Keeping Everything in Balance
The difference in ventilation delivery strategies.
In the last entry, we talked about some of the ventilation requirements in the Ontario Building Code (OBC). This time around, we're going to talk about ways to get the fresh air into your home. There are three types of ventilation distribution strategies in residential buildings - Exhaust only systems, Supply only systems and Balanced distribution systems. Each system has its own set of benefits and disadvantages, and we'll discuss each of these in detail.
Exhaust Only Ventilation
The most common ventilation strategy, especially in older homes. This system typically consists of an exhaust fan, such as a bath fan or vented range hood that is installed to remove air from the home. By exhausting air from the home, usually at a high airflow rate, the fan creates negative pressure in the home.
Let's look at an example. If a 300 CFM range hood is running and exhausting to the outdoors, and all of the windows and doors are shut in the home, then this creates a pressure imbalance that will be corrected by infiltration 300 CFM through cracks in the envelope. This outdoor air will come through opening in the window sills, baseboards or even through electrical sockets.
The advantages to this scenario are three-fold:
Low Cost: bath fans and kitchen range hoods are par for the course is residential construction, so this really is a low/no-cost strategy.
No Pre-Heat: As the incoming air through the envelope is infiltrating at multiple points and is uncontrolled, there is no way to pre-heat the incoming air.
Moisture Removal: As we've all experienced after a hot shower, a vented bath fan can clear out excessive moisture in a relatively short duration.
While this may seem like an appealing solution, there are several disadvantages that need to be considered:
No Heat Recovery: An exhaust only strategy will exhaust the conditioned air to the outdoors which means wasted energy. The incoming air is unconditioned and will have to be tempered by the space conditioning system.
Drafty: The air infiltration through the envelope may increase drafts around windows and doors.
Combustion Appliances: Create a negative pressure imbalance in the home increases the risk of combustion spillage from fuel-fired appliances. To mitigate this risk, a make-up air unit may be required.
Soil Gas: If the house in constantly under negative pressure, this may lead to soil gas problems or it may adversely affect a radon mitigation system if installed.
Supply Only Ventilation
As the name implies, a supply only ventilation system is the exact opposite of an exhaust only ventilation system. A fan is used to pull in outdoor air and distribute it throughout the home, thereby causing a positive pressure environment. The positive pressure in the home will force indoor stale air to exfiltrate through the envelope.
A house will always try to remain in a neutral pressure plane; if a supply only ventilation system is bringing in 250 CFM of outdoor air, then 250 CFM of stale indoor air will exfiltrate through cracks in the building envelope.
The advantages to a supply only ventilation system are:
Cost: While more expensive than an exhaust only system, it is a lower cost solution than the balanced ventilation system as distribution is only required for the supply air.
Soil Gas: Postively pressuring the home will push air to the exterior and may assist with decreasing soil gas problems.
Combustion Appliances: Inversely to the risks associated with an exhaust only ventilation system, a supply only system will not cause negative pressure that would affect spillage susceptible combustion appliances.
Low cost, mitigating soil gas and no concerns from spillage susceptible appliances, sounds great right?
It's not that easy, let's look at the disadvantages.
Moisture Build-Up: Positive pressure that forces conditioned air through the building envelope can also push moisture in cold weather. This limits a supply only system to a home with a good air barrier and situated in a milder climate than we Canadians experience.
Limited Options: A supply only system is an oddity in Canada, so you likely won't find an 'off-the-shelf' package system. This means slapping together a supply air, intake hood, filtration and distribution from individual components.
No Heat Recovery: Similar to the exhaust only strategy, a supply only system will push the conditioned air to the outdoors which means wasted energy. The incoming air will not have an opportunity to extract heat from the leaving air which leads into the next point.
Pre-Heating: The incoming supply air is unconditioned and, especially in the colder months, will likely need an electric pre-heater to temper the incoming air. This leads to higher energy costs. The supply air ductwork on the cold side of the fan and pre-heat will also need to be insulated.
Distribution: A supply only system will likely be distributed to multiple spaces in the home, which means dedicated supply air ducting or tying in to the existing forced air furnace distribution.
Okay, so lot's to consider so far, right? For each benefit listed on each system, there is an equally important disadvantage that may cause you to re-think your ventilation approach. Let's through a bit more into the mix....
Balanced Ventilation System
We've talked exhaust only, we've talked supply only, now let's talk about a balanced system (our favourite!).
A balanced ventilation system is a whole-home solution that uses a combination of both a supply fan and exhaust fan. These fans operate at the same airflow rate and at the same time. This approach ensures that the home is neither positively or negatively pressurized, maintaining a neutral pressure balance with the outdoors. While there are balanced systems that do not use any type of heat or energy recovery, the more common types of balanced ventilation systems, particularly in residential applications, are either Heat Recovery Ventilators (HRVs) or Enthalphy Recovery Ventilators (ERVs). We'll talk about the differences between HRVs and ERVs in a later blog post.
Examples of Balanced Ventilation Systems without heat recovery (left) and with heat recovery (right)
For the purposes of this article, we're only going to look at the pros and cons to a balanced system with heat recovery.
The benefits to a balanced ventilation system are:
Moisture: Will not push or draw moisture into the building envelope.
Pressure: A balanced ventilation system maintains neutral pressure with the outdoors, mitigating the risk associated with spillage susceptible appliances.
Energy: Incorporating heat recovery allows for reduced energy costs associated with tempering the incoming air as it captures energy from the exhaust air stream.
Common Place: Packaged balanced ventilation systems are available from multiple manufacturers and range is size, airflow capacities, static pressure ranges and configuration of the ports on the equipment. This allows designers to select the best equipment for each project.
Controls: Dedicated controls available that allow for independent control and boost timers throughout the home.
Comfort: Tempering the incoming air and balancing the supply and exhaust air streams are less likely to cause discomfort for the occupants.
No Preheat: These systems do not typically require pre-heat of the incoming air, which can cause condensation and other problems when required.
Distribution: Balanced ventilation systems can be configured in either a dedicated distribution system (whole home ducting) or a simplified configuration that ties into the existing forced air duct work.
So far, a balanced ventilation system has the most appealing list of benefits. So, what about the disadvantages?
Defrost: In our Canadian climate, the heat recovery core can be at risk for freezing in the winter months. A consideration for core defrost is required, which we'll get into in a later blog entry, but for now know that defrost can either be exhaust only, recirculation mode, or an internal pre-heater. Each of these defrost options have their own considerations that must be given.
Costs: To take advantage of the heat/energy recovery and optimize occupant comfort, a balanced ventilation system typically has higher equipment and installation costs.
Now that you have the background on ventilation strategies, what do we recommend?
At ZON, our mechanical design team always tries to create a balanced system that is in sync with the outdoor environment. This creates the least amount of risk with spillage-susceptible appliances such as wood stoves, gas ranges and water heaters or even a gas furnace. A balanced ventilation system also allows for energy savings through integrated heat and energy recovery, and we're all about saving energy! In our experience this is the best approach and, when we're talking about airtight, energy and environmentally conscious buildings, we're all in on balanced ventilation systems.