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Manual J wants 10 CFM to a room…possible?

jeffwatson | Posted in General Questions on

Some of the rooms in a Manual J heat load required air volume velocities as low as 10 CFM coming out of a duct. I’m not an HVAC pro or even a construction professional – just a homeowner having issues with temperature variation, high static pressure, lack of return air, negative pressure, high dust, and unsealed/inaccessible ductwork. I’ve done all these “energy efficient” things but at the end of the day, I feel less comfortable & temperature variation is a lot greater than what it was.

I’m trying to go by the book and follow the Manual J recommendations but the first few HVAC contractors have told me 10 CFM isn’t possible to achieve (I’m considering just redoing all the ductwork). I hate being in the position where the energy auditor tells me one thing & then the contractor says things don’t happen like that in “real life” and I have to either fend or give in. Feels like if you’re not building a house from the ground up with energy efficiency in mind, it’s impossible to get it right after the fact if you’re just a homeowner who has to go out & seek various contractors.

Going off on a tangent. I even have a small area that requires 1 CFM. So what do I do? Keep looking for an HVAC contractor who can read a Manual J & be confident in delivering what it says or settle because the Manual J has unrealistic numbers? Is 10 CFM even noticeable???

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jeff,
    Your HVAC contractors are correct that it's not realistic to expect a forced-air heating system to dial in 1 cfm or 10 cfm. It's almost impossible to commission a forced-air heating system to achieve these air flow rates precisely. Just a little bit of dirt on the filter, and everything changes.

    However, such precision is unnecessary. If you have a good thermal envelope -- one that is tight (with a low level of air leakage) and well insulated -- then you have managed 85% of the job to achieve tiny room-to-room temperature differences. Provide heat or cooling to the rooms that need 50 cfm or more; if a room needs less than 50 cfm, chances are that it will do fine with 0 cfm.

  2. jeffwatson | | #2

    Martin, thanks for your reply.

    This stuff is confusing for a regular homeowner like me then.

    My home is apparently tight. 800cfm @ 50 Pa, 1000sqft floor plan, basic rectangle. Single story + basement. R60 attic. R11 basement. Yet I have 5 degree variation from thermostat to other rooms. Cold air felt right after the heat comes off from return vents. Only 3 areas out of the 12 on the Manual J require 50+ CFM. 40 kBTU/hr 90% furnace.

    So what's my problem if it's not ductwork? Just following what I hear on these energy web sites & what energy auditors, but come into resistance once I talk to contractors...which it sounds like you're saying there's some truth in what they're saying. So confusing.

  3. Expert Member
    Dana Dorsett | | #3

    What means " Cold air felt right after the heat comes off from return vents." in 'merican?

  4. charlie_sullivan | | #4

    A 5 degree variation sounds like evidence of a problem that is not reflected in the manual J calculation. Is that other rooms hotter or colder than the thermostat, or both?

    Like Dana, I don't know what the cold air felt comment is about, but that sounds like a clue about a problem.

    We have a major problem in this country that most of our HVAC contractors are not educated to the level that they'd need to be to design good systems and troubleshoot problems beyond a system running or not running. A good energy expert (e.g. auditor) is the most likely resource to help you figure out what is really going on. It's often a problem to then convince contractors to do what the energy expert says, but an experienced energy expert should also be able to refer you to good contractors.

    But in your case, I think there might be more to what is going on than needing to match the system to the manual J calculations. It would be that there's a hidden air leak or thermal path that is making things worse than the manual J. Not because you did the manual J wrong, but because something is built wrong.

    An example: if there's an air leak from unconditioned space through the walls to the attic, a blower door test won't pick that up, but you lose heat, and the room(s) that are against that wall will be cooled.

  5. jeffwatson | | #5

    The energy audit considered my basement as conditioned space though there's no registers in the basement & the basement is just cold period. There's insulation on the interior foundation walls but no insulation separating the basement & main floor.

    Since the basement is partially finished, with the sheetmal ductwork that is exposed, it's not sealed with mastic so I assume the inaccessible ducts are the same & possibly have leaks.

    The "cold air" comment meant that with the heat off, I can wet my hand, put it up to a return air vent and feel cold air against my hand.

    The manual J included provisions for vents in the basement which is why I was just looking to tear it all out & redo it and make sure they get sealed.

    I've already dealt with an energy auditor. My house is now air-sealed like crazy to the point I have a carbon dioxide/IAQ issue.

    So if 10CFM isn't realistic, why does the Manual J even have this as the required airflow? Honestly it makes me question the whole Manual J report then.

  6. GBA Editor
    Martin Holladay | | #6

    Jeff,
    Manual J calculations show heating loads for every room in your house in BTU/hr. It's up to your HVAC contractor to figure out how to meet those heating loads.

    In theory, you can heat a room with a small electric-resistance heater, or a hydronic heating system, or a ductless minisplit. You don't need to heat a room with air-flow through ductwork.

    You need to focus on your comfort problem, not on trying to achieve a certain airflow rate through your forced-air ductwork.

    If your basement is cold, and if you suspect that you are uncomfortable because you feel cold air entering the upstairs room, I suspect that your basement wasn't air sealed well. You may have air leakage at the rim joist area. (If your basement walls are already finished with drywall, these leaks may be hard to see.)

    That's issue #1, it seems to me.

    Issue #2 concerns your preferences for basement conditioning. If you would prefer to have a warmer basement, that's usually easy to achieve. You can either install a register in your supply plenum or one of your main ducts -- assuming that this installation doesn't unbalance your supply air system -- or you can install a new duct and connect it to your supply plenum.

  7. jeffwatson | | #7

    Martin, I'm under the impression that the comfort problem is caused by poorly designed and/or sealed ductwork. Wouldn't the lack of air sealing in the basement just be accounted for in the air infiltration number used during Manual J calculations?

    Drywall does block access to the joist pockets in the brick, so I couldn't hit those areas to air-seal. I don't necessarily _want_ heat in the basement (because I'm almost never down there) but I assumed heating the basement would help alleviate the comfort issues & temperature disparity on the main floor. The (hardwood & tile) floors are super cold.

    This all just runs back into redoing the ductwork since that would require tearing down the drywall that seals them to the basement ceiling/walls.

    How can I test how balanced the current setup is? The room that the Manual J requires 10 CFM (bathroom, 5x7), currently that register probably blows the hardest of all registers!

  8. GBA Editor
    Martin Holladay | | #8

    Jeff,
    Q. "How can I test how balanced the current setup is?"

    A. If you have any HVAC contractors you trust, ask them whether they have tools to measure airflow rates through ducts or registers. If they don't, you would probably need to hire a good home performance contractor or energy rater to measure the airflow at your registers.

  9. jeffwatson | | #9

    We have a major problem in this country that most of our HVAC contractors are not educated to the level that they'd need to be to design good systems and troubleshoot problems beyond a system running or not running. A good energy expert (e.g. auditor) is the most likely resource to help you figure out what is really going on. It's often a problem to then convince contractors to do what the energy expert says, but an experienced energy expert should also be able to refer you to good contractors.

    While I understand your sentiment, as a homeowner who's simply just trying to solve a problem, in my experience a separate issue is also with the advice received by the energy experts that point out something & simply point a homeowner to "go find a guy or contractor who does this or that."

    We(homeowners) need more than just auditors or energy experts. I used to feel an auditor who has nothing to sell in terms of building/construction services is beneficial since they'd pride themselves on no-conflict, but after going through various contractors after issues raised by a single auditor, this is just a never ending process with no telling if things are being done the "right" way.

    Sorry, just had to rant a little bit after getting another response directing me to another contractor, no offense Martin. I don't have any contractors (that I trust). That's not to say that I don't trust contractors at all, just that as a new homeowner, I've never dealt with contractors so all I have is Yelp...and the internet...that's why I'm here...

    I still appreciate all the advice given thus far, but I'm looking to learn as opposed to spending another couple hundred dollars for information that ultimately doesn't take me far or can only be used once.

  10. GBA Editor
    Martin Holladay | | #10

    Jeff,
    If you want to educate yourself enough to commission an HVAC system, you can. I would start with one of the trainings offered by RESNET or BPI.

    Using information gathered on GBA, you can also find ways to experiment on your house without hiring a contractor. Install a balancing damper in each duct run, and adjust the dampers as an experiment. See if these experiments result in more or less comfort.

    Of course, if you skip the training, there are hazards to this approach. For example, if your use of balancing dampers reduces airflow through your furnace or airflow over your coil too much, you can damage your equipment.

    If you don't want to take either of the routes I've suggested, you'll have to look for a competent contractor, energy consultant, or mechanical engineer to help you. Good luck.

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