1d. variable water flow systems, Laars heating systems – LAARS Mighty Therm PH (Sizes 500-1825) - Installation, Operation and Maintenance Instructions User Manual
Page 4
LAARS Heating Systems
Page 4
Design Temperature Rise Across The Boiler
20°F 11°C
25°F 14°C
30°F 17°C
35°F 19°C
Indoor
Outdoor
Flow
H/L
Flow
H/L
Flow
H/L
Flow
H/L
Sizes
Sizes
GPM
l/s
Ft.
m
GPM
l/s
Ft.
m
GPM l/s
Ft.
m
GPM
l/s
Ft.
m
500
—
38
2.4
1.4
0.4
31
2.0
1.1
0.3
26
1.6
0.9
0.3
22
1.4
0.6
0.2
—
500
41
2.6
1.7
0.5
33
2.1
1.1
0.3
27
1.7
0.9
0.3
23
1.5
0.7
0.2
600
—
49
3.1
2.3
0.7
39
2.5
1.6
0.5
32
2.0
1.1
0.3
28
1.8
0.9
0.3
—
600
49
3.1
2.3
0.7
39
2.5
1.6
0.5
32
2.0
1.1
0.3
28
1.8
0.9
0.3
715
—
56
3.5
2.5
0.8
45
2.8
1.9
0.6
37
2.3
1.5
0.5
32
2.0
1.0
0.3
—
715
58
3.7
3.2
1.0
47
3.0
2.2
0.7
39
2.5
1.6
0.5
33
2.1
1.1
0.3
850
—
66
4.2
3.4
1.0
53
3.3
2.5
0.8
44
2.8
2.0
0.6
38
2.4
1.4
0.4
—
850
69
4.4
3.6
1.1
55
3.5
3.1
0.9
46
2.9
2.2
0.7
39
2.5
1.6
0.5
1010
—
79
5.0
4.7
1.4
63
4.0
3.4
1.0
53
3.3
2.7
0.8
45
2.8
1.9
0.6
—
1010
82
5.2
5.0
1.5
66
4.2
3.6
1.1
55
3.5
3.1
0.9
47
3.0
2.2
0.7
1200
—
94
5.9
6.5
2.0
75
4.7
4.8
1.5
62
3.9
3.7
1.1
53
3.3
2.6
0.8
—
1200
98
6.2
6.9
2.1
78
4.9
4.9
1.5
65
4.1
3.6
1.1
56
3.5
3.1
0.9
1430
—
112
7.1
8.9
2.7
89
5.6
6.5
2.0
74
4.7
5.0
1.5
64
4.0
3.5
1.1
—
1430
117
7.4
9.5
2.9
93
5.9
6.7
2.0
78
4.9
5.0
1.5
67
4.2
3.7
1.1
1670
—
*
*
*
*
102
6.4
8.8
2.7
85
5.4
6.7
2.0
73
4.6
4.7
1.4
—
1670
*
*
*
*
109
6.9
9.1
2.8
91
5.7
6.8
2.1
78
4.9
5.0
1.5
1825
—
*
*
*
*
114
7.2
10
3.0
95
6.0
8.0
2.4
81
5.1
5.5
1.7
—
1825
*
*
*
*
119
7.5
10.4
3.2
99
6.2
8.3
2.5
85
5.4
5.7
1.7
*Consult factory for recommendations
resistance of the boiler plus the entire circulating
system at the designated GPM (see Table 1). The
temperature rise across the boiler should never exceed
40°F (22°C). Primary-secondary pumping is the most
effective way to assure proper flow through the boiler
resulting in longer boiler life and greater system
efficiency (see Figure 2).
1D. Variable Water Flow Systems
Heating systems using zone valves, zone pumps
or 3-way valves can experience reduced water flow
through the boiler. This can result in an excessive
water temperature rise and unstable boiler operation.
If the system water flow is variable, it must not be
allowed to fall below 30% of full flow. The boiler
must be equipped with staged or modulated fire with
the temperature sensor installed in the outlet water.
State variable flow when ordering. Laars recommends
primary-secondary pumping for all variable flow
systems. Primary-secondary pumping is mandatory
for variable flow systems where minimum flows are
less than 30% of full flow conditions. 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 2).
Table 1. Flow Requirements.
A Special Note About 3-Way Water Valves
3-way water valves (2 position or proportional)
are sometimes used to divert water around a boiler
and control the temperature of water being supplied to
the system. Valves installed in this manner must be
supplied with an end switch or some other suitable
control to shut off the boiler when the flow is reduced
to 30%. The boiler must be supplied with staged or
modulated fire as indicated in Section 1D, Variable
Water Flow Systems.
It is often possible to accomplish excellent water
temperature control without 3-way valves through
proper application of staged or modulated firing
systems. Consult the factory or local Laars
representative for assistance with such systems.
System Pressure Requirements
The model HH and PH boilers are designed to
operate on closed, pressurized systems. A minimum of
12 psi should be maintained on the system where
boiler supply water temperatures are 200°F (93°C) or
less. If higher temperatures are required, the minimum
system pressure should be at least 15 psi above the
water vapor pressure corresponding to the elevated
water temperature.
The hydronic boilers are not suitable for open
systems unless the supply water temperatures are kept
below 180°F (82°C), and minimum of 5 psi static
head is maintained within the boiler.