Gas Heater Help

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O

ofcdduke

Member
Apr 22, 2020
15
Metairie, LA
A friend of mine just bought a house with a pool and I have been helping him get the pool in shape. So far I have got the pool water in good condition and cleaned the sand filter. I have changed the light bulbs and fixed his Polaris pool cleaner.

We are having a problem with the Laars Lite natural gas heater. The heater fires up just like it should and appears to be working. However, the water going to the spa/ pool is only luke warm at best. The water never gets to the temp that a spa should get to. From reading, I know that the heater has a safety feature that shuts it down if the water flow is too low and the heater does shut down if the pump turns off, so I know that works and tells me that the water flow is good. However, the water still does not get hot. If I turn the thermostat down, the heater turns off. The heater does eventually turn off when the thermostat is at its hottest setting. I am not sure if that is a safety feature that only allows the heater to run for a fixed amount of time, or if the thermostat is not reading the temp of the water properly. I'm not finding any solid information on what to check next. The things I'm reading tell me to check the high limit switch and make sure the gas flow is adequate. I am not sure how to test those things. Can I test the limit switches with a multimeter? Is there resistance on the limit switches that an Ohm meter could check? I am really not sure how to check for proper gas flow? All other gas appliances in the house work fine, including a standby generator installed right next to the heater. I'd appreciate any help y'all can share.
 
If the heater is running then the heat is getting into the water.

Is the heater shutting down before it gets to the set temperature?

What model and BTUs is the heater?

How many gallons are in the pool and spa?
 
A picture of the equipment pad and plumbing layout would help. The Laars manuals I’m seeing have a “temperature rise test” you should perform between the input and output temperatures of the water. If you have too much flow through the heater, then the temperature rise will be low.

If you don’t know your BTUs on the heater but you can get a good set of numbers from your gas meter, then let the heater run for an hour and check how much gas you are using. For example, a 400kBTU/hr heater should use 400 cu ft of natural gas.
 
I don't have any pictures of the equipment pad on me but I can get some this afternoon. The plumbing is pretty simple. The return to the pool/spa goes through the heater whether its on or not, as I assume like most pool heaters. Opening the heater panel shows me the ignition control, transformer, gas valve, and what I think is the high limit switch. I haven't found much info about testing the limit switches with a multimeter. Can it be done with the Ohm meter? I'm gonna test the transformer for voltage today. I think I saw that it should be putting out 24v to call the heater to turn on.
 
I’m not sure what type of limit switch they use as it may be a thermistor that’s has a resistance which varies with temperature. Without knowing the type, it’s hard to put an ohmmeter on it.

The HLS on most heaters usually isn’t the problem as they are fairly indestructible. When a heater throws and HLS error, it’s typically indicative of a water flow issue (and usually an obstructed flow). The HLS really doesn’t control the heater temperature, it simply acts to protect the heater from a dangerous situation.

Have you opened up the header to make sure the internal bypass (if one is present) is working and that the heat exchanger tubing is not corroded or fouled? I believe doing the “temperature rise” test as described in the owners manual on the inlet/outlet temps will tell you if the heater is performing properly. Given the plumbing setup, too much flow through the heater can limit its performance and might require installing an external bypass loop in the plumbing.
 
The problem got worse the other day. All of a sudden the burners would not fire up. So I did some troubleshooting tonight. When the heater is turned on, the heater goes through the process of checks and the igniters glow bright orange. I can hear the gas valve click but no gas flows. This process happens three times and the heater shuts down. I verified 24 vac from the transformer, the fusible link, pressure sensor, all the way through to the gas valve. From the control box terminal, I get 24-27 vac when the call for gas is made. When I put the wire back on from the terminal to the valve, when the call for gas happens, I only get 20-11 vac. The gas valve clicks but the gas never flows.

I am assuming the gas valve is bad and not opening which causes resistance on the wire and drops the voltage to the 20 vac. I also assume that this caused the gas valve to shut prematurely and not heat the water properly. Does sound like a bad gas valve?
 
I still think the gas valve is bad, but the other day, I turned on the heater. On the first light cycle, the gas valve opened and the heater fired up for a few seconds then turned off. Subsequent attempts to light were unsuccessful as the heater never fired up again. I am still leaning to a bad gas valve, but this had me thinking that the flame sensor might be bad. But I would assume a bad flame sensor would allow the heater to light on every attempt and then turn off after a few seconds, not just light on the first attempt.
 
Is there a test point for the flame sensor ? My pool heater has a Fenwal controller and it has test points for measuring the flame sensor current. Does your heater have a Fenwal? Just looking around those old Laars heater gas valves are very pricey...$300 new.
 

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Don't know what a Fenwal is but I'll google it and check. I actually found a replacement valve for $167.00 from a local supply house.

So I just looked it up and see its the controller, so yes there is a Fenwal controller on the heater. I don't know if there is a flame sensor test on it though.
 
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Fenwal is the name of the company but people often refer to the unit as “the Fenwal”. It is just a modern, microprocessor control module that checks and monitors all the switches and sensors on a heater to determine if it’s safe to fire up. They come in different varieties depending on the options the heater has. Those Laars units are pretty ancient so it probably doesn’t have one.

Nice find on the gas valve. The one for my heater is $300 new so I hope it never goes bad ?
 
Thanks for the help so far. FYI the model of the heater is LLD255. I pulled the heater apart today and for the life of me I could not find a flame sensor anywhere. The only wires I could find coming from the burners were white insulated wires. One going to the gas valve and the other to the Fenwal ignition terminal. I assume those are for the igniter. Can the igniter also act as a flame sensor? Do all heaters have a flame sensor?
 
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