CO

allie27

Have a one year old hot air furnace that is direct vented. Everything is correct and has been tested numerous times. CO showing up on detector sporadic, could go a couple of weeks in between. Detector new, fire department been over along with gas company and contractor. There is also a gas water heater close by that is still vented into chimney by itself. House not tight by any stretch, becoming a real PIA to correct.
Furnace on natural gas and 2 pipe vent, these are the only 2 gas appliances in the house. CO levels range from 35 up to 100 ppm. Any ideas aside from eliminating one gas appliance to determine the problem. No one has been able to pick up any CO readings around either appliance, venting, draft hood, etc. etc. Seems to happen more at night.Thanks

DZoro

Were you having CO issues before the furnace replacement?
How high are the CO numbers going to?

I have seen brand new furnaces with bad heat exchangers. A very careful combustion analysis must be done on the furnace. This should be done by a very experienced tech, video taping the combustion analyzer and furnace at the same time. Sometimes the numbers can change quickly and not be caught right away. May need to pull the furnace and put eyes on both heat exchangers also.
Have the combustion also recorded on the water heater. Record them, if still needed after that, post the recordings here for possibly more help with your situation.
D

neilc

I'll take a guess,
The house is not well insulated, or more importantly, well sealed for air flows, up thru the attic.
This occurs more on the colder or windy nights.
And that the small water heater can't overcome the stack affect coming DOWN the (now) over size chimney.
Don't ask me how I would know this.

lchmb

I agree with neilc..I'd suspect the water heater.. liner is probably over sized now that the furnace has been removed..

allie27

That is my belief also, no liner in the chimney, old tank and 60 year old house. House was empty and then rehabbed. Furnace has exhaust blower to vent so everything from burners and through heat exchanger is under negative pressure, if anything was wrong with heat exchanger it would be pulling air into it not the other way around as I understand

EBEBRATT-Ed

Shut furnace and water heater down one at a time and retest

neilc

wait a minute,
you said there were 2 pipes for the furnace,
one of them is outdoor air, piped in, confirm?
And if that new furnace is faulty enough, it could still be bad news, and it should still get checked.
And in your scenario, "pulling air into it" would still be pulling air DOWN the chimney, against the water heater.
And or bathroom fans, kitchen hood fan,
Did you renovate or was this a flip prior to your ownership?
One of those crazy commercial kitchen exhausts ?
https://heatinghelp.com/find-a-contractor/
get someone in there that knows what's what

Leonard

In a restaurant I've seen cooking exhaust fans blow so much air outside that they pulled a mild vacuum on building and a HUGE hurricane wind was coming DOWN the chimney. Nat gas fired 40 gal hot water tanks had no exhaust fans, chimney down draft was blowing their exhaust away from exhaust pipe, actually scorched plastic of nearby dielectric water coupling.

Anything that pumps air OUT of house has the potential to do this, clothes drier, cooking exhaust fan, high winds outside. But the restaurant kitchen fans were powerful, 2 HUGE roof top mushroom units, 30 inch dia .

John Mills_5

Exhaust hoods, radon vents...

Is the chimney lined with metal liner properly sized for the water heater alone?

When I was young (aka long time ago) I worked in a restaurant that was so negative from hoods that the boiler and water heater both had big inducers on them in order to vent.

Leonard

The Chinese restaurant "solved" the exhaust problem in heater room installing 4 inch exhaust fan venting that room's air to the outside, not from the heaters. Could still smell exhaust in that area of building. They didn't do anything right!!!.

allie27

No real exhaust vents, pretty basic old house. Chimney does not have a liner, not required when tank originally installed.
Yes furnace has air inlet and separate exhaust, no combined termination, 2 pipes to the outdoors
Problem with shutting off one appliance and waiting is it sometimes takes a couple of weeks for alarm to go off again.
Really believe it must be a dead chimney at times or has some penetrations upstairs that are allowing exhaust to come back into living space.

allie27

Both appliances have been checked numerous times, right at appliance, exhaust vent, ductwork, absolutely no readings and then a few days later alarm is going off. Confirmed by fire dept. with there equipment so not just a spastic alarm.

DZoro

What is the CO number going up the chimney from the water heater?
What is the CO number going out the furnace exhaust?
Before fan on?
After fan on?

What is the 02 reading in the furnace exhaust before fan on?
After fan on?
These are important numbers at those specific times.
D

Zman

You might try smoke testing the chimney. You could put a lose cap on top and use either a smoke bomb or a smoke machine designed for sewer lines.

Leonard

Might want a CO detector that can give you ppm reading even when they are too low to call for an alarm. Maybe wave it around to flow air thru sensor to scan for CO. Don't know what it's refreash rate is , might only read once a minute, you have to experiment with it.

Guys here have told me about a regular house CO detector that can do that , when you press it's button, ~ $26, link here
https://forum.heatinghelp.com/discussion/165963/need-recommendation-for-home-co-detector-want-ppm-readout#latest

EBEBRATT-Ed

@allie27 what @Zman said is a good idea.

Also the chimney may not get hot enough with only the water heater vented into it. It's called the 7 times rule.

For instance if the water heater flue connection is 4" then 4 x 4 x.7854=12.5 square inches. 12.5 x 7=chimney liner inside dimensions cannot exceed 87.5 square inches.

If the water heater is a 3" flue then the chimney liner inside cannot exceed 49.4 sq "

I would put a ss liner in the chimney

Fred

Have you removed the WH flue pipe to see if a squirrel or bird wanders down the chimney and tries to nest in the WH flue, then the heat runs it out until it forgets and tries again a few weeks later? With just the WH vented in that chimney, it is probably a comfortable place to be, until you get into the actual flue.

DZoro

If they didn't install a liner for the water heater, after removing a chimney vented furnace, then shame on them. You need a proper sized liner for the water heater, or you will experience CO, and chimney failure.
Pretty standard procedure for that to be done immediately.
D

SuperTech

The problem is obviously the chimney as @neilc explained. Have an insulated liner installed and the problem will go away. Do you have spill switches installed on the water heater? You should. Chances are the chimney is in poor condition and has needed relining for a while, recent changes with the installation of the new furnace have caused this to become the issue that it is.