Heater not heating to Temp

dphbmw

New member
Jan 16, 2020
4
Ripon, Ca
I have an older Teledyne Laars ESC 400 heater. I have built in hot tub/pool combo. Heater stopped working 5 years ago so was not used. Finally checked myself using test ladder in manual and discovered gas valve was bad. I replaced gas valve and lit immediately. The problem is it wont go to temp set on control. 100-102 as before. It wouldn't go past 96 deg and after replacing thermister, now it won't go past 88 deg. 135 and 150 hi limits have 12 volts going in and out on both so does that mean that they are okay? Heater exhaust is 275 deg. Control has pool/spa switch but same issue on both. While heating if I turn down temp heater shuts off and if I increase it relights so that seems ok. I'm at a loss on what to check next. Hard to believe both potentiometers would be bad. I have cleaned both with contact cleaner. TIA.
 

Jimrahbe

Mod Squad
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Jul 7, 2014
15,366
Bedford, TX
D,

I know squat about your specific heater, but it does appear that you limit switches are working fine.. If the heater fires up at all then you have good water flow/pressure.

My guess would be that the heater "thinks" that it is up to temperature, even when it is not. The water temperature thermistor is what I would have assumed was the problem. If not the thermistor, then maybe the circuit that the thermistor is connected to..

I could not quickly find any schematic for your unit. If you have a link to one, please post it here.

We have some "real" heater guys here, so let's see of one of them chimes in..

Thanks,

Jim R.
 

setsailsoon

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TFP Guide
Oct 25, 2015
1,895
Stuart/FL
dp,

Here's what you can do until one of our experts chimes in. I'm assuming when you say "won't go past 88" you mean the burner turns off prematurely If this is the case it can be a number of causes. First I'd check the safety device loop. Not sure what you mean with "both measure 12v in and out" on the hi low limit switches. I would check the voltage (AC volts not DC) from the black wire on the temp control board to the red wire on the transformer before and after the flame stops. Be very careful doing this. If you accidentally short this circuit to ground you can blow the fuse and/or transformer secondary wiring. The safety devices in this loop should remain closed (no voltage drop) during normal operation. There should be no voltage drop unless one of the hi lo limit switches or pressure switch is open. If it reads ~ 24 vac, one of the switches is opening prematurely. If this happens you need to isolate which one is opening. If this proves the safety devices are operating properly then I would expect the thermister in the temp control. Unfortunately it looks like the control is replaced as a unit including the thermister. You might be able to look for a manufacturer's part number to see if you can replace separately or maybe one of our experts knows if there's a way to replace just the thermister. In newer units it's just a replaceable 10K thermister.

I hope this is helpful.

Chris
 
Last edited:

Bama Rambler

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Jun 22, 2009
23,701
SouthWest Alabama
I suspect the high limits. I would Temporarily (and I mean temporary) jump across one and then the other and try to run the heater. If it gets up to temp then you know that it's a weak 'Hi Limit' switch. Quick and easy to test, and it doesn't cost anything.
 

dphbmw

New member
Jan 16, 2020
4
Ripon, Ca
Thanks to both. I'm out of town but will try later. Chris, the voltage in and out of 135 and 150 high limit switches is 12v so I'm assuming they're ok if theyre like a fuse and something disrupts voltage from in to out.
 

dphbmw

New member
Jan 16, 2020
4
Ripon, Ca
Okay. I tried jumping each high limit seperately then both and no change. I switched thermister wires and now goes to 97 deg. Anything else to try? I sure hate to spend money on board and doesn't change anything.Thanks again!!