Step 19c – Exterior Ducting and Control Wiring

The last step in installing the ERV was to duct the supply and exhaust to the exterior of the house. The manufacturer recommended using insulated flex duct for this portion of the install, since these ducts will run through unconditioned attic space. One duct will bring fresh air from the outside into the ERV, and the other will remove the stale return air from the ERV to the exterior.

First, I marked out positions for the vents and ensured they will be a minimum of six feet apart from each other as the manufacturer recommended. You don’t want to exhaust nasty air from the bathrooms only to have it go right back into the supply vent and back into the ERV. I drilled a hole through the wall and fed the vent and attached duct through. I cut a slit in the water resistant barrier and eased the top of the vent underneath. Much like a window, I applied flashing tape to the sides first, and then the top. Last, I folded the water resistant barrier over the top and secured it with vapor barrier tape. This method of “positively flashing” wall openings is paramount to ensuring any moisture will flow down and out, and not get behind the hole I made in the water resistant barrier.

Back in the attic, I used sheet metal screws to secure butterfly dampers to each of the vents. I faced each of them in opposite directions to ensure that they would only allow airflow in the proper direction. Next, I attached an elbow so the duct was pointed directly at the ERV. It is wise to avoid bends when using flex duct because it creates excessive friction inside the duct. Using the metal elbow ensures a nice, smooth airflow. I cut a piece of flex duct extra long so that the attached insulation would wrap around the elbow, damper, and vent up to the attic wall. It is very important to insulate the duct to eliminate the possibility of water condensing in the attic.

It is really important to stretch flex duct nice and tight, again to reduce friction inside the duct. I secured the flex duct using sheet metal screws, and then much as I did with the metal duct, I taped it with aluminum tape to ensure an airtight seal. Then I pulled the rest of the duct insulation over not only the end of the flex duct, but the metal duct elbow and butterfly damper as well, to ensure the entire run was insulated. The result is that the duct appears to be curved but only the metal duct is curving and the flex is only used on the straight part. I used a strip of metal ducting under the strap to prevent the insulation from compressing where the strap supports it. Fiberglass insulation is less effective when compressed because it works by trapping air between the fibers, and when it is compressed there is no space for the air to get trapped.

The last step in preparing the ERV was to wire the boost switches, main controls, and ‘Econocool’ function. The boost switches are wired with what is commonly known as “thermostat wire”. It is a relatively thin wire and contains two 18 gauge conductors (recall that a conductor is a wire that carries electrical current). I attached one end of the wire to the control panel of the ERV, and ran the other end to the closest boost switch near the master shower. From there I ran another wire to the water closet boost switch and the downstairs bathroom boost switch, and then last a wire from the downstairs bathroom to kitchen. For the main control, the unit came with a very short connection cable. I cut this cable in half, and then used thermostat wire with seven 18 gauge conductors to splice the two halves together. These splices were completed inside accessible electrical boxes as prescribed by the electrical code. Below you can see where a pair of wires terminate inside a gang box for the master shower ERV boost switch. On the left side of the box will be the boost switch, and on the right side will be the switch controlling the shower light.

The ‘Econocool’ function is a really nice feature that helps with summer nights when you don’t want any of the heat to be exchanged between the incoming and outgoing air streams. Much like the boost wiring, I used thermostat wire to connect the panel to a switch in the mechanical room. This switch will be flipped in the summer, at which time the ERV will cease to transfer heat when the incoming air is between 55 and 70 degrees Fahrenheit. This will cool the house faster in the summer evenings, improving efficiency.