makeup air w/HRV and sealed combustion ? - Page 2
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  1. #14
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,677
    Quote Originally Posted by bjherron View Post
    Do you think it will function with the AC coil on that way, or shoud I have them turn it?
    absolutely no issues with this, I guess I would have faced it to the front, they must be planning on the A/C coming from the back side and aren't very good with a bender
    You can't fix stupid

  2. #15
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,231
    Quote Originally Posted by bjherron View Post
    I
    I have a goodman 95% variable speed 115K BTU furnace and American Aldes 300DD HRV ducted to the basement and main floor. We are about to fire the systems, and the contractors put in the vent in today. They have a 3" furnace exhaust pipe running out, but they said the intake will draw air from the utility room. They also connected a 6" make-up air vent from the outside wall to the return air trunk. Keep in mind my HRV's fresh air port is also connected to the return air vent, and it exhausts from the bathrooms.
    I assume you are in a green grass cold climate and making a considerable effort to make your system the best. How about other devices like fireplaces, clothes drier, range hood, (size cfm), and central vac with outside exhaust? They all need make-up air.

    Quote Originally Posted by bjherron View Post
    I
    Question 1: Shouldn't the furnace get it's combustion air from outside? I also have a power vent water heater in the mechanical room, but I assumed it would also have pipes to get it's combustion air from the outside.
    pics attached
    Many power exhaust furnaces are not totally sealed. Although you can connect the fresh air to the cabient, air can come in though cabient vents. Not good or bad. Taking combustion air from the basement will slow the stack effect loses. Taking 100 cfm from the basements reduces warm air loses from the house by approx. 50 cfm. The same applies to the water heater and other exhaust devices. If you have an open fireplace that back-drafts when the exhaust air is need, you must address make-up air aggressively.

    Quote Originally Posted by bjherron View Post
    I
    Question 2: If the combustion air is taken from the outside, is there any reason to have a makeup air vent connected to the return? It seems almost pointless to have that with an HRV, especially an expensive dual core one if it's just going to draw air thru a 6" hole in the wall.
    ed
    Keep in mind, fresh air is only sucked in when the air handle is operating on high speed. This concept is required by some regional building codes and was intended to provide make-up combustion air for gravity chimineys. Heating/cooling fans operate a small percentage during the year. Therefore are not reliable make-up air providers.

    Quote Originally Posted by bjherron View Post
    Question 3: Should I go back and mastic all of the joints? I've heard it's good to do, but I am busy with other parts of this project and when I brought it up with them it sounded like they either didn't want to do it or would want to charge me a lot.d
    Because the air ducts are in the conditioned space, duct leakage is not as critical compared to attics/crawlspaces.

    As you know better than your suppliers, HRVs have nothing to do with make-up needs of your home. With the except of an open fireplace, your power exhaust devices will probably function. Also your air tight home does have some natural air leakage. I would expect 50-100 cfm on a cold windy day. Exhausting 100 cfm stops most of the natural exfiltration. This may be ok if no gravity exhaust devices are in the home. You need 75-100 cfm of fresh air in typical deluxe home, when the home is occupied. The fresh air purges pollutants including moisture, and renews oxygen. During the coldest windy weather, you need enough make-up to operate your exhaust devices and provide IAQ air. Your HRV is needed occasionally, but difficult to sequense without over-ventilating. As the weather warms (spring/fall), stack effect declines and balanced ventilation is better suited.
    During summer (no stack effect, only wind), make-up air is still a big issue. In addition in green grass climates, maintaining %RH by ventilating is no longer possible. 100 cfm of fresh air adds 100 lbs/day of moisture to your home. On hot days, your a/c will remove the mositure. But during cool, damp weather, indoor moisture builds to the point where dust mites and mold are an IAQ/comfort are effected. For ideal humidity control, a whole house dehumidifier is ideal. These high eff. dehu have enough moisture removal capacity to maintain <50%RH throughout the home without any a/c operation, fresh air ventilation, and moisture from occupants. If you were starting from scratch, a ventilating dehumidifier would be a better primary device to provide filtered make-up air, maintain <50%RH during the damp weather, and distribution through-out the home. Adding whole house dehumidifier to a HRV and co-ordinating the operation of both may be considered the ultimate mechanical systems. Provide make-up air when occupied and monitor %RH. Keep us posted. I work for a dehu mfg. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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