New Furnaces Need Love Too
January 29, 2007
In part one (Jan. 15), I told you that there is still a lot of mechanical satisfaction to be had from maintaining and fine-tuning today’s gas appliances. As an example, I described a service contract call for one particular customer, a retired English teacher named Mrs. Johnson.
Mrs. Johnson, a smoker with a dog, likes to follow you around on the call. You suspect, and rightly so, that she doesn’t get many other visitors. The call this year started at 8 a.m.
You remove the thermostat’s cover and blow the dust from the inside. You check the anticipator setting: 0.6 amp. Looking inside the cover, you see where you jotted the setting last year with pencil, and it hasn’t been changed. You set the temperature up exactly 10° and switch the furnace on. You want to make sure the furnace doesn’t satisfy and shut off during your inspection, and you want to make sure you set back the thermostat to exactly where you found it.
(Remember last year? You had to return hours after your inspection because the house was too cold. Mrs. Johnson asked you at what temperature she would be comfortable, and you had to adjust the thermostat.)
The inducer motor starts and you head down the stairs with a bucket of tools, an inspection light, and a battery-operated drill. As you pull the front cover off the furnace (a six-year-old, 80-plus AFUE model that your company installed new), the main burners light with a gentle puff. You bend down to watch them as the blower comes on. When it does, the burners remain stable and the flames are unaffected. A few more seconds and you switch the furnace off to proceed with the ventilation air test.
THE VENTILATION TESTYou ask Mrs. Johnson to close all of her interior doors except in the bathroom, where she will turn on the bathroom fans. You confirm that the fireplace damper is shut. Returning to the basement, you start the electric dryer. There are no kitchen exhaust fans, so you are ready to start testing.
You shut the gas cock on the hot water tank, located in the same room as the furnace. You close the basement door; Mrs. Johnson is ahead of you, dog in arm, to watch over your shoulder as always.
You inspect the furnace’s burners and crossovers for blockage and corrosion. A visual inspection of the heat exchanger with a flashlight reveals no apparent defect. You switch the furnace back on and watch the trail for ignition. Standing to the side of the furnace (you know from experience this is an ideal spot to avoid eyebrow removal), you watch the furnace light. The burners start, and the blower soon after. The flames remain unchanged.
Walking to the hot-water tank, you hold a lit match up to the draft diverter; the flame pulls gently into the flue. You light the hot water tank and the burner starts. Draft is established in seconds, and the appliance continues to draft properly - no floating, lifting, or flashback of the flame. Walking to the top of the stairs, you open the door and return quickly to the hot-water tank; no change. You are sure there is adequate combustion air. You again shut off the furnace.
At 8:17 a.m. you shut off the dryer and Mrs. Johnson turns off the bathroom fans. The vent pipe starts to cool. You look for signs of corrosion and verify the pitch. Carefully removing the screws in the pipe at the vent connector and the furnace, and using gloves, you remove the pipe. With a flashlight you inspect the liner for evidence of corrosion; all OK there, too.
At 8:23 you continue with the visual inspection with the vent pipe removed. You inspect the inducer wheel for corrosion. You give the inducer a light spin; the bearings turn freely. Using a fiberoptic scope, you look down into the housing and see a sheet metal screw that is easily removed from the motor with a small magnet attached to the end of the scope. It must have fallen in prior to the installation. No matter, it’s out now.
At 8:26 you reinstall the flue pipe and zip in the screws with your battery drill, three screws per section. You insert the combustion analyzer in the flue; it is still zeroed from the ambient CO testing. You restart the furnace and stand to the side again. The burners light and you visually inspect the flame. There is no floating or lifting. All is well. When the blower starts, you focus your attention to the oxygen reading (7.5 percent) on the analyzer; any increase would indicate a crack or hole in the heat exchanger. All OK there.
This completes the three-part inspection on the heat exchanger, visual, flame disruption, and chemical testing (O2 test).
FILTER CHECKAt 8:30 you start checking the filters for cleanliness, proper size, and correct flow. You want to make sure the type of filter meets the needs of the occupant of the dwelling. Better filtering systems can remove pet dander, dust mites, viruses, bacteria, tobacco smoke, and aerosols, plus normal contaminants like dirt, hair, and lint.
With the dog and the smoking, it’s especially important for Mrs. Johnson to change the filters more frequently. It’s a media-type air filter and it’s good practice to check the pressure drop across the media when it is new, so you can have a reference point to change the filter when the pressure drop becomes excessive. Initially it was 0.27 inches water column (wc); now it’s 0.32. It will be OK for a while. This method will not work with electronic air cleaners, as they become less efficient as they become dirtier. A visual inspection and regular cleaning are very important.
You check the filter rack to make sure a seal is formed around the filter and the duct; air leaking around the filter rack can cause a negative pressure at the appliance, affecting the combustion or ventilation air requirements.
The burners are clean, and you check the manifold pressure. The manufacturer calls for 3.5 inches wc. The actual reading is a little low at 3.2 inches, so you remove the regulator cap and make a fine adjustment. Using your meter, you check the flame rod: 3.7 µA. When removing your meter from the series circuit, the burners go out.
THE AIR MOVERAt 8:35 you move on to the blower compartment. The prime air mover, or furnace blower, is the fan that moves heat by convection through the conditioned space.
The furnace blower must overcome the resistance of the filter, heat exchanger, evaporator coil (if installed), supply air ducting and registers, and return air ducting and registers. The amount of work the blower can do is rated in inches water column, shown as external static pressure on the manufacturer’s label. It is most usually 0.50 inch wc. (The term external static pressure is used because manufacturers have already calculated the pressure the heat exchanger and a standard filter will use; this is the pressure left over.)
The blower speed must be set according to the temperature rise shown on the manufacturer’s data plate in the furnace, in this case 35° to 65°. The temperature rise should ideally fall in the upper middle of the range for this furnace (50°-55°). You let it operate for about 10 minutes; the temperature rise starts to stabilize. You know the rise should be checked during every startup and service period. It should also be checked if the type of filter used is changed, or dampers are adjusted.
While the furnace is operating, you record the supply and return static pressure and figure the total external static pressure. From the manufacturer’s label, you can tell the furnace is moving about 1,000 cfm. The temperature rise is 57 degrees, about what you would expect since the filter is not new anymore.
With your meter you verify voltage to the motor and the measured amperage; both fall within the required range according to the manufacturer’s label. You turn off the appliance and lock out the power source. With your inspection light, you make sure that the wheel and motor end are clean.
Next you take hold of the blower and/or motor shaft and feel for play in the bearings. There should be some play parallel to the shaft (thrust), but there should be no play perpendicular to the shaft. You clean inside the blower compartment with a vacuum and shop rag.
WIRING AND SAFETIESAt 8:45 you check the appliance wiring through all connections into the furnace, looking for loose connections, proper sizing, and proper installation. Then you make sure the incoming voltage falls within the specified range.
It is imperative that the polarity be checked on all equipment. Make sure the hot wire (120-V black) is carrying 120 V to the hot wire (120-V black) on the furnace. The neutral should also be checked. With the furnace off, measure the resistance between the neutral (white wire, 0 V potential) and any earth ground (metal). Is the resistance less than 2â„¦? Check to make sure the ground is securely fastened. Record the operating voltage and system amps. Finally, make sure the equipment does not operate with the blower door removed.
At 8:55 you start checking safety controls. You slide a piece of cardboard into the filter rack and allow the furnace to overheat until the limit trips. The limit is an L220-30, and it opens when the plenum temperature reaches about 235°. The maximum temperature allowed in the plenum is 250°, so the limit appears to be OK.
At 9:07 you install a tee between the pressure switch and the draft inducer motor. The actual cutout setting of the switch is printed on the switch. With your digital manometer on the branch of the tee, you start the furnace and let it operate until the burners light. Slowly you pinch off the tube close to the inducer assembly while watching the pressure on the manometer. The burners go out when the pressure drops to the setting on the switch. You allow the burners to relight and pull the tee from the switch; the burners go out and the furnace does not try to re-establish flame. You reconnect the tube.
At 9:13 you perform a combustion analysis. You start the furnace and allow the stack temperature to stabilize. The oxygen is 7.5 percent, stack temperature 374°, draft 0.02 inch wc, and CO is 17 ppm undiluted and stable. The combustion efficiency is running at about 81 percent. You print off the readings; one copy for her, and one to staple to the invoice.
The furnace is still operating; you remove the analyzer from the stack and reinstall the upper burner compartment door. A quick check of the hot-water tank for excessive CO reveals nothing, and the draft is good with both appliances running. You put your tools in the bucket and look at the stack temperature reading on the combustion analyzer. It has dropped below 100°, so it’s safe to put away. Mrs. Johnson follows behind as you trudge up the stairs one last time.
Back in the living room at 9:30, you turn the thermostat back to its original setting and wait for the furnace to stop. You put away your tools, finish your paperwork, and get your invoice signed.
For a brief moment you think you finally made it, but as always a spelling error and some punctuation are corrected at the door. You remember why you hated English, and thank goodness that other people are technical writers and journalists.
As for Mrs. Johnson, she’s OK. And with the level of service that you’ve just performed, you know she will renew that contract for at least another year.
Publication date: 01/29/2007