Chapter 3
Equipment Sizing
The various components that are used in
hot water heating systems were covered in
Chapter 1.
The proper sizing and installation of those
components (boilers, circulators, piping and
expansion tanks) is essential to ensure troublefree
comfort for your customer.
All hydronic system
installations need to
start with a heat loss
calculation for the
building. This establishes
20 National Oilheat Research Alliance
a target for
the heating system
and provides some
documentation for
the customer. There
are many heat loss calculation
methods–the
most recommended are the Hydronics Institute
guide H-22 and the Air Conditioning Contractors
of America’s Manual J, Figure 1. Boiler
manufacturers often provide free load calculation
software as well.
Programs can also be obtained online: www.
acca.org, www.wrightsoft.com and www.
elitesoft.com
Additionally, there are heat loss calculators
available in software form. They may differ
slightly, but will provide a reasonable estimate
of the heating needs of the building. All heat
loss formulas have a built-in safety margin,
so designers/installers should not adjust these
carefully derived estimates to add more heat!
The ONLY way to properly size a heating
system is to perform a heat loss calculation on
the building. Many authorities having jurisdiction
(AHJ) or building inspectors, require that
a heat loss be performed when installing a new
or replacement boiler.
Heat is lost two ways—the first is air infiltration,
or heat lost due to heated air leaking from
the inside of the house to the outside, being
replaced by cold air coming in. This replacement,
or infiltration air, will need to be heated
up to comfortable levels. Infiltration heat loss
is, along with window loss, the largest single
heat loss element.
The second method of heat loss is conductive
heat loss. Heat loss calculations also include
conductive heat transmission through walls,
ceiling and floors, doors, windows and skylights.
A house is extremely variable and these
heat load calculations will not be precise. Thus,
some rounding and estimating will still provide
for acceptable results.
In addition to a whole house heat loss evaluation,
it is also important to do a room by room
heat loss analysis. This enables correct sizing
of the terminal units—baseboard, radiators, radiant
floor heat—for each room, ensuring that
each room in the house is comfortable.
Example
A simple 10′ by 15′ room with 9′ ceilings and
two outside walls with windows.
When conducting a heat loss analysis, the
indoor design temperature (the desired indoor
temperature) needs to be known. In most cases,
the number to use is 70° F. The outdoor design
temperature, or the “coldest day of the year,”
needs to be determined. A heating system
should be able to maintain the structure at
70° F when the outdoor design temperature is
reached.
Figure 1