What should the CO count on a typical 90% furnace

rob

On same 90% FA units with accessible sidewall venting, I will read at the terminal, providing I can see all the vent piping. Since it is supposed to be positive pressure and gas/water tight. What do you think?? If there was a faulty joint it probably would leak condensate anyway?
Can I disconnect the piping at the IDB, assuming it hasn 't been glued in place by some ""installers"". Roof terminals I may drill and tap the PVC and install a nylon or SS T316 plug. What do you think? Some old Pulse furnaces had a pressure tap that was removable - only for test purposes -
since the PS was referenced a differential to prove blocked venting condition. Your comments are appreciated. Thanks..

Dameon Wright

In the flue

Josh M.

Typically the best I can get is about 20-30PPM @ 6% O2...

Dameon Wright

So you would say 250ppm would be excessive

Mark Hunt

250ppm


is cause for shutting down the equipment.

Anything above 99ppm means there is a problem.

Mark H

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S Ebels

Ditto

250 ppm is a surefire indicator that something is wrong. One thing I always watch on 90%, or any other F/A appliances for that matter, is if the O2 and CO2 change abruptly when the main blower comes on. If you see that happen, it's a very good indication that there is a HX problem as in crack, hole or piece fallen out. Sometimes a failure is in an area where the airflow doesn't effect it so it's not a 100% thing. We find a lot of them rusting through where the primary connects to the secondary.

J.C.A._3

Ditto,

To Steve's ditto.

If the O2 and CO2 numbers aren't changing when the fan comes on, I would personally be looking at the heat exchanger to see if something has "dropped" into the path of the flame making it screwy.

When/if there is ANY doubt, have the technician place the probe in the feed plenum. If there is ANY CO there, it's time for a new one.(they SHOULD be tottally separated from each other) Chris

Unknown

You should see less

than 100 PPM in the flue. Be careful to take your reading 18" to 24" away from the furnace. Readings should be for O2 at 6 to 9%. Flue gas temperature should be around 130 degrees to 140 degrees and the plenum temp should be close to that reading also.

Make sure the furnace is not sooted up.

What is the temperature of the incoming air when this reading is being taken?

Make sure the condensate line is clear and the secondary heat exchanger is not blocked in any way.

Connect your microamp meter to the sensing rod while you make adjustments. Flame stability has a lot to do with proper adjustments. You should read between 2 to 10 microamps normal is about 3 to 5.

Does the unit have Nitroous Oxide inserts in the burners? Is it on natural gas or LP gas? If it is on LP those inserts must be removed.

Does the furnace have a flue restricter inserted? Some come from the factory with those in place.

Is the furnace over gassed? This can be determined by clocking the burner at the meter. It is also important to check the gas pressure and for a start it should be set at the recommended pressure on the rating plate.

Armed with that information you want to set up your combustion analyzer and make adjustments to gas and air until the CO comes down to a safe level. Do not be suprised that an increase in gas pressure will bring the CO down. What you are doing is bringing the furnace to its "sweet point" (my term) what that means is it is at the design perimeters for the unit.

I find on many condensing furnaces the cause of CO is that incoming air is cooling the flame and increasing the CO level. This usually occurs because the unit is not firng at its maximum level.

I find that on most condensing furnaces I am able to get CO to around 25 PPM.

If you are not familar with what I just posted then call a professional gas technician to do these adjustments.

jim lockard

Pressure switch

Will this fault not be caught by the pressure switches?

J.C.A._3

Jim,

Any "small leakage" wouldn't be detected by the pressure switch but would be shown with a CO reading at the plenum.

Been there, seen it and will stick by it.

If I see any soot or dirt at any register on inspection, it's the first thing I look for. The digital analyzer has no reason to lie and I trust it fully. Of course, you should also check to see if the culprit may be another appliance having its exhaust sucked into the heated air for the home first. Chris

Bob W._3

Tim, this is a novice question, but how does one get a reading at that 18-24" distance in a PVC flue pipe? do you drill the pipe and then plug it or is there some other way? Sorry for the silly question but I have never seen this done.

Edit: The reason I ask is that I just had my auxiliary furnace checked, a 1988 Carrier. The tech replace the condensate collection cover, which was leaking, set the gas pressure to 3.5 and put a probe in the heat ducts to check for ambient CO, but I didn't see him do a combustion analysis (he may have done this while I was not looking).

S Ebels

Short answer

NO! We just replaced a 10 year old Luxaire 90% that had a bad HX connection between the primary and secondary HX. It never failed to light up. The HO called because his friend, who used to work for me, noticed an odd smell in the house. The unit was direct vented intake/exhaust and was making over 200ppm in the exhaust with all adjustments normal. When we tested the flue gas, the CO remained stable (high) but the telltale CO2/O2 reading fluctuated when the main blower came on. I suppose in a worst case scenario the pressure switch would drop out but I've often found that it doesn't happen.

jim lockard

Thanks Steve

and Chris. So once the furnace lights, its not likly to shut off on pressure switch fault. When checking a heat exchanger I use a Mag gauge and watch for movment when the fan comes on. Thinking about it and depending who's furnace (manf) I am checking perhaps I should check diff. pressure between pressure switch and exhaust flue.Agree? J.Lockard

Unknown

Bob W.

> Tim, this is a novice question, but how does one

> get a reading at that 18-24" distance in a PVC

> flue pipe? do you drill the pipe and then plug

> it or is there some other way? Sorry for the

> silly question but I have never seen this

> done.

>

> Edit: The reason I ask is that I just

> had my auxiliary furnace checked, a 1988 Carrier.

> The tech replace the condensate collection cover,

> which was leaking, set the gas pressure to 3.5

> and put a probe in the heat ducts to check for

> ambient CO, but I didn't see him do a combustion

> analysis (he may have done this while I was not

> looking).

Unknown

Bob W. one of my

good friends here on the Wall, Paul Lessard showed me how he taps a hole in an elbow on the PVC flue. Then when he is done he screws a stainless steel plug into the hole to seal it and for future testing. It would help if manufactueres of equipment gave us a place to test but unitil they do we have to be creative.

Bob W._3

Thanks, Tim. There is an opportunity for someone - create a sort of "Pete's Plug" for testing flue gas.

Gary Reecher

Lennox Pulse

the manual says that must be less than 50 PPM