In many instances, the decision will be made to replace the existing heating and cooling system rathe than rehabilitate it. The old system may be well beyond its expected life.

Many newer systems are more efficient and can quickly pay for themselves in reduced energy bills. The availability of fuels may have changed (e.g., natural gas may now be available) since the system was originally designed and installed.

If the old heating and/or cooling system in the house being rehabilitated is beyond retrofitting and needs to be replaced, there are two primary reasons why it should not simply be replaced with another system of the same size.

The old philosophy of “bigger is better” no longer applies. Systems were traditionally oversized, causing them to cycle on and off frequently. Cycling that results from oversizing is inefficient and hard on the equipment.

Also, rehab work may also include the addition of more or better insulation, and better performing windows and doors. This will reduce the heating and cooling loads and allow for a smaller capacity system to be installed.

A design load analysis should be conducted to determine the current heating and cooling capacity needs. There are various methods and levels of sophistication for performing these analyses. Most equipment vendors are equipped with worksheets or computer software to estimate the appropriate size of the system for the home.

They will typically perform a sizing calculation as part of the sales process. While such a service from the dealer is available at no cost, it should be remembered that the dealer is selling equipment, not efficiency.

Methods are often over-simplified with factors of safety built in, resulting in over-sized equipment. An alternative is to size the system yourself. There is a multitude of books available that provide instructions, data tables, and examples for performing system sizing calculations.

It is recommended that calculations be performed more than once with different methods and sources to provide confidence in the results. While sizing the system may cost a modest amount of time, lack of experience by the novice estimator may result in mistakes.

Basic estimating techniques may also not properly account for unique aspects of the home. Another alternative is to hire a consultant to size the system. Professional energy specialists and auditors can evaluate the home and provide recommendations on the size and type of equipment.

The advantage here is the benefit of an experienced professional who is focused on energy efficiency, but consulting fees may be hefty.

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