturers subcooling value.
- Measured line voltage and low voltage circuits for voltage and amperage. These values shall be within the manufacturer’s requirements.
If you have good air flow in some rooms and not enough air flow in other rooms, your home has an air balancing problem. Have your system checked and balanced. This is known as test and balance or TAB. I have done test and balance work. Generally, the air flows should be within 20% of the design or application requirements.