Pentair Mastertemp 400 short cycling, but other than an occasional "Service Heater" its cycles till it satisfies?

[iannecj]

Problem Summary:

Since I purchased the pool ~5 years ago the Pentair mastertemp 400 would heat in short cycles taking a very long time to heat (6+ hours) here in central Florida even on hot days. Now occasionally it will light “Service Heater” but that can be cleared by a restart and it will on subsequent tries go back to what I believe is anomalous short cycling. My goal is to remedy both things.

Problem Details:

Since I had this Pentair equipment installed approximately 5 years ago the Pentair Mastertemp 400 (NG fed unit) would cycle even before it reaches setpoint and it takes 4+ hours to heat. I thought this was normal.



Recently, I got a service heater light and it would lock out.

• With Service Heater on front panel the PCB has the green spa on and now RED LED10 but no other red LED. No lights on FENWAL
• In troubleshooting I found if I power cycled the unit it would often go back to what I believe is its original but still anomalous behavior with these short cycles. When it does this it never throws an error LED or code on the PCB or the fenwall LED…no red LEDs anywhere. It just short cycles until it achieves setpoint.
• After getting the service light I began to troubleshoot and If I understand the videos and the “Master Temp Further Reading” post on TFP, the short cycling is anomalous as is of course the occasional service heater LED. I am wanting to correct both.
• I started by replacing sensors ( HLS, SFS, and heater Thermistor without checking if they were bad. This did not solve and there was no difference in behavior. Here is source: https://www.amazon.com/dp/B09SHH5FHJ?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_4
• Then since I was in it I tried AGS replacement too from this source. No difference in behavior. Amazon.com
• Next I read on TFP posts Thermal Regulator or internal bypass. I pulled TR and put in hot water and then very hot water >170deg and it never moved. It seems to have some glue on it and it looks like some epoxy possible factory to keep spring and clip. I gather maybe this device never worked?
• While I had it out I put a borescope into hole and I can see cap of bypass valve and its there and feels relatively solid.
• I ordered on aftermarket ARU unit for TR despite warnings as I could not get a Pentair unit quick anywhere. That unit starts to open at 120F and is wide open around 130F. Amazon.com
• Thought I solved it, put it in. Initial power up gave me service heater but no other red LEDs as before and also as before a power cycle got me back to my typical short cycling.
• I have the bypass on order but have my doubts
• In the mean time I put a meter on ever sensor output to see if any were momentarily opening but I don’t see in contact open except the pressure switch which I can get to open if I turn off pump while heater running (as a means to test). All others seem to stay closed and even my meter on fast hold doesn’t detect a momentary open. These are latching? so no LED means no problems with them I would guess?
• This short cycling will occur when the in pool or spa more and that’s with the VSF pump registering 27GPM for pool or higher for spa at like 45GPM (SPA is elevated some 4 feet or so and this is through a freshly cleaned and serviced DE filter 48, plot below as I go between heat in spa and pool mode testing and timing short cycles and looking at stack flow temp. Both modes do same thing.
• The air is always moving and there is suction into white area at all times even , though the heating is in these modes cuts out, I can put my hand over exhaust and feel air but its clearly cooler when heat cycles off.
• These short cycles are running from 30 seconds to about 100 seconds in the 15 short cycles or so I have timed or so.
• I notice no heating light on when it is heating which is not how a recall it in past operations, I thought I saw the amber heating LED when I first got it.
• Here is a plot of temp in spa over same time period overlaid:
• Once it gets hotter the curve flattens out and it talks much longer to get the last 10 degrees than the first.
• So I cant seem to correlate the stop of heat cycle to any state change. No LED seems to even flicker when heat stops.
• Also when heat starts and then stops I have held the button to get stack temp and its reads like its is still rising and is low at the instant heat stops. It the nearly one minute its heating stack temp will start low and then slowly rise. At the instant it stops it’s at around 225. On longer cycles as hot at 250. If I use IR gun on black vent I’ll get close at like 230. When I watch temp its aways rising until it cuts out heat and exhaust still blows but blows like ambient air.
• If I let the heater cool for an hour or so, it I disconnect the SFS I read ~1-1.2Megaohms. I am in Florida, it’s a 95deg day and its in a black pipe so I figure could be right.

 

[ajw22]

This short cycling will occur when the in pool or spa more and that’s with the VSF pump registering 27GPM for pool or higher for spa at like 45GPM (SPA is elevated some 4 feet or so and this is through a freshly cleaned and serviced DE filter 48, plot below as I go between heat in spa and pool mode testing and timing short cycles and looking at stack flow temp. Both modes do same thing.

The minimum flow for the MasterTemp 400 is 40 GPM.

Run your pump to have at least 50-60 GPM when the heater is on.

See if it makes any difference.

 

[iannecj]

The minimum flow for the MasterTemp 400 is 40 GPM.

Run your pump to have at least 50-60 GPM when the heater is on.

See if it makes any difference.

View attachment 650460

VSF was set to 3000RPM, went up to max (3450) it upped flow to 51GPM, still short cycled, only watched 3 cycles but similar to first cold cycles at lower flow 45GPM and closer to 45 seconds of heat. I attribute to elevation of both spa and main basin this pump draws from which both have water levels above this pump 2-5 ft. I may be able bypass the DE with the multiport valve in recircle mode and expect it to go way up. Should I try that?

 

[ajw22]

I may be able bypass the DE with the multiport valve in recircle mode and expect it to go way up. Should I try that?

yes.

 

[iannecj]

yes.

Well not as much as I would have thought, bypassing DE only got me up to 57GPM, it is a clean filter with fresh DE so I guess there is that for an explanation of not much loss from DE..

Same short cycling. 2 I timed where 30 seconds and stack flue temp only to 225deg in those cases. Haven't seen the service heater light since first attempt this morning. So no red LEDs of any kind all day of troubleshooting.

Any other ideas? Flow is above minimum so maybe not flow?

Am I correct it should heat (and hence I can feel a hot exhaust) until first satisfying setpoint right?over an hour continuous heat right?

Also, should blower be going when heat cuts out? I do feel forced ambient air when heat cuts out. In fact forced air out of the vent is always there, its only very hot and ambient depending on heating on or not. That correct?

Other ideas?

 

[ajw22]

Post pictures of your equipment pad.

Your stack flue temperature is low. You have an internal water flow problem in the heater. Yiu may need to remove the manifold and examine the manifold, bypass valve, and heat exchanger.

Your pump flow is very low and indicates restriction in your plumbing.

Do you have a heater bypass to see what flow you can get when bypassing the heater?

You could have a scaled heat exchanger.

 

[iannecj]

Post pictures of your equipment pad.

Your stack flue temperature is low. You have an internal water flow problem in the heater. Yiu may need to remove the manifold and examine the manifold, bypass valve, and heat exchanger.

Your pump flow is very low and indicates restriction in your plumbing.

Do you have a heater bypass to see what flow you can get when bypassing the heater?

F could have a scaled heat exchanger.

Thanks for these comments I'll test and take pictures tomorrow. Until then can you help me by answering my questions above?

 

[iannecj]

Post pictures of your equipment pad.

Your stack flue temperature is low. You have an internal water flow problem in the heater. Yiu may need to remove the manifold and examine the manifold, bypass valve, and heat exchanger.

Your pump flow is very low and indicates restriction in your plumbing.

Do you have a heater bypass to see what flow you can get when bypassing the heater?

You could have a scaled heat exchanger.

OK....Here are the pictures, original hydraulic engineering, and the flow data both present and historical you wanted.

To Summarize:

-max flow possible has declined a small amount (72GPM vs 66GPM) from when pool was installed In April 2019 to now (also there is a new VSF pump (slightly smaller) in there which may be part of that delta). This max flow with both DE and heater bypassed is 73GPM today w/ VSF at 3450 RPM (max) and 38PSI across pump,
-with just DE bypassed (heater in but not heating) max flow is 66GPM and 39 PSI (Today).
-In April 2019 I did a similar flow test when pool first commissioned. I did'nt bypass heater at that time but did bypass DE. Max flow in Pool mode with DE in the loop was 56GPM and 37PSI across, with just DE bypassed 72GPM and 36PSI
*Note above flows are in pool mode where system only pumps a fraction of water to spa whereas data yesterday was spa mode and 100% of water pumped to most elevated tier. I guess this accounts for the plus up today as compared to yesterdays testing of flows.

-This is lower than original engineering estimates which prompted my tests in April 2019. They estimated ~90GPM max with old (slightly stronger) VSF pump.

-The run 30-120 seconds heating then cycling off with forced air always running is similar behavior since April 2019.

-At any flow level with heater not bypassed the heater behaves the same with cycling and no diagnostic LEDs save an occasionally "Service Heater" which never has any accompanying red LED on board or Fenwal.

-A DMM set to max hold never shows even momentary contact state changes on the interlock switches when the heater cycles.

So my question, since flows are above specification for the heater, and have declined but only marginally since install, and since we don't see HLS, PS or AGS, how would the heater know its flow is low and hence this be the cause of cycling? Shouldn't low flow, if it was going to be problematic for heater, cause an interlock switch to be set?


Table from April 2019:



Original Engineering



Pictures of Pad, zoom in on valve config when I bypassed Heater and DE and then manual pump run at max flow:

 

[ajw22]

So my question, since flows are above specification for the heater, and have declined but only marginally since install, and since we don't see HLS, PS or AGS, how would the heater know its flow is low and hence this be the cause of cycling? Shouldn't low flow, if it was going to be problematic for heater, cause an interlock switch to be set?

The heater never knows what its flow is. The heater only has a water pressure switch.

The heater only knows about heat in various areas. The HLS, AGS, and SFS measure the heat. If the heat is above limits the heater shuts down.

Some errors in the MasterTemp do not latch, and when the limit switch cools, everything resets, and the heater cycles. I think Pentair made the cycling and restarts a feature that makes it difficult to find the cause of the cycling.

We see many Pentair heaters short cycling here and manage to fix most of them.

Something in the water flow is overheating and tripping a limit sensor.

Have you removed the manifold and examined it and the heat exchanger?

My bet is that replacing the manifold with the manifold kit that comes with all new sensors, except for the thermal regulator, will fix the problem. Costs $500.

 

[iannecj]

One more question since I put a meter on hls and I never see those contacts open when I have it set to hold and Max voltage during the time the heater is operating and cycling doesn't that tell me the heater limit switch is not changing state? Is there something else I should be measuring?

 

[ajw22]

One more question since I put a meter on hls and I never see those contacts open when I have it set to hold and Max voltage during the time the heater is operating and cycling doesn't that tell me the heater limit switch is not changing state? Is there something else I should be measuring?

I don't know how you can see the contacts open when your instrument will bridge whatever you're trying to monitor.

The best way to test sensors is to jump one at a time and see if the heater runs normally. The problem is, with an overheat problem, the heater will keep on running and can damage itself.

 

[iannecj]

I don't know how you can see the contacts open when your instrument will bridge whatever you're trying to monitor.

The best way to test sensors is to jump one at a time and see if the heater runs normally. The problem is, with an overheat problem, the heater will keep on running and can damage itself.

when one opens you get the 12VAC across it, if closed you get near zero. HLS always has zero

on jumpering the HLS as a sanity check, you dont recommend?

 

[ajw22]

on jumpering the HLS as a sanity check, you dont recommend?

Watch the heater carefully and monitor the stack flue temperature using an IR thermometer.

If the heater runs longer than it cycles now call the test successful and shut it down.

 

[Bill1974]

Since there are no errors, it seems to be a low flow issue. It's cycling because the outlet water temp has reached it's limit or some other temp sensor has reached it's limit.

Double check all your valves are open (suction and return). A restriction on the suction side will look like everything is normal and the flow will be low. You won't see high pressure as if you had a restriction on the return side.

Have you tried back flushing the heater, maybe something isn't restricting the flow. Could be scale buildup in the heat exchanger if this has been happening for a while.

Also check the burner side of the heat exchanger to make sure it's clear of debris.

Have you checked your inlet gas pressure. Might be the regulator allowing too much NG in. It will act like a higher output burner and require more flow.

Have the heater checked with a combustion analyzer to make sure the air and NG are set correctly. If you look at you gas meter readings you can get a pretty good idea if the heat is over fueled. You will also need to somewhat accurately measure the time the heater is actually running using gas. An hour of run time should be 4 CCF.

Can you check the temperature of the inlet water and outlet water temperature. Compare it to what the heater controls are indicating. Since you change some components and there are no errors I don't think this is the issue.

 

[iannecj]

Watch the heater carefully and monitor the stack flue temperature using an IR thermometer.

If the heater runs longer than it cycles now call the test successful and shut it down.

OK I have checked out and confirmed the following:

1.) The HLS is not opening to cause the short cycling. (Beside the fact there is no light) I verified by disconnecting the HLS sensor leads jumpering them together and monitoring he now disconnected but still in place HLS sensor with a meter to ensure contacts of HLS switch remain closed throughup heating cycle. Heater starts and short cycles even in this mode with HLS wires jumpered. Meter says actual sesnor always stays closed throughout. Conclusion HLS not cause.

2.) The AGS is not opening. Same steps done with HLS above. Conclusion AGS not the cause.

3.) Water pressure switch has 12V across it until I start pump and begin heater. Then the contacts clsoe and goes to zero volts verifying PS funcitonal. Monitoring this PS during startup these contacts never open even when short cycle occur. Conclusion: sensor is verified not the cause and verified functional.

4.) Thermistor- disconnected leads, placed a 10K across leads . Started system...short cycles still happen. Display reads 76 deg on unit. Also BTW the table people post on here is wrong for 10k thermistor voltage. Resistance for that table is correct map to temp but the measurement circuit in the mastertemp 400 in the voltage divider used to make the measurement applies above half the voltage that whatever system was used to make that table. With a 10k resistor in place of my thermistor while system is running display on mastertemp reads 76 and voltage across it is 1.33V...roughly half table value. When I measure resistance on the in place thermistor with leads disconnected I get 7.75k and water is in fact 87 deg according to the other temp sensor in pool pump line. Conclusion both thermistor and input on control board for temp sense are working.

In summary there is no momentary switch change of state on and of these safety interlock switches while cycling happening.

Already discussed SFS above.

What other temp sensor should I check?

 

[iannecj]

Since there are no errors, it seems to be a low flow issue. It's cycling because the outlet water temp has reached it's limit or some other temp sensor has reached it's limit.

See my post above for steps to assess if this is occurring. Is there some other senesor I should check?

Double check all your valves are open (suction and return). A restriction on the suction side will look like everything is normal and the flow will be low. You won't see high pressure as if you had a restriction on the return side.

When you say doublecheck you mean position right? OK all good judging by handles. Is there a better way to do this?

Have you tried back flushing the heater, maybe something isn't restricting the flow. Could be scale buildup in the heat exchanger if this has been happening for a while.

How does a DIYer do that? Just garden hose after I remove manifold?

Also check the burner side of the heat exchanger to make sure it's clear of debris.

Not sure how you go about this without full disassembly. I placed a borescope in the exhaust and unlike others I see on youtube there is no water, no crusty salty blocks. I will try to repeat tonight so I can post a picture that you can see. Its me taking a picture of mu borescope screen. Seems in good shapre but you can be judge.

Have you checked your inlet gas pressure. Might be the regulator allowing too much NG in. It will act like a higher output burner and require more flow.

I dont know how to do this, have a reference that describes tools and meters I need? Are we now into cheaper to call a pro?

Have the heater checked with a combustion analyzer to make sure the air and NG are set correctly. If you look at you gas meter readings you can get a pretty good idea if the heat is over fueled. You will also need to somewhat accurately measure the time the heater is actually running using gas. An hour of run time should be 4 CCF.

LIke the one above this might be call the pros unless you can point to a reference on how to do and tools I need and thats cheaper than a pro vist.

Can you check the temperature of the inlet water and outlet water temperature. Compare it to what the heater controls are

I measure water at the pump and its very close to water temp on mastertemp screen. How would I get right at inlet and outlet while running?

 

[iannecj]

Repeating some of my questions above here since I think folks missed them in my long posts:

-Am I correct it should heat (and hence I can feel a hot exhaust) until first satisfying setpoint right? over an hour continuous heat is typical? Mine nver hearts for mort than 100 seconds in a cycle and that's not correct right?

-Also, should blower be going when heat cuts out? I do feel forced ambient air when heat cuts out. In fact forced air out of the vent is always there, its only very hot and ambient depending on heating on or not. That correct?

 

[ajw22]

Am I correct it should heat (and hence I can feel a hot exhaust) until first satisfying setpoint right? over an hour continuous heat is typical? Mine nver hearts for mort than 100 seconds in a cycle and that's not correct right?

Yes. My heater can take water from 60F to 100F in one cycle.

-Also, should blower be going when heat cuts out? I do feel forced ambient air when heat cuts out. In fact forced air out of the vent is always there, it’s only very hot and ambient depending on heating on or not. That correct?

The blower runs for a few minutes after the gas valve closes for heater cooldown.

The Fenwal controls the gas valve and blower.

Put your voltmeter on the gas valve 24V contacts and monitor that. You should see the gas valve losing 24V when heat cuts off and getting 24V when heat resumes.

Read here where @swamprat69 explains how the Fenwal works…

Another.....Mastertemp 400 with AGS LED + Service Heater LED

I thought so too but who's ever been done wrong off the interweb??? Great price! They also sell wall sconces, lamp shades, and perfume. Is there a problem with that?

www.troublefreepool.com

What you can try and figure out is if the Fenwal is cutting off gas on it own or because it is behind commanded by the PCB or AGS.