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Venting a Natural Gas Fired Boiler

I'm not all that familiar with HVAC issues and looking for some explanation and help with venting a natural gas furnace for hot water baseboard heat. This would be for construction with new house.
Is it necessary to vent directly into a brick chimney or if I do not include a chimney with the house, are there other ways to vent the system? The proposal I received from a plumber includes an 80 gallon Superstor Contender indirect hot water heater being supplied it's water from a Weil Mclain GV-5 gas fired furnace. I'm assuming the H2O heater would not need to be vented... it's just the furnace I need to understand about how it would be vented....so I can decide whether I need a chimney or can do without it........ Thank you for your help.

Re: Venting a Natural Gas Fired Boiler

You will have a boiler installed, not a furnace (a furnace heats and circulates hot air).

For new construction, a masonry chimney is usually omitted in order to avoid the labor costs.

The modern approach is to install a double-walled stainless steel chimney if the boiler needs such a chimney, as would the WM GV-5; the other option is a side wall vent, which is not recommended.

The present trend in modern heating units is condensing variable speed boilers (95-97% efficient) that require no chimney; the fuel is almost completely burned by the boiler, producing as its only byproduct a slightly acidic water condensate which is discharged to a cellar drain via a 2" pvc pipe.

I strongly advise you to get additional estimates from other heating contractors (Yellow Pages; 'Heating Contractors"); there are much better boilers around for the same money than the WM.

The Weil McLain GV-5 has only an 87% efficiency rating and has had recalls for faulty blower motor seals.

This unit also has elastomeric seals which have caused water leaks between boiler sections.

Better boilers would be the Triangle Tube Prestige, Dunkirk/Utica,Buderus boilers, Lochinvar, Peerless Pinnacle, Crown Freeport, Slant/Fin Eutotic, Biasi B10, Viessamann Vitodens.

Most of these are variable condensing boilers; some are triple pass boilers, which are also highly efficient in fuel savings.

The triple pass boilers would require the stainless steel chimney, the condensing boilers would not.


Re: Venting a Natural Gas Fired Boiler

Jack... thanks very much for your suggestions.
I'm thinking if I can get away with not building a chimney, and vent the boiler to the outside that would be very good for a number of reasons. But why do you say that venting through a side wall is not recommended and it's better to use a SS chimney? If I can vent though a side wall that would be better for my layout as opposed to running a SS chimney up through the whole house to go out the roof.... (or maybe I'm not interpreting correctly what you said).....

Re: Venting a Natural Gas Fired Boiler

Venting thru a side wall is completely within code, but objectionable exhaust fumes are almost certain to interfere with your enjoyment of the yard, especially in summer.

If you add an indirect hot water heater (strongly recommended), the boiler will periodically fire thru the summer (very little fuel is expended for the HWH in summer due to the extensive insulation these units have).

Any open windows are likely to let in a foul smell.

Side vents often also cause staining of the siding around the exhaust vent.

In the winter, some homeowners experience a lot of ice or frost formation in this area.

Stainless steel chimneys IMHO dress up a roof's appearance, and need only 10" X 10" of a raceway to get it up to the roof.

Prevailing breezes and the shape of the roof serve to whisk exhaust fumes safely away from the house.

Re: Venting a Natural Gas Fired Boiler

Jack... I see your point.

Is it necessary for the SS vent stack to have a direct line up to the roof to exit... or can it run horizontal for about 18 feet (to the other side of the basement) and then run straight up to the roof.

The scenario is I'd have the boiler on one side of the basement but would need the stack to exit from the roof on the other side of the basement (due to being located in a historic district and the Historic Committee would most likely not permit the stack being visible from the street, but would not object to it exiting the roof on the other side, not in view of the public)....

The other option would be to move the boiler to the other side of the basement, but I don't really have the space to do that.

Thanks again for you help.

Re: Venting a Natural Gas Fired Boiler

Only the installer of the heating equipment can answer the questions you pose, as to how the SS chimney can be effectively installed, and still comply with local fire code.

Generally the SS chimney has to be in a straight line from the cellar to the roof; there is some bend allowed in the connecting flue pipe from the boiler to the chimney, as long as at least 1/4" upward pitch per foot is maintained.

Moving the boiler to the side of the cellar where the chimney will go up to the roof is the most practical.

Boilers these days are the size of large suitcases, & thus require little room.

It's important to get several other estimates from heating contractors at this point; it's part of their job to be fully knowledgeable of local codes on how the boiler they install can be vented.

Don't ignore quotes from local oil dealers, who are also licensed to install gas-fired equipment, and may offer a better deal.

You can also check the Yellow Pages under "Chimney Builders"; they can give you a free estimate of where the chimney should be placed & approximate cost.

SS chimney requirements are covered under NFPA 211 (National Fire Protection Assoc.).

Your local town hall should also have a copy of NFPA 211, or even the local fire dept., or local library.

The site below has typical guidelines; the site describes installing a wood-burning hot water boiler, but the guidelines are similar, and there are important links.

For more sites, Google "NFPA 211" chimneys gas appliances.

You may save yourself lots of $$$ if you opt for a condensing boiler; not only will you save lots on your fuel expenses, but you will avoid the cost of installing the chimney.


Re: Venting a Natural Gas Fired Boiler

Jack and DwarfWytch, Thanks very much for the replies and info. I've been doing some research on your suggestions and will proceed by contacting a few other vendors for additional quotes and see what their suggestions are.

Thanks again. I appreciate your sharing your knowledge.

Re: Venting a Natural Gas Fired Boiler

Several houses in my neighborhood have boilers with house sidewall vents that are very noisy. They sound like someone is running a loud hair dryer. I assume the noise is from an exhaust fan, which is necessary since the fumes from condensing boilers aren't hot enough to flow up and out themselves.

Are there any condensing boiler or vent designs that aren't so noisy?

Re: Venting a Natural Gas Fired Boiler


Condensing boilers burn 95% of their fuel, so the don't use a vented exhaust, only a 2" condensate drain.

All of the other furnaces and boilers, however, need either a chimney vent or side vent.

since roof chimneys are more expensive & the stack temps of newer, improved units are often under 300 degrees, installers are doing more powered side-vent and auto draft-inducer installs.

Noise is another reason i don't like side venting; my preference is for a condensing boiler, or a roof vent using a 3-pass boiler or efficient furnace.

Re: Venting a Natural Gas Fired Boiler

I have trouble believing that the side-vent boilers in my neighborhood aren't condensing boilers. The exhaust is barely warm -- I can easily put my hand in it. It smells like gas exhaust and not like laundry. So what is coming out of those vents if not condensing boiler exhaust?

Re: Venting a Natural Gas Fired Boiler

who knows----- could be anything-----have you tried asking your neighbors?


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