Thursday, 05 May 2022 13:18

Where Can I Put My U-Tube?

Everyone in the radon biz has seen coutnless pictures and illustrations of U-tubes mounted on the side of the vent pipe on the suction side of the fan -- and why not? It is a convenient place to put it, provided the homeowner can see it easily.

Figure A

Common U-Tube Installation

Figure A manometer

A mitigator pops a hole in the basement slab and runs the exposed pipe up a basement wall and out the rim joist. The portion of the pipe in the basement is exposed and provides an easy place to mount the pipe as in Figure A.

But what do you do when the pipe is not exposed, or the only exposed portion is within the crawlspace? This often happens with active systems for new home construction where the vent pipe is behind a finished wall, and the pipe that one would normally mount the U-tube on, is inaccessible.

We have seen some interesting approaches that maintains the incorrect assumption that U-Tubes must be mounted on the vent pipe.

Figure B shows a U-Tube mounted on the vent pipe in a crawlspace. For the homeowner to view the U-Tube, they would need to jump down into the crawl (after moving stored boxes over crawl hatch) and belly-crawl over to the manometer. That would not fall within the definition of easily viewed location.

Figure C shows a vent pipe that was behind a finished wall, within a living room. To find it the wall had to be cut. Later a hinged door was installed-sort of like the “peak-a-boo” door of a Chicago speak-easy.



Figure B

U-Tube on pipe in crawlspace

Figure C

Vent pipe and U-tube behind dry wall with hole busted into wall

U tube 2

U tube 3

Neither approach as shown in Figures B and C are appropriate nor are they needed. That is because:


U-Tubes, or any pressure measurement devices such as a pressure alarm does just that. They measure pressure. There is no air flowing through the tube to the manometer and hence there is no pressure drop in the tubing. Consequently, the length of tubing from the device (u-tube) to the point at which the measurement is being made (connection to vent pipe) makes no difference. In other words, the length of tubing could be miles long, provided there were no leaks in the tubing or its connections.

To illustrate this point, we set up a demo radon system and installed two U-Tubes that were connected to the suction pipe at the same distance from the fan. In one case, a U-Tube was connected to a hole drilled in the suction pipe and connected with the 3-inch tube typically provided with a U-Tube. In the other case, a 50-foot length of 1/4-inch tubing was connected to the suction pipe with a 3/8-inch NPT thread to 1/4-inch serrated fitting. As you can see in Figure D, both U-Tubes measured exactly the same vacuum-even though one had a 50-foot length of tubing between it and the radon vent pipe.



Figure D - Demonstration

Figure D pic 1 Figure D pic 2 Figure D pic 3

Demo Set-up:

  • Radon fan with piping on inlet and discharge
  • Two U-tubes mounted next to each other for ease of viewing.

Right U-Tube:

  • Typical installation with 3-inch tube from U-tube to hole in pipe.

Left U-Tube:

  • Connected to port in suction pipe with 50 feet of tubing between U-tube and port.

Demo Set-Up View

  • U-tubes are located next to each other for ease of viewing but the left one could have been 50 feet away due to length of tubing (50-ft coil) shown in left corner picture.

Results with Fan Turned ON:

  • Both read the same vacuum!


  • U-Tube can be remotely located and tubing routed to it.

This means that in the case of a crawlspace, tubing could be connected to the exposed vent pipe in the crawlspace and routed up into a utility room or closet directly above the crawlspace and mounted on a wall. It would be even nicer if it was mounted on a board with a label next to it that would advise the occupant as to what it was and how to interpret it.

The same would be true in a non-crawlspace house where the vent pipe is totally concealed. You will need to plan ahead and install the tubing and route it to a proper location before the walls are sheet-rocked.

One might argue the connection should be made to an exposed portion of the vent pipe in the attic or outdoors. Be careful of this because if the tubing is routed through a cold space, condensation can occur in the tubing and cause whacky readings and unneeded service calls.

Oh, and one last thing on U-tubes. Read your label and locate the label appropriately. Many labels have warning arrows that are to be located such that they point to “zero” on the U-Tube. The verbiage says if the liquid levels get to that point, then call for service. Zero means the fan is likely dead. Often, we see labels that are located above or below the U-Tube where the liquid level can never reach the point where the arrows are located. See Figure E

Figure E: Incorrect Label Location Relative to U-Tube

Figure E pic 1

Arrows should point to "Zero"


Figure E pic 2

Label is incorrectly located below U-Tube where the liquid level will never meet the "call for Service" arrows


Doug Kladder


Read 222 times Last modified on Friday, 06 May 2022 10:19
Doug Kladder

Director of Center for Environmental Research & Technology, Inc. (CERTI).