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The Weil McLain HE II Gas Fired Boiler, How it works, what all the controls do, and in what order.

This page is not intended to instruct the non professional on how to troubleshoot and repair this boiler. All boilers gas or oil fired, can be very dangerous and should not be worked on by any one who is not qualified. Get a qualified service person to work on this boiler for you.

Weil McLain HE 2 Boiler

Click on any image to see a larger image. With the Jacket cover removed this is what you will see, your boiler may be wider and have more burner tubes but the controls work the same.

Weil McLain HE II

This is the plate that you should find on the side of your boiler. this will help the heating tech troubleshoot this boiler by counting the blinks on the control.

Weil Mclain HE II furnace

This is a side view you may notice the circulator pump does not look like the one on your boiler, as with all older boilers, parts and components get changed over the years, and are not replaced with the exact same parts. You will also see the pressure and temperature gauge. you may or may not have a service switch on the side of your boiler.

Weil McLain control board

This is the control module, yours may look different, This is the heart of the boiler, it controls everything. Most people and many heating techs make the mistake of assuming the control is defective first, sometimes it is, many times it is not. DO NOT MESS WITH THIS CONTROL DO NOT MOVE WIRES AROUND IN AN ATTEMPT TO FIX YOUR BOILER YOUR SELF, doing so can bypass important safety components, causing explosions, fire, and or carbon monoxide poisoning.

Weil McLain combustion motor

The black thing top left of this image is the combustion motor. The round silver disk is the pressure switch. The combustion motor blows toe boiler fumes out of the vent pipe to the outside. The pressure switch detects if the combustion motor is doing it's job. If the combustion motor fails, the pressure switch will not send the OK signal to the control module, and the boiler will not continue. 

Weil McLain HE 2 gas valve

This is the gas valve this gas valve does not have a pilot light, it's called an electronic gas valve, it receives power from the control module to turn it on and off,
Like the control module many people assume the gas valve is defective, when in fact it is not. The only thing a non professional should touch on this gas valve is the on / off knob.

HE II Transformer

The part on the left is the main transformer, this transformer provides 24 volt power to the module, if this fails nothing will work. But again don't assume.

Zone valve transformer

The gold colored rectangle transformer  with the black front and wires connected, was added to this boiler and was not part of the original boiler, it supplies 24 volt power to the zone valves but would not be able to operate the control module.

Furnace roll out switch

Remember you can click on any image to see a larger image. in the above image the blue arrow is pointing to a role out switch, if the boiler is plugged or if for any reason the flame can not go up , this switch will turn the boiler off, this part is not reset able, once it shuts the boiler down it will need to be replaced, Note: there are many different temperature settings for spill switches, they all look the same, make sure your tech replaces it with the correct one for your boiler.

Weil McLain HE II HSI

The red arrow is pointing to the hot surface igniter HSI yours may have been replaced with one that does not look like this one. The HSI gets cherry red and ignites the gas when the gas valve opens. This also sends a signal back to the control module indicating there is a flame, in other applications a separate flame sensor probe is used to verify there is a flame.

Weil McLain HE II aquastat

The Circled Item is your aquastat this controls the maximum boiler temperature, If you ad all of the above components together with a thermostat and or zone valves. you have a complete system.

How does it work? In most applications the boiler or furnace (depending on what you call it) is normally off and will cool to room temperature if there is no demand for heat, In some cases, when an indirect water heater is installed, or a garage unit heater is installed on the system, the boiler will maintain water temperature. In either case the boiler or furnace runs in the following order.

1 The thermostat calls for heat, this completes a circuit on the main control board
2 The control board power to the combustion motor, this starts the spinning of the motor.
3 Once the combustion motor reaches it's designed RPM Rounds Per Minute. The air flow sends a signal to the pressure switch.
4 Once the pressure switch senses a positive draft of air going up the vent, it sends a signal back to the control board.
5 Once the control board or module receives the signal from the pressure switch indicating there is a positive draft, the control module sends power to the HSI (hot surface igniter) for about 45 seconds.
6 After the 45 or so seconds the control module sends power to the gas valve, causing gas to go to the burners.
7 Once the burners are lit the HSI or separate flame sensor sends a signal back to the control module,
8 Once there is a positive flame signal the boiler will continue to run until the aquastat reaches the set temperature about 180 degrees Fahrenheit.
9 The circulator or pump starts to run as soon as the thermostat calls for heat.
When the aquastat reaches temperature and turns the boiler off, the pump or circulator continues to run.
when the thermostat stops calling for heat, the pump or circulator stop and the boiler shuts down, the combustion motor continues to run for a short time after to clear all the fumes.

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