Sta-Rite SR400NA Heater Blowing Fuses

[cpfox]

Hello All,

I have an old Sta-Rite SR400NA heater. When I say "old", I mean it was built and installed in 1998 - so yeah, it's an antique. I know I should just replace it, but it heats my 10,000 gallon pool/spa up so fast - I just hate to part with it. I inherited the heater in 2012 when I bought the house. Over the years, I have replaced sensors, heat-exchanger gaskets, and other parts since that time. The heater gets used at least 3x-4x / month, mostly to heat the spa and I've never had a problem with it until a few weeks ago...

A few weeks ago, the heater refused to start heating, even though the power to the control panel was on. The digital display has burned out a few digits and it is sometimes hard to read the readout, but I could tell it was throwing an error code. I bought a new control board (yes, it was probably a Chinese knockoff), along with a new membrane pad. The new control panel seemed to work just fine... The new control panel showed an E06 Error Code - indicating a bad Stack Flue Sensor. I replaced that sensor and no more E06 error code. When I tried to fire up the heater, the water temperature readout indicated 125-140 deg, when the actual temperature of the water was about 45-50deg. The heater would not come on because the logic board thought the water was already at 125-140 deg, so it wasn't going to start heating. I bought and installed a new Thermistor (#42001-0053S), and that seemed to fix that problem, as the reported water temperature was showing about 53 deg. So far so good...

After fixing the thermistor, I had all the covers and electrical boxes off when I tried to start the heater just to make sure everything was going to work. I turned on the spa temperature to 102 deg. The blower started up as it should have just fine. About 5-7 seconds later, you can hear the "click" sound that ignites the gas. At that very moment, I saw the 1.25amp fuse flash and blow, which killed the power to the control panel. After another 10 seconds or so, the blower shut down normally - clearly from a failed ignition sequence.

I have another ten 1.25amp fuses coming via Amazon in about a week, but I'm trying to get ahead of this problem to figure out why the fuse would blow at the moment of ignition? I suspect an issue with the gas valve or the ignitor, but without power to the control board (and a possible error code) I just don't know since the control board is powered through the fuse and I don't dare jump it for fear of frying the board. I've inspected every wire and connector - nothing chewed, frayed, burned, or damaged.

I called Pentair helpline to see if they had any ideas, but all they said was, "Wow! that heater should've died years ago and nobody here knows anything about units that old - just replace it". I called BS on that because this 1998 model has almost 100% parts commonality with modern SR400NA models, but Pentair doesn't seem to know or bother to want to help with it...

From my research on forums, it seems the cause may be a short to ground somewhere, but cannot find anything else about a cause. Any ideas???

Thanks in Advance!

 

[ajw22]

Welcome to TFP.

Pentair MasterTemp Heaters - Further Reading

www.troublefreepool.com

You can try removing the operating control board connectors at J1, J2 and J3 leaving only the 24V input to the control board and the firemans switch return to the control board at J7. Does the fuse blow? If so you have isolated the problem to the firemans switch and its wiring or the control board. If it doesn't blow, attach the connector at J1 and repower the heater. Does the fuse blow? If so, you have isolated the problem to the ICM or its associated wiring. The only connection at the ICM that should have zero or near zero ohms to ground should be the one lableled GND.

If the fuse blows with only the J7 connector in place, you can remove the J7 connector and check for continuity to ground on the 2 connections associated with the firemans switch. If they read as open, you have a problem with the control board.

 

[cpfox]

Thank you so much AJW22 for the detailed troubleshooting instructions - much appreciated! I will do this procedure as soon as I get the new fuses in and report back to you (the fuses are supposed to arrive on 12/20).

Studying your procedure, it seems to my electrical engineering challenged mind that you've focused in on the ICM as the culprit (#42001-0052S) - assuming the control panel / fireman's switch and associated wiring is good, right? In the meanwhile and waiting for the fuses, I will dig my Ohms meter out and do continuity testing on the ICM and get back to you with my findings.

I've noted that you didn't mention the ignitor or the gas control valve as possible problems (at least not yet!). Is your thought-process thinking the ignitor and/or gas control valve are probably NOT the source of blowing the 1.25amp fuse?

Thanks again!!!

 

[ajw22]

The fuse is on the 24V control loop.

The igniter is not directly on the 24V loop.

The igniter is controlled by the ICM which is powered by the 24V.

The gas valve is powered by the 24V and could be the culprit but that is low probability.

 

[cpfox]

Hello ajw22,

I did short to ground checks on all the terminals for the ICM and Fireman's Switch:

With all the wires disconnected from the ICM (except the ground wire), there were no shorts to ground at any terminal.

I've attached 2 schematic pics of the fireman's switch and the readings from the ohm's meter testing checking for a short to ground. Where "NS" is Not Shorted - meaning the ohm meter indicated "1". The other number readings are what the ohm meter read on the 200 scale with one probe connected to the heater ground wire. Instead of writing all those numbers and descriptions out, I figured 2 pictures were worth 2,000 words, right? The 1.25A fuse is blown and it'll be a few more days before I get new ones, so all readouts are without the fuse...

I can't make sense of the readings. Can you make sense of it?

 

[ajw22]

Let’s get @swamprat69 into the conversation.

 

[ajw22]

Is the heater connected to automation through the fireman’s switch or does it have the yellow jumper on it?

 

[cpfox]

Is the heater connected to automation through the fireman’s switch or does it have the yellow jumper on it?

It has a jumper - basic stuff and no automation

 

[cpfox]

Basically, just like my edited schematic shows. I edited out the optional field installed dotted line stuff.

 

[cpfox]

Is the heater connected to automation through the fireman’s switch or does it have the yellow jumper on it?

I just thought of something I should mention. I'm not sure if this has anything to do with anything, but - just-a-sayin'...

The evening the heater quit working when I tried to turn it on, my Malibu light system quit working too. Malibu lights are on a separate 12V DC / 300W transformer that is also quite old (probably a relic from 1998 or so), but the 110V / 300W lights transformer plugs into via a GFI plug in on the side of main electrical junction box that feeds the 220V for the heater.

I've since figured out the Malibu transformer is just plain dead, and I have a new one on the way...

There were no thrown breakers or obvious issues I can run down in troubleshooting when this double-electrical failure occurred with the lights and heater. However, I find it a really odd coincidence the Malibu light transformer AND the SR400NA heater both take a dump at the same time, or within a few minutes of each other... Maybe an odd coincidence, or the electrical gremlins attacked? Who can say...

 

[swamprat69]

Not sure what electrical meters you have? Basic multimeter? Clamp type ammeter? Sequence of operation goes... 24V in to IND on the fenwal starts the blower if the water pressure switch and high limit switch are closed... Then when the air pressure switch closes, you get 24V to TH on the fenwal which powers the ignitor for 20 seconds to heat it up... after the ignitor is powered for 20 seconds the gas valve is powered for 7 seconds through the VAL terminal on the fenwal to the gas valve to try for ignition. Am I correct that the fuse you are blowing is the glass fuse associated with firemans switch wiring? Your blowing of the fuse 5-7 seconds after the blower starts doesn't make sense with this ignition timing. There are only 3 things in the 24V ciruit at that point that could cause the 24V fuse to blow from excessive amp draw. First is the fan/combustion air blower relay... second is the ignitor relay... and third is the gas valve or asssociated gas valve wiring. You could disconnect the wires at S2 and VAL on the fenwal after cutting power to the heater and put electrical tape on the wires so that they are insulated and can't be grounded or touched by you. Run the heater. If the fuse blows you have a problem with the fan/combustion air blower relay. If the fuse doeesn't blow power down the heater and reconnect the S2 wire. Run the heater. If the fuse blows you have a problem with the ignitor relay. If the fuse doesn't blow power down the heater and reconnect the VAL wire. Run the heater. If the fuse blows you either have a problem with the gas valve or a shorted wire from the fenwal VAL terminal through the AGS switch to the gas valve. Safety first, do not play with or touch wires while the heater is powered! This should be the manual for your heater https://www.poolsupplyunlimited.com/Products/Manuals/96946_2013123110924.pdf with the conection and ladder wiring diagrams on pages 30-31.

 

[swamprat69]

P.S. Also make sure that all wiring connections stated above are dirt and corrosion free.

 

[cpfox]

Thank you swamprat69 - much appreciated! The fuse that is blowing is the glass 1.25A fuse in the fireman's switch. I've got 10 more fuses ordered and should be delivered on Wednesday. I will go through your procedure very carefully at that time and report back to you...

And, thank you for your service. My father in law was a 2-tour USMC Vietnam vet. I have the highest respect for anyone involved in that war...

 

[swamprat69]

Thank you swamprat69 - much appreciated! The fuse that is blowing is the glass 1.25A fuse in the fireman's switch. I've got 10 more fuses ordered and should be delivered on Wednesday. I will go through your procedure very carefully at that time and report back to you...

And, thank you for your service. My father in law was a 2-tour USMC Vietnam vet. I have the highest respect for anyone involved in that war...

TYVM and tell your father in law WELCOME HOME

 

[cpfox]

swamprat69:

Running through your procedure...

"You could disconnect the wires at S2 and VAL on the fenwal after cutting power to the heater and put electrical tape on the wires so that they are insulated and can't be grounded or touched by you. Run the heater. If the fuse blows you have a problem with the fan/combustion air blower relay."

This was fine - the 1.25A glass fuse did not blow from the fan/combustion air blower relay

"If the fuse doesn't blow power down the heater and reconnect the S2 wire. Run the heater. If the fuse blows you have a problem with the ignitor relay."

This too was fine - no blown fuse. And, I just replaced the ignitor after I broke the old one trying to take it out to inspect it.

"If the fuse doesn't blow power down the heater and reconnect the VAL wire. Run the heater. If the fuse blows you either have a problem with the gas valve or a shorted wire from the fenwal VAL terminal through the AGS switch to the gas valve."

This is where the fuse blew out. So, it looks to me like I'm now running down either a shorted wire from the Fenwal VAL terminal, a bad AGS switch, or a bad gas valve? Any thoughts?

Thanks again so much!

 

[cpfox]

swamprat69,

A little update here... all the wires going from the control board to the AGS and to the gas valve are good - no shorts. There is no ohms resistance across the AGS terminals (reads something like 1.2 ohms resistance). There is no ohms resistance across the gas valve terminals either. Is that normal?

I also cleaned ALL the terminal connection points with a good electrical contact cleaner.

Thanks!

 

[cpfox]

More updates...

I'm getting 27.3V from the transformer to the control board. I then disconnected the two terminals going to the gas valve and put the meter on those two terminals to check for power when the gas valve should open. I started the heater, and when when the 24V power to the gas valve kicked in, the 1.25A fuse blew. The meter for a brief second showed 4V just before the fuse blew. Just to be clear - the 24V terminals to the gas valve were not connected to the gas valve. I had both plugged into a volt meter...

So, I'm thinking the gas valve itself is not the culprit here. I'm trying to understand specifically how the AGS valve works... If I'm understanding this correctly, the AGS valve senses temperature? If the heat exchanger outlet temperature is <=140 degrees, the the valve should be closed and there should be zero, or almost zero ohms resistance across the AGS terminals? And, if the temperature is >140 degrees, the AGS will open and show infinite resistance across the terminals?

Can you think of any scenario where the AGS may be blowing the 1.25A fuse? Other than that, all I can think of is a bad control board - but I just put a new one in. Yes, it was probably a Chinese knockoff board. I'm flying blind here... any suggestions? Could the Fenwal control box be bad? If so, how do I check it?

Thanks!

 

[cpfox]

Welcome to TFP.

Pentair MasterTemp Heaters - Further Reading

www.troublefreepool.com

You can try removing the operating control board connectors at J1, J2 and J3 leaving only the 24V input to the control board and the firemans switch return to the control board at J7. Does the fuse blow? If so you have isolated the problem to the firemans switch and its wiring or the control board. If it doesn't blow, attach the connector at J1 and repower the heater. Does the fuse blow? If so, you have isolated the problem to the ICM or its associated wiring. The only connection at the ICM that should have zero or near zero ohms to ground should be the one lableled GND.

If the fuse blows with only the J7 connector in place, you can remove the J7 connector and check for continuity to ground on the 2 connections associated with the firemans switch. If they read as open, you have a problem with the control board

Hi ajw22, I tried your procedure...

"You can try removing the operating control board connectors at J1, J2 and J3 leaving only the 24V input to the control board and the firemans switch return to the control board at J7. Does the fuse blow?" No, the fuse is good after trying this.

"If it doesn't blow, attach the connector at J1 and repower the heater. Does the fuse blow?" No, fuse still good after attaching J1 connector.

Thanks so much, still trying to run down the issue.

 

[1poolman1]

More updates...

I'm getting 27.3V from the transformer to the control board. I then disconnected the two terminals going to the gas valve and put the meter on those two terminals to check for power when the gas valve should open. I started the heater, and when when the 24V power to the gas valve kicked in, the 1.25A fuse blew. The meter for a brief second showed 4V just before the fuse blew. Just to be clear - the 24V terminals to the gas valve were not connected to the gas valve. I had both plugged into a volt meter...

So, I'm thinking the gas valve itself is not the culprit here. I'm trying to understand specifically how the AGS valve works... If I'm understanding this correctly, the AGS valve senses temperature? If the heat exchanger outlet temperature is <=140 degrees, the the valve should be closed and there should be zero, or almost zero ohms resistance across the AGS terminals? And, if the temperature is >140 degrees, the AGS will open and show infinite resistance across the terminals?

Can you think of any scenario where the AGS may be blowing the 1.25A fuse? Other than that, all I can think of is a bad control board - but I just put a new one in. Yes, it was probably a Chinese knockoff board. I'm flying blind here... any suggestions? Could the Fenwal control box be bad? If so, how do I check it?

Thanks!

AGS is just a "dumb" temperature sensing switch. At the right temperature it opens and cuts power. The fact that the heater tried to light (sent power to the gas valve) means it is closed and not causing an issue. Check the wiring from the gas valve back to the source. There may be a short.

 

[cpfox]

AGS is just a "dumb" temperature sensing switch. At the right temperature it opens and cuts power. The fact that the heater tried to light (sent power to the gas valve) means it is closed and not causing an issue. Check the wiring from the gas valve back to the source. There may be a short.

Thanks for the reply 1poolman1.
I checked for continuity on the gas valve wires which run back to the control board in the J3 terminal block. The wires were good with no shorting.