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LAARS HEATING SYSTEMS
2G. Water System Requirements
2G-1. Flow Requirements
The Model HH boilers must have continuous
flow through the heat exchanger when firing for
proper operation. The system pump must be capable
of developing sufficient pressure to overcome the
resistance of the boiler plus the entire circulating
system at the designated flow (see Table 5). The
temperature rise across the boiler should never exceed
40°F (22°C).
2G-2. Variable Water Flow Systems
There can be reduced water flow through the
boiler in heating systems using zone valves, zone
pumps or 3-way valves. This can result in a high
temperature rise across the boiler. Laars recommends
primary-secondary pumping for all variable flow
systems. The boiler pump in a primary-secondary
system maintains constant flow through the boiler
even though the system flow is variable. In a primary-
secondary system the pressure drop of the boiler is not
added to the system (see Figure 5).
NOTES: Sizes 320M and 400M use 4-pass heat exchangers; 520M, 625M and 775M use 2-pass heat exchangers.
*Pressure drop (head loss) through the boiler, expressed in ft. of H
2
O.
Shaded area is the recommended flow and temperature rise. Minimum inlet temperature is 105°F.
Table 5. Temperature Rise.
TEMPERATURE RISE IN DEGREES (°F / °C )
MODEL
10°F
6°C
15°F
8°C
20°F
11°C
GPM
H/L ft.
LPS
H/L m
GPM
H/L ft.
LPS
H/L m
GPM
H/L ft.
LPS
H/L m
HH0320M
HH0400M
HH0520M
HH0625M
HH0775M
HH1000M
54
68
88
106
132
170
11.2
26.2
8.9
12.2
22.5
37.3
3.41
4.29
5.55
6.69
8.33
10.73
3.4
8.0
2.7
3.7
6.9
11.4
36
45
58
70
87
113
5.0
11.5
4.0
5.7
9.4
16.5
2.27
2.84
3.66
4.42
5.49
7.13
1.5
3.5
1.2
1.7
2.9
5.0
27
34
44
53
65
85
2.8
6.6
2.6
3.3
5.2
9.4
1.70
2.15
2.78
3.34
4.10
5.36
0.9
2.0
0.8
1.0
1.6
2.9
Flow
High
Normal
Low
Figure 5. Primary-Secondary Plumbing.
Boiler Circulation
Pump
Cold Water
Make-Up
System Pump
12"
Max.
12"
Max.
12"
Max.
12"
Max.
Thermometer
Temperature
Sensor
Globe Valve
Check Valve
Pressure Reducing Valve
w/Fast Fill Bypass
Expansion Tank
with Air Scoop and
Auto Air Vent
3-Way Valve
Valve
Pump
LEGEND:
Boiler circuit piping must be equal to or larger than
boiler water connection size.
Boiler circulation pump sized for flow through
boiler.
Dotted devices indicate alternate locations.
WARNING: This drawing shows suggested
piping configuraiton and valving. Check with
local codes and ordinances for additional
requirements.
Purge
Valve
