Thread: Test for 90% furnace

I inspected two Goodman 90% furnaces last week each had CO readings of 90 to 150 ppm from the outside exhaust. Each exhaust had substantial amounts of condensate dripping to the ground.

I call Goodman and got no answer.

Are there any general guidelines as to CO exhaust and condensate output for 90% furnaces?

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Jory,

I am not an HVAC guy, however, depending on how long the furnace had been on (since it was a condensing furnace) over 100 ppm could be an issue. When condensing furnaces first start up the CO level is pretty high. If you really want to learn HVAC combustion and carbon monoxide take the Combustion/CO course given by National Comfort Institute.

Originally Posted by JORY LANNES

...Each exhaust had substantial amounts of condensate dripping to the ground. ..

The exhaust pipes are supposed to tilt back to the units so the condensate does not drip outside, as far as I know.

OK, as I do not take CO readings I really have no idea what the proper CO should be at the flue exhaust on a gas furnace.

What should the CO levels be on a 90%+ CAT IV furnace?
What should the CO levels be on a 80%+ furnace?

I really have no idea what they should be.

As for the CO levels, it is impossible to give an accurate answer with the info given. Need to know the size of the units (BTU), single stage, two stage? As to the condensate, while the pipes should be pitched back to the unit a certain amount of condensate will be pushed thru the vent. The amount is dependant on a number of factors, humidity, return air temp, etc.

Originally Posted by Bob Spermo

Jory,

If you really want to learn HVAC combustion and carbon monoxide take the Combustion/CO course given by National Comfort Institute.

Only take this course if you are willing to re-think everything you thought you knew about combustion. Having taken the class, anything over 100ppm is not good. Now having said that, good luck finding an hvac contractor that can tune the furnaces to operate properly (if they haven't taken this course, they won't be able to).

Originally Posted by ray jackson

Only take this course if you are willing to re-think everything you thought you knew about combustion. Having taken the class, anything over 100ppm is not good. Now having said that, good luck finding an hvac contractor that can tune the furnaces to operate properly (if they haven't taken this course, they won't be able to).

This course is great for general information and knowledge but, just like any training course, I would caution against taking all the information presented as gospel. Some of the techniques and adjustments presented by NCI are against manufacturer recommendations.

The ANSI Z21.47 standard for gas-fired central furnaces allows up to 400 ppm of CO in the vent. Realistically, this is at way far edge of the design
standard and no manufacturer would ever design to that as a minimum level.

Because of the 400 ppm limit, and the fact the so many factors affect CO, there really never has been a published "acceptable" or "normal" level of CO in the vent. One thing causes high CO; incomplete combustion. But, many factors can contribute to that incomplete combustion.
The sensor(s) in a combustion analyzer are only good for X number of months. After that, the sensor(s) may not read properly. Also, the batteries should be fresh. A bad sensor or weak batteries can affect your results. Meters should be calibrated outdoors

When testing for CO, the following conditions must be first met, or your readings will be high. Most will need to be checked by a qualified HVAC tech, as they are out of the capabilities and responsibilities of even a good inspector)

The burners must be clean and lint free.
The unit must be fired within 2% of the rating plate. (high fire on 2 stage furnaces)
The unit must be at the mid-point of the temperature rise or
slightly above
The unit must be at steady state conditions, which means it has to have run for at least 15 minutes.
For 90% furnaces, the drain trap must be clean and condensate must run free. Water trapped in the secondary HX reduces the excess air levels and results in poor combustion performance.
You need to make sure you have sufficient combustion air to the room.

And for any CO test, you need a background reading of CO in the room or area from which the combustion air is being drawn. You may have high CO in the combustion air. This is especially true if combustion air is being pulled from a garage; very likely when dealing with an 80% non-condensing furnace.

For 80% furnaces, the sampling probe should be put in the vent pipe 12 inches above the draft hood or inducer outlet.
90% furnaces can be checked at the termination outlet outdoors, but more accurate results will occur if you drill a hole in the exhaust pipe 12 inches away from the inducer outlet.
You should seal the hole around the probe in the vent pipe to prevent room air from diluting the sample.

CO tests on site at the unit are not a good idea.
Test the CO away from the unit.

I read guys taking measurements from inside the hood and wonder why they are getting high readings ,for crying out loud.

Originally Posted by Bob Elliott

CO tests on site at the unit are not a good idea.
Test the CO away from the unit.

I read guys taking measurements from inside the hood and wonder why they are getting high readings ,for crying out loud.

Where exactly should the readings be taken Bob?

Originally Posted by Jerry Shipman

This course is great for general information and knowledge but, just like any training course, I would caution against taking all the information presented as gospel. Some of the techniques and adjustments presented by NCI are against manufacturer recommendations.

There are a lot of times that the manufacturers recommendations need to be analyzed a lot further than they are as they are taken as gospel by most.

We go by these criteria when checking carbon monoxide.

• • Test oil fired furnaces in the vent connector before the barometric damper.
  • Test low efficiency propane and natural gas furnaces (<80%) at the outlet of each heat exchanger section before the draft diverter or collector box, and before any dilution air. Treat drum type heat exchangers as one cell.
  • Test mid (80%) and high (90%) efficiency propane and natural gas furnaces in the vent connector after the draft inducer. Drill a hole in the vent pipe 12" to 18" above the outlet thimble. This hole must be sealed after testing is complete.
  • CO readings should peak, drop, then stabilize within 5 minutes. If CO readings continue to climb for more than five minutes the condition must be considered unsafe and corrected - even if CO levels are still below 100 ppm.
  • If highest single CO readings are:
    • 0 - 50 ppm conditions are acceptable.
    • 51 -99 ppm servicing is recommended
    • 100 + ppm the condition is considered unsafe and immediate review by an HVAC technician is necessary.

Drill a hole in the vent pipe? Are you serious? Minn allows this? This profession of Home Inspections is becoming scary.

Originally Posted by Philip

Drill a hole in the vent pipe? Are you serious? Minn allows this? This profession of Home Inspections is becoming scary.

We do it regularly. Much of the time we don't need to though since there's generally already a hole from the testing / tuning done after the initial installation. Just have to remove the plug for our test and replug it when we're done.