How a Pool Heater Operates


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Ever wonder what's under the cover of your heater? This guide explains the basic functions of a heater and gives you some idea of what might be wrong when your heater malfunctions. This guide is based on the design and operation of a smaller 100 BTU heater but the basic concepts are applicable to most heaters.

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Step by Step


Step 1

HEATER SPECIFICATIONS: This picture of the heater label shows the specifications for this heater. The heater is a Hayward model H1001D1 WITH a 100,000 BTU rating. Its output rating is 79,000 BTU which equates to a 79% thermal efficiency- on the low side for heaters but typical for this size heater. Most heaters will have efficiency ratings of between 79 and 83%. The gas orifice is selected at #44 which is standard for this heater’s designed altitude shown as 0-2000 ft. Heaters for higher altitudes would change out the gas orifices with a larger one. This heater is set up for using Natural Gas commonly designated as NG. The other gas option is propane commonly designated as LG.

Step 2

CONTROL PANEL – The only manual control on this heater is a control knob on the front panel that is connected to a thermostat to regulate heat. Larger heaters have multi-function LED displays to set and monitor heater operation and status.

Step 3

CONTROL MODULE – This internal circuit board collects and correlates signals from monitoring sensors and sends control signals to operational components like the ignitor. It monitors several temperatures and pressure sensors to automatically shut down the system if limits are exceeded.

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Step 4

GAS SIDE OVERVIEW - This picture shows the major components of the gas side of the heater. The gas enters through the gas valve and is burned in the burner box. The heat exchanger transfers the heat generated by the burners to pool water. The blower brings in forced air to the burners. Each of these components and other related components are described further below.

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Step 5

GAS INPUT PORT - The gas input port is located on the side of the gas valve. The gas line from a propane tank or house natural gas source is connected to the heater at this port.

Step 6

GAS VALVE – Regulates the flow of gas into the gas manifold and burners. This valve can be purchased with a transformer for 24V operation (as shown here) or without a transformer for 120V operation. The valve is adjusted at installation to correct for variations in incoming gas pressure and gas pipe size.

Step 7

GAS VALVE SWITCH - This switch allows the user to manually shut off the heater. It is access through the front panel.

Step 8

GAS MANIFOLD – Distributes the gas to the burners. Larger heaters will have larger manifolds to supply more burners.

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Step 9

ORIFICES - The fittings between the gas manifold and the burners. The size of the hole in the orifice controls the amount of gas input to the burners. These vary with altitude. Heaters operating above 2000' are fitted with orifices with larger holes.

Step 10

IGNITOR – This heater is set up with an electronic starter. A cable (removed) connects the ignitor to the power source on the control module. The control module sparks the ignitor when the heater is turned on. Some heaters use a continuously burning gas pilot system (called millivolt) to light the burners.

Step 11

BURNERS – This picture is taken looking down through the top of the heater with the heat exchanger and blower removed. The gas burners are the two cylinders seen at the bottom the insulated burner box. Larger heater can have as many as six burners. The gas valve in on the other side of the fire wall. This picture also shows the spark side of the ignitor discussed above.

Step 12

HEAT EXCHANGER. This is a picture of the heat exchanger flipped over. This unit sits above the burners to transfer the heat generated by the burners to the pool water. It consists of a continuous tube with fins. Pool water flows through the tube while the tube is heated. The fins connected around the tube help capture the heat to heat the tube and water.

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Step 13

BLOWER - Some heaters, notably Hayward, have forced draft systems which move air up through the combustion chamber at a precise flow rate. This eliminates all outside weather variables that can affect the heating performance, such as high wind conditions. Other manufacturers, like Raypak, eliminate these conditions thru their built-in wind resistant design.

Step 14

AIR PRESSURE SWITCH - An air switch is required with the use of blowers to monitor air pressure. If the air pressure becomes too high, the system will shut off.

Step 15

WATER SIDE OVERVIEW - This is the water side of the heater opposite the gas side covered above. It shows the other side of the fan, the heat exchanger and the burner box, and also shows how the pool water is integrated into the heater. Pool water flows into the heater from the left port, goes through a larger pipe called the header and exits to the right port back to the pool.

Step 16

HEADER - The header connects to three smaller pipes that go into the heat exchanger, loop around at the other side and return back to the bottom of the header. This header is actually a split pipe with baffles inside to direct the water out through the top three pipes (shown) and to direct the returned water out though the right port. The three return pipes are not visible. They are directly below the top three pipes shown.

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Step 17

HEAT EXCHANGER - This is a bottom view of the heat exchanger showing how the water tube loops around on the gas side of the heater. It also show the fins inside the heat exchanger that wrap around the pipes to help capture the heat.

Step 18

WATER PRESSURE SWITCH - This switch monitors water flow and pressure inside the header. If the switch senses that the water flow and pressure are either too low or too high, it shuts off the system.

Step 19

HIGH LIMIT SWITCH - These switches monitor water temperature in the header. If the temperature exceeds a set limit, either switch will shut off the heater.

Step 20

THERMOSTAT - The thermostat regulates the water temperature of the heater. It is wired to the knob on the control panel discussed at the beginning of this guide.

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Step 21

WATER OUTPUT PORT - The heater is connected to the circulation piping at this port to take heated water back to the pool. The black rubber flange around the pipe seals the hole in the heater housing through which the pipe exits. There is a similar flange on the other side of the header to create a seal around the input port.


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 Posted: 8/20/2020 

Water is leaking where the sensor wire from the thermostat enters the Header. What will it take to repair this leak?

InyoPools Product Specialist  Posted: 8/20/2020 

First, what is the model number of your heater? Different designs leads to different solutions.

 Posted: 8/20/2020 

The heater is a Hayward H100ID. It is leaking where the thermostat wire enters the header.

InyoPools Product Specialist  Posted: 8/21/2020 

Hayward H-Series Above Ground Heater - H100ID1 & H100IDP1 Parts Diagram, is the leak due to a deformity of the header and the sensor port or is it due to the flow sensor? Depending on what is causing the leak would determine if you need the header or if sensor needs to be replaced or modified.

 Posted: 2/2/2020 

Our equipment pad is near our patio. We are in need for a new heat pump. What is the quietest pump on the market. We will need a 140,000BTU How can we make it a bit quieter in that area? Thanks

InyoPools Product Specialist  Posted: 2/10/2020 

Currently, the quietest heat pump is the AquaCal Heatwave SuperQuiet 143,000 BTU - SQ225AHDSBNP

InyoPools Product Specialist  Posted: 5/22/2017 

allwet - Unfortunately, we do not have that diagnostic information. Call your heater's manufacturer for further assistance. Hayward is 866-772-2100. Raypak is 805-278-5300.

 Posted: 5/20/2017 

Can a water pressure valve be checked?? how

InyoPools Product Specialist  Posted: 7/31/2016 

Raypak thermostat - I have not removed this thermostat. Please contact Raypak at 805-278-5300 for information.

Anonymous  Posted: 7/28/2016 

I have RayPak electric heater mod# ELS1100-Z 11 kw 240 v and I would like to find out how to remove the thermostat sensor from the heating unit. I'd hate to create a leaking problem.
Thanks Larry


InyoPools Product Specialist  Posted: 7/19/2016 

Sherry - There are several control or monitoring devices to control the safe operation of a heater including: the heat controller itself, the thermostat, the air pressure switch, the water pressure switch, and the high limit switch. Any or all of these devices could be partially faulty. You would have to have a heater professional analyse your system to isolate your problem.

 Posted: 7/17/2016 

I have a hayward pool heater. It always comes on but sometimes the heat will not stay on. Is there a certain temperature that it shuts off? If so, can it be adjusted higher and if so, how?

InyoPools Product Specialist  Posted: 6/21/2016 

Antonia - Blower (idxl2bwr1930) is used on Low Nox H-Series heaters manufactured after 9-20-04. For further information, you will have to contract Hayward tech support at 866-772-2100.

 Posted: 6/19/2016 

Recently my heater blower was making noise and not turn on. Thinking to replace the blower IDXL2BWR1930 appears a new part with 1931 is required. Are the other part I should expect to replace?

InyoPools Product Specialist  Posted: 5/16/2016 

Drew - I would suggest calling Pentair directly at 800-831-7133. Will should have the experience to isolate your problem.

 Posted: 5/12/2016 


I have a Pentair Minimax 100. I replaced the thermopile last year. This fixed the heater, but then it began short-cycling. That burned out the thermopile.

From my research, to fix the short-cycling (I obviously bought another thermopile), I have seen either the hi-limit switch, or the flow valve. Any experience? Obviously I expect nothing here, this is no way to diagnose a problem.



InyoPools Product Specialist  Posted: 8/31/2014 

todd - I do not know the answer to this problem. Maintaining a gas heater can be involved. I would suggest calling a Jandy rep on this. Their number is 800-822-7933.

 Posted: 8/31/2014 

I have a new Jandy heater that worked for 3 months and quit. I noticed soot inside and on the bottom. I replaced the pilot generator and the fusible link. The heater lights and turns on but there is black smoke coming out of the top. It seems as though a valve in the heater is closed and not allowing water to pass through the exchanger. Any advice or help is appreciated.

InyoPools Product Specialist  Posted: 6/9/2014 

terry - Getting the right amount of gas to your heater can be tricky. I would call your heater manufacturer and ask him for specific instructions or to get the name of a heater rep in your area.

 Posted: 6/8/2014 

I had a heater installed but the heater won't fire up properly. I can hear the ignition start and the gas valve open but no flame. Gas supply line is about 60ft from the meter and is 1/2" piping and goes into heater which is 3/4" piping. Could the problem be not enough gas supplying the heater to get it started?

InyoPools Product Specialist  Posted: 8/23/2013 

luis - We have not written a guide on replacing a pool heater yet. You might go on the internet and look under " replacing a pool heater" for instructions.

 Posted: 8/22/2013 

how do i remove the old pool heater? do you have any instructions i can see?