Laars LT heater won't stay lit


New member
Jun 20, 2007
Hollister, CA
I just put in a new pool with a Laars LT 400,000 BTU natural gas heater. The pipe from the gas meter to the heater is 1 1/2" and is reduced to 3/4" at the meter and at the heater. The length of the 3/4" pipe is about 4" at the meter end and about 18" at the heater end.

When I turn the heater on, the blower comes on at high speed for about 15 seconds then goes to a low speed. About 15 later the heater fires up and I can hear a jet engine like sound. There is warm air coming from the vent and it appears like it's working fine. About 15 seconds later, the fan goes back to high speed and the burners go out. This cycle goes for 2-3 times and then it stops trying all together. The AGS (auto gas shutdown) light will flash red.


1 - Do I need a high-flow gas meter to keep up with the heater demand?

2 - Is the 18" length of 3/4" pipe at the heater too restricting? If so should I plumb the 1 1/2" pipe as close as I can get?



TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
400,000 BTU is a lot of gas. It seems unlikely that you would have a meter with 400,000 BTU of flow above your current usage available in a residential setting, though it is possible. Most meters have a peak flow rate printed on them somewhere. Are you running a low pressure system, or 2 PSI, or something higher? In a low presure system the 3/4" pipe could be a problem and preexisting 3/4" at the meter would also suggest that the meter wasn't intended for that flow rate.


TFP Expert
LifeTime Supporter
Mar 29, 2007
Coastalish 'down easter'
Try changing out the AGS switch, sometimes these things go bad and a new switch will correct the problem, if it persits in giving you trouble, let me know (I work mostly on Sta-Rite max-E-therms, but they all work pretty much the same)
[edit]Actually, first 'jump' the AGS and see if the unit operates correctly, then replace the switch - I 'throw' new parts at the heaters as the first step because i have a stock of them on the van :hammer: as a $ saving technique, you can use a piece of wire to bypass the suspected switch before investing in a part you may or may not need - sorry for my haste last night :oops: