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Laars LX400 Natural Gas Pool Heater- Will ignite but won't stay lit

Laars LX400 Natural Gas Pool Heater- Will ignite but won't stay lit

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  #1  
Old 03-27-17, 10:49 AM
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Laars LX400 Natural Gas Pool Heater- Will ignite but won't stay lit

Hello Everyone/ Anyone,

I bought a house w/ a pool and heater but the heater didn't work when I bought it or else I'd have my home warranty send someone out to throw parts at this thing until it works or they have to replace it. I think the seller was trying to do that prior to the sell as there are many parts that look brand new among the rusty screws in the actual cabinet.

I'll start by saying I have read the sticky to pool heaters, the flow chart to the LX400, and have checked all 24vac sets with passing on each one prior to getting to my problem. I've also read every thread that contains the LX models and LX400 that I could find, and while some seem to have the same problem as me, none ever come back and say what, if anything, fixed it even when suggestions to what needs to be done are thrown out.

My heater comes on, fan blows high, get a glowing red in the sight glass, fan goes to low, gas valve clicks open, and bam...I'm up and running. Then 2-4 seconds go by and the gas valve clicks closed and the fan clears the fumes by returning to high.

I have pulled the flame sensor and it seems to be brand new, with no blemishes on the rod itself. Also it is too hot to touch after the flame has been on for the 3 cycles prior to the "Service" warning on the control panel.

I put leads from my meter into the yellow and brown wires that control the gas valve and when it clicks, they receive 29VAC for the seconds prior to it being cut off and returned to 0VAC. For fun and the possibility of bypassing a safety feature that could blow me or my house up, I then kept the leads in the two wires so I'd know when it was trying to open the valve and at that moment jumped the 29VAC from the transformer to the pins on the valve which opened the gas and the burner came on and I let it run for 5 min before pulling the jumper and shutting it down. Water in spa went up 1 deg in that time.

I know that the valve opens and works fine when it has constant voltage fed to it. I know that there is some other feature that is shutting that voltage off after the initial start up. I've pulled everything I've seen others be told to do and cleaned/ checked for spiders etc. with no success of it still running.

Tonight when I get home I plan on jumping the pressure switch to see if it is flow that is inhibiting the valve from having the current sent to the valve. If that is in fact the case I would think I'd be getting a different error as others have had, but maybe the pressure really is low (cartridge type filter and they are clean with 20psi on the gauge) and I don't know it, or perhaps that switch is bad and I didn't do a good job of checking/ troubleshooting it there.

I am also aware that it could be the flame sensor even if it is brand new. All I have done with it is pull it and check that there is continuity between the pin and the female connector that terminates in the board.

If there are any other ideas or someone else has had this problem and applied a correct fix to it, I'm all ears. I am usually pretty handy and was enjoying learning about the heater while testing it, so I was avoiding a service call, but maybe that is in my near future.

Thanks in advance,

Tyler
 
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  #2  
Old 03-27-17, 11:24 AM
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Welcome to the forums.

It's not the flow switch. The unit will not light at all with no water flow.

The flame rod is nothing more than a piece of metal. Its job is to sit in the flame and be part of a circuit. The circuit is an AC voltage sent out of the control box, applied to the rod, converted to DC by traveling thru the flame and returned to the control box thru ground. The DC current is what the control box is monitoring.

If the loop has high ground resistance it will keep tripping the protection circuit.

What is needed to be done is a meter put in line (series) with the flame rod to measure the current. Do you have a meter with a microamp (ua) setting in it ?
 
  #3  
Old 03-28-17, 06:14 AM
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Thanks for the reply. I do have a couple good meters. I looked all over youtube watching people put their meter in series with the flame sensor, but either I'm doing it wrong or it is defective. I clipped one end of the meter on the actual metal rod of the FS and then put the other to the FS pin coming from the igniter module (Fenwal- I think that's what that box does). On DC and for MicroAmps I got 0.0 the entire cycle. I tried some different pin configurations trying to find that 5 microamps but never did.

I checked for continuity from the rod to the spade connector and it is fine.

Will you go through exactly how to put it in series where I should get a reading?

Also, I've seen for the LX400 that they list both a 1.5" and 3" version. Mine is the black wire 3" version. The 1.5" I've seen online has a red wire.

Thanks again.
 
  #4  
Old 03-28-17, 09:38 AM
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It sounds like you had it connected correctly in the heater.
There will only be DC current flow when the flame is present.

Check from the flame sensor connection at the board to ground for an AC voltage. Could be up to 50vac there or so.

I was looking at the unit schematic.... the ground connections are not very well marked but the ignition board needs to be solidly grounded so check into that too.
 
  #5  
Old 03-28-17, 05:09 PM
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Ground connection on the GUI looks good. I held a meter to the pin for FS off the GUI to ground on AC and got 0.35 VAC so not anywhere near the 50VAC you suggested I could get. I assume this means the GUI is bad?

From here I'm probably going with a pro to see how they assess the problem. I would think everything else working on the GUI would also mean the FS portion is working, but maybe I assume too much and that is the one part causing the cycle to shut down after ignition.

I can't imagine the FS being bad since it looks brand new and is just metal with a wire. Finally up against that wall and tired of banging my head on it.

Thanks for the help Pete.
 
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