FAQ's on the Importance of Commissioning a Zehnder HRV or ERV System


In the excitement of finishing a construction project or moving into a new home, it can be easy to overlook the final step in correctly installing an HRV (heat recovery ventilation) system: commissioning. Our latest blog article explains why Zehnder units need to be commissioned and what happens during the process.

Why do Zehnder Units Need to Be Commissioned?

All ventilation systems need to be commissioned, especially those that have HRVs and ERVs. With a Zehnder unit, commissioning and balancing is the way to get the most out of the equipment.

During the commissioning the flows are measured in each room.

When a system is initially installed, it’s probably pretty far out of balance. The Zehnder system is both simple and complex: ducts run from the outside of the building to the HRV, and then from the HRV; more ducts are run to the bedroom, living room, bathrooms, and kitchen. All houses are different shapes, so the length of the ducts and how much they twist and turn through the house varies from house to house. These differences impact the supply and exhaust flows by adding different levels of resistance pressure between the supply and return ducts. The resulting pressure loss makes it harder for some ducts to carry air than others. The Zehnder system has two fans which turn at the same rate, but because of the difference in pressure loss, the result is two different rates of flow running across the core, putting the system out of balance.

What happens in commissioning?

Basically, commissioning is the process of balancing the flows for incoming and outgoing air in the heat or the enthalpy recovery core. When the two flows move across the core at the same rate, the system operates at peak efficiency for both heat and/or enthalpy recovery.

In commissioning, both the supply and return flows are measured to find out how much they are out of balance. Once measured, they are then corrected by adjusting the fan speeds up or down to put them in balance. After this is done, normal fan speed settings for the home is set as well as the rate for the low occupancy rate, and the boost rate.

What is the boost rate?

The boost rate kicks the HRV or ERV unit into high gear, after a shower to pull the excess moisture out of the bathroom or to pull excess cooking smells or exhaust out of the kitchen. It’s just running the fan on high speed for a pre-determined amount of time, which could be 15 minutes or 45 minutes. The boost time is whatever time the occupants want to set their boost rate to run. It’s a decision that’s made early on in commissioning and once it’s selected, the system goes into boost for the selected amount of time every time the boost button is pressed.

Besides checking and adjusting air flows, what else happens in a commissioning?

In addition to balancing the airflow, especially with Zehnder systems, some other important steps happen in commissioning. One of the biggest is confirming and setting the fireplace program.

When a home has a fireplace or some other combustion device, it’s possible in colder climates, and overnight lows in some conditions, that the system can turn into more of an exhaust fan than a ventilation system. It does that to keep the core warm because it’s really quite cold (below freezing) outside. When this happens and there’s a fireplace, there is the risk of pulling wood smoke or other combustion gasses out of that combustion area and into the home, which can be uncomfortable or even dangerous. So one step of commissioning is to confirm the presence of combustion gasses. If so, we set a program to make sure there is adequate preheating; that way the system can’t go into imbalance and will operate in the cold temperatures in the project’s location.

Additionally, at commissioning, the commissioning agent will confirm that the system was installed as designed and is working right. The agent will also check that all filters are new and clean and that any other special applications (such as a ground loop defrost or other optional accessories) are there and are working correctly.

How important is commissioning?

Commissioning is really important for the functioning of the system. Without final commissioning the system is undoubtedly operating out-of-balance and the room to room flows are not going to be operate within the parameters of the system design. It’s vitally important that commissioning be completed so the system can do what it’s intended to do.

When in the building process does the commissioning happen?

Commissioning happens at the end of the project, just before occupancy of the building or just after occupancy. Some building codes require it prior to issuing an occupancy permit.

It’s best to commission just at the point where everything has been installed and is running right. The building should be cleaned up a little bit, so we’re not sucking in drywall dust and construction debris, and all the grills and diffusers should be installed.

How long does a commissioning take?

A typical commissioning for a 2,500 square foot home with three baths and three bedrooms can take three hours or more to complete.

Why is it recommended that someone other than the person who installed my Zehnder be selected to do the commissioning?

Small Planet Supply prefers to work with third party commissioning agents. Typically, installers can run into some challenges installing the system and they can make choices that don’t lead to the system running as well as it could. A third-party commissioning agent is more likely to identify these issues since they are a set of fresh eyes and can compare the job to the system design and then balance it as recommended.

Commissioning is really the homeowner’s final assurance that the installation is done correctly and that the system is balanced. On occasion we can have the same installer do the final commissioning. We don’t see that happening often and when it does we’re typically working with an installer that we know quite well and that we have high confidence in their work.

Who arranges for commissioning?
There are a number of people who can arrange for commissioning. It can be the HRV system installer, the project’s general contractor, or it can be the homeowner who wants to get their system balanced and commissioned. The process starts with filling out some questions on the Zehnder America website which triggers a chain of events. One of the chain of events is that the system plan is returned to the system engineer where it receives a final review. A copy of the plans is sent to Small Planet and then a commissioning agent is assigned to it.

The commissioning report details final air flow measurements and other information that assures the system has been installed correctly and has been adjusted for optimal comfort and efficiency.

How can commissioning be completed most easily and without extra cost and frustration?

Make sure that all the work is done before a commissioning agent comes to commission the system. One common frustrating situation is when the agent comes before the system is ready. The commissioning agent is not there to finish installing the system. The system needs to be up and running, with all controls, terminations and diffusers in place and functional before the agent arrives to commission the system. One of the pre-commissioning steps is a dialogue with someone on the project to make sure the system is ready, because if it’s not ready then the commissioning agent has to come out again, which triggers another charge.

How does a homeowner know that their commissioning has been done, or been done correctly?

At the end of a commissioning, we supply a commissioning report to the homeowner and the installer.

What if you’re unsure whether your home/project Zehnder unit has been commissioned?

Commissioning reports for Zehnder systems sold by Small Planet Supply are retained on file at our company and are available to the homeowner. Small Planet Supply just needs to know the address of the project and when it was installed.

If it has been a long time since a system’s been commissioned, another option to consider consider is to have the system looked over and commissioned again. This often makes sense after a remodel, a major event, or to just have the system inspected and re-balanced every three to five years.


Small Planet Supply
Small Planet Supply
Eva Rooks has been trying to save the planet since 1982. During her 30 year career she has worked in social work, public policy, public…

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