Demolition: Dealing with Mechanicals

Know what your HVAC, plumbing and electrical systems have in store before you start cutting.
By: Mark Clement

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In addition to the "operation" itself, plumbing (including supply lines, vent pipes and waste stacks), electrical and low/no-voltage wire, and HVAC (ducting and chases) all require pre-operative procedures on demolition jobs. Here's how it breaks down:


The easiest mechanicals to deal with are the obvious low-voltage stuff, such as phone lines, and no-voltage, such as cable. They are often surface-mounted and can be moved or spliced easily. If you need to splice phone line, say to lengthen or otherwise move the line, you can bury the splice in the wall or the ceiling. Or you can come out of the existing jack with a new line. It's worth pointing out, however, that even a good a splice is the weakest link in the system. If you can retain access to the splice, should something go wrong later, you won't have to dismantle portions of a wall to locate it. Whenever possible, move these wires before you start big-deal demo.

For the heavier-duty stuff, the best thing you can do for yourself before you tear up anything is to locate any mechanicals and decide what to do about them in advance. Say you want to take down a kitchen wall to expand the space. Grab a flashlight and a tape measure and start triage on your house. Measure the location of the wall from the outside walls. Then head to the basement and, assuming you have visual access to the floor system above you, locate where your to-be-demoed wall is, again by measuring. If there are mechanicals in there, such as a waste stack or an HVAC supply, you're likely to see them enter the wall through the bottom of the floor. It's sort of like taking an X-ray. Using a flashlight helps here.

The X-ray is crucial because it can tell you what's in there, and if you find several services in there, you'll have a very good look at the scope of the surgery you plan to perform—and may decide performing it doesn't suit the patient or the budget.

Also head upstairs. Look in the rooms above the room to be demoed for HVAC vents in the floors, walls and ceilings. If there's a bathroom above the wall to be demoed, you can be almost certain there are supply, drain and vent lines running in the interior wall.

The Operation

The good news is that you can move mechanicals—where there's a will and a good subcontractor, there's a way. But the moves must be planned. If you decide to open up that wall after all and find it loaded with stuff, here are some rules of thumb to follow in each area of the job:

HVAC. Air likes the path of least resistance. While you can move ducting, plan to move it so that the new line has as few bends and turns as possible. For example, moving a supply vent to the edge of a wall from the middle is possible as long as you have a straight chase all the way through your wall system from furnace to vent. And while you may be inclined to simply eliminate a cold-air return, that's a bad idea. That will make your furnace work harder to get makeup air (look at it like breathing through a straw), and you'll probably shorten the life of the unit, never mind affecting how well your space is conditioned.

If you come across a job where you're inclined to move the refrigerant lines, don't. You need special equipment for that and an HVAC specialist to do the work.

Electrical. Wire might be the easiest mechanical to move. The key is to either run new line (while eliminating the old) or correctly place any junctions (splices in the line). If that guy down the street who claims to know everything there is to know about houses tells you that you can bury a junction in the wall, he's what experts call "wrong." Any time you junction a wire, you need to have access to it. Proper ways to junction a wire, however, are many—in a light or a plug, for example. Or you can simply put the wire in a box and cover it with a blank face plate. You might also be able to turn the box around and add a plug to an adjacent room.

And for when the real world outsmarts code, there's a code-approved option for splicing wires in walls. It's a non-metallic union made by NSI Industries. Where and when to use such a device is, in my opinion, the bailiwick of an experienced electrician.

If you have old-fashioned knob-and-tube wiring (so called because the wire is surface-mounted to framing with a ceramic knob—when the wire passes through framing, it's sleeved with a ceramic tube), you're really going to want your electrician and, according to my electrician, an old one who has worked with knob and tube. Back in the old days, there was little rhyme or reason to how electricians wired houses, and you could have several rooms being fed by a single line. It takes someone with serious know-how to diagnose and cure knob-and-tube-itis.

Pipe. Plumbing supply lines, drains and vents are also movable to a point. The key is to have a chase(s) for them all to live in. If that chase is on an outside wall and you live where it snows, most plumbers would call it a bad idea. Water has a tendency to freeze and, in so doing, burst pipes. Read: disaster. Ideally, water pipes should run in interior walls as much as possible. Check with your plumber.

As for eliminating vents (the pipes that run up through the house and exit through the roof), that's also a bad idea. Your plumbing system needs its vents to breathe so that water can drain properly. While it may not seem as if a vent does much, it really is there for a reason.

And that's the point: All this stuff in your walls is there for a reason and acts as a system. Knowing what you're dealing with ahead of time gives you the information you need to make the best decision for your patient.

Mark Clement is a remodeler and author of The Carpenter's Notebook and The Kid's Carpenter's Workbook, Fun Family Projects! Find out more at

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