LT heater heats but won't re-heat

InvaderZim · Jun 21, 2015

I have a Laars LT heater (2006 vintage) that we only use a couple of times a month to heat the spa. A couple of weeks ago I noticed that, while heating, it was shutting off prematurely -- the first time when it reached 98 degrees, and the second time it reached the target 101 but wouldn't turn back on to reheat the water. The light was blinking so it was being asked to heat the water more, but wasn't coming back on and there were no trouble or service lights lit.

I noticed a lot of leaves under the exhaust grate, so I took that off after the last use and cleaned out the leaves hoping that was the problem.

Now, I noticed that the pressure switch light is lit. I had my pool guy (who cleans the pool and knows a bit about the equipment but doesn't do major repairs) look at the heater, and the cleaned out the pressure switch and put it back. Then I used the spa and it got to 100 before shutting off and refusing to come back on, again with no trouble lights.

We also cleaned the "flame sensor" and that was fine.

Now I see the pressure switch lamp is on again. Would it be worthwhile to replace the pressure switch, which looks easy to do as a quick fix? Or is this definitely a job for a pro?

Any tips for why it might not be coming back on? Is there some sensor that doesn't cause the trouble lights to come on?

Richard320 · Jun 22, 2015

If the pressure switch sees no pressure or too low pressure, it kills the gas right now so you don;t end up boiling the heat exchanger dry.

So... you either have low pressure or a bad sensor. What does the pressure gauge on the filter show? Remember, that's on the dirty side, so if pressure is high there, flow is restricted, and when that reduced flow hits the clean side, pressure will be really low. Maybe too low to close the switch. For that matter, a suction side air leak can cause low pressure and flow,too. A bad sensor can be jumpered and if the heater works right you've found the fault and you can replace the switch.

danpik · Jun 22, 2015

I will agree with Richard on this one. sounds like a possible flow issue...maybe. The indicator light for the pressure switch does not mean the switch is bad, it simply tells you that a certain, preset, criteria has not been met. Some flow/pressure switches are adjustable. If you do mess with it, be sure it shuts the burners down if the pump shuts off.

InvaderZim · Jun 22, 2015

Thanks so much for the replies. In spa mode, the pressure on the filter reads 25 to 30; 5-7 less lower in pool mode. Our spa is unfortunately pretty far from the equipment so the spa jets have never gone gangbusters, but they work adequately well. We also have a blower that barely works, but that's a separate topic.

When we took the pressure sensor off to clear the little pipe, the stream coming out wasn't a firehose -- more like a gentle 2-3 foot arc. Is that what's expected?

We did get the pressure sensor light to turn off after backwashing the filter and cleaning out the pump basket. But that only lowered the filter pressure a couple of PSI, so it was already pretty clean. Now I notice that the pressure sensor light is on in pool mode, but not in spa mode.

The odd thing is that this is all new behavior (the heater worked fine earlier in the spring but we hadn't used it for a month or more), so something changed. Is there a scenario where the pressure sensor would kill the heat after, say 20 minutes of operation and not let it come back on but without any trouble lights? That's what I'm seeing now.

To bypass the pressure sensor one time as a test, do I just short the two tabs? That sounds like an easy test to see if that's what is killing the heat once the spa reaches 98 or 100 degrees, or not letting it come back on without any trouble lights (with careful observation, of course).

Richard320 · Jun 22, 2015

Just jumper the two wires together. That tricks the controller into thinking that the pressure is there.

InvaderZim · Jun 22, 2015

Great! I'll give shorting that out a try and post the results.

InvaderZim · Jul 10, 2015

Sorry for the delay -- I shorted out the pressure sensor and that didn't help.

So I threw in the towel and called for service. Turns out it's the blower motor. They told me on the phone that there is no trouble light for this problem.

I suppose that might explain why the heater would heat and then not re-heat the water -- if the blower motor wasn't starting reliably, I suppose it might not have been able to restart.

At first, I was hoping that it was a startup capacitor problem (I previously fixed both a pool pump and an A/C compressor by replacing the capacitor), but I looked at an exploded diagram of the heater that I found online and I don't see a separate listing for a startup capacitor. So I'm guessing it must be built into the motor.

So I'm in for a pricey repair, but hopefully I can amortize the cost over another couple of years before the heater has to be replaced.