How best to install through-the-wall thimble in cinder block?

[deckeda]

The cinder block was previously knocked out when my inlaws installed a wood stove. I am removing the wood stove and all piping associated with it. I bought a DuraVent 3PVL-KHA vent kit.

I do not own a pellet stove yet, nor is the interior wall finished, but I want to secure the exterior pieces ASAP because the current setup is janky and unsealed. So my immediate question is about the exterior thimble piece, and the broader question is about the rest of the wall cavity.








I'm not sure if it's best to create framing inside the hole and somehow attach that inside the cinder blocks, or if I should punt and just attach a board to the exterior and somehow seal it to the sculpted concrete originally slathered on the outside of the cinder blocks (what you're seeing is not stucco.)

 

[SidecarFlip]

That is called Parged and it's used to covert exterior cinder block that is weathering and is decomposing from age. Don't know what to tell you except lay in new block and cut a new hole (carefully) with a diamond blade circular saw. Looks to me like you have sill issues above the cabbaged out hole for the vent pipe.

In laws were lucky not to have burned the house down, that single wall vent pipe is way, way to close to the wood joist and subfloor.

 

[deckeda]

Thanks. Actually the sill is fine, it's just that all the old fiberglas is still shoved in there. 1.5" XPS has been installed in the rims with spray foam sealer I'd have to eventually cover up.

I don't think that cutting a hole in a new cement block is an option. The 3" vent pipe kit has a thimble which requires ~ 7 inches diameter. That would not leave nearly enough meat in an 8" block.

 

[SidecarFlip]

Thanks. Actually the sill is fine, it's just that all the old fiberglas is still shoved in there. 1.5" XPS has been installed in the rims with spray foam sealer I'd have to eventually cover up.

I don't think that cutting a hole in a new cement block is an option. The 3" vent pipe kit has a thimble which requires ~ 7 inches diameter. That would not leave nearly enough meat in an 8" block.

...but a 3 1/2" hole would.... read on...
Probably going out on a limb here but..........you really don't need a thimble in cinder block. The wall thimble is to space the vent pipe from combustible materials via an air gap. Cinder block isn't combustible and Pellet vent, unlike wood stove venting, don't get that hot. Mine running wide open might see 400 degrees outer shell temps. If it was me (my install, I'd take the end of the vent, mark the diameter in the block, masonry drill holes around the marked line and knock the center out, install the pipe through the block, caulk the difference between the pipe and the bloc hole with high temp RTV, allow it to set up and call it good. Considering how close that wood stove vent pipe was to the joist and flooring, your pellet vent is child's play......

 

[deckeda]

...but a 3 1/2" hole would.... read on...
Probably going out on a limb here but..........you really don't need a thimble in cinder block. The wall thimble is to space the vent pipe from combustible materials via an air gap. Cinder block isn't combustible and Pellet vent, unlike wood stove venting, don't get that hot. Mine running wide open might see 400 degrees outer shell temps. If it was me (my install, I'd take the end of the vent, mark the diameter in the block, masonry drill holes around the marked line and knock the center out, install the pipe through the block, caulk the difference between the pipe and the bloc hole with high temp RTV, allow it to set up and call it good. Considering how close that wood stove vent pipe was to the joist and flooring, your pellet vent is child's play......

Hmm, didn’t realize that was an option, but then again I’d never come across an install video or directions that showed thru cinder block.

Presumably a standard cap block (4” thick) would be OK? I’d still want something around or near it on the inside, like mineral wool for a little insulation. Later, once the interior wall goes up I’d get a drywall trim piece of some kind.

 

[rickwai]

When you are ready to put a pellet stove in, pull old pipe out and put new pellet vent in at the bottom of the existing hole then mortar the hole closed.

 

[JRemington]

Did they ever use the wood stove?

 

[deckeda]

Yes, they used it for about one season. We moved into their house when we sold our place in another state. To make some room, they cleared out some furniture into storage and basically moved into the basement so that we could have the upstairs.

Being unfinished and drafty It was understandably cold, and so they found a what I presume is a used? Dutchwest 2462. There was an existing Empire ventless propane (CIVF-21-1) down there, but lets face it: ventless propane makes Baby Jesus cry.

The first few times they lit it they didn't get the draft going very well. The outside venting was far too short, and it kept blowing back smoke at one of the main floor bedroom windows. Hey! Knock that chit off! Now!

They got some extension pieces, this time leading to about the height of the roof's edge ... still not tall enough for proper safety and performance, but whatever. One night, I woke up around midnight on a cold night and the main floor was filled with smoke. I ran to all the windows and threw them all open wide.

 

[SidecarFlip]

The top of the venting or masonry chimmney must always be higher than the peak of any existing roofline or you will have a backdraft (venturi) situation develop, depending on how the prevailing winds are blowing.

That don't apply to a forced draft stove or furnace like a pellet or corn burner where the combustion gases are artificially exhausted which makes biofuel installs much easier and less costly. However, exhausting near the side of a house can and will discolor the siding eventually, from the byproducts of combustion.

In my case I followed the roofline rule with my venting anyway

 

[deckeda]

Right.
I’m currently paused here. What are the pros/cons of doing the smaller hole in cement block (the common 4” thick variety) and mortar vs doing a large hole, using the thimble, and swapping in 2x board piece to block up the surrounding hole?

What attracts me about the thimble is that you have two mounting points (each end piece) you can secure to whatever each surface is. A pipe just going through concrete block sorta sounds wobbly by comparison.

I’m still thinking of tackling only the exterior first since that’s the air-leaky hole, and because the interior finished wall does not yet exist.

 

[deckeda]

Or ... why wouldn't I just lay this in like so. Now I have a hole already made for the 3" exhaust, and another one if I want to use a fresh-air inlet. I can motar it in, including the small gap below where the top of the block sitting below it was previously chipped out.

Just need to figure out how to seal the gaps around piping as it goes through the block.

 

[Oldmanwinter]

Any of the listed options work. Keep it simple and there is less chance of failure. Lol

 

[JRemington]

Yes, they used it for about one season. We moved into their house when we sold our place in another state. To make some room, they cleared out some furniture into storage and basically moved into the basement so that we could have the upstairs.

Being unfinished and drafty It was understandably cold, and so they found a what I presume is a used? Dutchwest 2462. There was an existing Empire ventless propane (CIVF-21-1) down there, but lets face it: ventless propane makes Baby Jesus cry.

The first few times they lit it they didn't get the draft going very well. The outside venting was far too short, and it kept blowing back smoke at one of the main floor bedroom windows. Hey! Knock that chit off! Now!

They got some extension pieces, this time leading to about the height of the roof's edge ... still not tall enough for proper safety and performance, but whatever. One night, I woke up around midnight on a cold night and the main floor was filled with smoke. I ran to all the windows and threw them all open wide.

I have no idea why that floor joist isn’t scorched, or why the house is still standing. That single wall pipe should be 18 inches from a combustible material.

 

[deckeda]

Late start today. Bought the motar, but before mixing yanked out the old external pipe and thimble and test-fit the cinder block. Would not fit; too much parging or whatever in the way. on both sides of the opening. Ran out of daylight. Boarded it up for the night. Or for the week if it rains all day tomorrow.

 

[deckeda]

... That single wall pipe should be 18 inches from a combustible material.

Right!? I first looked into the DuraVent double-wall stuff that lets you get within half of that distance, and the local Ace has one of those steel insulator/isolator boards I could attach to the joists, but the current clearance is only like 2-3" max, so I scrapped that idea. We can get free wood from a sawmill, so the free-fuel was mighty tempting to stick with a wood stove that we already know from experience can run you out of the house with heat ... even on the main floor.

I was willing to spend the large green for all-new double or triple wall piping outside for max efficiency but the inside clearance was always a showstopper, and I'm not going to dig out the ground and make a NEW hole in the wall. I even considered other areas of the basement but the spot its in is the only practical one.

I am not looking forward to the maintenance and eventual repair costs of a pellet stove, let alone chasing down pellets in middle-north TN, but I've considered every alternative for this FrankenLego old house and all the other choices are worse.

 

[FirepotPete]

You have some good options given. I would only add that I would make sure that there is air around the pipe and insulate that area with fire proof insulation. You don't want to have to wrestle getting that pipe out to change it if it is mortared in. And any concrete product touching the pipe will eventually eat it up.

 

[deckeda]

You have some good options given. I would only add that I would make sure that there is air around the pipe and insulate that area with fire proof insulation. You don't want to have to wrestle getting that pipe out to change it if it is mortared in. And any concrete product touching the pipe will eventually eat it up.

All agreed. Although the cinder block openings are square there should be about 1/4" - 1/2" gap around a 3" pipe. I'll check that tomorrow. I'm wondering if there's some sort of flexible/silicon-y type gasketing I could use around the pipe(s) that I could secure to the cinder block openings. Something firm-ish like that could also locate the pipe within the opening so as not to have pipe touch cement. I thought I saw something similar available for pipe that comes up through roofing.

Or I could just splooge high-temp RTV or whatever in there to seal it up.

I'm thinking the pipe-through-the-wall installation would wait until the interior wall + stove come into play. I wouldn't know what horizontal length to make that just yet.

 

[JRemington]

Right!? I first looked into the DuraVent double-wall stuff that lets you get within half of that distance, and the local Ace has one of those steel insulator/isolator boards I could attach to the joists, but the current clearance is only like 2-3" max, so I scrapped that idea. We can get free wood from a sawmill, so the free-fuel was mighty tempting to stick with a wood stove that we already know from experience can run you out of the house with heat ... even on the main floor.

I was willing to spend the large green for all-new double or triple wall piping outside for max efficiency but the inside clearance was always a showstopper, and I'm not going to dig out the ground and make a NEW hole in the wall. I even considered other areas of the basement but the spot its in is the only practical one.

I am not looking forward to the maintenance and eventual repair costs of a pellet stove, let alone chasing down pellets in middle-north TN, but I've considered every alternative for this FrankenLego old house and all the other choices are worse.

I was talking about there being a fire there when there was a wood stove there. Pellet pipe only needs 2 inch clearance to combustibles.

 

[JRemington]

I’m not trying to be a dick but that’s about an hour or two job. Take all that piping out of there but leave the sheet metal on the outside. Your now going to have an 8 inch round hole on the outside that a duravent thimble will fit into and cover and screw into the sheet metal. You could actually leave it just like that and run your pipe through, but seeings you have the other half of the thimble you may as well finish it. Get a 2ft by 2 foot piece of plywood, a half dozen tapcons and the size concrete bit for the tapcons. Put the plywood up there and secure it with two tapcons on both sides and the bottom. Now cut out a 7 1/4 by 7 1/4 inch hole to match the outside. Stuff some insulation on both insides, put your thimble together and screw it to the plywood. Now run your pipe up and out a foot and cap it or go up, out and up three feet if you like. You’re going to need an adapter T or a regular clean out T and an appliance adapter. Then a section going up and an adjustable piece. Then a 90 and a 2 foot piece going out and a cap. If you give me a measurement from the center of your stove flue to the center of the thimble I can tell you exactly what you need.

 

[deckeda]

I was talking about there being a fire there when there was a wood stove there. Pellet pipe only needs 2 inch clearance to combustibles.

Understood. I was giving you the backstory of what I first considered doing prior to the decision to find a pellet stove, since you had commented on the wood stove setup that'll be removed.

 

[deckeda]

I’m not trying to be a dick but that’s about an hour or two job. Take all that piping out of there but leave the sheet metal on the outside. Your now going to have an 8 inch round hole on the outside that a duravent thimble will fit into and cover and screw into the sheet metal. You could actually leave it just like that and run your pipe through, but seeings you have the other half of the thimble you may as well finish it. Get a 2ft by 2 foot piece of plywood, a half dozen tapcons and the size concrete bit for the tapcons. Put the plywood up there and secure it with two tapcons on both sides and the bottom. Now cut out a 7 1/4 by 7 1/4 inch hole to match the outside. Stuff some insulation on both insides, put your thimble together and screw it to the plywood. Now run your pipe up and out a foot and cap it or go up, out and up three feet if you like. You’re going to need an adapter T or a regular clean out T and an appliance adapter. Then a section going up and an adjustable piece. Then a 90 and a 2 foot piece going out and a cap. If you give me a measurement from the center of your stove flue to the center of the thimble I can tell you exactly what you need.

You're right, didn't take long (next post). The metal on the outside wasn't reusable for anything; it's just thin flashing and not structural at all, but I get your meaning. I decided to just go through the existing hole in a new cinder block and will find a way to seal it all when I do the actual stove install. Like I said before, I don't own the stove yet. That being said, the kit I bought does have many of the pieces you mention. I'll resell the thimble perhaps.

Fun fact about the thin aluminum flashing you saw on the outside. I added that a year ago ... they had just stuffed the gap with fiberglas, with nothing to hold it in or out on either side, with fiberglas falling out, getting wet, animals walking in. It was ... not good.

 

[deckeda]

a test fit, including a sample pipe




A local building inspector arrived to sign off on the project:




I mortared the large bottom gap as much as possible prior to sliding the block in, then proceeded to backfill and push as much in to fill the gaps inside and out. It's not pretty but it doesn't need to be pretty.




For now, I covered the outside with EternaBond (butyl) tape.

 

[deckeda]

Here is my idea for sealing up the exhaust pipe when the time comes; a high-temp silicon boot that can be secured to the cinder block. An air gap between pipe and cinder would be maintained so as to protect the pipe. It's non-permanent yet should be water and air-proof.

Amazon product ASIN B07ZZG3YFM
Any better ideas?

 

[tlc1976]

Here is my idea for sealing up the exhaust pipe when the time comes; a high-temp silicon boot that can be secured to the cinder block. An air gap between pipe and cinder would be maintained so as to protect the pipe. It's non-permanent yet should be water and air-proof.

Amazon product ASIN B07ZZG3YFM
Any better ideas?

Looks like a nice idea but I wonder if mice would chew through it. I would probably take a piece of flashing and cut a close fitting hole, or 2 overlapping pieces with close fitting U shapes. Fasten them in place then high temp silicone around it. Just cut and peel off the silicone and redo it if you ever have to change the pipe.

 

[rickwai]

Here is my idea for sealing up the exhaust pipe when the time comes; a high-temp silicon boot that can be secured to the cinder block. An air gap between pipe and cinder would be maintained so as to protect the pipe. It's non-permanent yet should be water and air-proof.

Amazon product ASIN B07ZZG3YFM
Any better ideas?

Thats actually a great idea. I never gave those a thought for a horizontal term