Pentair Mastertemp 400 Common Problems: How To Solve Them?

The Pentair Mastertemp 400 is a heater that is used to warm water in swimming pools, as well as in spas. Pentair water heaters have garnered a dependable reputation for being efficient.

Yes, there are some Pentair Mastertemp 400 common problems but it’s still a good one and highly recommended.

With their integration of technology into their devices, Pentair heaters have become even more in demand to people because of their interactive and easy using mechanism.

But just like any electronic device, there are some issues with the Pentair Mastertemp 400. So, here we will look at some issues and their solutions. 

Common Problems With Pentair Mastertemp 400

Pentair Mastertemp 400 common problems

Before I proceed any further, let’s have a look at those issues:

  • Heat Exchanger Corrosion
  • Sweating
  • Scaling Waters
  • Boiling in Heat Exchanger
  • Low Circulation of Water

Now, let’s talk about those problems in detail.

Heat Exchanger Corrosion

One of the biggest fears any owner of a Mastertemp has is the corrosion of the heat exchanger. This erosion occurs due to a variety of reasons:

  1. Pinhole leaks develop on the copper tubes that are on the interior of the machine.
  2. Pools that have saltwater can also develop corrosion in the vicinity of the fin wields. This could lead to leakage.
  3. Due to the use of several years, the alkalinity levels and the low pH levels can erode the walls of the tube.

How To Fix The Corrosion Problem?

To solve the problem of saltwater pools, you can use heat exchangers made out of cupro-nickel. Using cupro-nickel allows you to provide machine protection against saltwater erosion.

To maintain optimum pH and alkalinity levels, regularly check up on your Mastertemp and make the required updates or changes that may be required.

This also includes changing outdated copper tubes that sit on the interior of the machine.


Sometimes the heat exchangers leak. People often confuse or term this leakage as sweating. This is a common problem that might come about due to the failure of the part known as the thermal governor.

Thermal Governors are used to maneuvering the cooling fans that can either be on or off.

How To Fix The Sweating Issue?

Thermal Governors stop working primarily due to years of usage. They usually cannot be fixed. Even if they are fixed somehow, they will stop working after a while.

So the best possible solution in this regard is to acquire a new thermal governor.

Scaling Waters

Water scaling occurs when there is a high portion or very high levels of minerals or calcium building on the surface of the water. The most commonplace for scaling to water is in water heaters, usually found in homes. 

Not only for Pentair, but it also happens to other brands too. I have seen people are struggling with the DE pool filters.

When the amount of scaling is very low, it might often be beneficial. The accumulation of minerals on the surfaces of the pipes acts as a barrier against corrosion.

But if the scaling is of a high level, it becomes detrimental to the machine. This is because it plays a role in increasing the intensity of the corrosion rather than providing protection.

Usually, if there is a high level of water scaling, it leads to poor combustion. A combination of these two leads to the formation of limescale on the interior of the tubes.

There are certain effects of scaling, such as:

  • Reduction of efficiency of the heater.
  • Valves become leaky.
  • The flow of appliances becomes blocked.

How To Fix The Scaling Water Issue?

Change the heat exchanger and get a new one. This is the best possible solution to this problem.

Boiling in Heat Exchanger

Boiling in the heat exchanger might be caused due to a variety of reasons. Some of these problems and their solutions are mentioned below:

Reason 1: The flow of water to the heater has reduced.

Solution: Servicing on the pump or on the filter will fix this issue.

Reason 2: Plugging of the exchanger.

Solution: Alter the chemistry of the water and reach an equilibrium. You could also take the heat exchanger for servicing.

Reason 3: Bypass Valve has remained opened.

Solution: Take the bypass valve out for servicing.

Reason 4: Thermal Governor has got stuck and is not opening.

Solution: Replace the thermal governor with a new one.

Pool/Spa Water is Overheating or Underheating

The correct temperature of water depends on the user. Something mild to someone might be too hot or too cold for another.

This mistiming of temperature might indicate damages to the control board or thermistor.

How TO Fix Spa/Pool Water Over/Underheating?

Acquire a 10k ohm resistor. Use this resistor to test the thermistor. If the thermistor tests are bad or faulty, replace them with a new one. If the thermistor is in good condition, then replace the circuit board.

Low Circulation of Water

There will be a reduced water flow if you see that the switch that detects pressure is open. This happens due to a plethora of causes, such as: 

  • The external bypass is left open
  • Absence of sufficient amount of water
  • If the machine that circulates is not turned on
  • Damage to the switch which controls the water pressure

How To Fix Low Water Circulation Problem?

First, ensure that the machine that pumps water is turned. Check for stuck objects, such as worms, dirt, lint traps, etc.

Make sure that the filter of the heater is cleaned as well. If you deem it necessary, then clean the entire filter system. It is also advised to change the entire system if required. Close the external bypass if open.

Test the resistance of the pressure switch. If damaged, broken, or open, then replace it. Do this test with the water still running. Otherwise, it will be difficult to verify. 

There might also be damages to the suction air leaks. Check and rid the pump impeller of any dirt stuck inside.

Low Voltage Supply to the Control Board

Control Board of Pentair Mastertemp 400

The Mastertemp 400 requires a voltage of 24 VAC. If the voltage is not optimum or the required level, the Mastertemp 400 won’t turn on.

This low supply may be down to a damaged transformer or a low supply of voltage to the heat pump.


Enquire about how much voltage is being supplied to the heat pump. The optimum level of the voltage supply is between 208 to 260 volts. 

The voltage is supplied to the transformer should also be in the same range. The output of the transformer should be between 24 to 28 volts.

Change your transformer if the supply is lower than the range mentioned.

Voltage checks have the potential for a lot of danger. Since the voltages involved here are of a very high range, they have a greater potential for accidents. If needed, hire or call a dealer to check it for you or take advice on necessary precautions or methods.

The Unit is Left with Icy Residue on the Exterior of the Air Coil

This residue is usually formed at the bottom. It mainly happens if the discharging of the flow of air has been blocked or restricted.


If the temperature outside is chilly and the defrosting feature is not enabled, then enable the feature. Make sure that the air coil is being sprinkled upon during cooling.


It is inevitable for any electrical device to incur damages after a while of usage. This should not be a cause for concern, though. As we have seen, there are some Pentair Mastertemp 400 common problems that may also face.

Following this article, you can get solutions to some of these issues. Make sure to maintain the device properly.

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3 thoughts on “Pentair Mastertemp 400 Common Problems: How To Solve Them?”

  1. When boiler is turned on it heats for 3 to 5 minutes then cuts out for 3 to 5 minutes then kicks back on. It does this over and over .

  2. Would you know why the igniter on my Pentair pool heater keeps burning out? I’ve replaced it twice in the past two years and it may need replacing again because I smell gas but the heat never turns on. Would this be caused by too high of a gas/air mixture?

  3. We have a Pentair 460734 300 Master Temp SN/1117205200351Z.
    I get a small explosion in the exhaust pipe after the unit lights. The problem isn’t constant and only happens from time to time. We have checked gas pressure on both sides of the gas valve. Everything seems to be in spec. I’ve has 3 companies look at this as well as the gas company. I do have a number of pics that show the install and application in detail. No one has been able to explain or fix this issue. I’m sold my HVAC company in 2020 after 50 years. Point is I have some experience but certainly don’t know everything, far from it! My name is Larry Ferris 614.989.1096. Could you have some one give me a call?
    Kindest Regards, Larry

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