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fresh air inlet ‘frosting’

Muteability | Posted in Energy, Heating & Insulation on December 7, 2009 01:45am

Hello,

Had a new 95% efficient furnace installed last month.  Apparentyly, as a code requirement, a 5″ cold air inlet was installed.  We’re now getting our first real taste of winter temps (-20*C forcaseted tonight).  I just noticed that there is frost on the damper and the insulation around it is wet.

When the blower door test was done, I had 5.9 ACH @ 50Pa.  Should be a bit better now since I’ve sealed up a lot, but the windows still suck.  Do I really need a fresh air inlet to my 1950’s bungalow?  Can I just block this off?  If not, how do I keep the insulation dry?

Thanks,

Adam

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  1. BoJangles | Dec 07, 2009 04:39am | #1

    Does your new furnace have combustion air supplied by a PVC pipe that takes air from outside your house?

    1. Muteability | Dec 07, 2009 05:31am | #2

      yes. Intake and exhaust through PVC. Gas hot water is burning room air. Electric stove.

      1. DanH | Dec 07, 2009 05:45am | #4

        OK, I see you have a gas water heater. It needs some combustion air, but the setup you have (I assume that duct is leading into the cold air return) is pumping in more air than you need. The best solution might be to disconnect the duct at the furnace and just leave it to dangle near the water heater. Without the furnace fan to "pump" the air, the water heater will only draw what it needs.
        A society that presumes a norm of violence and celebrates aggression, whether in the subway, on the football field, or in the conduct of its business, cannot help making celebrities of the people who would destroy it.  --Lewis H. Lapham

        1. Muteability | Dec 07, 2009 06:08am | #5

          The old furnace and gas hot water heater ran with out the intake in the past. The house is still pretty leaky.Your comment about the fan pumping in a lot outside air did make me think though. The inlet does go in to the cold air return and I turned the fan from auto to on earlier today. I've probably been air conditioning the house, when the burner's not going. I'll turn the fan back to auto and see what happens.Thanks

          1. DanH | Dec 07, 2009 07:12am | #7

            Yeah, with that arrangement (and a leaky house) you don't want the fan running continuously. The fan creates a mild vacuum, so the duct draws a lot of outside air in, slightly pressurizing the house. If the house weren't so leaky this wouldn't be a problem, but if you were to use an infrared camera on a leaky house with this setup running you'd see jets of hot air exiting from every crack in the house.
            A society that presumes a norm of violence and celebrates aggression, whether in the subway, on the football field, or in the conduct of its business, cannot help making celebrities of the people who would destroy it.  --Lewis H. Lapham

          2. wane | Dec 07, 2009 04:14pm | #8

            you need it, and you don't .. this is a common sense approach to humdity control w/o an HRV, the damper only opens when the furnace is on a suppliments the stale house air w ext air.  It's more $ in the long run than the expense of an HRV and running a motor to power it.  That said it doesn't look like yours is done right, check to make sure the damper swings the right way, check the vent also swings the right way or is just a screen, hard duct the supply line and slope to the o/s ..

             

          3. Muteability | Dec 07, 2009 06:22pm | #10

            The damper is a manual open or closed affair. The inlet is just a screen. Too much humidity is rarely an issue in this climate. The humidifier runs most of the time. The blower door test came out at 5.9 ACH (air change per hour). Caulking around windows and sealing attic penetrations probably brought that down some, but there's still a lot of space for air to be pulled in. I'm leaning towards closing it up.

  2. DanH | Dec 07, 2009 05:42am | #3

    Frankly, I don't know why you'd need the thing in a 50s house that hasn't been extensively reworked to seal it, unless you have other combustion appliances that need the air. (Do you have a gas water heater, perhaps?) Don't know that I'd remove it entirely, but blocking off about 3/4ths of the air intake should eliminate some of the frosting. Also, you should try to seal the area around that damper lever better, to keep moist air in the basement from getting in there.

    A society that presumes a norm of violence and celebrates aggression, whether in the subway, on the football field, or in the conduct of its business, cannot help making celebrities of the people who would destroy it.  --Lewis H. Lapham
  3. BoJangles | Dec 07, 2009 06:46am | #6

    You don't need that thing.  It will freeze you out of your house when it gets cold!

    I would completely eliminate it and plug the hole for good.

  4. rdesigns | Dec 07, 2009 05:56pm | #9

    I doubt if the duct that's giving you trouble was intended for combustion air--instead, it was probably for bringing in outside air to improve indoor air quality, and in your dry climate, would reduce indoor humidity in the summer.

    I believe you could simply close the manual damper for the heating season. Your house is leaky enough to provide all the combustion air you'll need for the water heater and for indoor air quality.

    As a safety test, close the damper, close the door to the furnace room while the furnace is running, but with the water heater not firing. Now feel along the bottom of the door to the furnace room from inside the furnace room. If you feel a lot of air rushing into the furnace room under these conditions, there is danger of the water heater backdrafting, and you check further by holding a match at the edge of the water heater's draft hood to see if air is being sucked down thru the flue into the room--the match flame or smoke should be drawn up into the draft hood if the WH is drafting properly.

    I doubt you will see this problem, but if you do, shut off the furnace blower and do the match test again. If it drafts properly with blower off, it means you have an improperly balanced duct system that is de-pressurizing the furnace room.

     

    1. Muteability | Dec 07, 2009 07:43pm | #11

      The damper is already in the closed position, but there's still a lot of air being pulled in. I went outside with the fan spinning and could feel the draw at the inlet from 4 inches away.There is no door to the furnace room. Almost the whole basement has open air flow. There's only one small room with a door. Thanks for the match test idea. I wedged a square of 1/2" rigid foam in the inlet (outside), turned on the furnace fan and smoke is still getting drawn up the hot water tank chimney.Within 30 minutes the frost was melted and dripping to the floor.My return 'duct' is two runs of at most 7 feet of the 16"OC floor joists. No bends The fan is powerful and must have been pulling in a ton of outside air.I think I'll seal it up for the winter and maybe open it for the summer. Nights usually get cool even on the hottest days. I can turn the fan on at night to circulate the cool air.Thanks,Adam

      1. rdesigns | Dec 07, 2009 08:29pm | #12

        "My return 'duct' is two runs of at most 7 feet of the 16"OC floor joists. No bends The fan is powerful and must have been pulling in a ton of outside air."

        This opens another question:  What is the opening into these joist bays? Is it the typical wall grille in the wall of the floor above? If you remove the grille and look inside, you may see that the true size of the opening is just a small cutout of the wall plate, about 3" x 14" at best.

        If so, you likely have undersized return, which would explain why the outside air duct was drawing so hard. Undersized returns are very common, and they reduce the efficiency and airflow significantly, while also shortening the life of the furnace's heat exchanger because reduced airflow results in too-high temperatures across the heat exchanger. It also increases air leakage into and out of, the house, by creating pressure imbalances in various rooms and spaces--some will be pressurized, while others will be correspondingly de-pressurized.

        1. Muteability | Dec 07, 2009 09:14pm | #13

          "What is the opening into these joist bays?"The return inlets are two wall grills on the floor above. They're in the central hallway, pretty much dead center of the house. The opening behind the grill is 5"x27", and open to the joists below(no bottom plate). There is a small grill grate in the basement (8x12 maybe) that has some limited flow back to the cold air duct. Just a few holes in the top plate from the previous home owners reno. It's blocked off at the moment, because I'm replacing the 60's luan paneling with drywall and don't want to suck in the dust. The furnace that was replaced was 53 years old. There's definitely a LOT more air flow with the new blower."It also increases air leakage into and out of, the house, by creating pressure imbalances in various rooms and spaces--some will be pressurized, while others will be correspondingly de-pressurized."I don't know what I can do to fix that.

          Edited 12/7/2009 3:22 pm by Muteability

          1. rdesigns | Dec 07, 2009 11:53pm | #14

            If I understand your description, your return air consists of an opening of 5 x 27, or 135 sq. in.

            That seems very small, but, of course, I don't know the BTU rating of your furnace--my guess is that is will be about 100k. With a high-efficiency furnace, you ought to have about 150 CFM for every 10k BTU's (that would be 1500 CFM for a 100k furnace), and there's no way you'll be getting that much air through your current return air opening.

            The fix may not be all that difficult.

            You can use joist spaces for return air if you seal them well, so you could add more of the type you already have.

            If you heat the basement, you could open a large return air grille in the underside of the return trunk, and then open a pathway for air to get from the main floor to the basement.

             

          2. Muteability | Dec 08, 2009 12:56am | #15

            "If I understand your description, your return air consists of an opening of 5 x 27, or 135 sq. in."It's two openings that size but that's still small by your numbers.80K BTU furnace. The return plenum goes straight up from the furnace an is only the two joist bays.From the pics you can see the cold back of the basement return closed off with core plast. The return for the first floor is above that beam. They are only 2x4 walls so the numbers might be a bit smaller.

          3. rdesigns | Dec 08, 2009 01:52am | #16

            An 80k HE furnace should have about 1000 CFM. Each of the wall returns would need to deliver 500 CFM, which is doubtful, but not impossible, depending on what other restrictions the duct system has.

            If you have access to a manometer, a static pressure test would tell the story. The total external static pressure should not be more than about 0.7, and 0.5 would be better.

            However, you could do a simple test to see if the main floor is being pressurized:  Assuming you have a door separating the main floor from the basement, close that door while the furnace blower is running, and, standing on the basement side of the door, feel for airflow under the door. If there is much of a breeze coming thru, it means that the main floor supplies do not have adequate return pathways, and that the air is pushing into the basement.

            Another useful and simple test is to check the temperature rise across the heat exchanger, which is done by measuring the air temp at the supply plenum and the return plenum after the burner has been running for 5 minutes or so--if, say, the return side was 70 and the supply was 150, the rise would be 80, and that is too high according to most manufacturers, who want the rise to be in the range of 35 to 75. High temp rise = too little airflow. The temp rise can also be used to get a fairly accurate estimate of the total CFM. I can give you the formula if you feel that would be useful.

          4. BoJangles | Dec 08, 2009 03:42am | #17

            Not to worry....with that air inlet hooked up, he will be lucky if he can get his supply temp up to 70* when it is cold out. 

          5. Muteability | Dec 08, 2009 04:50am | #18

            "Each of the wall returns would need to deliver 500 CFM, which is doubtful, but not impossible, depending on what other restrictions the duct system has. The only restriction would be the filter, which is only a low cost FG (MERV 1 or 2??) "The total external static pressure should not be more than about 0.7, and 0.5 would be better"I do have access to a digital manometer. What is the test set up and what units of measure is the .5 to .7 you refer to? My flow meter only goes to about 120 LPM and has a 1" round inlet. Won't be much use for ducts.I'll be out of town until Thursday, so all testing will need to wait until then.Thanks

          6. rdesigns | Dec 09, 2009 05:57pm | #19

            The values of 0.7 and 0.5 refer to "Inches of Water Column" (IWC). A digital manometer will likely have a scale for IWC, as well as one for Pascals--250 Pascals = one IWC.

            You want to know the total static pressure (resistance) for the system. You need two readings to get this: the supply side and the teturn side. For the supply side, insert the probe in a small hole that you drill between the furnace outlet and the AC coil, being VERY careful not to drill into a refrigerant line. The return measurement is taken at a point in between the filter and the furnace. The sum of these two is your total static pressure for the system.

            If the supply was 0.25, and the return was 0.35, that would be a total of 0.6, which is slightly high, but not a problem. If you find the total to be much more than about 0.75, you could have airflow reduced enough to cause the heat exchanger of the furnace to overheat, or the AC coil to have insufficient airflow.

            You should be able to get the manufacturer's blower chart, which will corellate static pressure to CFM. The higher the static pressure, the lower the airflow, or CFM.

            In Calgary's dry climate, combined with its high elevation, you need much higher cooling airflow than the standard 350-400 CFM per ton of AC that is correct for sea level and high humidity. You should have at least 450-500 CFM per ton of AC to correct for the less-dense air and lack of humidity. (a "ton" of AC capacity = 12,000 BTU's.)

          7. Muteability | Dec 11, 2009 10:48pm | #20

            rdesigns,

            I hope you see this in the new format. I've had a bugger of a time finding things.

            I don't have AC installed, so drilling in to the coil is a non issue.

            So if I understand correctly...

            With the fan running on high,

            I drill a small hole after the filter, insert the tube attached to the digital pressure meter and I will see a negative pressure.

            I then drill a hole on supply side (the plenum right above the furnace?) insert the tube and I will see a positive pressure.

            The difference between the two should be around 0.5 IWC.

            I might give that a try this weekend. Getting back after three days with the fresh air inlet loosly blocked has resulted in the frosting disappearing and the moisture in the FG insulation drying up.

            I did try the closing the basement door with the fan running and could not detect any flow from the first floor to ground floor, even with the smoke from an extinguished match.

            Thanks again for all the help. If I can figure out a way to email directly. I'll try that.

          8. rdesigns | Dec 14, 2009 11:22am | #21

            New format = bad, but that's
            New format = bad, but that's another problem.

            Since you don't have AC, much of what I was concerned about is not really a problem, and frankly, I was wondering why you'd need AC in Calgary--its summer design temperature is only 80 degrees, meaning that it's warmenr than 80 only 1% of the time.

            Lower airflow on heating is not as critical as on cooling. Low heating airflow is mainly a concern when the temperature rise exceeds the manufactuer's acceptable range, which is quite large, 35-75 degrees being common. So the thermometer test I mentioned earlier is adequate to show this.

            If the temp rise is within limits, the only other question is: are certain spaces or rooms not adequately heated by the time the t'stat is satisfied and shuts down the system?

            If so, it means insuffient airflow to that space. The first thing to check is return air: is there unrestricted return from that space? If so, then the supply branch is the next thing to check, along with air balance for the whole system.

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