This document discusses sizing boilers for space heating and domestic hot water needs. It recommends using a sizing factor from the ASHRAE Handbook to account for offsetting peak demands, rather than adding the full domestic hot water load to the boiler plant size. An example is provided where the hot water load is half of the space heating load, so the sizing factor of 0.5 is used to determine only 50% of the hot water load needs to be added. This allows downsizing the boiler from a 3 million BTU/hr unit to a more appropriate 2.5 million BTU/hr unit.

1. SIZING A BOILER FOR SPACE
HEATING AND HOT WATER

2. INDIRECT WATER HEATER
• Indirect tanks are often used
in combination systems,
particularly residential
applications
• An internal heat exchanger
coil separates boiler water
from domestic water
• The domestic water surrounds
the coil for efficient heat
exchange
• A pump located on the supply
line circulates hot boiler water
through the coil

3. STORAGETANK WITH EXTERNAL HEAT
EXCHANGER
• In commercial applications,
an external heat exchanger
with a larger surface area
might be used.
• Same concept as using an
indirect tank

4. USE A SIZING FACTOR
• Source: ASHRAE Handbook
• Only PART of the hot water heating
load should be added to the boiler
plant
• This is in part due to offsetting times
for peak hot water and space heating
demands
• The space heating boiler is also sized
for design day conditions – a rare
occurrence

5. CALCULATING HOTWATER RECOVERY
BTU/HR =
GPH RECOVERY X TEMPERATURE RISE X 8.25
THERMAL EFFICIENCY

6. AN EXAMPLE
 Let’s Assume:
 1,000,000 BTU/hr = Hot Water Heating Load
 2,000,000 BTU/hr = Space Heating Load
Hot Water Heating Load
Space Heating Load
=
1,000,000
2,000,000
= .5 Ratio
 According to the Chart, a .5 Ratio (x-axis) has a Factor (y-axis) also equal to .5
 .5 x 1,000,000 = 500,000 BTU/hr additional boiler capacity

7. RESULT
 Rather than sizing a 3,000,000 BTU/hr boiler plant,
 ASHRAE’s factor allows for downsizing to a more appropriate 2,500,000 BTU/hr
combination boiler
 This saves on upfront material cost, operating cost, and physical space