Goodman home heat unit trouble - please help

[scotso]

Thank you in advance to anyone who can provide some assistance on this. I have a Goodman GMNT100 gas heating unit with electronic ignition. When the electronic thermostat calls for heat the unit will fire and run fine for several minutes. Then it will shut down prematurely before the needed temperature is reached. Immediately the HSI will glow again but the unit will not fire. I replaced the flame sensor so that is not the issue. It seems that some other check in the system is cutting off the gas and locking it down. If I reset the unit with the emergency switch it will fire right up and run since the thermostat is still calling for heat. Then it will cut off after several minutes again. Any help is greatly appreciated as my guess is that just a cheap part needs to be replaced and I am trying to avoid an expensive house call.

[Java_Bob]

install book on unit

http://www.alpinehomeair.com/related/Goodman%20GMNT%20Install%20Instructions.pdf

how the safeties work

SEQUENCE OF OPERATION
This appliance is controlled by the thermostat. Within this
section, the term lockout is referenced. This lockout is a
“soft” lockout, which will reset after one hour. It is the
obligation of the installer to educate the user on the proper
use of the thermostat and the sequence of operation in both
the heating and cooling modes. It is also important that any
repair or service be performed by a QUALIFIED service
person, not by the user.
Heating Mode
• The furnace control checks for an open main limit (this
limit is normally closed). If the limit is open, the furnace
will remain inoperable until the limit is closed. During an
open limit, the circulating air blower will be energized. The
status light will blink four (4) times.
• The room thermostat reacts to a demand for heat.
• The control will then check to insure that the vent pressure
switch is open. If, at this point, the vent pressure switch is
closed, the control will blink two (2) times and will remain
inoperable until this situation is corrected.
• The venter blower is energized.
• The vent pressure switch will close when it detects a
pressure in excess of its setting. If the pressure switch
fails to close, the status light will flash three (3) times. The
sequence cannot continue until the pressure switch closes.
• After a pre-purge of about fifteen (15) seconds, the
electronic ignition device will be energized.
• The flame rollout switches are then checked to assure
they are in the closed position.
• After a slight delay, the gas valve will open if the flame
rollout switches are closed.
• The burners will ignite and the flame sensor will detect the
presence of flame. The ignition device will deenergize. If
the sensor does not detect the burner flame, the gas valve
will close and the ignition cycle will be repeated for a total
of three attempts. If, after the third attempt, the presence of
flame is not detected, the furnace will go into a lockout
condition for one (1) hour. It will then repeat the ignition
cycle. This one (1) hour lockout and retry will occur

• Thirty (30) seconds after the main valve is energized the
circulating air blower will be activated.
• The furnace will remain in operation until the demand for
heat is satisfied.
• Once the demand is satisfied the venter will shut off, and
the circulating air blower will shut off after the field
selectable time off is attained.
• The furnace will remain dormant until the next demand for

[scotso]

Thank you for this Bob. According to the manual : "the electronic ignition device will be energized...the flame rollout switches are then checked to assure they are in the closed position...after a slight delay, the gas valve will open if the flame rollout switches are closed." This all goes fine the first time furnace fires up in the morning when the thermostat calls for heat. Then after running for several minutes it dies, the HSI glows immediately but the gas is not injected. Does this mean that the flame rollout switches have somehow been tripped by the furnace running for several minutes? Should I jump the pair of wires to check each one?

[Java_Bob]

Flame roll outs should be a manual reset, a little button will pop out. I would be more inclined to check the high limit.

[scotso]

Thank you for focusing my attention on the safety checks Bob. None of the buttons had popped out on the flame rollout sensors so I dismissed those as the source of the trouble. I noticed that the stack overtemp sensor was something else that can cutoff the supply of gas. The vent fan runs strong with great airflow out of the exhaust and all condensation drains fine so I thought that maybe this sensor was bad. I pulled the connectors to it and jumped them together. Unit ran fine that way with no premature cutoff. Put the connectors back on the sensor and back to cutting off and locking out.

[Java_Bob]

get yourself a new high cut out disk and good to go then, don't leave a jumper in there

[dosser]

New to this sight and not sure if you have your furnace running yet, but there is a pressure switch call "front door cover" that has, in the units I have looked at, two gray wires. One goes to the gas valve and the other to the logic board. This air hose that goes to this switch goes to the bottom of the black cover next to the condensate drain hose. This air hose can get plugged with condensate water and cuts off the air to the swicth and you will go through the ignition cycle with no fire, three strikes and lockout. Make sure the drain hose has as much verticle flow as you can, and check the small air hose to the switch for water. Remember that all the air switches are negitave pressure. This is a common problem with many furnaces.