Zehnder Interview Series: Aubrey Gewehr, Technical Sales Engineer

Interviewer: Could you introduce yourself, Aubrey?   

Aubrey Gewehr: My name is Aubrey Gewehr. I’m the Northeast Technical Sales Engineer for Zehnder America. I am a licensed mechanical engineer with a background in HVAC consulting work.

Interviewer: What kind of green building trends are you seeing in the HVAC industry?

Aubrey Gewehr: All kinds of exciting things are going on. Specifically to HVAC, we’re seeing more and more whole-house balanced ventilation systems being included and most of those are HRVs or ERVs. As buildings get air-tight, mechanical ventilation becomes critical. Also, we’re seeing a better understanding of the importance of decoupling the HRV from the central HVAC system. Traditionally, the default method to install an HRV by just tying it into an air handler or furnace and let that central HVAC system do the distribution. People are realizing the most effective way to get good ventilation is through separate distribution. By having an independent network for the HRV, you get the air into and out of the ideal rooms, have easier and more consistent balancing of the fresh and exhaust air, and higher heat transfer efficiency as well.

Outside of ventilation trends, we’re also seeing a lot of heat pumps being used for heating and cooling needs, especially air-source heat pumps. That’s a real growing trend for green buildings, where one efficient system will provide both the heating and the cooling. Where the loads are appropriate, it’s quite easy to accomplish this.

Interviewer: What’s the difference between an HRV and an ERV?

Aubrey Gewehr: Well in a word, the difference is moisture. An HRV transfers heat energy from one air stream to another. An ERV transfers both heat energy and moisture. So where heat energy always flows from hot to cold, moisture works in kind of the same way. It always goes from a high-moisture environment to a low-moisture environment to equalize itself out. As air goes through an ERV, it’s both transferring heat from the warm airstream to the cold airstream and also transferring moisture from the wet airstream to the dry airstream. ERVs are predominantly used in humid climates where moisture in the intake air is a big concern. An ERV will both cool down and dehumidify that air thus minimizing the load on the AC system of both the sensible (heat) load as well as the latent (moisture) load. The trade-off is that ERVs have a lower thermal efficiency than HRVs. Traditionally HRVs are used in heating-dominated climates where ERVs are used in humid cooling dominated climates.

Interviewer: How does the Zehnder HRV system help allergy sufferers?

Aubrey Gewehr: The biggest way is that you’re controlling the air that you’re bringing in the house.   With a standard exhaust-only system, the air that makes up for the air that’s being exhausted gets pulled into the house through all the cracks in the building. Often cracks are full of dust, debris, and other unwanted contaminants. A Zehnder HRV controls the air that’s coming into the building. Then as that air gets brought into the building, it’s filtered before it goes to the heat exchanger. Our standard filter is about a MERV 7/8 grade filter which keeps most things out. Particularly for people who suffer from very bothersome allergy issues, we can upgrade that to a MERV 13 filter. The MERV 13 rated filter removes at least 90 percent of particles down to a third of a micron in size, which is extremely tiny. This level of filtration takes care of most of the pollen that would otherwise be drawn into your house.

Interviewer: How does the efficiency of the HRV affect the temperature coming in and comfort?

Aubrey Gewehr:   The higher the efficiency, the higher the supply temperature because more heat transfers into the incoming air. With a less efficient commodity-grade HRV, the efficiency is often in the mid 60% range so when you’re bringing that into the house, the temperature range is commonly in the mid-50s degree range or so. When you feel that temperature air blowing on you, it feels cold.

With higher efficiency, when we’re getting into the mid-80% to mid-90% range, now we’re getting air temperatures coming out of the unit in the mid to high 60’s degree range – assuming about a 70-degree set-point in the house. The warmer that incoming air is, the more comfortable it is. This higher temperature coupled with a careful supply air delivery design makes all the difference. We’re supplying up at the ceiling level and not having air blowing directly at you. This approach makes for a more comfortable environment.

Interviewer: How important is it to regularly change air filters?

Aubrey Gewehr:   Changing the filters regularly is quite important. We recommend at least once or twice a year. Particularly when a system is first installed, it’s good to watch how quickly the filters fill up in the first year going through your four seasons. You can get a good idea of how quickly they’re loading. Filters are really dependent on two things: the outdoor environment where you are located and also the indoor environment in which you live. The loading of intake air filter is going to be dependent upon how much particulate there is in the air around you outside. Similarly, the rate of exhaust air filter loading will depend on the particulates in your indoor environment.

For example, if you’re in a major city, the air is unfortunately dense with pollutants so those filters on the outside load up quite quickly. Similarly, you may think that in a rural environment it isn’t quite as much of a problem but you tend to have a lot more pollen out in those areas so you need to pay attention to that as well, especially in the summertime. Then on the interior side, it’s really dependent upon the occupancy of the house. If there are just two people living there that are very neat and clean their house regularly, then the filters are going to be slow to load up. If you have four kids and three dogs and two cats, then you’re just putting a lot of dust in the air with all of those people and animals and that filter will load that much quicker. So it’s important to pay attention to your particular environment when dealing with that.

Interviewer: What kind of tips would you offer architects when selecting an HRV/ ERV system and maybe some tips for the homeowner as well?

Aubrey Gewehr: The first thing I would say to architects is that quiet systems make for happy customers. That’s one of the biggest differences between a really high-quality HRV and ERV system and a lower quality one. A high-quality HRV or ERV has very quiet motors and fans. Our HRV systems also include sound attenuators as a hub to the whole distribution system. Our customers really like that their system is energy efficient but they love the fact that it’s quiet. Keep in mind as well our discussion of how higher efficiency relates to higher supply temperatures and a more comfortable indoor environment.

Also, we recommend architects plan out the location early on. Planning from the beginning to allow space for the HRV and for the distribution is really important. Whenever possible the HRV should be located in a mechanical space that is centrally located in the house and on an outside wall. This approach will somewhat balance out the lengths of indoor ducts and minimize the length of the outside connected ducts.

For homeowners, I think it’s important to think long-term when deciding on an HRV or ERV system. When you’re building a house, you install an HRV or ERV system and after that, the wall is sealed. If you install a less quality and perhaps less expensive HRV that blows cold air and is noisy, it will be harder to replace and affects your comfort in the home. Similarly, saving money upfront by joining the HRV with the heating and cooling system ductwork is really hard to change later once the walls are closed up.   It’s often worth it to spend a little more money on the things that are buried in the wall and maybe a little less money on the things that are easy to change. If you need to make the compromise, you might be better off to get some less expensive fixtures. Invest more in the HVAC system you have in the walls, and then, later on, you can easily upgrade fixtures.

Interviewer: Thank you for your time, you’ve provided a lot of helpful information on HRVs and ERVs.  

Aubrey Gewehr: Thanks for the opportunity, have a great day.