Pentair MasterTemp 400 burning fuel but not heating water, 8 months old.

MUPPPP

Gold Supporter
Feb 7, 2019
73
Missouri
Hello,

We opened our pool last July and ran it through mid-November. We had the pool as hot as 95 degrees in 35 degree weather. We opened first of this month and pool heated from 48 degrees to 85 degrees in about 24 hours. All of the sudden, the heater won't heat.

Pool builder/HVAC/plumber are at a loss for what is going on. Temperature sensor seemed bad, as heater thought the pool was 95 degrees when it was only 65. So if I cranked up temp past 95, it would kick back on. But even then, the water coming out of the return jet was barely warmer than regular water, whereas last season, the return jet water was super hot to the touch. They fixed this sensor/thermostat and it is now reading correct temp. They also said there wasn't enough gas getting to the unit so they changed a flexible pipe to a straight pipe and a gas regulator (didn't make sense to me b/c it all of the sudden went bad).

So anyway, they say it isn't throwing off any error codes and at 10am today they had it all running and they thought it was good to go. It's been running since 10am (9+ hours) and it has only gone from 68 to 72 degrees, and the water coming out of the return jets remains lukewarm at best. From online calculators, this heater should heat my pool up 1 degree every 37 minutes, so it should have gone to at least 82 degrees by now. The heater is still firing, exhaust is very hot, thermostat/temp sensor works, but the water isn't getting heated. Water isn't bypassing heater, as the only return jet pipes go through the heater.

Please help, as my pool builder is out of ideas. I'm wanting to swap the unit out entirely for warranty, but if there's a quick fix, I'm all ears.

Thanks in advance.
 

JamesW

TFP Expert
Mar 2, 2011
18,492
Check the meter to make sure that you're using the correct amount of gas.

While running, press and hold the On button to make the exhaust temperature display.
 

MUPPPP

Gold Supporter
Feb 7, 2019
73
Missouri
Exhaust temp is 316 degrees. Not sure how to check the meter to see if it's using the correct amount of gas. Is that something that would change suddenly?
 

MUPPPP

Gold Supporter
Feb 7, 2019
73
Missouri
Found this youtube video. Sounds to me like the water is totally bypassing the heating coils in the heater and being pumped right back into the pool due to a broken manifold bypass valve. See especially 5:25 on this video. Sound possible?
 

JamesW

TFP Expert
Mar 2, 2011
18,492
The heat has to go somewhere. If it's not going in the water, it has to be going out the exhaust.

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.

So, the heat isn't going out the exhaust since the temperature is in the normal range.

The heater should use 400 cubic feet of natural gas per hour. You can check this by noting the reading at the start of the hour and again after an hour assuming nothing else is running.

Most likely the heater is cycling off and on.

Maybe due to a high limit error, which can be due to a bad thermal regulator or a bad internal bypass or an open external bypass.

There are lights on the circuit board that you can check if the heater shuts off. The high limit led might be coming on.

Note that the water coming out of the returns should never be hot.

What is the gpm flow rate going through the heater?

Stand by the heater for about 10 minutes and see if it cycles.
 
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MUPPPP

Gold Supporter
Feb 7, 2019
73
Missouri
Thanks for your help. I stood by the heater for 20 minutes just now, no cycling on/off. Continuously running. It sounds normal and I smell no natural gas. Checked exhaust again and it's 318degrees. Started monitoring gas used from the meter and will check in 60 minutes. Thanks!
 

JamesW

TFP Expert
Mar 2, 2011
18,492
The regular MasterTemp is about 84% efficient whereas the 400hd model is about 82% efficient.

The regular mastertemp puts about 336,000 btu in the water every hour.

The 400 hd puts about 328,000 btu in the water every hour.

35,000 gallons is 291,900 lbs, which means that you should get about 1.15 degrees per hour from the regular mastertemp or 1.12 degrees per hour from the hd model. You also have to account for heat loss, which can be significant if the pool is uncovered and the air is cold and dry.

Heat rise from inlet to outlet depends on the gpm. For example, at 40 gpm (the minimum recommended flow rate), the heat rise should be 16.8 degrees for the regular 400 mastertemp.
 

MUPPPP

Gold Supporter
Feb 7, 2019
73
Missouri
Interesting.

Pretty certain it's not heat loss, as it was about 77 degrees here today and 66 now.

I was not aware of the 40gpm recommended minimum flow rate. I was running at 2200 rpm which was a 27gpm flow rate (but I've never noticed a problem with this before). I cranked it up to 2800 rpm and am now getting 41gpm. I'll see if that makes a difference while I'm waiting to see how much gas I'm using.

Are you pretty certain that the fact that my exhaust temp is normal that I don't have the problem I discussed above regarding a potential broken manifold bypass valve?
 

JamesW

TFP Expert
Mar 2, 2011
18,492
If the bypass was bad, there would be too much water bypassing the heat exchanger and the high limit would shut off the heater.

Also, if the heat is not going in the water, it would have to go out of the exhaust and the exhaust temperature would be too high.

If the heater is running continuously and the exhaust temperature is correct and you use 400 cubic feet per hour, I would say that the heater is working correctly.

If you're using significantly less than 400 cubic feet in an hour, there is a problem somewhere. However, the flame would probably be too weak to get flame rectification and the heater would shut down.

If your pool is uncovered and you can see water vapor coming from the surface, you're losing a lot of heat.
 
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MUPPPP

Gold Supporter
Feb 7, 2019
73
Missouri
There's no vapor coming from the surface. It's relatively warm and humid out (69 degrees now) and water is 73.

Looks like we used a hair over 300 cubic feet of gas in 60 minutes. The heater is running continuously and the exhaust temperature is 318 degrees. The temperature has gone from 68 to 74 in 10 hours, and the water from the return jets is barely warmer than the standing pool water (recall when I said it used to be almost too hot to put your body in front of). Also ambient temperature was in the 40's when we opened the pool 3 weeks ago and got thw water temp from 48 to 85 in 24 hours.

I guess I'll keep it running all night and see if we've made any significant progress overnight. Low temp is 63 degrees here tonight.

Thanks again.
 
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JamesW

TFP Expert
Mar 2, 2011
18,492
In 10 hours, you would expect to gain about 11 degrees.

Heat loss is difficult to determine. You gained 6 degrees, which could indicate a loss of about 5 degrees, which is possible with an uncovered pool.

If you are using less than the expected amount of gas, you might have a gas pressure problem or something causing a weak flame.

Do you have a 400 hd model or a regular 400 model?

With the water at 73 degrees, the return temperature should be about 90 assuming a 17 degree temperature rise from inlet to outlet. That won't feel hot.

You can install a temperature sensor downstream from the heater to see what it is.

Keep an eye on the amount of gas used. If you're only using 300 cubic feet instead 400, you might have a gas pressure problem.

You can have the pressure checked to verify that it's correct and adjust it if necessary.

What is the gas volume rating of the gas meter? Maybe it's too small?

What size is the gas line from the heater to the meter?

How far is the heater from the meter?

Is it a single stage or two stage line?
 
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JamesW

TFP Expert
Mar 2, 2011
18,492
Also ambient temperature was in the 40's when we opened the pool 3 weeks ago and got thw water temp from 48 to 85 in 24 hours.
That's about 1.5 degrees per hour, which is more than we would expect for a 35,000 gallon pool, even with no loss.

Are you sure about those numbers?
 

RDspaguy

In The Industry
Mar 21, 2020
595
Cabool, Mo
I have had similar problems from a bypass valve, as well as partially clogged heat exchanger. I can't recall the brand, but the sensors were manifold mounted right beside the bypass valve and it thought it was doing great.
I don't know about all that stuff James was talking about, I am just an old tech and that sounded alot like science to me. If I were charging you $95 to pull in your driveway, I would start by pulling the bypass and checking the few tubes I can see for calcium buildup. If the tubes look good, get a valve. If the tubes look bad, get a heat exchanger or rebuild kit. Rebuild is a PITA, so new is worth it just for the warranty.
 

JamesW

TFP Expert
Mar 2, 2011
18,492
If they're only using 300 cubic feet of gas instead of 400, then they're only going to get about 8.25 degree per 10 hours instead of 11 degrees per 10 hours.

Since they gained 6 degrees, we can assume a 2.25 degree loss relatively confidently.

Using 3/4 of the expected gas points to a weak flame, which could be from an undersized meter or gas line.

If there is anything else using gas at the same time, that can make the problem worse.

Definitely check the meter size and the gas line size to make sure that they are good.

Check the gas pressure.

I don't think that it's a thermal regulator or internal bypass issue.

Make sure that there isn't an external bypass open.

The thermal regulator is easy enough to check if you want to do that.


I wouldn't start tearing apart the manifold as the issue doesn't suggest a bad bypass or a scaled exchanger. Those would cause a high limit error and an excessive exhaust temperature.

You can check the internal bypass by looking in the inlet for the spring part.

You can see the round disc part by looking in the hole when you remove the thermal regulator.
 
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