Sunday, August 9, 2015

Whole House Ventilation

Whole House Ventilation
I think the biggest question / complaint I hear from builders today is - Why do I need to make a house this tight, then add a fan to bring in fresh air? If I made it leakier I could save the homeowner some money.

While to a certain extent they are right. They can sell you a less expensive, leakier house that is missing any kind of forced ventilation for the house. After all, your parents house never needed ventilation, their parents never needed ventilation, why do we need it?

The fact is, for our climate in Rochester, NY, it is more efficient in the winter to run a 45 watt machine that recovers 90% of the heat, than to heat up all of the 10 degree air leaking into the house with your furnace, plus your furnace fan.

The air leaking into your house is much drier (lower relative humidity) than the air inside your house. This means you either increase the thermostat temperature or add humidity to the air with a humidifier. Also in order for air to be coming into the house, some of the air inside the house must be leaving. So where does it go?

Green Building Advisor has some GREAT visuals of why infiltration and exfiltration is bad
The exfiltration of the indoor air with its high relative humidity cools and condenses inside the wall. This can lead to extensive moisture within your wall and in extreme cases, extensive wall rot as seen above. This moisture problem can be masked by common building materials like Tyvek, and Vinyl Siding since they will not show any problems until it is too late (with wood siding moisture in the wall is be evidenced by peeling paint or breakdown of the wood siding).

Passive House takes the Building Science theme "Build it Tight and Ventilate it right" to a whole new level.

The pictorial  from the CDC below illustrates some common concerns with building a tight home. Proper ventilation is essential to prevent compounding of these issues which can result from tight building practices. . Reducing the amount of infiltration / exfiltration is also healthier for the building.

You cannot rely on a leaky, non ventilated house to ventilate itself via infiltration/exfiltration, exhaust fans (bathroom, range hood). Bringing in fresh air is just as important as exhausting stale air. Opening windows is a great ventilation solution, but these sources for poor indoor air quality (above) still exist in the winter (and summer when you are air conditioning) when you have the windows closed.

The bath fans and range hoods in a standard house provide spot ventilation by mechanically exhausting air. The fresh air is then drawn into the house through a different 'path of least resistance'. A passive house, however, is equipped with an air tight structure and a whole house ventilation system.

In a passive house, usually the best indicator of indoor air quality is carbon dioxide concentration.  In our area outdoor carbon dioxide levels are in the 400-500 parts per million (ppm). The whole house ventilation system is designed to keep the indoor air at outdoor levels. Some of the most common sources of carbon dioxide in a home are:

  • Respiration (A typical human breath contains about 100 times the outdoor levels)
  • Cooking with gas (by product of combustion - along with water vapor)
  • Unvented Gas Furnaces (THESE ARE NEVER A GOOD IDEA!)
This is a graph from a 2012 publish paper that was assessing CO2 levels and inhabitants decision making

As you can see, in an office setting it only take 50 minutes before CO2 levels can rise to a level leading to poor performance.

To keep CO2 levels reduced in a passive house we always focus on the sources first. So it is easiest to no have any combustion appliances in a passive house (That includes, fire places, gas stove, gas dryers, furnaces, gas water heaters, etc) - it doesn't mean you can't have them, it just means it is much better if you don't. Then all we are worried about is the CO2 from respiration.

Ventilation System Design
This is where the design of the ventilation system comes in.

We want to supply fresh air to bedrooms and living spaces.
We want to remove odorous and stale air from bathrooms and kitchens (and laundry too).

We keep the supply and extract registers within the house separated to allow proper penetration of fresh air throughout the home. If the fresh air is being supplied into the bedroom, the extract register in the bathroom will draw that fresh air through the house toward bathroom where it will be extracted. If the registers are too close together the fresh air will not be as efficiently distributed.

See our proposed ventilation system design from Zehnder America

The blue dots indicate supply air. The red dots indicate the extract air. We will be supplying air to all of the bedrooms, and the office and we will be extracting air from all of the bathrooms, laundry room and the kitchen (the placement of the extract in the kitchen is intentional, this will allow efficient extraction of the cooking smells while limiting grease buildup on the register by avoiding close proximity to the range). We will run a charcoal recirculating hood above the range to remove the cooking grease.

The Zehnder America System will be supplying about 120 CFM continuously of fresh filtered air. In contrast the standard furnace fan runs at about 1200 CFM, and is just recirculating the air already inside your house, not supplying fresh outdoor air. The air flow for the Zehnder System is so low, you will not feel the air moving through the registers.

See this video for more details on the Zehnder Comfosystem

I should mention that the earth tube idea mentioned in the video is not really a good idea in our area due to the much wider temperature range and higher humidity levels than in Central Europe (where Zehnder system was designed for). There is another option that we will be using from Zehnder. It is called Comfofond. It is actually a small geothermal loop installed around the footer of the house. The loop is filled with brine and flows though a heat exchanger to temper the incoming air in the summer and winter. For example, in the winter the 5 degree outdoor air will be raised to about 40 degrees using the heat in the ground which is delivered by this brine loop, this will make the ventilation heat recovery more efficient.

I should clarify that this geothermal system is not a heat pump like those systems you commonly see advertised or installed in our area, it does not require wells or loop field or any extra excavation to install. No refrigerant is needed. It is not designed to heat or cool the house, it is only designed to temper the incoming air to make the ventilation system more efficient. There is also a significant cost difference when using the Comfofond system as compared to a full geothermal heat pump system for whole house heating. Geothermal heat pump systems also do not provide ventilation as the Zehnder system does.

So this diagram shows how the heat exchanger works
The Comfofond system is installed directly in front of the 'fresh air in' to ensure maximum efficiency.

The Zehnder system is up to 90% efficient and when combined with the Comfofond you can be extracting 68 degree air from the home and supplying fresh outdoor air at 66 degrees when it is only 5 degrees outside!

So now a single air source heat pump inside the house will only have to heat the incoming air 2 degrees to maintain the temperature in the home. In a standard house, with leaky walls, your furnace is trying to heat the incoming 5 degree air back up to a comfortable temperature and meanwhile your heated air continues to escape through the walls-incredibly inefficient!

In the next post we will talk more about how the air source heat pumps improve efficiency over a standard build.

1 comment:

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