Exhaust furnace pipe discharge gas

[Boris Rozenblit]

It is very good documented on internet that in high efficiency furnaces exhaust pipe discharge some gas. In many posts it says it is normal. In my case I have two windows right above exhaust pipe (first and second floors). Sometimes wind blow gas smell to windows. I understand it is normal but still very uncomfortable. I was thinking to extend exhaust pipe above the second floor window. Somebody told me that condensate may freeze the exhaust pipe. Do you know if anything can be done in my case to get rid of gas smell. Thank you.

 

[Jadnashua]

First, you might want to have a tech run an exhaust gas test to verify that the mixture is right.

THen, you'd want to review the installation instructions. Those typically give specs on what is acceptable for routing of the exhaust, the lengths, position of the outlet, the required slope, etc. One thing that is also usually spelled out in the installation instructions is the minimum clearance from openings like doors, windows, or other intakes or exhausts. If you have a window directly above, that may conflict with the installation instructions, and you'd have a case against the installer to fix it on his dime rather than yours. That sort of thing SHOULD have been caught in the inspection, but people don't always pull a permit, either, and not all inspectors are actually inspecting to the appliance's requirements, but those generally require installation to the manufacturer's instructions in addition to any 'normal' code requirements.

Running it up the wall outside probably isn't a good idea. Finding a path inside to run it up might be possible if the overall length is allowable. The condensate can freeze if you don't follow the instructions, and I found out the installer didn't on mine on a really cold day when the boiler shut down because it couldn't get any combustion air as it became blocked by the installer not following instructions. They fixed it for free, but I caught it after researching what happened and why, not them that should have been able to follow instructions (that's what you pay them for!).

So, can't give a definitive answer without the specs, and the exact location and distances of the piping involved. On some, depending on the distance, longer ones may require a larger diameter pipe, and that can make finding a path harder. The maximum distance is referred to as the effective length. That takes into account the linear measurement, but adds effective length for any changes in direction. For example, a right-angle fitting might be 1' in real length, but it might add 5' of effective length to the distance because of how it restricts the airflow. Those numbers are just for illustration, not exact...the documentation will assign the values you'd need to do the calculation. The fan and gas volumes involved will determine how much restriction the burner can work with.

 

[Sponsor]

 

[Fitter30]

Both intake and exhaust have to be on the same side of house if planning to move them.

 

[wwhitney]

Both intake and exhaust have to be on the same side of house if planning to move them.

Can you expand on that, as to the reason and provide a reference? I may have to adjust some plans.

Thanks,
Wayne

 

[WorthFlorida]

It is very good documented on internet that in high efficiency furnaces exhaust pipe discharge some gas. In many posts it says it is normal.

Where is the gas meter? If its near these windows, do a simple soap test on all of the fittings. I once had a bad diaphragm on the regulator and it was leaking from the weep hole.

One area that most do not understand is in which direction is the prevailing wind on your home. Depending on which windows are open and wind direction it can create a negative pressure inside the home and fumes can be pulled into the home. A very common problem when people run portable generators after a storm and many have been overcome by carbon monoxide.

 

[Boris Rozenblit]

Thank you for posts

 

[Jadnashua]

Condensing appliances rely on some internal pressure sensors to determine if the pipe(s) may be blocked. It expects the relative lengths of the pipes are the same. Those can trigger a fault and shut down the burner if they exceed certain limits. That is also a reason why there are effective length restrictions...longer effective length means it may not be able to pull in enough air for combustion, or to exhaust the fumes properly.

Over the years, I've had that happen to mine (on different devices) twice...one when a pipe actually broke, and on a second one when the exhaust cap was installed incorrectly by the dealer, and on a super cold day, it iced over, blocking the intake. No combustion air, or not enough, prevented it from running and in the first instance, a significant imbalance in the intake/exhaust.

FWIW, people sometimes complain about smelling sewer gasses, and that can be related to the winds and where the vent stack is relative to the windows, or other openings. Same is possibly true with burners and their exhausts and, their relative location to any other opening in the dwelling.

 

[Fitter30]

Can you expand on that, as to the reason and provide a reference? I may have to adjust some plans.

Thanks,
Wayne

If the two pipes are on different sides of the house they could be in two different pressures. One with the prevailing wind blowing on it the other in a negative changing the burner set up.

Can you expand on that, as to the reason and provide a reference? I may have to adjust some plans.

Thanks,
Wayne

This is out of AO Smith manual
• Terminating the venting through a sidewall is recommended for the direct-vent system. • Running the exhaust vent and the intake pipe parallel is recommended. • Terminating the exhaust and intake on the same wall/surface is recommended. Terminating in the same pressure zone allows for pressure balancing, which prevents nuisance shutdowns.
If the two pipes on different sides of house one could have wind blowing on it the other in negative pressure which could effect burner setup. Some manufacturers say you can do it so check with them.

 

[wwhitney]

If the two pipes are on different sides of the house they could be in two different pressures. One with the prevailing wind blowing on it the other in a negative changing the burner set up.

Thanks, makes sense.

Is this only a problem when the atmospheric pressure differential is against the normal airflow path (intake is in lower pressure zone than exhaust), or can it also be a problem when the pressure differential matches the normal airflow path (intake is in higher pressure zone than exhaust)?

Cheers, Wayne

 

[Fitter30]

Either way it can effect performance. When setting up burner in a boiler you can see o2, co and flue temp vary from a cold to up to temp boiler. Small adjustments of air or gas will make big difference in readings.

 

[wwhitney]

Either way it can effect performance. When setting up burner in a boiler you can see o2, co and flue temp vary from a cold to up to temp boiler. Small adjustments of air or gas will make big difference in readings.

Ah, now that you point it out, it's clear that too much combustion air may be a problem just like too little air.

So in bathroom exhaust fans, there are now ECM models (like most or all Panasonics) that purport to adjust fan power to maintain the set CFM exhaust rate and compensate for different exhaust duct static pressure losses. [Not sure how the controller knows the current exhaust rate, is it as simple as knowing the fan RPM and fan housing area?] Are any forced draft gas combustion appliances using similar technology to provide a steady combustion airflow in a way that would compensate for different vent lengths and wind-induced pressure differentials?

Cheers, Wayne

 

[Dana]

Can you expand on that, as to the reason and provide a reference? I may have to adjust some plans.

Thanks,
Wayne

Wind can create substantial pressure differences across the house, pressures high enough to interfere with the internal system controls. If you look at the installation manual of just about ANY condensing furnace or boiler (preferably the manual for your equipment) you'll find minimum & maximum separation between intake & exhaust. See Figure's 15 through 18 starting on p.18 of this typical installation manual.

Most still work fine taking combustion air from inside conditioned space, if routing both intake and exhaust to the same side of the house isn't viable for some reason. ( But find and read the manual first. ) In tighter than average houses with high cfm exhaust fans (like commercial kitchen grade exhaust without make-up air) the induced pressures can be high enough to interfere with the operation in much the same way as high winds can when the intake & exhaust are on opposite sides of the house.