Is this furnace venting/leaking problem as bad as it looks?
I’m visiting my parents’ place for the holidays and using my newfound amateur knowledge of building science to find everything wrong with their old house. 🙂 I’d like to get some confirmation on the most worrying thing I’ve found: a leaking furnace flue.
The furnace is a 30 year old 80% efficient model in the basement with a round metal flue that goes into a masonry chimney and exits about 40 feet higher at the top of the chimney. The connection to the furnace in the basement curves down at one point:
And the flue is leaking nasty-looking brown goop and water, which is running down the wall:
…and collecting on the carpeted floor, causing a moldy smell:
My amateur diagnosis is that the flue is mostly blocked from 30+ years of no cleaning, excessive length, and poor flue design.
My amateur prescription is replacement of the furnace with a high-efficiency sealed combustion model that vents through the rim joist, along with replacement of the 30 year old atmospheric combustion water heater with a high-efficiency gas condensing water heater with the same kind of venting so that we can seal up and abandon the old flue.
Does that sound correct?
Also, until we can get the furnace replaced (hopefully < 1 week), what would be a good temporary solution to stop or collect the condensation?
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Replies
That doesn't look like the correct vent material to me. I saw one just last week like that, made out of singlewall pipe and elbows, taped together with foil tape.
If they have the budget to replace the appliances and venting, it would be a smart move.
Nathaniel ,
Go to the local hardware or big box store immediately . Purchase a basic pair of tin snips and a couple of the fittings and pipe that are there . Do away with that reverse pitched section of pipe right away while paying attention that proper pitch from the water heater and furnace are maintained . Do this before you leave your parents' place to insure that this flue will not be the cause of a lack of future visits . . While you have that piping apart , at a minimum get whatever garbage is collected at the bottom of that clay thimble the hell out of there . Don't forget to use a metal tape on all joints from appliance to chimney and seal the annular space between the metal flue and chimney at the joint .
If this is not an option call a professional and seek his assistance .
Thanks for the advice, Richard. I've considered doing that. They've agreed to replace the units with new sealed combustion ones as the existing ones are 30 years old anyway and they're planning to sell the house soon, and have the impression that prospective buyers won't be too thrilled about such geriatric gas appliances. However, that might not happen for a week or two (I'll be here to supervise). Do you think it's important to clean and repair the flue before then?
As a amateur "Building Scientist" isn't it premature to replace the furnace before proceeding with proper air sealing practices first (blower door test) and then size the furnace properly? Most utilities provide this service for a minimal cost.
This is what a potential buyer will look for and can be used as a selling feature.
Most likely a new efficient furnace will use a power vent rather than the existing chimney. As for your condensation issue, I'm betting the chimney liner (if there is one) does not go down to the floor and is directed straight to the boiler. This may be why the moisture is coming out at the seams or the chimney is filled with years of debris.
If it's not lined, I would be concerned with elevated CO level's in the home.
Another step aside from the advice given above is to open the cast trap door at the bottom of the chimney and clean out all of the brick shavings, mortar and other debris to allow any pass by moisture to land where it should be going rather than draining back to the boiler.
If your adamant about replacing the boiler in a week without air sealing the house first, leave everything alone for a professional who is licensed to deal with it. After all this is not a new problem and has been on going. Just my opinion.
There is no chance of air-sealing this house. Aside from the substantial cost (28 80 year-old super leaky windows with leaky storms, daylight visible all over the place, asbestos attic insulation, multiple additions with poor detailing, etc.), the house is a building science disaster (rotting wood windows, humidifier and dehumidifier running at the same time to maintain comfort, second floor that is intolerably hot in the summer, unvented cathedral ceiling with nothing but fiberglass batts in it with can lights in it) and my folks are unwilling to undertake all the required ancillary projects. They are typical homeowners who care about safety and comfort 100 times more than efficiency. Inasmuch as reducing air leakage would also reduce the safety of this old house in a rainy, humid climate without doing a bunch of other stuff, it's a non-starter.
Before you ask how such a house is even salable, the entire housing stock in this highly desirable neighborhood is like this and they all sell very quickly. I don't really get it either, but people vote with their dollars about what they care about...
Richard McGrath, I'm not sure about the recommendation for tape on the joints. I believe they should be fastened with screws. What tape is listed for flue pipe joints?
Well that's an awful lot of critical information. Definitely leave this home to a professional to deal with. Good luck!
The job--whichever is it--will definitely be left to professionals.
It sounds like the two options are:
1. replace the equipment with sealed combustion units with new venting and abandon the existing chimney stuff
2. remove the metal flue pipe, clean the chimney, line it properly, create new flues with correct slope, and hope the furnace and water heater last
Sorry I forgot to mention using screws david . Something I take for granted , 3 per joint . The tape I was referring to was metal tape to cut down on leakage . I will provide manufacturer and SKU if you require . Does not matter if you're sure , it's good practice , the tape is metal . Prevents leaks that may happen , can you ever do too much to protect the well being of occupants dwelling within the built environment ?
Nathaniel,
It's hard to tell if this is a dangerous problem or not without more investigation. But I suspect that an oversized flue or a cold flue is part of the problem.
For more information on this issue, see When a Flue is Too Big.
Richard, I would like to see whatever you have on taping singlewall joints in an application like this.
The furnace fuel is indeed natural gas.
Flue pipe generally, while the exhaust is drawing up a chimney, are under negative pressure, meaning that at joints air leaks into the pipe not out. That's why you see flue pipe screwed together at joints but not taped. Look at a thousand installs. I have, no tape. I have never ever taped flues.
All exhaust flues should be serviced, oil fired should be serviced once to twice per year. Gas flues that I do are double wall to keep temperatures up and stop condensation in old style non condensing systems. New condensing systems use what the manufacturer states to use.
I guess all are assuming natural gas is the fuel?
And yes flue pipes have to be piped upward when horizontal, the manufacturers today give all the proper piping specs with the install manual or PDF.
Oh and yes the chimney may be the wrong chimney if at one time it was built for coal or oil. For natural gas to not condense it would have the proper liner added.
Get proper trusted pros onsite. They will love your desire to upgrade, the costs will shock you though. If selling I would sell the home as is after servicing the flue pipe and changing it's slope and cleaning the chimney all via a pro of course.
David ,
I don't know exactly what it is you are looking for but here is a link to what I use to lessen the likelihood or level of harmful gasses that may cause a problem . As I stated before it was my misfortune not to mention screwing said pipe also , sometimes I forget where i am posting . This FOIL ( aluminum) tape can do no harm .
http://www.amazon.com/Nashua-Aluminum-Multi-Purpose-Foil-Thick/dp/B00B28GJFO/ref=pd_sim_sbs_indust_5?ie=UTF8&refRID=0ZTQSTNNA7GV14PT6NDQ
Richard, my thoughts on tape. If applied to a proper set up of flue it can do no harm but if the tape hides problems like no draft or plogged up chimney flue base or a condensing situation filling the flue with liquid instead of seeing the problem via liquid oozing out. No tape for me. I want the draft to return and the condensation to stop. That is key to proper. Agree? It is for me.
And yes there is proper flue tape made for that purpose though I do not use it.
Richard, I'm not sure I can prove I'm right, but I think tape on single wall vent connectors is unnecessary and could possibly even be a risk if the adhesive melts or burns. The link you posted states the tape is good to 200F. I'm betting that draft hoods and other metal near the outlet of an appliance gets hotter than that. I'd have to dig into code books I don't have to find out if there is anything in there that covers this.
I see tape on vent connectors periodically, and usually they have other signs of trouble. The last one I saw was single wall right through a wall thimble, with tape all over the place.
I use a lot of that tape myself, but only when installing dryer, bath fan, or range hood outlets, and the occasional forced air duct repair. That's what I think it's intended for. I agree with AJ that vent connector piping is under negative pressure in use, and if there's any positive pressure the byproducts are just going to blow out at the draft hood anyway.
Gentlemen , in this type of masonary chimney any condensation is going to happen in the masonry and not in the metal portion of the vent because as we have stated that portion of the flue is certainly going to be hot enough . The condensation in this particular flue has ended up exactly where one would expect , in the bottom of the assembly at the cleanout and from around the pipe . Again , in a properly installed vent the condensation should it run back through the pipe is much better coming out of the draft hood as opposed to the pipe , people will know something is wrong and seek assistance ,people tend to ignore drips but sizzles and water from what most would deem the boiler , furnace or the like get folks attention . Kinda like the code that says a drain from an emergency relief pan must terminate over a door or window or other conspicuous location so the fact that there is a problem can be identified . That is in a code , International Mechanical Code current edition and everyone for the last 2 decades at least .
When we do non condensing stuff the flues get taped , but keep in mind I don't install garbage and my stack temps are not near 200*F ,because the bulk of the energy in systems I design and install have very little stack loss due to the energy being transferred to the heating medium efficiently . Thsi is something not found in a Crown Aruba boiler but easily accomplished using Buderus or Veismann . Yes we even install residential oil boilers that can be vented with PolyPropylene
If you come across a boiler producing temps higher than 200*F in the stack , replace it with something made in this century .