Jandy LT400 - need next level troubleshooting

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bmwhd

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Jul 4, 2015
14
Dallas/Texas
I've had no issues with this unit for years up until last week. It fires but shuts down after 10 seconds. Repeats sequence twice then triggered Air Flow fault.

I'm fairly handy and do a lot of appliance repair so I've tried all the usual suspects. Flu was a bit dirty but is clear now.

Working thru the troubleshooting flowchart I see 24VAC at the purple wire of air pressure switch at all times but I'm only getting 24 VAC on the other side of the switch when fan is on high (purge cycle) not at the lower speed it's running at ignition.

I'm replacing the air pressure switch first (after verifying vacuum hose is fine) but I'm doubtful this is the issue as the switch appears to work (continuity test) when applying vacuum manually.

Any ideas? I hate to replace the ignition control module on a whim as they're pricey.
 
I'd also check the blower motor to make sure it's intact. It might seem like it is but many times the squirrel cage is partially rusted out and not throwing enough air for the system to recognize it thus the air flow switch error. Also, that motor as a start/run capacitor, and if it's on the way out, it might not be getting it up to the correct speed. Have you also checked the air tubes from the blower to the switch to make sure nothing has chewed a hole in them?
 
Thank you. I have checked the air tube and it's fine. I will check the run cap. my HVAC units eat those like candy. I'm skeptical only because the blower comes on high to purge and seems to be blowing strong as ever.

I do notice that when the gas fires for 10 seconds each cycle that the hot exhaust air is extremely moist. That's happened in the past after rain but we've been dry for some time now.
 
Not familiar with that heater but it could be that your exhaust fan is on the way out... Heater fires, exhaust temp rises and fan slows down because of heat, then pressure switch drops out... Had that happen with my furnace at home
 
Tried a different run cap on the blower. No difference. It still runs fine on the purge cycle at full blast with no unusual noises. When it drops down for the ignition cycle it stays there and never kicks back into high.

The unit acts like a furnace with a bad flame sensor. May try cleaning it despite the Air Fault error code.t

edit: no luck there either.
 
Have you tried jumping out the sir switch? Let it start up and then pull the wires on the switch and connect them back together with a jumper wire. If it stays running and lights like that, bad air switch.
 

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Have you confirmed that the blower motors squirrel cage is intact? Yes the Fenwals can and do go bad but I have not seen one go bad and throw an AGS error code.
 
Yes. Cage is intact. And it's not showing AGS. It's air flow fault though the force of air coming out is as strong as ever.

Edit: I'd add that the only strange thing I see is there appears to be more moister in the exhausted air (during the brief period when it lights the burners each cycle) than usual.
 
bmwhd said:
Yes. Cage is intact. And it's not showing AGS. It's air flow fault though the force of air coming out is as strong as ever.

Edit: I'd add that the only strange thing I see is there appears to be more moister in the exhausted air (during the brief period when it lights the burners each cycle) than usual.
Click to expand...

Sorry I was thinking AGS for some reason. Well if the cage in intact and the air switch is good, it could be the control board.

You say moisture coming out. You don't have a leak inside the unit do you?
 
I don't see any leaks and the exhaust air has always felt a bit moist when it's running but it seems more so now. I would think a manifold leak would leak all the time but maybe the flame triggers it?

I ordered the ICM so we'll see if that helps.
 
It would leak all the time if the heat exchanger was leaking. Not sure why you would have moisture coming out of the blower when it runs. Also, if I am not mistaken, the air flow switch has nothing to do with the Fenwal module. The module only controls the gas valve and the ignitor.
 
With all the cost of parts you threw at it to see "if" that would fix it, you could have hired a local tech and found out hopefully relatively quickly what was wrong. Sometimes you are better off using a professional and learn from them.

Just my two cents.....
 
My experience is the opposite actually. I've become very adept at appliance repair after too many attempted ripoffs by even companies that come highly recommended by people I trust.

My favorite examples include my HVAC variable speed blower motor recommended at $1k that actually required a $2 thermistor and the $900 power steering pump on my Honda that actually needed $0.60 o-ring as specified by a Honda TSB. I could name a dozen more.

I tell my boys they either need to get rich or get good with tools in this age of throw away products.

Including the flame sensor I'll be in to this heater less than $200. That would never cover a service call in my area.
 

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