we are building a new home in New Hampshire, ranch style 1400sq.ft. with finished walkout basement, R 40 walls R 60 ceiling dense packed cellulose, anderson 400 windows, Mitsubishi ducted mini split with ERV .  So after pointing for a relatively tight , energy efficient shell we face some decisions. We have a propane cook top with a range hood that has variable speeds 100, 200, 300 cfm, two bathrooms with 30, 50, or 100cfm panasonic fans with passive make up air ducts capable of 18 cfm in 960 cu. ft. bathrooms and would like to have a wood stove mainly for esthetic reasons. How do we provide make up air for the wood stove and prevent back drafting when the range hood, bathroom fan, gas dryer are all running.  There are several wood stoves that are designed for separate make up air connections but it is difficult to get exact cfm numbers on them, they all have 3" diameter passive air supply via a dedicated through the wall connection that connects directly to the stove. Is this a significant problem or are we just being worry worts ??  Our range hood does have an optional make up air system with an electronic controlled damper that is designed to open when the hood exceeds 300cfm, it will open to allow air into the shell but does not duct the fresh air directly to the hood.  thanks  Craig

You have an ERV. A range hood, a bathroom, a gas dryer and a wood stove are all areas where you should be running a return.

Talk with the HVAC person doing the install.

Generally when you're using a cooktop, dryer, taking a shower, fireplace, you set up your ERV to up the number of air exchanges, either on a timer or for the duration.

Based on what I've seen in our homes, the option to a direct air intake for the wood stove is to open the window when you are using it, or smoke up the house. The danger with the gas dryer is that the ERV can pull the carbon monoxide exhaust into the living space, so the best option here is to shut off the ERV when you are drying clothes. The bath fan and range hood may also pull in unburned gases from the dryer or wood stove so caution is urged. You are absolutely right to worry, especially if your house is very tight*, but there are other serious problems that may occur if the house is not very tight. Buy several carbon monoxide detectors and keep them up-to-date.
(*gas dryers and stoves are strongly not recommended in these homes!)

The wood stove should utilize outside combustion air to a sealed firebox. It is critically important.

My hood draws up to 1200 cfm and I placed an opening skylight directly above the range to provide makeup air in close proximity so as to avoid evacuating the entire house when it runs.

With regard to using ERV/HRVs for makeup air, we discussed this topic at length a couple years ago:

Using ERV/HRVs for Range Hood Makeup Air

A typical ERV/HRV is a balanced system (i.e., brings in the same amount of air as it exhausts) and has a much lower flow rate capacity than a typical range hood. So as wonderful as ERV/HRVs are, they won’t help with this issue.

The minimum code flow rate requirement for range hoods is 100 CFM. Makeup air is only required for exhaust range hoods having a flow rate of 400 CFM or larger and the code states:

“Exhaust hood systems capable of exhausting in excess of 400 CFM shall be provided with makeup air at a rate approximately equal to the exhaust-air rate. Such makeup-air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system.”

So some would interpret the “...automatically controlled to start and operate...” language to preclude the use of manually opening windows, etc. to satisfy this requirement. However, there are several other ways to satisfy this issue:

First off, if you can get by using an exhaust range hood less than 400 CFM, you don’t need to provide any makeup air at all. If you can get by using a re-circulation range hood (even one having a flow rate much greater than 400 CFM) in lieu of an exhaust range hood, you also don't need to provide any makeup air at all since you are NOT exhausting any air.

If you need an exhaust range hood greater than 400 CFM, you can add an outside air duct that has an electrical/mechanical damper that is automatically controlled to open when the range hood is used. Some jurisdictions and inspectors like this air duct to be located in relative close proximity to the range hood it services. Some jurisdictions and inspectors are okay having this air duct be located anywhere in the building and this allows having this cold or hot outdoor air enter the building where it causes a less noticeable adverse comfort effect on building occupants and allows this outdoor air to get tempered as it flows through the building.

If you need an exhaust range hood greater than 400 CFM, you can also use a mechanical MAU (Makeup Air Unit) that is automatically activated whenever the range hood is activated. MAUs can also temper the cold or hot outside air using a heat exchanger like ERV/HRVs, but MAUs can be quite expensive like ERV/HRVs.

NEVER ever install naturally vented fuel burning appliances into an airtight house. Ask a ventilation expert if you are not sure and have them sign off on the project as well.

Posted By pacificstart on 17 Aug 2017 04:11 AM
NEVER ever install naturally vented fuel burning appliances into an airtight house. Ask a ventilation expert if you are not sure and have them sign off on the project as well.

Gas dryers , gas ranges, ventless gas fireplaces are installed all the time in new tight houses ,
the code requires an outside make up air source for combustion and carbon dioxide detectors are also a code requirement

Chris - yes, the code says that and people do it as you said. But the MUA can become obstructed and then your are relying on sensors to warn you that you are in trouble. The probability that both MUA and sensors will fail at the same time is small but not impossible. This is how accidents happen - two or more things go wrong at the same time. A building life expectancy is 100+ years or much more for durable construction such as ICF so that probability increases given the time. This is not paranoia but simple math.

Many gas and propane indoor appliances these days have O2 depletion sensors that will turn the appliance off. All ventless fireplaces certainly do. CO detectors are code in most places and using multiple detectors provides a high level of warning capability. I think the highest risk of generating CO is from wood burning appliances and these should always have sealed combustion chambers that are directly vented to outside the building.

Most CO poisoning occurs when people do dumb things like using their outdoor BBQ to heat their house when the power goes out. CO poisoning is also higher risk in red states that don't have any or adequate building regulations and builders who are more profit motivated than safety conscientious.

yes I think it is paranoia , the simple math says there is greater chance I will get struck by lightning