Heat Recovery Ventilation systems are a must for Net Zero energy homes!
Heat recovery ventilation provides a pretty appropriate description for the type of system we will discuss in this article. First I will provide some basics to explain the need from having a heat recovery ventilation (HRV) system.
This Net Zero Energy in Montana site tries to convey information that supports attaining the Net Zero claim for readers. First, we must explain what “true” Net Zero energy really means. Perhaps others will give a different definition, but here comes mine. Net Zero energy describes a home that produces all the energy necessary for a comfortable lifestyle. Those needs start with providing comfortable temperatures for all. Said differently, the home must provide an adequate heating, ventilation and air conditioning (HVAC) system. You will notice that in the common HVAC mnemonic, Ventilation appears second so it must be important.
Most people have never heard of a heat recovery ventilation system. However, every reader needs to know the importance of such a system. A heat recovery ventilation system can assist the reader in achieving a Net Zero home status. I have inserted a picture below of the basic HRV system we use in our home. You will note in the picture the Hepa filter box sitting on top of the heat recovery ventilation unit. More details following will describe how and why this wonderful system works.
We need to understand how a heat recovery ventilation system works.
First, a heat recovery ventilation system consists of two basic parts; the actual heat exchanging unit and the ceiling ducting network. The air exchanging unit recovers heat from exiting stale warm room air. The unit effectively transfers that room heat into the incoming fresh air.
A unique heat exchanger allows the cold outside air to pass by the warm stale house air being exhausted. No air is mixed but the heat of one flow is conducted into the other cold air flow. The two air flows pass by each other through two honey combed plates. One air flow comes in contact with one side of a thin membrane. The other air flow (in the opposite direction) travels through the other side of the thin membrane. Thus, the heat transfers from one side of the membrane to the other, but not any actual air.
The Heat Recovery Ventilation System performs several functions.
You must employ extreme home insulation to achieve your Net Zero energy goals. If you operate a very large active solar or wind mill facilities this might not be quite true. Active solar prices have fallen significantly in the past few years. Accordingly, you might not have to achieve a super sealed home to prevent heat loss. If you do over insulate and seal your home it is possible to deplete oxygen and increase carbon dioxide levels. Ventilation is really necessary to replace other dangerous toxins including carbon monoxide with fresh outside air. Anyone installing a heat recovery ventilation system should enjoy better health and alertness. This should be especially true if the HRV intake air is filtered before entering the house.
My wife and I found another feature of the heat recovery ventilation system we own which might sound counter intuitive. Our HRV does not yield the high efficiency as the most recent and more expensive designs. Our system produces fresh air at about 75% energy heat recovery efficiency. A 100% efficient HRV unit returns fresh air that is just as warm as the stale air leaving the house.
We actually appreciate getting cool but not cold outside air when the Heat recovery ventilation system is running. The system running during the day drys our eyes because of the low Montana humidity . Thus, we only run the heat recovery system at night. We prefer having fresh cool air enter our bedroom anyway. The cool air give us a better night’s sleep so it is worth the extra heat loss. Besides, extra solar panels really makes up for the slightly extra loss in heat recovery ventilation system efficiency.
Let us discuss the ducting installation of our heat recovery ventilation system.
Now we know why you should install a heat recovery ventilation system if you want a Net Zero energy home. I would like to explain how we installed a heat recovery ventilation system in our own home. We started by installing air intake and air exhaust piping throughout the house portion of our building. I personally wanted to install the HRV by myself as part of a learning process. The process commenced with installing four inch PVC (sewer) piping through out the house. This install had to take place when the rooms were all framed and not before the ceiling drywall was installed. You will see why this was important from the pictures below.
I attached the piping to various trusses and studs by using foam rubber isolation pads for noise and vibration reduction. As a result, that technique worked very well because (thankfully) those fears were never manifested.
My wife expressed concern that she wanted really clean fresh air in our home, and not smoke or dust. Hence, we added a Hepa filter unit in line with the fresh air piping coming out of the HRV unit. An extra duct fan overcame the extra air resistance imposed by the added filter. I terminated the outside ends of the intake and exhaust pipes with right angle PVC couplers. These couplers faced straight down to avoid rain penetration. I also added a screen inside the very end of the couplers to keep insects and birds out.
What about installing the heat recovery ventilation main exchanger unit?
I installed our HRV unit on a high shelf in the shop (see the picture at the top of page). We chose the location because it was central to all the ducting coming through the high vertical wall. I could most easily connect and service the main heat recovery ventilation unit to the duct work in that location . Though we do climb a ladder to reach the HRV main unit, at least the grand kids can’t reach it. That location serves us well as being convenient for routine maintenance such as cleaning filters. At least this location is more convenient than in a crawl space like to many people have to deal with.
I hung the heat recovery ventilation main unit from the ceiling with nylon straps. Those straps attenuate any vibration and sounds emanating from the fans. Surely direct contact with any walls would amplify those sounds and vibrations. We placed insulation batting material around the HRV unit as another measure to reduce sounds from the air flow.
My connecting of all the flexible duct hoses was the most difficult job for me in spite of the location. Frustration ruled the day when it came to the actual connection and routing of those large hoses. The flexible ducting connected the HRV unit to the duct work on the other side of the shop/attic vertical wall.
Introducing a different kind of ventilation system.
I am going to stray from the subject of heat recovery ventilation systems for an interesting side note.
We had already been using a large whole house fan in the shop for cooling purposes. Then we discovered another great benefit the fan offers us. I needed to move my lawn tractor out of the shop one day. The tractor had been sitting in one location in the shop for maybe six months. After I started it up, it bellowed out so much smoke that I could hardly see through it. Obviously the smoke left a terrible odor throughout the shop and it was probably not even safe to inhale it.
Instantly I headed for the whole house fan controller next to the entry door and I turned it on. Simultaneously I raised the largest overhead door several feet. The shop really needed some fresh air and this was the way to get it fast. That 5500 CFM whole house fan replaced the shop air with fresh outside air within about five or ten minutes. I instantly provided the desperately needed emergency ventilation at the right moment. Heat recovery proved not to be an issue due to the emergency situation. However, the radiant floor shop heat made the space very comfortable very quickly. Fast comfort recovery is one distinct advantage of that type of heating system.
I bet you thought this would be a short dissertation because I surely did. There was more to a good heat recovery ventilation system than I had originally thought as well. Please let me know what you think about the site. Maybe I won’t respond to many comments because of time restraints, but I will read them all. Thanks, Mike.