Plenum for natural gas furnace in a slab foundation.

JackW

So I have a situation that I have let go of for too long now, and I need to fix it. The sewer line from the bathroom runs over the top of the plenum. Unbeknownst to us, it had been leaking for several years when we bought the house. I only discovered it when I noticed sewer flys around the vent next to it. Upon inspection, the whole plenum had rotted away and collapsed. I cleaned it all out, laid plastic and sheet metal for the bottom, then used foil-backed foam insulation board for the sides, no top, I just screwed a sheet of OSB over the hole to the utility room floor.
As you can see in the picture, it's a mess. I'm considering using the foil-backed foam insulation board to make a new plenum. Is this acceptable? If not, what would you suggest? As you can see, having the sewer and water line running over the top won't make it an easy fix.

Thanks,
Jack

Jamie Hall

Foam backed insulation board will do fine. But -- before you seal things up, take a look at as much of the length of the ducts leading off it as you can. They may need work as well.

If I may say so, a remarkably poor layout from the beginning...

JackW

@Jamie Hall yes, it is; it's a terrible design. After cleaning it out, I bought several lengths of ductwork, lined them with plastic, then ran them in the ducks as far as I could. I will have to replace all of the ones shown in the photo. What do you recommend gluing the parts together with? Thanks for your help.

EBEBRATT-Ed

Don't know how to make that right.

mattmia2

EBEBRATT-Ed said:

Don't know how to make that right.

Maybe build the plenum enclosed under the plumbing and bring it up on one side or the other?

kcopp

EBEBRATT-Ed said:

Don't know how to make that right.

A ductless system.... Abandon this.

EdTheHeaterMan

I currently live in a home with Slab or Grade construction. My duct system is in the attic. It is great for cooling, Not so much for heating. I really want to put some baseboard heating in here, but the natural gas is about 5 streets away and I do not see it getting to my home any time soon. I don't see any real solution to this situation. I would look at alternative systems like the attic, ductless, or hydronic for heating. This is a real problem with the person to blame being the construction official that approved this mess in the first place. If no permit was taken out for this work, then the construction office is not even at fault.

You are stuck between a slab and a sewer line, which is a lot like being stuck between a rock and a hard pipe.

random12345

I admit I know nothing about forced air systems, but isn't this against code? Not trying to get anyone in trouble but I would like to know what the builder's thought process was when they decided to run a sewer line through a plenum or vice versa.

bburd

Hot air ducts in on-grade or subgrade floor slabs went away for a variety of reasons, not the least of which was that subterranean termites absolutely love them. But one of my relatives owned a slab on grade raised ranch house in Mass. built in 1972 with the ground floor heated by ceiling registers, and the family room on that level of the house was not usable during the winter. The slab probably had no edge insulation.

Probably for reasons of construction cost, the second (main) floor of the house had floor registers and was very comfortable.

BenDplumber

Absolutely against code however when you have unskilled labor that can't read or interpret blueprints this unfortunately is what you get. Slam it in and get your draw. I would open up the floor more and offset the main drain around the plenum with hard pipe and approved transition fittings, not the ferncos that you have pictured. No since in setting a unit on top of a repair that also isn't code approved. Now's the time to make it right, although costly to move, I would move the domestic waterline as well, since the trench would be open to move the sanitary drainage. Because you'll always know that it's there, and sleeping at night is very important.

pecmsg

Also against code here to have anything in the return. Electrical wiring needs to be in Conduit.

I would increase the size of the box and sleeve the drain and water lines or reroute around.

JackW

@mattmia2 that's what I plan on doing. See how it goes.

JackW

The house was built in 1974; it was a crappy build, I get it. Please stop commenting about it not being up to code; what good does that do me now?

JackW

@pecmsg, there is no electrical wiring.

JackW

@EBEBRATT-Ed, I have a ductless system. It works great in the summer, but not in the winter. I'm in NW Ohio, doesn't produce enough heat when the temp drops into the 20's and below.

pecmsg

JackW said:

The house was built in 1974; it was a crappy build, I get it. Please stop commenting about it not being up to code; what good does that do me now?

Wrong is Wrong. 

It’s open do it right. 

Sleeve the drain line so IF there’s an issue it’s not a major issue. 

That water line needs a sleeve also. 

JackW

@pecmsg, I appreciate your input, but sleeve it to where?

mattmia2

Put the pipes inside a larger piece of pipe that extends a foot or so beyond the plenum on both sides. thinwall pvc would be fine.

JackW

@mattmia2, the water line is dead, and the sewer line continues under the slab. Nowhere to go.

JackW

@Jamie Hall, here's what I came up with; not the best, maybe, but a heck of a lot better than it was. Thanks for your help.