Laars Heater is getting TOO HOT!

[heatherinthemoor]

hey..
I have a 7 year old laars lx400 low nox heater that is causing me soooo much trouble! I was hoping for some advice. The heater was working ok, but then started getting AGS errors and blowing the EXTSW1 fusible link sometimes. I noticed some blow back - so I took out the burners and cleaned and replaced ones that were burned through. A bunch were clogged up and nasty. I reassembled the heater and it lit and ran fine - no blow back - nice and quiet - blue flame - but after a few hours it burns out the EXTSW1 fusible link. I don't think it is blowing back - in fact, I used an IR thermometer and think it is just running really, really HOT! The fusible link is doing its job.

What could cause this? I know the burners are new and clean. Could there be soot or something on the heat exchanger? The vent stack and lid are really hard to get off, making getting to the heat exchanger really hard - so tell me that's not it, please. I did have to swap out the 3HP Northstar pump with a lower performing one - a 1.5HP pump - so the flow rate is lower than normal. This would lower the rate that the exchanger removes heat so it could cause it to overheat... but it shouldn't if the pressure is high enough to trip the heaters flow rate sensor, right?

I need to get a replacement pump that is sized right, but in the meantime, I would like to heat the pool! Any suggestions?

thanks so much!

 

[mgarf33]

A fusible link is designed to fail due to a drafting issue. Whether it be clogged HX, draft motor problems, blocked flue pipe, low water flow etc...
Flue gasses will rollout due to insufficient combustion air. With draft induced heat exchangers, secondary air is pulled into the combustion cell to aid in more complete combustion. If the draft motor isn't pulling enough air through the HX, the combustion is incomplete and will likely soot. The flames will seek O2 to stay alive, and rollout toward the fresh air inlet.
You can test the flue gas and measure the CO output to confirm the issue.
If the unit is in an enclose room, be sure there is plenty of fresh air for combustion.

I would suggest calling a pro for a full service cleaning and inspection. Also, have him check the gas manifold inlet and outlet pressures.
Your issues could be anything from rusted draft motor wheel, leaking draft motor gasket, over firing gas valve, blocked HX or flue, not enough fresh air in the space, this list goes on...
Don't count on the draft pressure switch alone to shut the unit off.

 

[heatherinthemoor]

Thanks for your reply!
Yes - I understand how the fusible link works and what it is supposed to do - I guess I am trying to get out of taking the flue venting down to get into the heater. There are no professionals who come out to this remote corner of the universe, so I'm it.

Regarding combustion air - that was an original design flaw in the pool (just one of many) that I've since corrected. The pool was built in 2001 and this is a mechanical room in the accompanying pool house. I figured that one out when I took over maintenance some years ago and cut a sufficiently sized inlet vent for air. The gas pressures were tested and verified by a gas pro - so I think that is ok. I guess I just need to give in and take the whole thing apart and clean it again. I had done so at the beginning of the season and it takes me a long time to do it. I was suprised that the burners were so bad so quickly. They were also cleaned and some replaced in May. So something else is going on. The fan seal is a good candidate to check - the fan seems to be working fine but perhaps it is causing incomplete combustion as you say by air leaking through a bad seal.

I don't count on the pressure switch to turn off the system - I meant I hoped the minimum flow rate to kick on the switch should be enough to pull heat out of the exchanger to not overheat the unit. I calculate that I should have 60- 70 gpm through the heater with reduced HP pump, if my TDH numbers are right. The normal flow rate is considerably higher.

I guess I wanted to hear something else, but your advice is good. Thanks!

heather

 

[ps0303]

Soot or a down draft are usual causes of the fusible link blowing do to the flames kick out towards the front of the heater. However if the heater is getting too hot and causing the link to go, I would question whether or not the refractory material has degraded to the point that it's allowing the unit to heat up excessively. Even though you changed the pump I would think you should still have plenty of flow for this unit. These and other units operate just find with even a small 1HP pump. You mentioned you replaced some burners. Are you have a condensation issue and the water that is dripping on the burners is what's rusting them out?

 

[heatherinthemoor]

I'm not sure what is going on with the burners, I cleaned and replaced a few in the spring, so how could they be so funky already? The heat exchanger may be leaking when it is hot. It doesn't when the heater is cold. I had the burner tray out and watched for leaks thoroughly. I don't know if there is excessive condensation. I haven't seen any evidence of that.

Refractory? Reflective? Do you mean the inside of the combustion chamber? It's all black in there- so no refraction or reflection. I guess the black body absorbs some heat energy. I guess I don't know what you mean.

Thanks so much for your input. I really appreciate it.

I'm trying again to find someone to look at this. But there are just so few technicians of any sort out here in the third world country that is Appalachia.

Heather

Soot or a down draft are usual causes of the fusible link blowing do to the flames kick out towards the front of the heater. However if the heater is getting too hot and causing the link to go, I would question whether or not the refractory material has degraded to the point that it's allowing the unit to heat up excessively. Even though you changed the pump I would think you should still have plenty of flow for this unit. These and other units operate just find with even a small 1HP pump. You mentioned you replaced some burners. Are you have a condensation issue and the water that is dripping on the burners is what's rusting them out?

 

[tim5055]

Franklin is not out in the middle of nowhere, you have a great Sushi restaurant!!

 

[ps0303]

The refractory is a white material that lines the side walls of the unit. It's kind of like insulation. If yours is black that tells me that you probably had/have a sooted heat exchanger.

 

[heatherinthemoor]

Ahhh yes I remember that now. It is white-ish. Not terribly dirty.

I got up on the roof and looked down the flue. It has some soot but nothing terrible. It's what I would expect from a 13 year old flue. I'm going to take the vent pipe put and get on to the heat exchanger today I hope. It is just really difficult because I'm not very strong or big. I'll let you all know what I find... Thanks for all the responses!

Heather

- - - Updated - - -

Franklin is not out in the middle of nowhere, you have a great Sushi restaurant!!

Oh please! You'd eat that?

Franklin is the address... We're a good 45 minutes from franklin.

 

[heatherinthemoor]

Update:
I got into the heat exchanger with the help of two burly propane guys... It was very sooty. Cleaned it out... And viola! It works! I think it validates the theory that the efficiency was lowered by the lower water throughput, taking a marginally performing heater and making it overheat. Now that the HX is better, the heater is working within its limits. When I get that new pump installed, I should be good to go. They adjusted the flame as well and predict less soot buildup.

Thank you everyone who chimed in on this!


Xoxo heather

 

[tim5055]

Oh please! You'd eat that?

Franklin is the address... We're a good 45 minutes from franklin.

Yes, every chance I get.

I'm going to guess you are either out twords Standing Indian or Silva because 45 minutes south on the GA road would put you below Clayton. I have a place just south of Franklin that I visit as often as possible. Just trying to think if we should build a pool when we move there full time.

 

[ps0303]

Update:
I got into the heat exchanger with the help of two burly propane guys... It was very sooty. Cleaned it out... And viola! It works! I think it validates the theory that the efficiency was lowered by the lower water throughput, taking a marginally performing heater and making it overheat. Now that the HX is better, the heater is working within its limits. When I get that new pump installed, I should be good to go. They adjusted the flame as well and predict less soot buildup.

Thank you everyone who chimed in on this!


Xoxo heather

Poor water flow doesn't cause a heater to soot up. In fact I've not seen it. Poor water flow would cause the heater to simply overheat and thus steam would build up in the heat exchanger causing a system to knock and bang or sound like it has gravel inside.

Glad you were able to get the issue resolved.