Sizing, Installing, And Maintaining An HVAC System
When installing an HVAC system, calculate heating and cooling loads using a recognized method (ACCA Manual J), and ensure you install the smallest sized system that will meet these load requirements (ACCA Manual S).
When systems are not sized correctly, many problems can occur. If a system is not big enough for the home’s heating and cooling demands it will not satisfy the customer’s comfort level.
That being said, bigger is not always better – oversized equipment can also create a number of issues.
First of all, larger (capacity) systems cost more to operate than smaller systems even when the efficiency ratings are the same.
In addition, oversized equipment will short cycle, and much of the stress to HVAC equipment occurs during the startup process. Therefore, systems that are too large will incur wear and tear, which can lead to premature failure, costly repairs, and shortened life expectancy.
Another consequence of short cycling is that if a system only runs for 10 minutes and then shuts off, it never reaches its rated efficiency. Modern systems must run a minimum of 15 minutes (sometimes longer) before they actually achieve their rated efficiency. Therefore, to maximize utility savings, you need long, even cycles because this is when the system is performing at its best.
Additionally, properly-sized systems enhance comfort in a variety of ways. Longer, smoother run times promote more even room temperatures. For rooms to cool properly, air must be circulated throughout the whole structure. The longer a system runs, the more effective it is at circulating air in and out of a room. This can prevent homeowners from over-conditioning some areas of the house to compensate for a single area that is uncomfortable in comparison. This practice creates a much higher utility bill than if all the rooms were consistent.
Longer run times also improve humidity removal, so in humid climates many people find that they can keep the temperature setting higher since the humidity level is lower (ideally in the 45-55% range). If a system is oversized, customers will run the temperature very low to compensate for feeling “clammy”, which causes unnecessary run time and hence more utility consumption.
Finally, by making structural improvements to the house, such as adding in proper levels of insulation and sealing/repairing/replacing ductwork, the load requirements of the house will sometimes drop enough to justify downsizing an existing system. This can have huge benefits in energy savings and, in a lot of cases, a smaller system will save more than just small improvements to the efficiency rating.
Primary Efficiency Terms
There are many ways to measure a heating and cooling system’s efficiency. The main metrics we use are as follows:
SEER (Seasonal Energy Efficiency Ratio) – SEER is the total heat removed from the conditioned space during the annual cooling season, expressed in Btu’s, divided by the total electrical energy consumed by the air conditioner or heat pump during the same season, expressed in watt-hours. SEER is used in comparing Cooling Efficiencies.
EER (Energy Efficiency Ratio) – EER is a ratio of the cooling capacity in Btu/h to the power input value in watts at any given set of Rating Conditions expressed in Btu/(W*h). EER is used in comparing Cooling Efficiencies.
HSPF (Heating Seasonal Performance Factor) –The total space heating required during the space heating season, expressed in Btu’s, divided by the total electrical energy consumed by the heat pump system during the same season expressed in watt-hours. Used in comparing Heat Pump Heating Efficiencies.
AFUE (Annual Fuel Utilization Efficiency) – The percentage of the heat in the incoming fuel which is converted to space heat instead of being lost. Used to compare Furnace Efficiencies.
Generally speaking, we use these ratings as reference points to compare different equipment combination match-ups. The higher the rating the more efficient the system has been measured to be (under a standard set of conditions).
These metrics can often get confusing to home-owners and technicians alike. Heating and cooling efficiency ratings are similar to cars’ MPG ratings. When you purchase a car, they use the best case scenario for MPG ratings, but when you actually drive the vehicle, it never really hits that MPG level unless it is under the most favorable conditions.
The day a new heating and cooling system is installed is the most important day in its life. If a system is installed improperly, it doesn’t matter what the certified efficiency rating is, it will never work the way it was designed and it will not last as long as it should either.
Here are four critical areas of which you need to be aware:
Proper Charge Adjustment – It doesn’t matter if it is undercharged or overcharged, if the system does not have the proper amount of refrigerant, it will not work properly, breaking down more often, and will not last as long as it should.
Proper Airflow – If the point of an air conditioning system is to condition the air, then you would think contractors would be aware that ensuring proper air flow is critically important. All too often, however, this is a secondary thought. During our system change-outs, we have often found restrictive airflow issues that have robbed the old system of its cooling capabilities and the utility savings that had been sought by the customer when upgrading the efficiency.
Specifically, make sure to pay attention to adequately sizing the return and supply, and avoid restrictive filters and improper blower settings.
Poorly Sealed Return – The equipment will draw air from anywhere it can and this air often comes from areas other than the living space. If the return runs through an attic or crawlspace and has leaks, you can end up drawing contaminated and unconditioned air directly into the system and distribute it throughout the house. Having to condition this extra load causes the system to work harder, affects the air quality of the home, leads to excessive repairs and can shorten the life of the equipment.
Poorly sealed supply – Conditioned air that should be going to rooms in the home end up going to the attic and crawl spaces, wasting energy and money. No one in their right mind would leave a duct just blowing air outside the house. Sadly, though, we see this all the time in the form of leaks in the ductwork.
Maintenance Best Practices
The cleaner a system stays, the better it will retain its efficiency. Here are a few key elements to consider when maintaining a system:
1) Replacing Filters – As discussed earlier, dirty filters restrict airflow.
2) Dirty Coils – A dirty evaporator coil restricts airflow. Dirty condenser coils, on the other hand, restrict the air movement across the coils that reject heat. As these coils get dirty it inhibits the capability of transferring heat and diminishes capacity and efficiency in addition to causing stress to operating components of the system and shortening the overall life expectancy.
3) Improper Charge – Leaks can develop causing the system to lose capacity and efficiency in addition to stressing the compressor, shortening the equipment’s life expectancy, and causing permanent/irreversible damage to the system.
4) Loose Electrical Connections – As a system operates, the electrical connections can become loose. This may not cause a major draw on electrical consumption, but it can cause premature damage to electrical components in the system.
5) Repairing weak/worn components – helps prevent major breakdown, and keeps the system operating like new.
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Posted In: Building Performance, Residential Buildings
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