Mastertemp 400 Short Cycling

dehn0045

Member
Jun 7, 2018
13
LEAGUE CITY
I've been lurking here since building my pool a couple years ago. I've finally run into a problem that I haven't been able to resolve with info from the TFP archives.

I had the dreaded mouse hotel in my mastertemp 400. I got it cleaned out and mouse-proofed, but the heater has started short cycling. I'll try to lay out all of the timeline as best I can:

After cleaning out the mice, the heater seemed to run fine, but every once in a while it would stop and show the "service heater" light. I would cycle power to the heater and it would work fine for a couple weeks. I did notice a few times that the heater would randomly kick off and restart, but it was infrequent. This issue progressively got worse until the heater wouldn't light at all.

My first serious troubleshooting (beyond cycling power) I found the heater would go through an ignition sequence 5 times and if it did not light then it would turn on the "service heater" light and stop. There are no LED error codes under the control board. When it failed to light I didn't hear any click from the gas valve and no mercaptan smell, so I assumed that the gas valve was bad. The mice had peed all over it and it looked pretty bad. Well, after replacing the gas valve it is still doing the same thing.

I then decided to replace the thermal regulator, but given that I wasn't getting the HLS led I assumed it wouldn't do the trick, it didn't.

I have a 2psi gas line to a step down right at the heater. The liquid drain hole on the heater exhaust blows hot flue gas right onto the step down regulator, so I sort of suspect maybe low gas pressure. I was able to test the outlet pressure of the gas valve during operation, it was negative 0.75 inches. During the prepurge it was about negative 5 inches. I need to get a fitting before I can test the inlet pressure to the gas valve. I was able to watch the meter while it was running and it showed about 5.5 cfm, so maybe a little bit low but probably in the ballpark.

I tried to run a separate ground wire for the ground wires near the FENWAL, but this didn't seem to make any difference. I'm also not getting any error codes on the FENWAL.

I tried running at higher pump speed (3450 vs 3100), no noticeable difference. I don't think it is a flow related issue.

I have noticed a few times where the heater seems to lose flame but immediately reignites, this seems pretty strange.

I worry that maybe there is a coil leak, but I couldn't find any drain on the combustion chamber side, I'm not sure what the best way to check this.

I wonder about the thermistors, but not really sure how to test other than simply replace them.

Other than checking the static/dynamic inlet gas pressure, I'm not really sure where to go next. Any advice would be greatly appreciated.
 

dehn0045

Member
Jun 7, 2018
13
LEAGUE CITY
I'm starting to wonder about the control board. I went through the troubleshooting flowchart in the manual and replacing the control board is where I landed, although the flow chart doesn't match exactly to my scenario (no error codes, sometimes doesn't light, but sometimes does light but cuts out prior to reaching setpoint)

One other tidbit that might be useful is when it fails to light it cycles the blower off in between each attempt. You can hear it slow down but within a couple seconds it ramps back up for the purge. The AFS light comes on when the blower slows down, but it almost immediately clears when the blower ramps back up.
 
Last edited:

swamprat69

Well-known member
Mar 30, 2019
328
Las Vegas, NV
Your gas pressure at the gas valve outlet is outside of operating range per manual. Outlet pressure at the gas valve outlet should be negative 0.20"WC. Inlet pressure at the gas valve should be between 4"WC and 14"WC
 

JamesW

TFP Expert
Mar 2, 2011
20,822
I found the heater would go through an ignition sequence 5 times and if it did not light then it would turn on the "service heater" light and stop. There are no LED error codes under the control board.
I'm also not getting any error codes on the FENWAL.
I need to get a fitting before I can test the inlet pressure to the gas valve.
If the “Service Heater” LED is coming on, there is going to be an error LED on the back of the board or on the Fenwal. So, I suspect that you’re missing something.

Verify the inlet gas pressure while the heater is off and then when it is trying to light and when it is running. If the inlet gas pressure is correct, the manifold will probably be correct.

I suspect that you’re not testing the gas pressure correctly. If you don’t have the fitting for the inlet pressure tap, you don’t have the fitting for the manifold pressure tap.

See the manual for the proper test procedure for testing the manifold pressure.

This appliance is equipped with an unconventional gas control valve that is factory set with a manifold pressure of –.2" (–0.5cm) wc. Installation or service must be performed by a qualified installer, service agency, or the gas supplier. If this control valve is replaced, it must be replaced with an identical control. The combination gas valve incorporates dual shut-off valves and a negative-pressure regulator. For proper operation, the regulated pressure at the outlet manifold of the valve must be –0.2" (–0.5cm) wc below the reference pressure at the blower mixer inlet, and the gas valve ‘VENT’ tap must be connected to the end cap air orifice as shown in Figure 7.

Check the flame current when the heater is running.


See this reference for more information.


Can you show a video of the bootup and heater start and run?

Does the water temperature on the display change quickly or is it correct and steady?

When the heater runs, press the "Pool On" button until the display changes to the stack flue temperature and report what that is.
 
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Killer95Stang

Well-known member
Aug 17, 2012
874
Sunny SoCal
I know your heater is fairly new... and you probably already physically looked at this piece, but trust me. I have had the same symptoms as you twice since 2013 when the pool was built. Both times I changed the Thermal Regulator and it fixed my issues for 2 -3 years. They are $17.99 on Amazon and worth a try.
 

Attachments

bongardjm

Gold Supporter
Aug 12, 2014
45
Canton MI
Had the same issue with my Mastertemp 300..never got error codes and slowly started failing like yours. Replaced a lot of the sensors like you did, thermal regulator etc.., nothing worked. Mine was only a few years old as well.
Always have balanced chemicals..took it apart and saw this below!!! Once I replaced the tube sheet and cleaned it up all works great.

See my post ..https://www.troublefreepool.com/threads/mastertemp-300-solved-cycling.194996/

1598282745054.png
 

dehn0045

Member
Jun 7, 2018
13
LEAGUE CITY
Your gas pressure at the gas valve outlet is outside of operating range per manual. Outlet pressure at the gas valve outlet should be negative 0.20"WC. Inlet pressure at the gas valve should be between 4"WC and 14"WC
I think you are supposed to remove the "vent" tubing that goes to the blower suction to get a proper test of the outlet side running pressure. I left it hooked up. I will check this again when I have time.
 

dehn0045

Member
Jun 7, 2018
13
LEAGUE CITY
@JamesW the FENWAL shows a solid LED but only after the 5 light attempts fail. I think that it blinks once in between each light attempt but I think this coincides with the AFS light that comes on when the blower ramps down.

I was able to use the 90 degree barbed fitting from the old gas valve on the outlet side, but need to get a straight one for the inlet side.

Need to get a true RMS for checking the flame sensor. I was under the impression that if the flame sensor is the issue then I would be getting error codes on the control board and/or FENWAL each time it failed.

Boot up is normal as far as I can tell, I'll get a video. The display shows the correct temperature and seems solid.

I wasn't aware you could get other readings on the display, will get the stack temp reading and report back.
 

dehn0045

Member
Jun 7, 2018
13
LEAGUE CITY
I know your heater is fairly new... and you probably already physically looked at this piece, but trust me. I have had the same symptoms as you twice since 2013 when the pool was built. Both times I changed the Thermal Regulator and it fixed my issues for 2 -3 years. They are $17.99 on Amazon and worth a try.
Just replaced yesterday, didn't resolve the problem
 

dehn0045

Member
Jun 7, 2018
13
LEAGUE CITY
Had the same issue with my Mastertemp 300..never got error codes and slowly started failing like yours. Replaced a lot of the sensors like you did, thermal regulator etc.., nothing worked. Mine was only a few years old as well.
Always have balanced chemicals..took it apart and saw this below!!! Once I replaced the tube sheet and cleaned it up all works great.

See my post ..https://www.troublefreepool.com/threads/mastertemp-300-solved-cycling.194996/

View attachment 159471
Thanks! I feel like I looked at all the threads a dozen times, but don't remember seeing this one, will check it out.
 

JamesW

TFP Expert
Mar 2, 2011
20,822
I was under the impression that if the flame sensor is the issue then I would be getting error codes on the control board and/or FENWAL each time it failed.
If you're not getting ignition, then flame sensing does not come into play. If you are getting ignition, but the flame current is too low, the heater will shut off with no LEDs on the control board. The Fenwal will show 3 flashes for ignition lockout after several failed tries for ignition.

the FENWAL shows a solid LED but only after the 5 light attempts fail.
A solid LED indicates a control fault of some kind, which probably indicates a bad Fenwal ignition module.
 
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JamesW

TFP Expert
Mar 2, 2011
20,822
How old is the heater?

Has the control board or the Fenwal ever been replaced?

You can remove one of the bottom manifold bolts to see if water comes out, which would indicate a leak in the heat exchanger.
 

dehn0045

Member
Jun 7, 2018
13
LEAGUE CITY
It's about 2 years old, installed new with the pool build.

Control board and FENWAL are both original

The meter is the one going to the house. It has a 1/2 foot dial and a 2 foot dial. Nothing else in the house was running. Checked a few times with a stopwatch.

Stack temp topped out at about 285

The boot up flashes 888 then 128 then r14 then shows the temp which is close to expected temp.
 

JamesW

TFP Expert
Mar 2, 2011
20,822
Stack temp topped out at about 285
Here is what the normal exhaust temperature should be:

Below 250 degrees...very low
250 to 290 ..................low
290 to 350.................acceptable
350 to 480 ..................high
Above 480...................error/shutdown.

Note: HD models can be up to 75 degrees higher. HD models use a cupro nickel exchanger and the efficiency is slightly lower resulting in less heat transfer and more waste heat.

Your stack temperature is a little bit low and the cubic feet per hour comes out to about 330,000 btu/hr, which is also a little bit low, which might indicate a gas supply problem.

Verify the gas pressure at the inlet static (heater off) and dynamic (Heater on).

What is the size and length of the gas line?

Do you have a regulator near the heater?

Is the heater a “HD” model?
 

dehn0045

Member
Jun 7, 2018
13
LEAGUE CITY
I believe it is 1.25" and is about 60 ft (the protective sleeve that comes out of the ground is about 2.25" diameter and is labeled 1 1/4". I think it is a 2PSI line to the heater as there is a separate regulator to the house after the meter, the regulator upstream of the meter says 2psi. There is also regulator near the heater. The regulator near the heater is very close and gets hit with exhaust gas that comes out of the weep hole on the exhaust stack. I am wondering if maybe the regulator got overheated and failed. I put up a piece of flashing yesterday to keep the hot gas off of it, but that didn't seem to help.

It is not an HD model.

Not sure if they are going to cover me on warranty. I had the pool builder from ****, project started on 5/24/18 and the punch list completion and final payment wasn't until 12/31/18. Actually filled the pool with water on 9/24, but as you can probably guess the builder didn't supply much in the way of documentation. Actually had to get a lawyer to draft a letter to get him to finish the dang thing. Now I'm pretty sure he is out of business.

Once I get the fitting I'll test the static/dynamic pressures. I might be able to check the drain tonight, but might be on hurricane watch the middle of this week so not sure how far I'll get.
 

dehn0045

Member
Jun 7, 2018
13
LEAGUE CITY
Yeah, called them and he said to register online, wait 24 hours and then call back. I did the registration, will see what they say when I call tomorrow. I wish I would've been more on top of the equipment stuff, but it was like pulling teeth just to get the necessary stuff done.

Will report back when I get the supply pressures and check the drain. Thanks for the help so far!
 
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