Carrier 48MA - Manual (Page 6)

GV - Gas Vaive (Zone Module)
MS ~ Main Burner
MGV -
PS -
Main Gas Vaive
Pilot Sorrier PV ~ Pilot Valve (shutoff;
MOTSS:
t
Pirst stage of gas valve is a 115-vott sok-noid; second stage is a
24-voit soienoid with .60% gas bvpess.
Gnits 48MA034 ano 040 fi ave one pilot shotoff valve feeding all
pilot burners.
Unit is eouipped with a forced-draft blower and: the foiiowing
safety devices: forced-draft airfiow switcfi, tiame rcll-oot pro­
tection switch, combustion dtarnbet access door switch, heating
lirrtt switches, and soark-ignitecl automatic pilots. Al! of these
switches are iccateci in the heating section and rrtust. be in safe
condition before tfie inain burners can ignite.
Fig. 3 — Gas Piping Schematic (10-Zone 48MA Unit Shown, 8- and 12-Zone Units Similar)
6. Heating limit switches.
7. Flame rollout protection switch.
In special applications where natural gas supply
is limited, units must be modified to operate under
derated input/output conditions. The 48MA modu­
lar multizones can be derated by changing the zone
module burner spuds and gas valve orifices as
follows:
NATURAL GAS FIRED UNITS
T O T A L M O D U L E
D E R A T E D I N P U T ( % )
H i g h F i r e / L o w F i r e
90/45
80/40
70/35
S P U D S I Z E
No 36
No 38
No 41
No 43
G A S V A L V E
O R I F I C E S I Z E
No 36
No. 38
No. 41
No 43
Under these conditions, the units still have 2 stages
of derated heat input. Derating below these limits
is not approved. If single-stage heat is acceptable,
disconnect high fire stage to permit each module
low fire input only (% as shown under low fire).
Contact Carrier Service Department before de­
rating to the above limits.
ELECTRIC HEATING SYSTEM (SOME) - The
SOME electric heating system contains single-phase
Nichrome wire coils (see Fig. 4), wired and phase
balanced to provide 2 or 3 steps of heat control.
Each zone has 2 or 3 steps of strip heat available,
controlled simultaneously by the zone thermostat
and the outdoor air thermostat. When heat is
required, the first stage of zone thermostat ener­
gizes the first step of zone heating. The second step
(on 3-step units) of heating is controlled by the
second stage of the zone thermostat. The second
(on 2-stage units) and third step of heating is
controlled by the outside air thermostat (OAT.)
and operates simultaneously with the second stage
of the thermostat (on 3-stage units) when the
outside air temperatures are below OAT. setpoint.
The setpoint on the outside air thermostat is
adjustable from 0 to 55 F.
Safety features include:
1. UL certification on entire unit, as well as
electric heat section
2. Manual reset circuit breakers
3. Klixon high-temperature protection
4. Airflow safety for indoor fan motor
5. Fusible links in each heater phase
6. Two-pole contactors on each element
Fig. 4 — Electric Heating Unit (50ME)