Munchkin Tech Tips

This is the first of an unofficial series of technical notes from JTG/Muir meant to assist Munchkin dealers in boiler and system troubleshooting. Throughout its history, the Munchkin boiler has endeavored to provide the most reliable, most cost effective, most advanced boiler available. Whether the problem is caused by the system’s installation or a Munchkin malfunction, the Munchkin’s diagnostic features will provide clues/answers to the symptoms of the problem. The following is an attempt to assist in the evaluation and steps to take corrective action.


Please be sure to read the entire installation manual. The manual is updated as the Munchkin is updated and there are important helpful details you could be overlooking.


The Munchkin has an evolution of three series to date, plus Vision Controls:


1st series (A) uses a flashing light on its control board to help diagnose faults. Note: this series A uses a Honeywell gas valve, an air pressure switch, and requires an orifice to change a NG unit to LPG. For 4 pulse faults, see FO9. For 7 pulse faults, see F16. For 8 pulse faults, see F15.


2nd series (B) uses the 925 digital control, Honeywell gas valve, has an air pressure switch, requires an orifice to convert from NG to LPG. Is Vision Control compatible.


3rd series (C) uses the 925 digital control, Dunns gas valve, has no air pressure switch, and only requires a screw adjustment to convert from NG to LPG and back again (CO meter however is required to readjust fuel to air mixture to around 100 ppm CO on high fire).


Series B & C with 925 display. Remember: Press S4 to reset boiler control.


Basics: Electrical: Do not connect neutrals of the incoming power with the central heating pump, and, or the DHW pump. The result is usually a blown fuse. The 925 control supplies both live and neutral to its pumps, it is not a switch leg like Series A.

The total 925 amp capacity is 4 amps, or 3 amps for the hot surface ignitor (HA) for all pumps and alarm; so count up the full load amp draws of each pump/alarm. External relays may be necessary. Infuriating nonsensical lockouts especially with jobs with nonstandard power require a direct Munchkin ground to grounding rod connection (the grounding rod is driven directly into the earth).


NC    If this appears on the display it is usually the result of an improper neutral connection supplying the boiler. Check all wiring, assuring that the neutral is connected and that the ground has no stray current in it.


FOO    If you experience an FOO code on startup on a new boiler, disconnect the two gray TT wires form whatever they are attached to and wire them together. Reset boiler. The FOO code had appeared as a result of a reversed polarity on the gray wires. This will also appear if the TT wires going to the Munchkin or the gray Munchkin wires are shorted out to water pipes, conduit, or the like.


FO1:    Is the Safety Limit Flue indicator. First check the flue for blockage. Second check the condensate for blockage or frozen. Third, check the combustion chamber for signs of recirculation, or incomplete combustion (see FO9, Fifth and Sixth).


FO2:    Is the Supply Thermister. Check the sensor wire’s continuity and wiring connections, and the thermister’s reading (it is based on 10K ohms). Replace malfunctioning component.


FO3:    Is the Return Thermister. Same as FO2.


FO5:    Supply Temperature is too High (locks out at 230 degrees F). Make sure you have water flow. Does the Munchkin shake, make a gurgling sound, and then lock out? You have a water flow problem. First check the circulators to see if they are working. Check to make sure pumps and check valves are installed in the correct direction, not backwards. Make sure there are no obstructions in the plumbing such as ball valves turned off, or almost shut off. No three-way or two way valves should be used on the boiler primary loop without a pressure-by-pass valve. If you have a pressure by pass valve, check its setting, and check that it is not installed backwards.


FO6:    Return Temperature is too High (locks out at 230 degrees F). Check your piping. If your return temp is higher than the supply, your pump is probably installed backwards.


FO9    If a FO9, flame failure, appears on initial start-up, first check if the gas lines have been properly bleed of air and that the gas is turned on. Or in the case of LPG installations, check to see if you still have gas left.

Second check with a manometer your incoming gas pressure for natural gas or LPG is at least 7″ water column, but no greater than 14″. Too low a pressure will cause it to flame fail, too high a gas pressure will jam the gas valve closed and will damage it. The gas regulator may be defective, call your gas supplier. LPG can be problematic, in cold climates moisture can get in the regulator diaphragm and freeze. It runs during the day, but fails at night. Condensation or moisture in the regulator could also be the cause. The test port on the Munchkin gas valve is shown on page 11 of the manual and says “IN” on the gas valve. Loosen the screw on the inlet gas pressure tap and slid your manometer hose over the tip. If you suspect that the gas valve is defective, here is a simple test of the gas valve; place your manometer over “OUT” gas tap (first loosen the screw). The manometer must not be of liquid type. Turn on Munchkin and simulate a demand. Watch the manometer indicator go into the negatives (we are drawing a vacuum against the gas valve). If the valve opens your manometer should read zero and it is good. If it does not open the reading will remain in the negatives.

Third, make sure you are at least 10′ from your gas meter or regulator and that you are supplying at least ¾” gas line to the Munchkin. If flex lines are used they must be full size interior ¾”. The Munchkin draws a vacuum against the gas line and must have the full volume of gas to ignite. If it is impossible to have a full ¾” gas line, an oversized drip leg like 2″ x 12″ should work.

Fourth check the through the viewing port that the ignitor is actually sparking (both are in the 11:00 position on the boiler face plate) and whether the ignitor is directed downward toward burner. Gentle tweaking of the ignitor is recommended to direct the ignitor toward the burner.

Fifth, make sure that the ignitor gap is ¼”. Note: the Munchkin FO9 is commonly the result of an improper air/gas mixture. Every unit is fired at the factory before delivery and it is set at 100 ppm at high fire, but because of differing gas caloric and atmospheric site conditions, field gas regulator adjustments on the Munchkin will be necessary. See page 11 of manual for location of the “Throttle Adjustment Screw.” To start a challenging boiler, cover the air intake located on the left side of the blower ½ to ¾ of the way with your hand (essentially choking the air intake). The unit should fire. Temporarily place tape over the air intake. Once a solid flame is established, you should be able to set the CO level to 100 to 110 ppm on high fire by adjusting the throttle adjustment screw. Check the rpm’s on your display board at high fire. To get the Munchkin go immediately to high fire, go into Test Mode, and press S3 & S4 simultaneously for on second. Then press S2 key and adjust the fan speed to its highest setting. Your CO ppm should be dialed in at this point. Press S1 to get to low fire and you can set you low fire ppms. If you are unable to fire with out choking the air inlet and adjusting the gas valve throttle, replace the gas valve and blower. Something is interfering with the air to fuel mixture that could be lodged inside the gas valve or blower..

Sixth, if the Munchkin has been working for a while but now FO9’s once or twice a day, check to see if your condensate is properly draining. Check the whether the condensate neutralizer is plugged and needs replacing. Check inside the combustion chamber for water marks. Sometimes you could get wet when you open the chamber, so watch out! If the condensate is plugged and combustion chamber is sooted up, this could be the result of the fuel to air mixture either being operated too high or too low for incomplete combustion. Adjust throttle screw as previously indicated to set CO around 100 ppm’s. Clean combustion chamber according to manual instructions on page 43. Or it could be due to construction debris such as sheet rock dust, saw dust, landscaping dust, or excess pollen from pine or oak trees. If it is a dust/pollen problem, clean combustion chamber as on page 43 in the manual as well as use your Shop Vac and vacuum the 3″ PVC air inlet pipe from inside the Munchkin, the top of the Munchkin insides and with the power off and gas turned off, place the shop vac hose over the fresh air inlet which is connected to the gas valve. This will clean the inside of the blower and burner. Construction debris is a non-recurring situation, so this need only be done once in this situation. Pollen is at least a yearly event, we currently do not have an approved filtration system, so an evaluation of the termination must be taken into account. Mostly what we have seen are applications which have roof terminations adjacent to pine and/or oak trees, so treat the air intake on the roof as if it were in a snow load, so locate the air intake on the roof 18″ to 24″ above the roof. If the termination is a two pipe, install two street 90 degree ells facing the roof into the air inlet tee 18 to 24″ above the roof line.

Seventh, make sure the flame rod is clean and that it stays in the flame even in low flame conditions. Dial in the flame rod to a 4.1 ionization (using the Test Mode, d7, and adjusting the gas throttle valve).

Eighth, see F10.


F10    F10 is indicative of a flame failure while the Munchkin is running.

First, check that the condensate drain is clear, and that the condensate can flow freely from the boiler. Condensate back-ups create steam which suffocates the flame, throws off rectification, or in the worst cases, soaks the refractory plates, causing the spark to ground out instead of sparking.

Second, check the flame rod by depressing and holding the S4 button, the display should read d1. Press and release the button until d7 appears. The d7 reading should be at least 4.0, and is directly related to the CO output of the boiler.

Third, check your vent termination for exhaust being recirculated back into the fresh air intake. Two pipe horizontal termination the exhaust must be off-set at least 18″ farther away from the intake air, for the vertical termination, the off-set must be at least 24″. With a concentric flue, make sure that the PVC kit is completely glued. Also check for, overhangs, newly installed fences, vegetation which has overgrown, anything that can trap the exhaust where the intake is. Exhaust gas recirculation will cause the unit to fire then fail almost immediately and will eventually damage the blower assembly.

Fourth, on the Munchkin 80M’s, check that the brass vent on the rear of the gas valve is not blocked by the rear of the boiler cabinet. If necessary, gently shift the boiler slightly forward in the cabinet.


F11:    Flame Rod sees flame when there is no actual flame. Check wiring to gas valve and flame rod.


F13:    The Fan is running too Slow. Check the wiring of the blower. Check the rpms of the blower. The series A 199M’s polarity could be the reason. Older blowers, series A, with a non molex connection should not be used with series B & C 925 controls. Usually, this means the blower needs to be replaced. However, check the termination for exhaust air recirculating back into the intake air. This will destroy the blower and the next blower if not fixed.


F14:    Fan is running too fast. See F13, but not a recirculation or deteriorating blower problem.


F15:    Air Pressure Switch is stuck Open. This applies only to series A & B Munchkins. Note: the Munchkin must start from the open position. First check if the Air Switch is good. Do this by placing your finger over the air switches’ other air connection while the blower is going (the one not connected to the plastic hose). If the switch closes, then the air switch is good. If it does not close, replace air switch. If the air switch is good, then adjust the switch while the fan is running until switch closes by turning the center screw in ¼ turn increments counter clockwise. If the Munchkin continues to lock out of F15 intermittently, it could be the blower starting to fail. The blower is not able to have enough suction to pull the air switch closed. Replace blower. Also check your vent termination for recirculation of flue gases. Blower failure is often the symptom of recirculated gases. Also check for blocked flues.



F16:    Air Pressure Switch is stuck Closed. See F15 for helpful hints. The Munchkin will not start until the air switch starts in the open position, so you must adjust the center screw in ¼ turns clockwise until switch opens.


F18:    Gas Valve fault. The gas valve wiring is not connected. Check wiring.


Alarm Contact, The 925 control is equipped with an alarm contact that will provide 110 volts to an alarm or light whenever a fault code is encountered. When considering an alarm/light, remember the limitations of amp draw of the Munchkin, 4 amps for spark and 3 amps for hot surface ignitor. So count how much your system, and DHW pumps draw, plus your alarm amp draw.


Freeze Protection: The Munchkin boiler has built in freeze protection; when the boiler senses that its internal temperature is 45 degrees or less, the central heating circulator is energized and recirculates the water. If the temperature continues to fall, at 38 degrees F, the burner will be energized until the boiler’s internal temperature reaches 50 degrees F.


If blower runs only on high speed and does not modulate. Check the blower low voltage wires in the rear of the blower and at the control board. A wire is not plugged in correctly or is broken.


Other blower helpful hints: The 80M has a high blower fan speed of 4600 rpm’s on some models and 5200 rpm’s on others. The 140M has a fan speed of 3900 rpm’s. The 199M has a fan speed of 5500 rpm’s. The blower is the same for each 80M, 140M & 199M, but each one has a separate control. In a pinch, a 140M board could be used temporarily on the other boilers.


Pump Exercised: The circulators wired to the Munchkin controls are exercised 20 seconds every 24 hours. Be aware of this when using the unit in snowmelt applications that the freezing water does not affect other parts of your heating system.


Water Capacity:

80M = 2 Quarts

140M = 3.7 Quarts

199M = 4.5 Quarts


Glycol/Anti-Freeze Notes:

30% propylene glycol to 70% water ratio, figure a 15% drop in boiler BTU capacity and a 5% increase in pump head.

50% propylene glycol to 50% water ratio, figure a 30% drop in boiler BTU capacity and a 50% increase in pump head.


Amp Draw by Munchkin only, not inclusive of circulators or alarm.

0.87 amps draw upon start up.

0.37 amps draw running load.


Vision 1: The basic principle of Vision 1 is the ability to operate two different supply water temperatures from one control. Offering two distinct thermostat/aquastat sources to control their respective circulators. It is important follow the installation instructions to split the two distinctive temperature loops so as to not send too hot of water into the low temperature loop.

A high temperature loop fixed at 194 degrees F with priority is used to generate domestic hot water through a heat exchanger or indirect water heater utilizing either the sensor that comes with the Vision package or a mechanical aquastat like a Honeywell L40808 for desired DHW tank temperature default factory setting of 119 degrees F. Priority timing is adjustable form 1 to 60 minutes.


Vision sensors: if a sensor should fail or its connective wire is broken, the Munchkin will ignore the function of that sensor. So, if the DHW sensor or wire fails, there will be no DHW, but there will be space heating functioning normally.

The sensors are 10K ohm based.



If the DHW works, but the central heating pump will not energize, check the following:

The blue low voltage wires for the DHW sensor can pick up interference from line voltage and send false signals back to the control board. The domestic hot water can be up to temperature and satisfied, but the central heating pump does not energize, because the false signal is saying DHW is calling and the priority is locking out the space heat relay. The easiest way to diagnose this is to simply disconnect both blue wires from the control. Then trace your DHW wires and move them away from the high voltage wires. Also, check that the #1 function in the Vision programming is de. If it show as dI for dO, use the S1/S2 button to return it to de. EO3: will appear if the setting is set to dI.












Vision installers are able to check the current status of the Munchkin parameters by pressing the S/4 key for 3 seconds. For example: the display will show d1 alternating with the actual outlet temperature. Actual values are displayed for each function. To view the next value, press the S4 key. The following are the values which can be displayed but not changed:


Function Value:

    D1/ Current Temperature of Boiler Outlet sensor.

    D2/ Current Temperature of Boiler Inlet sensor.

D3/ Current Temperature of Domestic Hot Water when using Vision sensor in tank, or if using a mechanical tank aquastat, the control will display closed for call for DHW, and open for DHW demand satisfied.

D4/ Not used.

D5/ Current Temperature of Outdoor sensor.

D6/ Current Fan Speed in RPM’s (times 10). Example: 410 rpms x 10 = 4100 rpm actual fan speed.

D7/ Current Ionization current reading from Flame Rectification probe. A reading of 4.1 is the ideal reading.

D8/ Actual Status of the Central Heating Circulator “Off=”, “On=”.

D9/ Actual Status of the Indirect Water Heater Circulator “Off=”, “On=”.

D10/Actual Status of bus communication “=connected”, “=not connected”


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