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MHU45LP won't fire

Heater is flashing 3 times indicating pressure switch failed closed before CAB is energized wondering what this means.Is this gas line pressure

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Just purchase the 75k btu natural gas Mr. Heater. It would light and run until it reached the setting on the thermostat, then wouldn't relight and had the code 3. I found when assembled from the factory, the service door was install unside down. This heater can be installed right side up or inverted. Once I turned the access door 180, the heater works like a swiss watch. This heater works perfect to heat my 750 square garage. Very satisfied.

Posted on Apr 01, 2012

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The pressure switch to which you are referring is the switch connected to the suction side of your induced draft fan. This pressure switch will have a hose connected between the switch and the induced draft fan. Please let me know if you need any more assistance.

Posted on Feb 17, 2009


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1 Answer

Pilot does not ignite?

Look for the pattern of flashing lights when it fails after you turn the power off/on. Then look at the wiring diagram / instructions on the inside of the panel. The flah pattern should be listed. For example, two green flashes and pause, repeating may mean that the exhaust flow is not sufficient, or another flash pattern may indicate pilot is not lighting.

Likely ly a pressure switch is bad or exhaust is blocked.

May 17, 2015 | Water Water Heaters

1 Answer

Furnace problems

On - OK
1 Blink - Ignition failure. Check Gas flow, gas pressure, gas valve, flame sensor.
2 Blinks - Closed pressure switch. Check pressure switch (stuck in closed position).
3 Blinks - Pressure switch - failre to close. Check venter, pressure switch, vent blockage.
4 Blinks - Open limit switch. Main limit or aux limit are open. (My comment - Means overheating in chamber. Often result of blocked intake air. Bad filter. In my case, plastic wrap had been sucked into the intake. )
6 Blinks - False flame sensed. Check sticking gas valve.
Rapid continuous blinking - Incorrect twinning, incorrect primary polarity. Reverse transformer secondary leads, reverse primary wires.

Closed pressure switch can be from dirty combustion vents, bad chamber pressure - like missing screws, maladjusted gas pressure, failing pressure switch, loose hoses, moisture in the hoses or fan blades The switch need to be closed to ge the unit to work

Jan 23, 2008 | Goodman GMS90703BXA Heater

1 Answer

Ignition control led 3 flashes

First, make sure that combustion fan comes on and there is nothing obstructing air flow in the combustion chamber. Then, check the tube between the pressure switch and the induction motor to make sure it is connected and not obstructed. Then, replace the pressure switch, part number 60030.

Mar 02, 2014 | Mr. Heater MHU45LP

1 Answer

Low pressure indicator flashing and current temp reading missing. Any ideas?

low pressure can mean from either water supply or gas. make sure the pressure switch is closing at 2psi.

Aug 03, 2013 | Jandy Laars LX Pool Heater / 400,000 BTU /...

1 Answer

3 guick flashes from the indicator

On this model 3 flashes means that the pressure switch is open. Verify that there is no blockage in the flu pipes, check for obstructions in the pressure switch tube(s) and orifices. Make sure the wires are all securely attached and that the inducer motor is running too. If all checks out ok then it may be the switch itself. In rare situations this error can be related to gas pressure at the manifold.

Jan 02, 2011 | Goodman GMS90703BXA Heater

1 Answer

I installed a new goodman unit this past spring. It seems to work ok to good in the day time but the tempature is warmer so we do not need as much heat. At night I know the thermostat is coming on. I...

Heating Mode (Goodman Service Manual)

· 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

· 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

· After a pre-purge of about ten (10) seconds the electronic
ignition device will be energized.

· After a slight delay the gas valve will open.

· 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 indefinitely.

· 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 heat.

Dec 03, 2010 | Goodman GMS80903BNA Heater

1 Answer

Furnance is flashing 3 times indicating pressure switch failed

The 3 flashes is an error code. Most likely you have either a vent blockage, the inducer motor is not spinning fast enough to draw a vacuum on the pressure switch, you have a bad pressure switch or a blockage in the hose going to the pressure switch. You can **** lightly on the hose going to the pressure switch to close it or you can jump it out to test it.

Jan 16, 2010 | Mr. Heater MHU45LP

1 Answer

Furnace was not coming on red light was blinking once now its working wondering what the one blink means cant find my manual

One blink does not tell you much, it tried 3 times to start and did not.

On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

Jan 10, 2010 | Goodman GMS90703BXA Heater

2 Answers

New furnace, installed last October - has run approximately 3 months at the most. Have had to replace pressure switches, and again it is not working....will not ignite, but blower fan runs a pushes cold...

It's possible that the pressure switch is doing what it was designed to do, shut the burner off if there is a restriction in the vent system. Your problem may be intermittent, and is caused by wind blowing across your roof in a specific direction. Rarely, but on occasion I have seen this happen, the wind creates pressure on your vent pipe and prevents the pressure switch from closing. A directional cap (similar to  a wind rotating chimney cap) may be required. Good luck and let me know if this helps.

Jan 02, 2010 | Goodman GMS90703BXA Heater

1 Answer

Meaning of rudd furnace trouble codes

Error Codes
A green LED indicates system faults
Steady On: System OK
Steady OFF: Board control fault or no power applied
1 Flash: Ignition failure (control in one-hour lockout)
2 Flashes: Pressure switch fault, fails open (switch is
stuck open)
3 Flashes: Limit switch open (main or auxiliary)
4 Flashes: Pressure switch fault, fails closed (switch is
stuck closed)
A yellow LED indicates flame status
Steady on: Normal flame is sensed
Flashes: As flame weakens, frequency of LED flashes

Dec 12, 2009 | Heating & Cooling

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