Swimming Pool Blog

Pool Heater Repair: My Pool Heater won't Start!

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by Myles McMorrow, April 1, 2009

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Swimming pool heaters are great, they make the water warm and extend the pool season. They also can be the most ornery piece of equipment around a pool. What do you do when the heater will not start up leaving you with a cold swimming pool? I will cover the most common reasons why a pool heater will not start up in this article.

  The most common issue with a swimming pool heater when it will not start up is also the simplest one, a dirty swimming pool filter. A dirty pool filter or pump basket will restrict the water flow, so always ensure that everything is clean and all valves are open.
multimeterThere are several types of pool heaters; for this article, we are focused on gas and propane pool heaters. For more advanced trouble shooting on a pool heater you will need a power meter or multi-meter to check the current as it flows through the various heater components.  "The problem lies where the power dies..." All swimming pool heaters have what we refer to as the control loop. This is a simple circuit (think school science fair, with the battery, switch, and light) that goes through a number of safety controls. Only if all the safety switches are enabled will the heater turn on, or "fire" and begin to heat the water. As stated the most common issue is the pressure switch, but let’s start from the beginning.


All heater control circuits run on electricity. Your heater will have one of two power sources, live voltage from a breaker box, or millivolts that your heater makes on its own. If you have a standing, always on pilot light that is manually lit, you have a millivolt unit. The heat from the flame heats two dissimilar metals in a bulb next to the pilot that produces the milli-voltage (less than 1 volt) to run the gas valve and control circuit.

Millivolt pool heaters have lost ground to electronic pool heaters. These heaters are supplied with power that runs through a transformer as it enters the heater. The reduced voltage, usually 24 volts, is then sent into a control module, aka IID or Fenwal. This gray box disperses power to the control circuit. Once you have established you are getting power through the transformer (24 volts) or from the Pilot Generator,  aka thermopile, (400-600 mv), then we "follow" the power with our multi-meter. We are looking for a Voltage Drop at any one of the components in the heater.  

"The Problem Lies Where the Power Dies"

honeywell s8600FYou will need to follow the power to each safety switch. The first one is the pressure switch. This “switch” makes sure water is flowing through the unit before it will let the pool heater turn on. This means you must have the pump on and a clean filter for this switch to close and allow voltage to pass through. With a multi-meter, you should get voltage reading through the pressure switch if it’s working right.

A word on bypass valves. Most pool heaters have an internal bypass valve that is located in the front header of the heater (where the pipes go in and out). This spring loaded device allows excess water pumped into the heater to return, immediately, right out of the heater, without being heated. This is necessary to keep the optimum flow of water through the heat exchanger. Too much flow can cause condensation, erosion, and poor performance. If you have a high flow pool pump, you may want to consider installing an external bypass. This is a plumbing manifold that diverts a percentage of water "around" the heater, bypassing it back to the pool.

Pressure switches and other heater components can be tested by "jumping out" the switch. By using a jumper wire, or sometimes a screwdriver to make good contact with both terminals on the pressure switch, on/off switch, hi-limits, this will bypass the switch. With the on/off switch is on, thermostat up, water flowing, etc., hold a jumper wire on both terminals for 2 or 3 seconds (be careful of flame roll-out). If the heater fires, this may indicate a bad switch, but first make sure that the switch is not just doing it's job of telling you that the water flow is insufficient.


hi limit switch The next stop for the power is to the high limit switches. Most heaters have two high limits and they will are usually located side-by-side on the outlet manifold. These are designed to turn the heater off if it gets too hot. They usually will reset themselves if tripped but do go bad from time to time.  If your high limits go bad, there could be an issue with the bypass valve in the in/out header causing the unit to overheat.

 From the high limits the power then goes to the thermostat. This is what controls the heater and lets the unit know what temperature to heat up to. When testing your heater, turn the thermostat all the way up. You should find voltage going in and out of this if the probe that goes to the in/out header is working properly.

fusible linkThe next safety switch inline is the fusible link. These sense heat near the gas valve. If the flame from the burners comes into the front of the heater (flame roll out),  a small wax pellet inside the link melts, breaking the circuit. The fusible link cannot be reset and if yours has failed, the heater may have a blockage of the burners or improper venting / exhaust from the top of the heater. Be sure to check all possible causes for this, as it could indicate a hazardous situation.

Next the electric goes to the gas valve and, depending on if you have a 24 volt electronic heater or a millivolt heater, the power then goes to an electric ignition box or IID that will light the pilot. 

Checking the safety circuit is a good place to start, you should get power into and out of every safety switch in order for the heater to come on. On the newer electronic heaters, many have a circuit board that will give you an error code telling you what switch is not working keep the heater from coming on. Check in your owners manual for a list of error codes.

Be sure to be careful working around gas pool heaters as they can be deadly dangerous! Do not bypass safety circuits to operate the heater, or you could be creating a very costly future repair. For more info on heaters and how they work, please check out our pool heater info pages.