Required Ventilation for tightly built homes

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Old 02-20-17, 08:56 AM
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Required Ventilation for tightly built homes

Hi,

I have a newer house that was advertised as being energy efficient and was build pretty tight. I do not have a heat exchanger, but I do have a fan that runs constantly to help the house pull in the required amount of fresh air. It is my understanding that you have to replace the air in your house every couple hours or so to prevent the buildup of carbon dioxide (CO2).

I live in Michigan and the winters are pretty cold. I am guessing that this is sucking a bit of my heat out. My question, do I need to run this fan when no one is at home? If the major source of the stale air is from people and cooking, is it safe to turn this fan off when no one is at home? We work during the day so I could turn it off during this time. I was thinking of installing a zwave switch to automatically turn this on and off.

Thanks for any advice!
 
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Old 02-20-17, 09:59 AM
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Hi dan,
There should have been some test results and a good explanation left near the electrical panel for all future home owners which would answer your question. Given that few builders actually provide that info, we can help.

Now, even though they advertised it as being pretty tight that doesn't tell you how much fresh air you need. Did they provide any supporting numbers, like the results from a blower door test, a CFM50 number or ACH50 number?

Although excess CO2 is considered a small part of indoor air pollution, it is not the deadly variety like CO. Basically, you can turn that fan off anytime you want. One of the more common concerns is moisture and if your house is REALLY tight you will soon see condensation on your windows, assuming a cold climate. No condensation, then most likely the house isn't as tight as advertised. It may still be energy efficient (for which they should have given you an energy report) but to get it really tight is a step above energy efficient.

Here's a link that may help. I just skimmed it (read many) but I'm familiar with the author so it should be good.
Why Do Airtight Homes Need Mechanical Ventilation?

Basically, what you need to do is go through the determination process to see IF you need fresh air and then determine how much and how it should be achieved.

Bud
 
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Old 02-20-17, 11:37 AM
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Thanks Bud! Yes they provided a packet of information including the results of the blower door test. I will find it when I get home this evening and post the numbers.

I believe I do need fresh air. I did a test last summer and turned the fan off and watched the humidity level and it did keep increasing until I turned it back on. In the winter, we just try to keep enough humidity as we have to humidify to offset all the forced air heating.

I was looking at a couple indoor air quality monitors like birdi or awair. Would these or similar air quality monitors give me enough of a measure on how often I need to run the fan? I see they watch VOCs, CO2, dust and pollen, as well as Carbon Monoxide. I sent one of them an email to see if they can detect mold spores and formaldehyde.

I think I should be ok on a couple other pollutants. When I moved in, I had to put a radon fan in that sucks the air from around the outside of the basement. If I remember correctly it was at a 6ppm before and went to equal to the air outside afterwards (.05 or under I believe). The furnace has a direct vent so I don't think much pollution should get inside and we have an electric water heater but we do have a gas stove.
 
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Old 02-20-17, 12:36 PM
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I just went to awair site and they said you could hook it up to a nest and other units. A would guess that you could connect to fan and have it automatically.
 
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Old 02-20-17, 02:21 PM
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If you find you need to run the fan to keep humidity and pollutants at bay, you could install a heat recovery ventilator. Smaller unit are not terribly expensive, and you apparently already have the inlet and ducting....
 
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Old 02-20-17, 03:58 PM
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The idea that forced hot air heat causes low humidity is half myth. The half that is true is when the forced air system pulls in outside air which is much dryer. A well sealed and well balances system does not increase the air exchange so has nothing to do about the air being dry. The fan however tries to make the inside air the same as the outside, plus or minus the heat or cooling we add. In Summer the outside air can be more humid than inside and the reverse is true for winter, outside air is usually drier.

Note, outside air at 30 and 50% RH sounds humid until you warm it up to 70 when its RH drops to 11%. If you are having to add humidity in the winter, turn that fan off, it is lowering the RH.
Compare dew points. Temperature, Dewpoint, and Relative Humidity Calculator
Use outside temp and RH to calculate a dew point. Then use that dew point with the new temp that air will be warmed or cooled to once inside. That will give you the new RH that air is contributing to the house.

Sorry if I'm a bit disconnected here, helping someone buy a house. Those numbers from that report will tell you how much fresh air you need.

Bud
 
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Old 02-20-17, 05:22 PM
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Interesting, I didn't know that about humidity. I will try turning the fan off and see what the humidity does. It is unseasonably warm right now 30 - 50 so the humidifier is not running much but I will try turning both of them off.

The report says the infiltration rate of: Htg:1017 Clg:1017 CFM50 Method: Blower door test.
I also see it list a ventilation syste: Exhaust only 60 cfm, 65 watts. I am guessing this is the fan.

Are these the right numbers? How would you interpret them as how often to run the fan?
 
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Old 02-20-17, 07:45 PM
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If I'm reading those numbers correctly they say the blower door test results were 1017 cfm at a negative pressure of -50 pascals. That would be on the tight side. What I need in addition would be the area of your home. Actually the volume but area provides that if all ceilings are 8'. So how big is the house?

Bud
 
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Old 02-20-17, 07:53 PM
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Looking at the energy report it says a conditioned area of 2988 sq. ft and a conditioned volume of 23904. It is a 2 story house with 8 ft ceilings. I believe it is 2200 sq. ft. finished and an unfinished basement. I believe the energy report's 2988 sq. ft must be including the unfinished basement.
 
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Old 02-21-17, 05:20 AM
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Well, I now have to agree you have a rather tight home. Rough calculations suggest about 8 hrs for a complete air change, that's using the average numbers. Unfortunately our homes, the weather, and our lifestyles rarely follow that average value. If you can get through that link above it can give you some guidance as to when you need lots of ventilation and when less will do. But I tend to agree with the builder that you do need to be using the exhaust fan. You could install a HRV (heat recovery ventilator) to achieve the desired ventilation with less heat loss, but they are a bit expensive.

Your thoughts of cycling the unit off can work but remember the ventilation is for more than human generated pollution, again read that link.

I'll ease your concerns a bit about heat loss. When you turn on an exhaust fan, we'll say 60 cfm, you slightly depressurize the house. That change in house pressure increases the infiltration as we expect, but it also decreases the natural exfiltration. There is a long explanation but they named that effect the "half fan rule". A 60 cfm exhaust fan will result in only 30 cfm of new infiltration.

I tend to agree with Dr Lstiburek that our guidelines call for too much ventilation and we really don't know what math your builder used. The solution is to dig into the numbers and decide what it right for you and your home.
How Much Fresh Air Does Your Home Need? | GreenBuildingAdvisor.com
Looks like I omitted the link before, a result of shortening that response. So there it is, see what you can get from it.

Bud
PS their offer on the house was accepted so less distracted this morning.
 
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Old 02-21-17, 07:35 PM
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Hey congrats on helping them find a house!

Good article. I liked some of the comments too. What do you think about the one that said "We need to abandon wild ass guessing about iaq. We need to continuously measure and manage accordingly". This is exactly what I was wondering. It seems like air quality would vary based on the time of year/weather conditions and location.

Do you agree with this comment? Would it be better to buy a monitor and watch the air quality numbers rather than guessing (educated guess) if that fan is pulling in the right amount of air? Most importantly, I do want to make sure I am getting enough fresh air.
 
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Old 02-23-17, 06:42 PM
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@ dan ""What do you think about the one that said "We need to abandon wild ass guessing about iaq. We need to continuously measure and manage accordingly". This is exactly what I was wondering.""
Well, it can be done, but is it necessary and is it going to be cost effective?
I attended a conference on iaq and the speaker, who was a strong advocate of iaq, brought along his CO2 monitor and had it running while he talked, about 50 attending. He kept checking the reading and finally excused himself for a moment to talk to the conference room manager. When he returned he asked the people in the back to open a window while he opened one in the front. Then he chastised the building owner about their failure to install ANY form of ventilation. A few minutes later another check of the reading and he opened more windows.
To explain his action he said the level wasn't high enough to cause health issues, but high CO2 level have been known to put audiences to sleep. He said he could have brought his methane meter along (flatulence), but people really don't like to know what they are breathing, but if CO2 is high you can bet methane is as well.

Well, no one got up and left, but there was certainly no one complaining about the windows being open with the cold air flowing through. So your concerns are valid, but doubtful your house will ever see sufficient pollutant levels to require anything more than that fan running intermittently.

Wish I had a better answer, but you can tell from the article that the guidance on this topic is still out to lunch.

Bud
 
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