This is my full Heating/AC stack. As you can see, there isn’t much room left to move to a double condensing system, so I’m stuck with an 80% unit.

This is a downdraft system.

At the very top is the return air duct coming down through the ceiling.

The air cleaner is a Honeywell. As there was no vertical space, the contractor elected not to use a disposable filter at its inlet. This means I must follow a rigorous schedule of washing the Honeywell’s elements every month. The risk is a plugged air conditioner evaporator and eventually enough dust on the furnace’s heat exchanger to produce a fire hazard. I don’t recommend doing it this way. Fight to keep that disposable filter.

Below the air cleaner is a short piece of ductwork joining it to the furnace. Its purpose is to provide service clearance to the air cleaner without interference from the flue.

Finally, the actual furnace. The flue pipe moves to the left before heading through the ceiling to the roof.

There’s a water heater to the left sharing the flue pipe. The flues are joined by a ‘Y’.

The tan box below the furnace is a York evaporator for the air conditioner.

The very bottom is the entrance to the whole house ventilation ductwork.

This is a closer look at the furnace itself, covers removed.

The unit itself is an Amana

The flue pipe is already double walled, but an additional layer of insulation is used when it passes through the blower compartment.

The wire harnesses pass between compartments by two channels:

• A plug in the bulkhead
• A grommeted hole about 1” in diameter.

A look at the blower compartment where the controller lives.

The controller in its housing can be swung to the side by removing a few screws.

Two more screws and the blower slides out on guides for service and oiling.

• Blower
• Controller

The controller is a White/Rodgers 50A50-206

It is a hot surface ignition system.

This is not the original controller. The contractor swapped them out to get the necessary connections for the air cleaner.

A look at the lower compartment where we can see:

• Upper Over-temp sensor
• Burners
• Gas valve
• Induction sensor
• Lower Over-temp sensor
• Flue
• Induction blower

Here is the gas valve. I did not obtain manufacturer or model.

The base of the igniter can be seen.

The induction blower is made by FASCO. I do not have the model number with me.

You can see the induction sensor port at the top of the insulated box. The red hose goes from there to the pressure switch.

The induction sensor (pressure switch) is manufactured by Tridelta who sold their sensor business to Honeywell.

The label has the following 3 numbers:

FS6111-366

01073/1892

C645 6504

The electrical connection consists of a SPDT micro-switch. The closest terminal is common and gets the orange wire. The center terminal is the NC terminal which is not used. The one closest to the diaphragm is the NO terminal which gets the yellow wire.

When a vacuum is sensed, the diaphragm is pushed by air pressure to close the switch.

There are no markings indicating the vacuum it is intended to trip at.

The schematic label from the blower compartment cover.

There is water damage, probably due to condensation, and is hard to read in the first place. The picture quality could help too.

In the lower compartment, lower left side, this specification label is affixed. Although you can’t read all of it here, the pertinent line says “For operation at an external static pressure of -0.5  inch W.C.” It is unclear if this refers to the induction system or the whole house blower. Interestingly enough, it is about 2/3 of the pressure read at the induction sensing port.

This is my water column pressure measuring device made from instructions Scott Meenen provided me.

To build a WC measuring device like mine:

Obtain the following materials:

• A clear plastic cup
• A clear flexible tube that will fit the induction sensing port and reach from there to a location below the port where the cup will be placed (Mine was a 6 foot length of 3/16” medical hose used to supply oxygen to a patient)
• A steel rule. Mine was 16” long. Width is unimportant as long as you can read the scale with the tube attached. The ruler should have a hole at one end which the tube will fit through. Mine came with a 3/8” hole, but you can drill or punch one anywhere near an end.
• 3 zip-ties

Assemble the parts and attach them to the furnace as follows:

1. Insert one end of the tube into the hole at the end of the ruler. This will force the tube to remain near the bottom of the cup without touching. The end of this tube must remain submerged, yet never touch the bottom or side of the cup where it could be plugged. Induction pressure will need to push water out the end, or induction vacuum will permit ambient air pressure to push water into the end.
2. Fasten the tube to the ruler using two zip-ties. Use one at  each end of the ruler such that the tube remains straight and parallel to the ruler’s scale.
3. Connect the free end of the tubing to the induction sensing port. This must have a good fit that doesn’t leak. If your tube is too large, leakage will prevent getting a meaningful reading.
4. Place the cup in a convenient place where it is unlikely to be spilled. This position really should be below the induction sensing port. It can be on the floor, on a shelf or chair, held by an assistant, or inside the furnace as I did. If you can’t get the cup low enough, the hose may be routed up to a convenient attachment point and then back down to the port. A vacuum system will pull water up, so make sure the highest part of the tubing is a greater height above the cup than the inch/WC you are expecting. Example: You expect -10 inches WC. Make sure the top of the tubing is more than 10 inches above the top of the cup to avoid pulling water into the induction system.
5. Place the ruler in the cup, with the open end of tubing at the bottom. It is important that the ruler and its attached tube remain vertical. Hold the top of the ruler with any convenient object while avoiding any bare terminals (remember, steel conducts electricity and care must be taken to avoid a shock hazard). This may be a clamp, your assistant, or part of the wire-harness as I used.
6. Coil the excess tubing and use the third zip-tie to keep it out of the way.

Fill the cup with water to a convenient mark on the ruler that also guarantees the end of the tube will remain submerged. If you have translucent tubing, or can’t see the water in the tube very well, some food coloring could be added to increase the contrast. Most food coloring is water based, which shouldn’t affect the reading.

After doing whatever is necessary to engage the induction blower, a pressure system will push the water down, while a vacuum system will permit ambient air pressure to push the water up.

In my case, the water in the tubing rose, indicating a vacuum. If you have a pressure system, and it pushes the water completely from the tube (you will see bubbles), you will not be able to take a measurement until more water is added to the cup. If you have a high vacuum system, water may be pulled all the way to the sensing port. To prevent this, you may need to use a longer ruler and set the cup lower or raise the highest point in the tubing.

Although this isn’t a very good picture, you can see the water level in the cup as measured by the ruler.

If the water surface is no longer at the convenient mark you chose, you can adjust it by adding or removing water to or from the cup.

Just above the rim of the cup, you can see how far the water in the tube rose, where it can be measured on the ruler. The difference between these two readings is the WC value produced by the induction system.

In my case, I filled the cup to three inches. As the water rose in the tube, it fell slightly in the cup. I added just a little water to bring the surface back to 3 inches. The water in the tube went to the 3&13/16 inch mark. Subtracting the 3” water surface height from the 3*13/16 water in the tube height revealed my induction system was producing 13/16”WC vacuum.

If you have something near the expected WC value, it should be safe to permit a flame to ignite.

This picture shows the flame of my system while running normally. In this case, as my induction sensor is bad, I had to bypass it to enable gas flow. The chief symptom to look for is flame rollout which my system did not exhibit.

Flame rollout is caused by insufficient induction or draft flow. The flames stay near the burner, and sometimes “roll out” into the lower compartment, a very dangerous condition. In some cases, the induction or draft is so low that the gas will not ignite, or if it does, the flame will be partially or completely yellow in color and may produce soot due to the lack of oxygen.

A low WC value can be caused by a plugged induction sensor port, a bad induction blower, a restriction or blockage in the flue pipe, or a leak in the heat exchanger.

While a mechanically adept homeowner may be able to fix the first three of these symptoms, do not attempt to continue repairs if you suspect a leaking heat exchanger. The risk of CO entering the home is too great, and eliminating this risk is one of the primary reasons for monitoring induction. Leave that one for the professionals.