I’ve never seen figures for the maximum length of insulated flex pipe you can use with an HRV, but I do know that airflow rate is significantly reduced with ducts longer than 10 feet. That said, if 18 feet is as short as you can accommodate in your installation, I’d still go for it. One of the reasons for this is that the airflow through an HRV is slow and low pressure. In fact, most Canadians have to run their HRVs gently, or only part of the time, to prevent their homes from becoming too dry during cold winter weather. Do all you can to keep your insulated flex pipe straight and with minimal bends as it’s installed. Also, be sure to protect this pipe wherever it goes through areas where the plastic outer layer could become damaged. Any kind of tear or puncture in this layer will allow warm moist indoor air to infiltrate the fiberglass insulation and condense next to the cold inner part of the pipe. The best approach I’ve found for protection and support uses a piece of 8-inch diameter round furnace duct, not snapped together along the central seam. Opening up this metal and putting it over the insulated flex creates an armored metal covering that prevents damage. You can hold this metal cover in place with perforated metal straps screwed to the surrounding wall or ceiling structure.