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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