Jeff Wilson replaced his HVAC system for the same reason most homeowners do: It broke. Five years ago in the heat of summer, his air conditioner died — and this was well before he embarked on an all-out Deep Energy Retrofit (DER) to green his 70-year-old Cape Cod. Still, he used this opportunity to upgrade the old, standard unit. "We had to keep a little air conditioning on so we didn't have so much mold," Jeff says, describing a major pitfall of the home: high humidity that produced damaging condensation, causing woodwork deterioration and poor air quality. Jeff got three bids for the HVAC project, ranging from $3,500 to trade the old HVAC unit for a comparable model to $12,000 for a state-of-the-art system. (These price estimates are from the time of installation in 2005.) "We expected gas prices to go through the roof, which they did in 2008," Jeff says, weighing the options. "And we expected this new HVAC unit to last." The 26-year-old unit Jeff replaced was 80 percent efficient — not bad for 1979 technology.

But we've come a long way since then. The standard option Jeff considered as a replacement would basically restore the system to status quo for $3,500. But Jeff was looking for more. The midrange unit was priced at $4,200 and would boost the furnace efficiency to 90 percent, with a 16-SEER air conditioner. The lower the Seasonal Energy Efficiency Rating (SEER), the more energy it takes to cool the home. The high-end system would cost Jeff $5,800, but for that $2,300 more, he got more features and optimum, 97 percent efficiency. Because the Wilsons mainly used their air conditioning to defuse the thick humidity typical in southeast Ohio during summer, a variable-speed blower was worth the investment. This mechanism allows Jeff to set the humidity level on his thermostat. "Instead of cooling your house down to get humidity levels low, you can do a bit of dehumidifying [with the blower] to keep the temperatures lower," he explains. Jeff could get this feature plus 7 percent more efficiency on the gas furnace.

"We were looking toward the future," he says of the price tag and, when comparing systems, the estimated five-year payback. The choice was relatively easy for Jeff. He chose the highest-end unit and sized the HVAC system exactly to fit his home so no energy would be wasted. However, he did not figure in extra capacity to supply heat and air to the new home addition, which would come to fruition five years later. Because this addition is built using the latest green technology in new construction — and solar panel energy — he doesn't expect to need the extra power for this part of the house. Immediately after installing the new HVAC, Jeff noticed energy bills that were about 20 percent lower. "That's pretty significant," he remarks, noting that as HVAC systems age they become less efficient. The old system was probably running at 50 percent efficiency by the time it was replaced in 2005. Invest in the best. "This is a long-term decision; it's not an appliance like a microwave," Jeff says.

"By buying the highest efficiency rating possible now, you hedge your bets against the high energy prices that will come later."It may sound like a good thing if an HVAC unit kicks on and cools down a house in five minutes. "If you put in a unit that's too big, you'll fry the compressor," Jeff says. Ask the HVAC professional to size the system just right for your home.air conditioning units for pop up campersJeff asked three contractors for estimates and made an educated decision based on the cost benefits of each. goodman packaged hvac unitsShop around before you settle on a system or a contractor.air conditioning units for sale nottingham The heating and air guys that we work around want the circuit conductors sized according to the maximum circuit capacity given on the unit.

The units give a minimum and maximum circuit ampacity. I was reading in article 440 and my interpretation is that you size your circuit conductors according to the minimum circuit ampacity, and your overcurrent protection, whether being a fuse or a circuit breaker at the panel, would be the maximum circuit ampacity. For example, a unit that has a maximum circuit ampacity of 40amps and a minimum of 27amps would have a number 10 copper conductor with a 40amp breaker. Is this correct or what is the correct way? It sounds like the HVAC guys may be confused about the code rules. Many folks who haven’t taken a close look at Articles 430 and 440 of the NEC usually fall back on the basics of Article 240 – match the wire ampacity to the circuit breaker or fuse size. If you use #10 wire, then you can't have larger than a 30A breaker or fuse. If you use #8 wire, you can't have greater than a 40A breaker or fuse, etc. However, for motor and compressor loads the NEC rules have a little more substance than that basic view.

Here are the basic items you find on HVAC units: MCA - minimum circuit ampacity. This is the calculation of what the minimum size wire is for the particular unit. The MCA is actually calculated out of UL 1995 - the standard for HVAC equipment. It generally follows our 125% of the largest motor plus the other loads rules. This value is taken as is on the nameplate (in other words, you don't multiply it by 125%, etc.). So if the MCA on a unit is 30A, you can use a #10AWG copper wire for the unit. MOP - maximum overcurrent protection. This is the maximum size fuse or breaker permitted to protect the unit. Again, the manufacturer has already done the calculations to arrive at this value. If it says 50A maximum overcurrent device, then you can use a 50A breaker or fuse. So, if I had a unit that said MCA = 30A, and MOP = 40A, then I could use a #10AWG copper wire and protect the unit with a 40A circuit breaker. This is usually where, if folks are not familiar with how motor loads are handled in the NEC, you will hear “You can't put a 40A breaker on a #10 wire”.