20
MZ INSTALLATION & MAINTENANCE
Is there flame failure?
Likely flame failure causes: bad gas-air mixture (too little or
too much gas), no spark, reverse polarity or poor ground.
Check spark by looking up into sight glass at sharp angle.
Check for proper gas pressure with manometer. Check power
for grounding and polarity with volt-amp meter. Check ignitor
electrode gap.
Proper gas-air mixture plus spark equals ignition. If you don’t
have ignition at this point, re-check everything.
When you realize that the MZ is a heating system that requires
nothing more than certain occurrences in sequence to operate,
troubleshooting becomes very straightforward. There are no
circuit boards (other than what may be in the Honeywell flame
control module), only a few relays that dictate exactly what
the MZ is doing, and if it not working, what it needs to do.
Here is the sequence of events that occur during normal
operation:
1. Power on to the unit
2. Call for heat - both T1, T2, & T2, T3 circuits must be closed
or there must be a DHW or other priority call.
3. Adjustable limit, leaving water and flue limit switches
closed.
4. DHW aquastat closed if priority call.
5. Fan full speed.
6. Pressure differential switch must close.
7. Gas valve open, spark.
8. Flame module proves flame and keeps gas valve open
until high limit is reached, DHW aquastat is satisfied or
call for heat is over.
Other than those sequential steps, there is nothing else that
affects the MZs operation. Here is a troubleshooting chart:
PROBLEM
MZ not running.
Power switch
not lit.
Power on,
nothing
happening.
No heat or
hot water.
Call for main
heat or priority
heat has been
verified. All
limits are
closed. No fan.
Pump is
running, fan is
running, unit
won’t fire.
POSSIBLE CAUSE
1. Power turned off, breaker off.
1. Check for high limit trip on top of heat
exchanger or where flue exits at bottom.
2. Check that adjustable operating limit is closed.
3. Make sure there is a call for heat or hot water.
4. Transformer failed.
5. Make sure pump is running (this can be done
by feeling for vibration or turning switch off and
on and watching for movement in the pressure
gauge).
1. No water flow through the unit. Not enough
flow to satisfy switch. Air in the system, or
return water strainer clogged.
2. Obstruction to flow outside MZ (such as stuck
zone valve).
Check position and integrity of tubes at air pressure
switch. Make sure that tubes are not cut anywhere.
Long tube from fan should see greater than 1.9
inches of water column and should be connected
to + (plus) port on switch. If p is less than 1.9, clean
and check fan.
If the pressure difference between the tube from
the fan and the tube from the combustion chamber
is greater than 1.6 inches of water column (IWC)
for the MZ25 or greater than .9 IWC for the MZ40,
the switch should close.
If pressure switch is OK, after blower has reached
full speed, switch should close. About 20 seconds
later, gas valve should open (loud click) and spark
should commence (spark visible through sight glass
when viewed at sharp angle from below).
REMEDY
1. Restore power.
1. Push to reset. If this occurs regularly, see below.
2. Check thermostat, jump T1, T2 & T2, T3. Make
sure switch is on winter (snowflake)
3. Check wires
4. Check for voltage on 24V side, replace if none.
5. Check for power to pump with tester and / or listen
for second relay from left to engage when power
is on. If there is power but no movement, remove
the silver slotted screw from the pump body and
turn the shaft manually. If the pump starts, replace
the screw. If it turns easily, replace the pump.
1. Jump red & orange wires to flow switch. If unit
starts and runs normally, either flow switch is bad
(unlikely) or there is no or low flow.
2. Find and clear flow obstruction. To clean strainer,
remove large nut behind fill/slash drain valve and
pull out strainer. Clean or replace.
Listen for click for gas valve relay after fan has
reached full speed. Watch for spark thru glass in
chamber. (If there is no click or spark, remove the
short clear hose from the black pressure switch. If
the unit fires, the venting is restricted. If no change,
jump the switch. If unit doesn’t fire after 30 seconds
of full speed fan operation, replace Honeywell control
unit. If there is a click but no spark, test for 110 volts
to spark transformer. If there is voltage, test for high
voltage with spare igniter. No high voltage, replace
transformer. If there is high voltage, replace igniter.
If the gas valve clicks and the igniter sparks, remove
P1 on the gas valve a make sure there is gas to the
unit.
TROUBLESHOOTING
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