The residence will be built at 6500ft in Wyoming with very cold winter temperatures. It will be radiant heated. It is 40x36 timber frame/SIP paneled structure creating a very tight environment. There will a 9t daylight basement, an open second floor and a loft above covered by a high ceiling. Volume of the basement is 4536 and above the main floor 8528 creating a total volume of 13,000 cu ft. A zero-clearance, wood burning fireplce with a dedicated air intake to the exterior will be on the main floor. The fireplace is basically an opening with flue and will not function as a stove.
The exchnger would only be needed in the Winter. I am wondering if an air-exchanger exhausting air from the ceiling area will cause flue-related problems with the fireplace, and whether I could avoid this and accomplish the same effect as an air exchanger by ducting air from the ceiling area to the ducting leading to the fireplace intake?
Negative pressure from the various exhaust appliances during normal operation may backdraft the fireplace regardless of where the make-up or exhaust is located. Need make-up fresh air ventilation/supply for the bath fans, clothes drier, kitchen hood, and floor/water boiler exhaust. The minimum amount of make-up air is +100 cfm. Expect .1-2 air changes per hour for a tight home during average winter weather. This amounts to 30-50 cfm of natural exfiltration/infiltration. Operating the clothes drier, 100 cfm exhaust, creates enough negative pressure to stop the 50 cfm exfiltration, and increase the negative pressure to increase infiltration from 50 cfm to 100 cfm all leakage points. Any leakage in the fireplace doors results in odors or smoke getting sucked into the home. Also any gravity chiminey may backdraft. A balanced flow HRV will not change this. Operating a HRV @ 100 cfm during the extreme cold weather may over ventilate while not resolving the negative pressure. To get 100 cfm of ventilation, explore make-up air ventilation of 50-75 cfm which will net 100-125 cfm on a cold windy day while helping your exhaust appliances and fireplace function. The lost savings is less than 75 dollars per year. Several make-up air system are available, example Filter-Vent.
The use of the word tight home seems to be very relative.
To Canadian Standards, the natural infiltration rate of something deemed tight could be .07 to 0.35 natural air changes per hour.
Taking a midpoint of 0.21 air changes per hour, with 13,000 cubic foot volume means about 2,730 cubic feet per hour or 45.5 CFM. In a cold climate, this low of an infiltration rate will result in window condensation problems.
An Heat Recovery Ventialtor (air to air) or other ventialtion will be required to control winter humidity levels. For only 13,000 cubic feet I would suggest something that moves approximately 65 CFM.
The first winter after construction the HRV will run continously to dry out the home. Use one with controls that will allow intermittent ventialtion.
In the intermittent mode, you set a Relative Humidity level. If the indoor humidity rises above this point, the unit turns on and ventilates the home until humidity drops. Your humidity set point will depend on the quality of your windows. The better the windows, the warmer the inside surface temperature of the glass, and therefore the higher indoor humidity level permitted.
This avoids over ventilation concerns that others have.
The window frames are also very important, as many frames can short circuit the effect of the insulating glass, resulting in U shaped condensation patterns.
Depending on exactly how cold it gets where you are, good windows could allow you to maintain 30 to 40% RH in the dead of winter.
With out an air distribution system, it will be difficult to provide make up air schemes.
The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.