View Full Version : B-Venting thru existing masonry chimney
09-17-2005, 04:54 PM
I have an old farmhouse that has a masonry chimney running up through the center of the home and eventually (obviously) through the roof. The previous owners had installed a Trane forced air natural gas furnace, and tapped into the existing chimney via a b-vent that lines the entire chimney to the top for exhaust. I would like to add an add-on woodburning furnace, but I don't know if I can tie that into the chimney and use it for its intended purpose (woodsmoke and ash) with the B-vent running inside as well. Is this ok, or do I need to find somewhere else to vent the gas furnace to before converting the chimney back to wood use?
09-17-2005, 04:58 PM
Install a new 90% gas furnace and vent it out the wall via plastic pipe.
Remove B-vent, and consider a stainless liner for the wood.
09-17-2005, 07:59 PM
For various technical and safety reasons, a gas furnace and a solid fuel furnace (aka wood stove) should not be vented via the same chiminey. A chiminey is designed for high-temperature flue gases (like those from a wood stove) while B-vent is designed for cooler, but more corrosive flue gases (gas furnace).
The B-vent was installed inside the chiminey by a previous HVAC contractor (probably) because gas furnaces should not be vented directly into a chiminey (flue gases are too cool and too acidic) without a liner or some other way of adapting the chiminey to accept a gas furnace.
Since your chiminey has been "converted" for use by a gas furnace, you will have to either restore the chiminey to solid fuel operation (removing the B-vent and re-venting the gas furnace) or creating a new solid fuel vent for the wood stove.
Which you do depends on how practical and expensive this work is. If the chiminey is in good shape (many old chimineys aren't) you might find it easier to use the chiminey and get a new vent system for the gas furnace. My guess is that it might be easier and cheaper to keep what you have and have the wood stove vented through a new solid fuel vent .
You should seek out a competent local professional to do either job. Improperly installed wood stoves can cause structural fires, carbon monoxide poisoning or creosote fires. Check out John Vivian's "Wood Heat" (available at most libraries) for more info.
09-17-2005, 09:53 PM
Originally posted by jack legg
Improperly installed wood stoves can cause structural fires, carbon monoxide poisoning
Most people don't put these two items together very often.
Most people think you have to burn fuel (gas) to have CO.
09-17-2005, 09:56 PM
sure masonary chimney does not have 2 flue chases up thru it already?
09-19-2005, 12:11 PM
How can I find out if I have two chases already? I suppose getting up on the roof and looking down would be a start, but is there an easier way?
My current train of thought is to pull the b-vent from the chimney and run a side-wall vent instead, keeping the requisite 1/4" per foot rise on the way through the floor trusses out to the side wall. This would free up my chimney for solid fuel venting. The run for this side vent would be approximately 12-15', and would also have a water heater vent tied into it. Because of the length and orientation of the vent, I'm not worried about the pilot light on my water heater, but more so about proper drafting. With the inducer fan running on the furnace, it would provide enough draw to also keep the water heater vent flowing, but obviously the point of adding this wood furnace is to cut back on the use of the gas furnace, which cuts down the amount of time the inducer fan is running. Heat rises, as we all know, but does it rise enough to properly vent this water heater? Are there any other problems with my thinking on this route?
09-19-2005, 12:48 PM
What you are proposing to do is dangerous, call a pro.
I am doubting that you are a serious poster, I suspect you are trolling in here. This kind of nonsense makes posters in here more reluctant to help legitimate people looking for help as they have no wish to be strung along by a troll looking for some cheap thrills.
09-19-2005, 06:18 PM
I'm not sure if I should be angry at your accusation, or glad that you take your profession seriously. I am a well educated manager in the constrcution industry. I work for the largest homebuilder in the country, and have taken many classes on many topics including HVAC. I even have a degree in construction managment. However, I do not claim to know it all, and often use available resources (such as this one) to gain information and insight into topics in which I am lacking. So while I can tell you how to properly run a b-vent from a new furnace through the roof on one of my new homes, I cannot tell you if my hundred year old farmhouse with its new furnace can be retrofitted to utilize multiple fuel sources. With utility companies estimating a 30 to 50 percent rise in natural gas prices this winter, I'm sure you'll find a plethora of questions such as this coming through in the next couple of weeks. What I'm trying to do here is nothing new. This is all deja-vu from the early 70's, when millions of homeowners installed wood heating in their homes due to the fuel shortage. Again, I appreciate the fact that you are taking your job seriously, but there are a lot of us out here who are looking for good reliable info to add to our knowledge base when considering options and making decisions, and empty accusations will get us all nowhere fast. If you claim to be a reputable HVAC professional engaging in threads on a discussion site such as this to help people and answer their questions, then it seems as though something has missed the mark here.
Back to the discussion:
Dangerous, possibly. Flues, stovepipes, high temperatures, fires, and combustion gases can always be dangerous. I will have a pro complete the work if I decide to go this route. However, I don't even know if this is a viable route at this point or not. Seems most people in the biz lately have all been installing the same furnace on the same floorplans over and over (myself included) and no one knows much about retrofitting anymore. Even the inspector in the city I work in shrugs and tells me that all furnaces vary. This I know, but I can't even find a customer support line for Trane on the internet. I know that a 90+ efficiency unit can be side vented via pvc pipe, but I'm wondering if it is possible with the 80% efficiency unit that is currently in my house. Our gas fireplaces that we install on our new homes use a side vent, so it seems like something that would be a possiblity.
09-19-2005, 06:39 PM
You would most likely need an approved side wall vent kit for the 80% efficient furnce, such as made by tjerlund, it would include a double acting barometric and a remote inducer fan.
You would need to go sidewall direct vent with the water heater, a coaxial sheet metal pipe with in a pipe with water heater installed along an exterior wall OR use a power vented water heater that vents similar to the 90% furnace, or go with an electric water heater.
So unless that 80% furnace was brand new, better off using a new 90% gas furnace.
If you had two tiles on the masonary chimney, you could see them if you were up on the roof. Usually two tiles would mean a furnace in the basement and a fireplace on the main floor.
You are asking dangerous stuff, and we have a few individuals pretending to be home owners, who like to ask dangerous questions, to string the regulars along. They enjoy people wasting their time to respond, and if this is not your case, sorry. Our resident troll will enjoy me even thinking that you are him, its a sad way to get your kicks.
09-19-2005, 07:42 PM
Ditto Carnak and Jack
Wood must vent into its own flue-period! You can run a liner to the UL1777 listing, which includes proper insulation not you cannot run a liner for wood without insulation and still meet the listing. The chimney needs a Level II inspection. The sweep or whomever you hire can advise on the suitability for a liner and help with sizing.
As for the gas venting, the draft inducer fan is not recognized as a power vent. Also, you cannot connect Cat1 appliances to a vent that is under positive vent pressure. If the unit is so listed, you might install a Tjernlund or Field external power vent that sucks the gases out rather than pushing them out. The pipe must be run to strict instructions and the fan must interlock to both appliances. Either way, you need professional on site advice on both the wood and gas and this means two separate contractors.
I agree with Carnak's caution. Just put yourself in our shoes and don't complain about free advice.
09-19-2005, 09:57 PM
Don't believe you could vent the water heater and furnace on the same power vent. If you have a lined chimney (tile liner) I think you could vent the water heater and woodstove in the same chimney, we used to do it all the time with water heater furnace and woodstove, gas must be installed higher in the chimney than woodstove. Of course this was in the old days before fan assisted draft on the furnaces. We did try side venting a 80% furnace once to see if it would work, the gas inspector was also curious as to whether it would vent, didn't work. The furnace pressure switch kept dropping out. The purpose of the inducer fan is not to assist venting but to draw the heat through the heat exchanger.
I would say venting any gas appliance with a wood stove is stupid
09-20-2005, 12:38 AM
May be stupid but it is legal in western canada, although in most towns now woodstoves are illegal due to polution concerns.
09-21-2005, 08:14 AM
Get a chimney pro in there. You might find that the Chimney's so large that 2 liners could be supported: one for the gas applianceand one for the wood burner.
09-21-2005, 09:53 PM
I appreciate everyone's help w/ this! I got the chimney inspected, and it has no liner in it - just straight masonry. At this point I think the cost to make all of this work will outweigh the benefits. I'd need at least a thousand dollars of chimney work, a new furnace, and a new water heater. I may look into other avenues instead, such as a more efficient gas furnace, or even an outdoor wood boiler. Again, thanks for all your help!
09-22-2005, 01:07 PM
If you upgrade the furnace to a Cat.IV, you can run PVC power vented out the side instead of the chimney. Regardless, you need to spend some money on the chimney to make it weather proof if nothing else. Does it have caps? Is the flashing properly cut into the mortar joints? Is the exterior masonry treated with a 100% vapor permeable water repellent Tx?
I hope that "thousand dollars" for the chimney did not include a liner. Just remember, you get what you pay for.
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