Gradient syncFURNACE Fault Codes: What Each One Means

Gradient syncFURNACE units show a fault code and turn the controller screen red when they lock
out. The codes are ALM 1 through ALM 6, plus NO FLOW and LOW PRES. ALM 1, ALM 2 and ALM 3 point
to the combustion blower, the chimney or a safety switch. ALM 4 and ALM 5 point to gas supply or
ignition. The most common cause we find in Calgary homes is a blocked condensate trap, which is a
cleaning job rather than a parts job. Some faults reset themselves after an hour, others need a
manual reset, and one of them means the heat exchanger has to be replaced. This page explains each
code in plain language, using the manufacturer’s own manual as the source.

If you need someone to look at it, we service Gradient units across southeast Calgary.
Call 403-404-9975.

Every code, in one table

The controller screen turns red when a fault has occurred. If your screen is red and you cannot
find a code, look again at the display: the code sits with the fault message.

ALM 3 is the one that matters most

ALM 3 means one of three safety switches has opened. The three have very different consequences,
and any page that tells you ALM 3 is a single problem is not being straight with you.

The vent limit switch

This one means the heat exchanger temperature went above the switch setpoint.

Gradient’s manual is unambiguous about what happens next. The switch is not resettable, it must
not be bypassed, and the heat exchanger must be replaced. Gradient Thermal should be
contacted.

That is the expensive outcome. It is also rare, and we would rather tell you it exists than let
you find out from someone who is trying to sell you something.

The water supply high limit switch

Supply water temperature went above setpoint. This one auto resets once the temperature drops
about 10 degrees Fahrenheit below setpoint.

Causes worth checking: a failed pump, a combustion blower stuck at maximum speed, a heating zone
that was sized wrong, or a failed pressure or supply sensor.

The condensate trap pressure switch, which is the common one

Gradient lists the cause as an undersized drain, or a partial or full trap blockage.

The corrective action, in the manual’s own terms, is to make sure the condensation drain is the
right size and slope, check the internal trap for blockage, clean the trap and remove all
blockages from the heat exchanger outlet through to the floor drain, then confirm the condensate
is running freely again.

That is a cleaning procedure. Gradient asks for the condensate trap to be inspected quarterly and
cleaned as required, and the condensate line checked twice a year. A unit that locks out on this
switch has usually missed that maintenance.

Here is the useful conclusion, and it comes from the manufacturer’s own documentation rather
than from us. A locked out syncFURNACE is more often a maintenance problem than a parts problem.
The correct first move is to inspect and clean, not to start replacing components.

ALM 4, ALM 5, ALM 6 and NO FLOW: the gas and ignition checklist

Gradient’s manual gives a fourteen point checklist for these faults. It is written for a
contractor, and we are publishing all of it, because you are entitled to know what a competent
visit actually covers.

1. The gas shut off valve is open.
2. The gas line is at the correct pressure.
3. The gas valve test ports are closed.
4. The differential pressure tube is connected, combustion air intake port to gas valve governor
   port.
5. The chimney is not partially or fully blocked.
6. The gas valve is wired correctly and receiving the open signal from the ignition module.
7. The wire harness has continuity.
8. The combustion blower has not failed.
9. The gas valve air and fuel mixture may need adjustment, per the manual’s combustion testing
   procedure.
10. Chimney lengths and elbow counts are within the manual’s limits.
11. No wind is blowing directly onto the vent termination.
12. The vent termination is in an approved location.
13. The trap pressure switch has not failed.
14. The ignition module, flame sensor and spark ignitor are functioning.

Four of those fourteen items are venting: numbers 5, 10, 11 and 12. In a Calgary winter the
most common venting fault by a wide margin is ice or snow blocking the outside termination.
Gradient tells owners directly to “monitor and remove any ice buildup from exterior pipes”. If
your unit only faults on cold or windy nights, start outside at the vent before you start
anywhere else.

What you can safely do yourself

None of this requires a licence and all of it is worth doing before you call anyone.

1. Write the code down. It tells a technician where to start and saves you diagnostic time.
2. Check the outside vent terminations for snow and ice, and clear them. This is the single
   most common winter cause in this city.
3. Check the furnace filter. Gradient says check monthly and change roughly quarterly. Any
   standard furnace filter fits, and Gradient names Aerostar, Clean Comfort, GeneralAire and
   Honeywell as examples.
4. Confirm the gas shut off valve is open. It happens more often than you would think,
   particularly after other work has been done in the house.
5. Stop resetting it. ALM 2, ALM 3 and ALM 4 retry automatically, up to three trials a day. If
   your unit keeps cycling through that, it is not fixing itself. It is hitting the same fault
   over and over, and each reset hides the problem for another hour instead of solving it.

Point five is the one almost nobody tells you, and it is the one that prevents a small fault
becoming an expensive one.

What you should not do

• Do not bypass a safety switch. Gradient says this explicitly for the vent limit switch, and
  the principle holds for all of them. Those switches exist because the alternative is a
  combustion or a scald event.
• Do not open the burner door. The burner door and combustion chamber insulation contain
  refractory ceramic fibres, which Gradient identifies as a potential carcinogen requiring
  protective equipment. Gradient cites IARC, CCOHS, OSHA and NIOSH on this.
• Do not work near the high limit safeties. They run on a 115 volt line, which the manual
  flags in bold.

Gradient requires service by a qualified, licensed technician. That is the manufacturer’s
requirement rather than ours, and it is the honest answer to whether you can fix this yourself.
You can clear ice off a vent and confirm a gas valve is open. Past that it is a licensed
gasfitter’s job.

Why your unit locks out in winter and not in summer

Because in summer the only thing it is doing is heating water, and in winter it is running the
heating too, in weather that ices vents. Cold weather only lockouts almost always come back to
items 5, 10, 11 and 12 on the checklist above, which are all venting. Snow drifts against a side
wall termination, a freeze thaw cycle glazes the intake, or a north wind blows straight into the
vent.

Clearing the termination is free, safe and often the whole fix.

Getting it looked at

We service Gradient syncFURNACE units across southeast Calgary, including units we
did not install and units sitting in tight builder framed closets that other companies have
declined to work on. Red Seal Journeyman Plumber and Certified Type B Gasfitter.

Call 403-404-9975.

How we price this work

We charge by the job, not by the hour. Hourly billing punishes you when a job turns out to be
complicated and punishes us for being quick, so we price the job. You get one number for the
finished work, agreed before anything starts, and you will always see it in writing before we
begin. Parts, labour, disposal and permits are in the number you agreed. If we find something else
once we are in, we stop and re-quote, and you decide.

On larger work you will see three options rather than one: the straightforward fix, the one we
would recommend, and the one that deals with the underlying cause. We will tell you which one we
would choose in our own house.

Frequently asked questions

What does ALM 4 mean on a Gradient furnace?

ALM 4 is an ignition module lockout, which means the burner tried to light and failed. The unit locks out for one hour and auto resets, up to three trials per day. Common causes are a closed gas shut off valve, incorrect gas pressure, a blocked chimney or vent, wind blowing onto the vent termination, or an ignition module or flame sensor fault. In Calgary the most common seasonal cause is snow or ice blocking the outside vent termination.

Why is my Gradient syncFURNACE screen red?

The controller screen turns red when the unit has registered a fault and locked out. The screen shows a code: ALM 1 through ALM 6, or NO FLOW, or LOW PRES. Write it down before doing anything else, because it tells a technician exactly where to start.

What does ALM 3 mean on a syncFURNACE?

ALM 3 means a high limit safety switch has opened, and there are three possibilities. A condensate trap pressure switch fault is usually a blocked or undersized drain and is resolved by cleaning. A water supply high limit fault auto resets once the temperature drops about 10 degrees Fahrenheit below setpoint, and points to a pump, blower or zone sizing problem. A vent limit switch fault is serious: that switch is not resettable, it must not be bypassed, and Gradient’s manual states the heat exchanger must be replaced.

My Gradient furnace keeps locking out and resetting. What is wrong?

ALM 2, ALM 3 and ALM 4 auto reset after an hour, up to three attempts a day. If your unit keeps cycling through that, it is not fixing itself, it is repeatedly hitting the same fault. Repeatedly resetting hides the problem rather than solving it. The most common underlying cause we find is a partially blocked condensate trap, which is a cleaning job.

Can I reset a Gradient syncFURNACE myself?

Some codes clear on their own after an hour. ALM 1, ALM 5 and ALM 6 need a manual heat demand reset, which usually means adjusting the thermostat to call for heat again. Doing that once is reasonable. Doing it repeatedly is not, because the unit is telling you something is wrong and each reset buys another hour rather than a fix.

What does NO FLOW mean on a Gradient water heater?

NO FLOW and LOW PRES both indicate low water heater fluid pressure. The system has lost pressure or fluid, and it will not reset until it has been refilled and re-pressurised. That is a service call, because something caused the loss and refilling without finding it just delays the next lockout.

Does ALM 3 mean I need a new heat exchanger?

Not usually. ALM 3 covers three different safety switches, and only one of them, the vent limit switch, means the heat exchanger must be replaced. The most common of the three is the condensate trap pressure switch, which is resolved by cleaning the trap and the drain. A technician can tell you which switch opened.

Why does my Gradient furnace lock out only in cold weather?

Almost always the vent. Gradient’s own troubleshooting list includes a blocked chimney system, vent lengths and elbow counts, wind blowing directly onto the termination, and termination placement. In a Calgary winter the usual culprit is snow or ice building on the outside termination. Gradient tells owners to monitor and remove ice buildup from exterior pipes, and clearing it is something you can safely do yourself.

What filter does a Gradient syncFURNACE take?

It depends on the model. The 05L, 08L, 06H, 10L, 12L and 10H take a 16×25. The 16L, 15H and 20L take a 20×25. All are 14 inches high by 23 inches deep. The model number is on the rating plate on the unit.

How often does a Gradient syncFURNACE need servicing?

Once a year by a professional, and Gradient’s schedule is specific: burner and combustion chamber cleaned annually, condensate line and trap cleaned semi annually, condensate trap inspected quarterly, and the flue, air intake, igniter and flame sensor checked annually. Gradient also asks for an annual water system flush and descale.

Written by Desmond Neufeld, Red Seal Journeyman Plumber and Certified Type B Gasfitter. Last updated 23 August 2026.