I've been doing HP assessments for 12 years, but find myself a little limited in my knowledge of this issue.

Here's the scenario: I do an initial assessment where I find a 7-year old NG water heater with 1500 ppm undiluted CO. The homeowner brings in his plumber to "fix" it. The plumber, who doesn't own a CO monitor, cleans the burner and air intake grill, and proclaims it good. After work is done on the home, I re-test and find the CO to be even higher this time, and the draft weak.

What typically can be done to a fairly new water heater to fix the CO problem? The gas/air ratio must be out of wack, I'm thinking. Is it usually corrected by replacing the gas control valve?

The homeowner is left hanging, the plumber is without a clue and me telling him it needs to be corrected. I would recommend a different plumber, but I want to find out what a knowledgeable individual would typically do to correct this, short of replacing it. One thing I know is I need to align with a knowledgeable person to address this in the future.

John Shillito

Tags: CO, heater, plumber, water

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I have to agree with Kurt. 2000 ppm is exceedingly high. I have tested hundreds of hot water heater (NG, Oil) and mostly they come in around 16 ppm. There is clearly something wrong and we all know that a gas hot water draft can easily be compromised. 

hey thanks for the links. Very informative on FVIR units

Hello John (and everyone else in the forum),

I'm a new visitor at this forum and it this is my first post. I look forward to more sharing and learning in the future.

The first condition that I would investigate the "weak draft". Is the flame lazy too? Is the equipment in place the same as what the flue was originally designed for?

I find the best place to contact when at a loss is the manufacturer's technical support.


1. Inspect flame .. nice blue is perfect.. white is a sign of carbon monoxide and incomplete combustion..orange too much oxygen but does not mean CO,,  If it consuming entire chamber and larger than normal flames it most likely overfired. could just need to be adjusted or replace valve. 

2,  Inspect burner and inspect primary air is at least open.. May have to be opened more but leave it to a pro.. . also look for debris or some material interfering with burn like a mouse or bird.. if debris is on burner , have it cleaned.. .  (HAVE PRO DO THIS). Make sure burner is seated correctly.. 

3. check for over firing ..clock gas meter.. I think KRIeger's residential Home Energy book has a section on clocking  Gas meters,, . as for auditing this is probably the best least intrusive  test for you.You need a stop watch and a calculator.... You do not  want to break rule  #1.. DON'T GET IN OVER YOUR HEAD!!!!

But out of box this should not happen.. check gas valve and manufacturers plate to make sure its not Propane,, Then you have to install natural gas valve or at least change orifice  (PRO again).. 

You can have a professional plumber test the pressure on the gas valve with a water Manometer or digital Manometer..It should be set 3.5 i.w.c or a little lower.. check manufacture plate for this rating and input BTU;s..  . and there is a adjustment screw to adjust pressure on the gas valve,  but leave that to a licensed service man... If he does not have these basic tools or know how to adjust gas pressure,, FIND A DIFFERENT PLUMBER,,, 


1 can not tell by flame.   New water heaters you look though a small little opening -  you can not see the flame.    We need a flue gas meter to read the O2 and CO  and flue temp.   

2. right on - must look at rust and stuff on burner.  impingement bring up high CO

3. clock gas meter?  have tried this?  what is the BTU of gas?   this can change each min or each hour and each day.   I have seen Nat gas go from 800 BTU per Cub foot to 1050 BTU's  in same hour but not normal.  Must use flue gas meter!   I have seen gas meters off 36% with the "tar" off Nat gas that's why they get rebuild each 10 yr.    When testing try keep each burner the same presser

So CO is a sign of incomplete combustion.  There are lots of things that can cause that. First cause is cold combustion surfaces.  If you put a cold pot on a gas stove top the CO spikes.  You need the burner to be rocking for a few minutes before you test. Also, 1500 is stupid high.  I would bring along either the old school glass tube tester or the CO alarm. 

The next thing that causes CO is insufficient oxygen.  This might be a lack of combustion air (fresh air). That would support the weak draft. The other thing that doesn't help this is doing this test in the peak of summer. Doing a combustion test could show low oxygen (or no oxygen) in the combustion gas.  Cleaning everything should help this. I assume there are no fans or motors, so check the pitch on the venting. 

The other cause of CO is burner issues. Insufficient oxygen can be caused by too much fuel. This is causes by someone messing with the burner or replacing parts. The other burner problems can be caused by rust, spiderwebs, damage, etc. 

The fix for all these things, power vent or direct vent water heater. Unless you find the venting issue or burner issue. 

CO of 1500 Into the Atmosphere is NOTOK any time or anywhere

Replace the water heater ! the Pressure Valve Might be off, the Burner Might need adjustment or might be defective. By the time you fix them you will probably spend as much as a new Water heater. You should also look at the piping connections and the Shut Off Valve and if they are not same age as HWH Replace them.

Caz Testing should reveal the cause of the Weak Draft. Several times I have Non Passing draft Flue #'s on clay lined Masonry Chimneys. It did not pass or barely passed, I told the Homeowner They had to get a Chimney PRO to check it it out to make sure it was clear. More than a dozen times the chimneys were almost completely Blocked !! 

Where I work if you orphan a HWH  (Convert the Furnace to Sealed Combustion) You are required to install a Stainless Steel Liner in the correct size. Also if a HWH is 40,000 BTU or Higher it requires a 4" flue. Max 3 90 Degree Elbows. Make the Flue pipe on top of the heater ( I call it the HWH Chimney) as tall as you can while keeping a 1/4 inch rise to wall Thimble. Max run is 10'.

While the Homeowner is responsible to make the required repairs & it may not be in the scope of the audit, I learned most of the above knowledge in my BPI training class 7 years ago,and I always talk to HVAC pros to gain more knowledge. Whenever I see a problem I will not leave the House until drawing a picture of the problem and what I recommend to correct it. I also offer the phone # of a Professional who I have worked with and make it clear they don't have to use my guy, But they need to fix it promptly.  And I let them know I will not do any work done until the problem is fixed and reinspected.

Hope this helps,


Forgot to add

Make sure you have sufficient Intake air for the room. You should have 1 sq inch per 1000 BTU's of all appliances in the CAZ, Preferably 50%  6" from the ceiling and 50%  6" from the floor.  Remember Louvered Grills have 40+% less clear space than their Dimension size. And it is OK to make the area larger


You can get combustion air from from the supply air from blower.   If blower is on 24/7/365 and seal the return air duct and filter slot.  Make the furnace room postive and add air to combustion.   I have seen to much combustion air grills go the wrong way.   Chimney back drafting are common with unsealed return.    


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