Intertherm M1B Series, Installation Instructions Manual

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5. The circulating air blower will energize after the 

temperature fan switch closes.

6. The furnace runs until the call for heat is satisfied.

7. The circulating air blower will de-energize when the 

temperature fan switch opens.

Gas Gun Furnaces (M1B Models)

1. On a call for heat, the furnace control begins an ignition 

sequence which lasts approximately 45 seconds.

2. After this sequence, the control module tests to see if 

flame is sensed. If it has, the furnace continues to heat 

until the thermostat is satisfied.

3. If the burner has not lit, the ignition sequence is repeated 

a maximum of two more times. If flame is not sensed 

after three attempts, the control enters Lockout and 

no further attempts to light the burner will occur. If 

Lockout occurs, contact a qualified service technician 

for assistance.

4. When call for heat is satisfied, the thermostat contacts 

open and the gas valve shuts off gas flow.

 WaRnInG:

should overheating occur, or the gas supply fails 

to shut off, shut off the manual gas valve to the 

furnace before shutting off the electrical supply.

 aVERtIssEMEnt:

En case de température excessive, ou sail est 

impossible  de  cooper  l’alimentation  en  gaz, 

fermer le robinet manuel d’alimentation en gaz 

du  générateur  d’sir  chaud  avant  de  cooper 

l’alimentation électrique.

FuRnacE contRols & FunctIons

Furnace  on  /  off  switch:

 This switch turns electrical 

power to the furnace on and off. The switch must be set 

in the ON position for the furnace to operate. If blower 

operation is not desired, the furnace On-Off switch may 

be set in the Off position to cut the electrical power to the 
furnace. See 

Figure 33

.

Limit Control:

 This furnace is protected by two high 

temperature safety limit switches. The auxiliary (upper) 

limit switch and the high temperature (lower) limit switch 

are automatic reset types. If either limit trips, the burner 

will shut off. If either limit switch trips off again soon after 

resetting, set the furnace furnace On-Off switch to the 

Off position and call your authorized serviceman. See 

Figure 33

.

Blower off Delay:

 When the blower fan operates under 

cooling mode, there is a 40 second delay after the call 

for cooling (from the thermostat) is satisfied before the 

blower turns off.

Gas Valve:

 The gas valves for the gas furnaces are a 

100% shut-off type and will fail safe, if for some reason the 

gas is turned off. The valve is a “slow-open” for M1M and 

M1B models which means it opens to a low-fire position, 

and after a few seconds, “steps-open” to high-fire.

oil Burner Primary control:

 The primary control for oil 

gun furnaces starts the burner, monitors a safe operating 

cycle, and shuts the burner off at the end of a heating 

cycle. The control uses a light sensing transducer to 

determine if fuel ignition has been successfully attained. 

If ignition is not attained by the end of the safety ignition 

timing period, the control shuts the burner off and enters 
“lock-out.” See 

Figure 33

.

summer cooling - (B, c, & D series):

 Your furnace is 

A/C ready, equipped with A/C relay and transformer. The 

unit is equipped to use a 4-wire thermostat. When using 

a 5-wire thermostat, RC and RH should be jumped (see 

instructions included with thermostat).

Figure 33

Furnace

On-Off 

Switch

Oil

Pump

Fan

Control

Lower 

Limit

Switch

Upper

Limit

Switch

Oil

Burner

Control

Figure 33. Furnace controls - oil Furnace