tube in a tube

Problem with your idea is it will not pass inspection

>>886444
don't they make int/exh combo tubes?

>>886444
what inspection?

>>886441
3 machine screws every 4' would center and support the inner sleeve.
get tapping

>>886454
Lel, enjoy your CO poisoning

No. The intake will take in the exhaust. Bad idea.

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>>886441
They make pipes for that anon.
I would not make something myself though. Use the correct stuff or you could wind up with a fire.

>Coaxial Direct Vent
> pic related

Now for the tricky part. You can't have that vent within 8ft or so of any outside window/door. (depends on the local code) but the idea is to prevent any CO from getting pulled back into the house.

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>>886510
that thing:
1: is expensive
2: doesn't solve the problem of "merging" the intake and exhaust which means some diy anyway

the "tricky" part is not that much so; the actual exhaust is less than 8ft away from 3 windows and the main entrance BUT the furnace is natural gas, forced air and high efficiency (and lots of sensors inside) meaning that if there's anything more than CO2 and water vapor coming out (beside a lot of air), everything stops and I get some error code

What I'll probably do is try to build pieces first (using screws to hold them coaxial like >>886457 said), check the seal as much as I can and if there's any chance I can't make it perfect I'll just say "fuck it!" and do them separately

why not just put two tubes touching and insulate around them? it'd be like a billion times easier.

>>886669
You need to do some more research there.
They make an adapter box just for older setups.

>>886441
You need to do a lot of research on this!!!
It is already 90+ furnace. that means the flue gasses are already cooled and almost no additional heat from the flue can be gained. It is OK to pipe the air intake to the outside but you must use the proper sized pvc according to the manufacture directions. Get the install manual for that model furnace. If there is already fresh air available like in an old house it would be a waste of time to pipe in combustion air. If you use any metal other then stainless it will rust out and you will die.

>>886441
Instead of trying to get almost no additional heat from a 90+ furnace why look at additional insulation and sealing up the drafty windows and doors. R-32 min insulation and you can not use faced insulation over old insulation as it traps moisture.

>>886510
Wrong setup for a 90+ furnace. That is a vent for a 80% or less wall furnace. If you use it on a 90%+ it will rust out and you will die.

>>886669
If you are getting error codes and it is not running it is because of other reasons, like over heating or a stopped up drain. The flash code tells you what has failed if you look up that model furnace flash code.
I have never seen a CO detector built into a furnace.
pro tip: look at the inside door with the schematic and see if you can find the error code list there.
Also if you don't use a 1/4 inch per foot rise on flue it will fill with water and block the flue shutting down the furnace.
There is a whole lot of stuff you have to know before you diy on a furnace.
you will die if you mess up and from what you are saying , you had better, in the very least get the install manual.

>>886830
P.S. All that crap sensors and stuff in that furnace are put there for a reason. If you find out what is wrong DO NOT JUST JUMP IT OUT!
I did that once when I first started in this business 40 years ago. I was a smart ass installer helper and the home owner ask me to fix another furnace in the house while we were doing another install. I knew enough electrical to find the problem. It was a pressure switch on
a flue vent booster fan. So I wired it our and the furnace ran. I was so proud of myself.
5 years latter the home owners bring every hvac co to court that ever visited that house because of CO poisoning and saying they got brain damage. So my company went through all their service records and called every one of us that were working back then telling us we may have to get called into court. I was a helper and did not write up an invoice on the repair I did. They had no service record except for the install we did. We were never called in and I kept my mouth shut. I still feel bad about it 40 years latter. I have seen in the past 40 years many many things that were installed and repaired wrongly. I have gone out my way ever since to make damn well sure every thing I do has others safety in mind.

Many of the DIY furnace and A/C vids on YouTube are fucked up information. You can not rely on them for correct information. Some are good but watch out!

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OP here
Turns out that mid efficiency water heaters have coaxial intake/exhaust
High efficiency ones don't need that (the exhaust temperature is just a bit above the temperature of the hot water coming out of the unit)
So I'll just stick with extending the intake