SS Relining with intermediate thimble

[ppanish]

I'm having my chimney relined and have a few questions. My house is relatively new (15 years) and the chimney is a well constructed centrally located (no exterior wall exposure below roofline) 2 story design (basement plus 2 floors) with complete fireblocking and required clearances to any framing. Unfortunately an 8x8 ceramic flue liner was used in the original construction (I didn't know any better). I have some minor cracking in the flue tile, probably due to an uncontrolled burn a few years ago, and on the recommendation of my sweep have decided to have the ceramic liner removed, and the chimney relined with a SS system.

The single flue is being used for a wood boiler in the basement which I installed a couple of years ago. I have an intermediate thimble on the first floor from the original wood stove, which is closed off, but which I would like to keep available in case I ever lose power for an extended period and need to keep the house warm (I'm off grid). I also seem to quickly build up a creosote glaze in spite of having my peak burn flue temperature between 420 and 500 degrees F.

The contractor I'm talking to wants to use a 7 inch smooth-wall flexible SS liner with an insulation wrap. They have yet to get back to me on whether they can handle the intermediate thimble. I asked about using an 8 inch liner with poured insulation, but they claim it's difficult to get the mix to uniformly surround the liner.

My question is whether it's possible to retain the intermediate thimble with a SS liner, should I push for an 8 inch liner, and is the smooth-wall flexible system a good way to go for my situation?

Thanks for your help.

 

[pen]

Unfortunately for your setup, even if it were possible, each appliance is supposed to have it's own flue.

NFPA 211 are the national guidelines for these appliances.

pen

 

[pen]

NFPA 211 (these are the 2003 guidelines, I don't have access to anything newer but I have not heard of a change to this section 9.8.2 )



Here is a link to a document about installations from New Hampshire that also makes reference to this situation not being permissible. http://www.nh.gov/safety/divisions/...nVentingofResidentialHeatingAppliances_2_.pdf

 

[ppanish]

Only one appliance would be connected to the flue at any given time. This is only in the event I have an inverter failure in the winter, which could take a few weeks to replace. It's a matter of 1/2 hour to swap out the respective stove pipes and would not violate code.

 

[pen]

Even in that circumstance, the guidelines being what they are means the components you would need to make this happen may not be in production (In my time looking at components, I've not seen a thimble that accepts a liner on top and bottom).

Also, you mentioned glazed creosote in your current setup even though you run solid flue temps. Any chance that the upper thimble is not 100% sealed and air is being pulled in here by the draft of the chimney while the basement unit is being run? If so, that would cool exhaust gasses and help contribute to that glaze.

In all, even if you don't have a local codes enforcement officer to worry about, I'm surprised your homeowners insurance didn't pick up on the double entrance to that flue and red-flag you until it was permanently sealed. My point in saying that is even if you make changes now and get it by your current insurance company, if you ever have to / want to change in the future, a setup like this might make that impossible to get coverage.

Sorry to be a dark cloud here, as I know where you are coming from and believe you in how you'd operate things, however, they (insurance companies and similar folks who check on this stuff in our homes) work off the principle that we are a liability from the start, and an exception like this often doesn't go over well.

pen

 

[ppanish]

The upper thimble is well sealed, and the glaze extends the whole length of the flue, from well below the upper (unused) thimble. I don't think thimble leakage could cause a situation such as this without being pretty apparent in terms of air movement. It appears more likely that the ceramic liner has enough thermal mass that it is relatively cool much of the time. I have a highly efficient building envelope, and typically need to run only one fire/day for heat and hot water unless it's unusually cold. The high temperature portion of the burn probably lasts two hours at most before dying down to embers. My hope is that the stainless liner will resolve this by heating far more quickly, and have much less adhesion.

I'd appreciate it if we could limit the discussion to whether or not this can be accomplished with available liner components, and leave the rest to the respective code and insurance authorities. I appreciate your mention of it, and I'll make sure there is no objection to what I'm trying to do. If you're sure that the stainless tee's can't be used in this way that's what I need to know.

 

[transmogrifire]

Class a tees accept pipe on all three sides. There is a way of attaching class a pipe to a liner. That does not mean it should be done.

 

[ppanish]

Any thoughts on the 7 vs 8 inch flue, wrapped vs vermiculite insulation part of the question?

 

[pen]

What wood furnace are you working with? What is the size of it's flue collar? How tall is the chimney? Does the furnace manual state that it is OK to run a smaller diameter liner than it's flue collar?

I'm going to move things over to the furnace/boiler room.

 

[ppanish]

It's a custom boiler made by Hydro-to-heat-convertor with a collar size of 7 inches. Chimney height is 35 feet.

The designer has allowed a reduced collar size (BTU output of the device is only ~40,000 at peak burn), however the problem is that the door on the boiler is very large and the draft is insufficient to prevent smoke from being dumped into the basement when the door is opened too wide at certain stages of burn. I'm going to make an insert to reduce the door opening which should help this significantly, but I'd still like to maximize draft.

 

[bholler]

well changing the size of the liner will not actually change the draft. but a smaller liner will increase the velocity of the draft that you have I would say that you should use 7" since that is the size of your outlet. just by going with a round insulated liner you will be increasing your draft as well as your velocity. also most manufacturers tell you to open up the draft before opening the door in order to clear the smoke you should try this. as far as the insulation we usually use poured insulation but there is nothing wrong with wrap at all we use it many times also it just depends on the situation. and yes physically an intermediate thimble could be put in but we would not do it. and most reputable pros I know would not either. I am wondering what type of smooth wall they are planning on using. if it is the double wall stuff I would recommend against it in your situation because if you continue to get glaze built up it will require some pretty aggressive cleaning to get it off and the light double wall stuff wont hold up very long. I would go with heavy wall flex which is also classified as smooth wall.

 

[ppanish]

bholler, I realized after reading your reply that I hadn't stated things quite correctly, but I get the gist of what you're saying. The collar has already been reduced from 8 to 7 inches to fit in the old liner. Am I correct in assuming this means that if I were able to use an 8 inch collar without reduction into an 8 inch flue I would increase the draft?

I always open the draft for a bit before opening the door, but it's still problematic. The door is way too big, especially with the reduction collar...

Thanks for the information on the different liner types. Double wall wasn't under discussion, but otherwise the only thing I've been told so far is Stainless 316 smooth wall flex. Do you know what gauge would be classified as heavy wall, or is all smooth wall flex heavy gauge?

 

[BoilerMan]

With 35' you should be fine with 7", even 6" for that matter. Smoke spillage has a lot more to do with door opening/height from flue collar than most people realize. I believe your glazing is due to the high mass inherent in a clay flue, that will go away with a metal flue.

7" gives more insulation room, and vermiculite is a great insulator. I'd go 7" w/vermiculite, which is essentially a class A.

TS