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Furnace Vent Condensation Concerns

Posted by BTooth (My Page) on
Thu, Jan 9, 14 at 13:11

We had a new HVAC systems installed a little over a year ago. I was in the crawlspace this morning, and noticed that the concrete wall was damp with some efflorescence right beneath where the furnace vent enters the base of the chimney. See attached image.

I've already scheduled an HVAC tech to come out to the house tomorrow morning, but I'm wondering if I should also get a chimney inspector involved? I just want to make sure that I'm not getting a band-aid solution or incorrect solution.

I don't beleive we had this issue prior to the new furnace. I've done some research, and it seems that perhaps the higher efficiency furnace may be the culprit and that the exhaust isn't exiting the flue prior to condensing. If that is the case, what are the potential solutions? Is there an active element that can be added to the path to force the exhaust out more quickly?


Follow-Up Postings:

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RE: Furnace Vent Condensation Concerns

I'm not a pro, or even close, but here is my experience. Lets see what the pros think and if I'm close or not!

I have had a high efficiency furnece since we bought our house 17 years ago. Mine vented through a pvc pipe up through the attic. We have ALWAYS had condensation on that darn pipe in the attic. The hvac people blamed the roof, though it never never got wet in the summer, only when the heat ran.

We got a new furnace this year and my installer was only "ok". I did figure out that the last few feet of the pvc were not insulated and maybe it was condensation. Stuck one of those foam pipe covers up there and this year there is no condensate leaking! Guess I was right to blame the hvac.

I would say you have condensation. My first thought is to ask if that pipe should be pvc, and second is if it should be insulated, , is there a leaky joint? (that's what I thought our problem was, but turns out the condensation was probably on the outside of the pipe.) Is that horizontal run too long? I think it might be if it still has to travel up and out rather than through a wall and right out. Mine goes almost straight up and out the roof (through 2 stories--we don't have a chimney in the basement to use)


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RE: Furnace Vent Condensation Concerns

It definitely feels like a long run, but I'm not an expert, nor am I familiar with the NC code (though I guess I could look it up). The furance is in the same location as the original that it replaced (circa '91 home). I'd assume the house passed code when built, and it was inspected when replaced. It's probably a good 12' from the furnace to the wall, then it's gotta be a good 30' to the top of the chimney. I have no clue how the flue runs up the chimney. I don't know the material. I don't know if it's actually 2 flues running up the chimney (1 for the fireplace, 1 for the furnace), or if they meet somewhere. There is a metal cage/cover on top of the chimney, so I can't tell if it's 1 or 2 stacks. If you can't tell, this is my first home, and I picked one that is teaching me a lot!!

I don't think the condensation is on the outside of the pipe. I guess the theory behind the higher efficiency furnace as the culprit is that with less emission the exhaust isn't forced out of the vent fast enough, so basically it has more time to cool off and condense before it can even escape the flue.

My fear is that I don't know if it's an HVAC system design problem or a chimney/flue problem. I'd hate for it to be a chimney/flue problem, and the HVAC folks do some uneccesary work that just masks the root problem. I guess it can't hurt to get the chimney inspected anyways. We've lived in the home 3 years and never had one, but we never use the fireplace.

I'm sure we'll get it figured out, but I like to be armed with knowledge.


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RE: Furnace Vent Condensation Concerns

What furnace do you have? If the efficiency is 90%+ then it can vent out the side of the house with a PVC pipe. You don't need the chimney.

It looks like the installer tried to make use of your old chimney exhaust. The connection is not water tight and is leaking water into the crawl space. The installation does not look correct to me, but I am not sure.


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RE: Furnace Vent Condensation Concerns

It's 80% furnace, so not high efficiency, but higher than what it was replacing. Here's a pick of the outside of our chimney, which I rarely see. I can't believe all the efflorescence ! Something is not right.


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RE: Furnace Vent Condensation Concerns

My furnace was. 90% and now 96%, so my comments aren't relevant. I think only 90% and higher can use PVC. I would get the chimney checked. Maybe something came loose.


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RE: Furnace Vent Condensation Concerns

Well I throw my 2 cents in on this ... I think that when it exhausts into the brick chimney, the chimney is creating an environment for condensation like cool or cold bricks rather then having it exhausts out to the direct atmosphere (Plus that seal on the inside doesn't look good to me either). I think what you see on the outside of your brick chimney is the direct impact of the furnace exhaust hitting those bricks (on the inside) in that spot. OK I'm done just me 2 cents.


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RE: Furnace Vent Condensation Concerns

Big Al, that's what I thought. The water is coming from 1 of two places, or perhaps both. It's either rain water that is somehow penetrating from the top, and/or exhaust condensating before it exits the chimney. The fact that the concrete in the crawlspace looks quite damp, that it hasn't rained recently, and that I don't remember seeing it in the summer when the furnace wouldn't be running, leads me to believe it's the latter. I'm hoping the HVAC tech coming today has the ability to investigate it further, with a scope or something.

If it is the exhaust condensing before exiting the chimney, does anybody know the solutions? Would a smaller vent pipe/flue liner force it out faster?


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RE: Furnace Vent Condensation Concerns

I think the proper fix would be to install a chimney liner. No matter how fast the exhaust moves, it will cool when it hits the top of the chimney and forms condensation.


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RE: Furnace Vent Condensation Concerns

HVAC tech just left. Nice guy, but no clue about what was going on. His supervisor is supposed to be coming by Monday.


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RE: Furnace Vent Condensation Concerns

This is a two stepper. First, by code you can only run 1.5'/inch of exhaust diameter, so if you have a 4" vent connector you must not exceed 6' of single wall vent connector. The whole vent connector from furnace to chimney should be B Vent. As for the chimney, I am assuming it is not lined with a properly sized chimney liner. I am not talking about the chimney tile liner. The National Fuel Gas Code has prohibited this type of installation for almost 20 years. Since 91 when this was coming into effect the subsequent editions of the code have become tighter and tighter in this regard. Sorry to say, but installing a metallic liner should be standard operating procedure for your contractor. I'd have a chimney sweep certified by the Chimney Sweep Guild install a new liner. I wouldn't trust the current installer to do it correctly. He's probably not insured for that kind of work.

Had you known this going in, it would have made sense for you to go with a 95% eff condensing gas unit and abandon that chimney. With the additional cost of the liner it would have been cheaper and better in the long run. Masonry chimneys are 11th century technology venting 21st centuryappliances.

Oh and another thing. Your old furnace was probably Pre-91 and therefore about 60% efficient. The new one is 78%. I'll bet he installed the same size unit as the rating plate on the old one giving you a grossly oversized furnace. Wish I had better news for you. Also, don't let yourself get hustled further on this by having someone come up with a workaround. What I've told you here is correct.


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RE: Furnace Vent Condensation Concerns

"our old furnace was probably Pre-91 and therefore about 60% efficient. "

Until I replaced my 38 year old furnace three years ago I had it service annually. The measured efficiency was always between 81% and 82%. My new furnace is essentially the same efficiency.

I don't understand "60% efficiency?" Not arguing, just don't understand!


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RE: Furnace Vent Condensation Concerns

Thanks for your feedback Jackfre. I feel like the intelligent thing to do at this point is determine what county inspector signed off on it and ask them to come back out, after all, my taxes and fees pay them to keep my home and family safe during these types of projects. Unless I can get referred by a friend to a decent chimney contractor, I'm equally likely to get hustled there. Unless of course I can find a chimney inspector that is not in the market for remediation.

Honestly, I don't know what the heck I thought about the furnace. I guess I thought it just vented out the side of the house like a direct vent fireplace or something. I was very happy with the HVAC salesman. He did a sizing analysis and I recall talking about how many BTU I needed, etc. Still doesn't mean it was sized correctly, but I did have other contractors that didn't do a sizing analysis before quoting me. Unfortunately nobody mentioned to me that I may need modification to the chimney.

BTW, the HVAC company I use is a huge nationwide company, and I'm sure they do tons of installs on homes in my area, many of which were built from the early 80s, into the 90s, and many of which have masonry fireplaces. I've had some very competent techs show up that I am very confident with, and other times I'll just ask for someone knowledgable to come back out as nicely as I can. Unfortunately today, I had to do the latter.


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RE: Furnace Vent Condensation Concerns

The horizontal vent pipe running from the furnace to the chimney is ~12' total. The output at the furnace appears to be 4", and the diameter of the pipe is 5". According to the code venting tables, it seems that 75k BTU falls in the acceptable fan min max range? Also, I guess it's supposed to be double wall B vent in crawlspace, but I have no clue what they actually used. Also, the first 5 feet of the run from the furnace is not sloping 1/4" per linear foot. It's actually level to potentially slightly inclined toward the furnace. I'm almost positive they didn't do squat with the chimney when it was installed, and the only all caps bold print in the code that I read states, "SINGLE APPLIANCE VENTING OF A FAN ASSISTED FURNACE INTO A TILE LINED, MASONRY CHIMNEY IS PROHIBITED". As Jackfre said, it has to be lined with type B vent. Looking at the connection from the vent connector to the chimney, it looks like it's just a clay pipe coming out of the chimney. Also, is it normal for the condensate line from the condensate pump to have water in it? What if it gets cold enough in the crawlspace and freezes? Anyhow, I'm foregoing contractors for now, and calling the county permit/inspection office Monday morning when they open and requesting somebody to come out. If it's wasn't installed according to code, it should be on the HVAC company to fix it, and have it re-inspected.


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RE: Furnace Vent Condensation Concerns

Did the contractor apply for a permit and was a final inspection done after the work was completed?


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RE: Furnace Vent Condensation Concerns

Hey Mike, Yes, I looked the permit up last night. Both the mechanical and electrical inspections failed initially (they don't say why online), but then passed the following day (I assume after things were addressed and re-inspected).


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RE: Furnace Vent Condensation Concerns

National Company! I'll guess Delta Mechanical. It all depends upon the guy doing the work. You should not have 5" on the vent connector. You should have 4" B-vent. The inch diameter you pick up will allow for proper grading of the flue. Your best bet on liner will be to install B-vent all the way down. You should have a bottom capped tee at the bottom of the stack as a drip leg. Yours is the worst condition for a Category I furnace, long vent connector and exterior chimney.

If you go with a chimney sweep for the liner, as I said, chick out the Chimney Sweep Guild for a "real" contractor. You are correct about getting taken on this. There has been all kinds of fraud in this field. Perhaps it would be best to make the installing company come through on this. MAKE SURE, if you have them do it with B-vent that they screw the joints together with screw that does not penetrate the inner aluminum liner. A seemingly small but important detail. Also have them use a metal tape on the joints. They can drop the tee and vert portion into the flue and work back to the furnace, finishing with an adjustable length at the furnace.

I am glad that you liked the salesman, but I would not let him off the hook on this. If he is a good guy you can help him become better by insisting that he come out and look at the system. You never install a Cat I furnace where a masonry chimney is going to be used. It is "the" reason to go to the condensing furnace using PVC for the side wall venting. As well, you pick up another 15% efficiency. You will be helping this man if you have this conversation. Nationally, once you cross the Mason-Dixon condensing furnaces are 95+% of the market. As you move south, the view is the heating load is not as high so, why buy the more expensive (efficient) equipment.

Net/net, regardless of intent, you got stiffed here by nice but not well informed people and your inspector pencil whipped the permit. Perhaps that is a bit tough. they did fail the inspection initially.

Salti, heck, we don't argue. I enjoy reading your posts. In my experience, if you were hitting 80% with the old furnace you "grabbed the brass ring", unless it was an oil furnace and the better brands have almost always hit the 80% mark. Is it oil? If it is gas and old, I really am surprised.

The reason the codes changed since '91 is that by being mandated to deliver a min efficiency of 78% up from those 60% figures is that by putting the heat into the house there was less energy available to drive proper venting of the new appliances in masonry. Proper venting meaning getting the flue gases out as a vapor and not having them condense (approx 3.2 Ph) on the walls of the flue. You can hide a lot of design issues with a high stack temperature. With high exhaust temps you gained velocity. With 78% you didn't have a chance. Even at that, it was interesting seeing the code changes over the years. Every four years with the new edition of NFPA 54 the restrictions became more and more stringent until about 10 years ago they finally said, "masonry chimneys do not work and do not use them".

Also, be aware, if you are hitting the 80% combustion efficiency" that is not remotely close to system efficiency. You have probably seen my fairly frequent posts on googling, "doe duct leakage". If you want to get the best out of your heating dollar, seal the duct system. Residential duct systems in the US are terrible. It is the "crazy aunt in the back room" that no one in the industry wants to talk about. It is a big deal and it is finally coming to light. What I have always said is the unitary manuf (Carrier, York, Lennox, Trane, etc) can put all the technology they want in the box. They simply cannot deliver it…due to duct leakage

Happy Heating!


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RE: Furnace Vent Condensation Concerns

The county chief mechanical inspector and a rookie field mechanical inspector stopped by today. They said that the venting did not meet code, and for me to ask the HVAC company to issue a new permit, vent the furnace correctly, and have it re-inspected. If the HVAC company refuses, the inspector will report the company to the inspection board. No clue how intimidating that is, so we'll see what the HVAC company does. They are supposed to come out tomorrow afternoon.


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RE: Furnace Vent Condensation Concerns

Jackfre,

" In my experience, if you were hitting 80% with the old furnace you "grabbed the brass ring", unless it was an oil furnace and the better brands have almost always hit the 80% mark. Is it oil? If it is gas and old, I really am surprised. ..."

My old furnace, and its three-year-old replacement are/were both oil... both have/had efficiencies in the low 80's.

I gather from your response that older 8o% gas furnaces are not so efficient?


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RE: Furnace Vent Condensation Concerns

"Both the mechanical and electrical inspections failed initially (they don't say why online), but then passed the following day (I assume after things were addressed and re-inspected)."

"They said that the venting did not meet code, and for me to ask the HVAC company to issue a new permit, vent the furnace correctly, and have it re-inspected."

You posted contradicting information in two of your posts. How can the mechanical inspection first fail, then pass, and then later fail again? Something fishy is going on. In my opinion you or the contractor should not have to pay for another permit for the same installation.


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RE: Furnace Vent Condensation Concerns

I'll let the HVAC company and the inspection office figure out the cost for the permit. The HVAC rep responsible for permits said that the company would either line the chimney or replace the furnace with an equivalent high efficiency model that can be vented out the crawlspace with PVC. I assume the latter is probably the easiest solution. Supposed to meet with a supervisor this afternoon.


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RE: Furnace Vent Condensation Concerns

I think, even if you had to contribute some dough to the equation, your best bet is to get the high efficiency unit. B-vent is a good product, but long horizontal runs of B-vent into an exterior chimney line with AN INSULATED LINER, meaning more B-vent or some other insulated product MAY still condense. This is very common with external chimneys. They are very difficult due to the mass of the masonry. DO NOT allow a single wall aluminum liner to be installed. It will condense, especially so if the furnace is oversized and short cycles. It takes a lot of heat for a external chimney to draw properly. In the mean time it is condensing. Push for the 90%'er. You want done with this and, again, even with the liner, on that external chimney you MAY still have problems.

I was in the rep business for 25 years representing chimney and chimney liner manuf. I have seen a great deal of this. Sorry to "flash a badge" ;) but try to get the high efficiency unit. You and the contractor will be happier.

Salti, yes oil furnaces, the good ones, have always been more efficient than the old gas units. After '91 the lowest efficiency gas units and oil were more or less neck and neck. The hi-efficiency gas units gained 12-15% over oil.Oil is great heat, but unfortunately, it is declining rapidly. FYI, I had a lot of people with oil heat install mini-splits and run the H&C with them and only run the oil when it got really cold. A friend of mine did this in a big old VT farmhouse overlooking Lake Champlain. He ran the MSHP's down the 20* and only below that would he run his oil fried system. He reduced his oil consumption by 80% and was perfectly comfortable…and happy! A lot of times my suggestion was, "don't change…add"


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RE: Furnace Vent Condensation Concerns

Getting the 90+ furnace, so I suppose a happy ending. We've also had pressure issues upstairs with the master BR door slamming closed (half the output is to that room/bathroom and the return is in the hallway) so they are installing a return vent in that room at no cost to us.


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RE: Furnace Vent Condensation Concerns

Is the plan to now vent out the side of the house?


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RE: Furnace Vent Condensation Concerns

Make sure they run both intake air and exhaust to the outside. You want the combustion air coming from outside too.

Please come back and let us know how this all goes.


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RE: Furnace Vent Condensation Concerns

They used a concentric kit, so there's one pipe coming out the crawl space wall. I have to call them to schedule an inspection.


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