by Rob Cox September 25, 2013
In our second in a series of pool parts pictorials, this time we take a high-res look at parts for gas pool heaters.
Pool heaters have no shortage of parts, in the control system, the burner tray and the heat exchanger - or the electrical, heat and water systems.
Each of these 3 systems in a gas pool heater can comprise dozens of parts. Today we bring you close-up images of the most commonly replaced pool heater parts within these 3 areas, to help you identify and inspect more easily.
Thermostats: Pool heater thermostats on Millivolt heaters are mechanical, with a capillary bulb that slides into a well. Electric thermostats are circuit board controlled, with potentiometers used for the dial. Thermostats can be single or dual, with two dials, one for pool and one for spa. If one of the two thermostats fails, you can operate the heater with the other.
Control Boards: The central nervous system of an electronic pool heater, control boards handle all aspects of a pool heater operation, troubleshooting and safety controls. The most expensive pool heater electrical part, control boards are not serviceable, unless you happen to be a skilled electro-genius, familiar with troubleshooting and fixing PCB's, or printed control boards.
Ignition Control Module: The module, sometimes called a Fenwal (manuf. name), or IID (intermittent ignition device) controls the spark ignition of an electronic pilot. It also monitors and controls temperature and pressure of the water, and air flow. The module is the brain of your pool heater, handling all executive functions, allowing or preventing the heater to operate. When something is amiss, it generates error codes displayed on screen.
Transformer: The transformer regulates the voltage coming into an electronic pool heater. Gas heaters can be wired with 110 or 220 volts, and this voltage is reduced at the transformer to 24-28 volts. Transformers are fairly simple devices, which operate on electromagnetic induction. Power into one side, is allowed to roll over to the other side, at a specific rate. Power surges or input voltage fluctuations can overload a transformer, and affect output voltage.
Fuse: A fuse is a very simple device, designed to monitor for voltage fluctuations in the safety circuitry of a pool heater. If your heater has no lights or life at all, this is a good area to start. A blown fuse however, is usually indicative of a larger problem. Check the transformer voltage. A fuse can be tested for continuity, if you're unable to visually see if the fuse is your pool heater problem.
Thermister: A thermister is a temperature sensor, and is used on electronic pool heaters that don't use a capillary bulb type of thermostat. It allows the module to monitor and control the heat inside of the cabinet, and thereby the amount of heat transferred through the heat exchanger. Thermister problems include incorrect temperature display, or operation inconsistent with thermostat settings. Check for wire, sensor and connector integrity and continuity.
Pilot Assembly: The pilot assembly for a pool heater delivers gas to a waiting spark, which once lit, lights the burners. Pilot assemblies are known for failure from rust, small insects or age. Millivolt pilots have a 'standing pilot', one that stays lit all the time, and electronic pilots ignite only when the pilot flame is needed, to light the burners.
Pilot Generator: Also called a thermopile, a pilot generator sits next to the pilot - only on millivolt heaters. The purpose of the generator is to produce enough millivolts to operate the pool heater's safety circuitry. When the two dissimilar metals inside a generator are heated up from the pilot, they produce 700 mv +/-. Low mv output will keep your heater off. Generators can be tested with a test meter for millivolt output.
Pilot Tubing: The pathway for gas from the gas valve to the pilot assembly. Typically made of aluminum, pilot tubing is soft enough to crimp during installation, or fall apart after years of moisture. Both ends of the pilot tube are secured with the use of ferrule nuts, which seal the soft metal tube into the bottom of the pilot, and the top of the gas valve. If gas is not reaching the pilot, the tube can be disconnected from the valve or pilot to check gas flow.
Gas Valve: The main gas valve of a pool heater delivers gas to the pilot and the burner tray, when signaled by the controller or thermostat. Gas valves (and pilots and burners) have different sized orifices for propane or natural gas. Connected to the gas valve are wires to control the solenoids inside the gas valve, coming from the module on an electronic heater, or the control circuit on a millivolt pool heater.
Fusible Link: Sensing flame roll-out is the job of a fusible link, used on Laars heaters, or the 'Thermal Cut Off' used on Pentair pool heaters. Flame roll-out is when flames of the burner tray come out the front of the heater, either from gas build up before ignition, or from restricted exhaust through the heat exchanger or through the vent stack. When a fusible link heats up to sufficient temperature, a wax pellet melts inside, which opens the circuit.
Igniter: An igniter is used on electronic pool heaters that have done away with the traditional pilot assembly. I call them 'glow plugs', because they are heated up until they glow, hot enough to ignite the burner tray. Another use or definition of a pool heater igniter is the sparking device located next to an electronic pilot, which sparks on demand to ignite the burner tray.
Front or Rear Headers: The headers are the end caps or manifolds of the heat exchanger. The front header has the flange connections for the pipes bringing water in and out of the heat exchanger. The rear header directs the water that has traversed the heat exchanger, in the opposite direction, back towards the front header, where the water exits the heater. Thermoplastic or brass headers are a bit more durable than cast iron headers, but all can break easily if full of standing water during freezing temperatures.
Heat Exchanger: Copper finned tubes that absorb the heat from the firing burner tray below. Cupro-Nickel heat exchangers are recommended for salt pools, or for pool owners who keep destroying their heat exchangers with low pH conditions. Heat exchanger pitting and corrosion can occur with poor water chemistry or a chlorinator located upstream of the heater. Very hard water conditions can cause build-up on the inside of the tubes, like plaque lining an artery. Bad combustion and overheating can cause the tubes to sag.
Automatic Bypass: Inside of your front heater header, where the water enters the manifold, it is pushed through half of the tubes to the other side of the heat exchanger. When it reaches the rear header manifold, it's turned around and sent back across the flames in the other half of the tubes, where it exits the front heater on the return pipe. To maintain proper water flow, excess water pressure is diverted in the front header, by means of an internal bypass device. These devices are interesting contraptions, and fall ill with bad water chemistry, or years of being battered by water.
Drain Plugs: Not a glamorous pool heater part, but the lowly drain plug serves an important purpose. When temperatures dip below freezing, and the pump is not pushing water through the heater, the heater headers will burst from ice expansion. Plugs for cast iron headers are brass, and can be lubed with the 'green' lube. Thermoplastic headers will have a plastic plug. Both are 1/4" NPT (Nat'l Pipe Thread), same as pump plugs, which can be used in plastic headers, but not in cast iron or brass headers. Some heater drain plugs have o-rings, an important part, of a part.
Pressure Switch: The pressure switch senses water pressure or water flow. Most heater pressure switches are set at 2 lbs - although systems at high altitudes may need to operate at 1 lb, and systems with flooded suction (heater below pool water surface level) will need a higher setting of 3 lbs or more. The pressure switch can be easily tested by jumping out both terminals with a jumper wire. If the heater fires, find out what is restricting the water flow - dirty filter, pump basket, impeller, or closed valves. Some pressure switches are adjustable from 1-5 lbs, or psi, and they do fail at some point, with some regularity. If you winterize, check to be sure that your heater type does not have a siphon loop that needs to be drained.
Hi limit switches sense when the temperature of the water is becoming too hot, which may scald someone, or cause pipes or other heater parts to melt, and they shut down the heater. Hi Limits is plural, because there are typically two of them, which could be called the low high limit, and the high high limit. A bit redundant safety, they serve to make sure the heater shuts off when temps of 150 degrees or more are sensed. Hi Limits usually are mounted into the header, and are essentially the same as a thermister. They can be tested for continuity, or jumped out for testing purposes on older heaters.
I hope you liked this pictorial gallery of pool heater parts. Kind of boring stuff, you know - I had to jazz it up somehow! Hopefully you are now better equpped at maintaining your heater when you know a little bit more about what these heater parts are, and what function they perform, and how many pool heater parts relate to each other in the 3 systems: Electrical, Combustion and Heat Exhange.