OSB
Series 2 Oil-Fired Steam Boilers –
Boiler Manual
Part Number 550-110-772/0816
9
Figure 8
Recommended piping for parallel-flow
systems with optional reservoir pipe
Modern steam boilers are designed to steam for less time than
older, larger boilers. When replacing an older steam boiler the
system condensate return time may be longer than the steaming
time. This could cause the following problems:
1. Boilers fitted with an automatic water feed could overfill.
2. Units fitted with only a low water cutoff would shut down and
cycle while waiting for condensate to return.
Following is a simple method for determining whether or not
a reservoir pipe is required to lengthen steaming time for a
residential installation:
1. Disconnect condensate return line at existing boiler.
2. Heat boiler and allow to steam for 10 minutes. Turn off boiler.
3. Measure length of time from when boiler started to steam to
when condensate begins to return through condensate line.
4. Measure length of time from when condensate begins to return
to when it stops returning. Divide this time by 2.
5. Add time measured in step 3 to time calculated in step 4. This
sum is the average time required for condensate to return to
the boiler.
6. If this total time is 10 minutes or less, no reservoir pipe is needed.
If total time for condensate to return to boiler (from step 5) is
more than 10 minutes, a reservoir pipe (or boiler feed system) is
recommended. See Table 5, this page, for suggested reservoir
pipe size. Install as shown in Figures 8 and 9, below.
For larger systems (as noted in Table 5), use a boiler feed system
with a condensate tank and feed pump. You will have to install a
low water cutoff/pump control on the boiler to operate the pump.
See “Low water cutoffs” on page 20 and Figure 18 on page 26.
Use Table 6 to size boiler feed systems. (The use of a combination
condensate tank and float-controlled condensate return pump
is not recommended. These devices do not supply water to the
boiler on demand from the boiler, and cannot compensate for long
condensate return times.)
For most residential installations a reservoir pipe may be all that
is necessary to ensure proper operation.
Condensate return
Table 6
Boiler feed system sizing
Table 5
Reservoir pipe sizing
4 Connect steam piping
continued
Figure 9
Recommended piping for parallel-flow
systems with optional reservoir pipe
Boiler
model
number
Net
output
Time from initial steaming to average condensate return
(boiler steaming capacity based on 970 Btu per pound of steam)
15 minutes
20 minutes
30 minutes
Steam
MBH
gallons
Pipe
length
(feet)
gallons
Pipe
length
(feet)
gallons
Pipe
length
(feet)
OSB-3
114
1
1
2
2
Use boiler feed
system
OSB-4
144
1-1/4
1-1/4
2-1/2
2-1/4
OSB-5
174
1-1/2
1-1/2
3
2-1/2
OSB-6
210
1-3/4
1-3/4
Use boiler feed system
Designed full capacity steaming time of modern boilers is 10 minutes.
Boiler
model
number
Net
output
(pounds
steam
per
hours)
Condensate
(gallons per
hours)
Minimum condensate
receiver capacity gallons
for steaming times
(minutes) of:
(note 1)
Suggested
feed pump
capacity
(GPM @
15 PSI)
(note 2)
15
min.
30
min.
45
min.
60
min.
OSB-3
118
14
4
8
13
17
0.5
OSB-4
148
18
5
11
16
21
0.6
OSB-5
179
22
6
13
19
26
0.7
OSB-6
216
26
8
16
23
31
0.8
Notes:
1. Maximum time to when condensate returns to boiler.
2. If pump exceeds capacity shown, pump can be throttled with globe or ball valve.
