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Thread: Mitsubishi Air Handler and Ducts in the Attic

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    Mitsubishi Air Handler and Ducts in the Attic

    Hi everyone, several months ago I posted about serious issues with a Mitsubishi minisplit system. That thread is here, for reference. I had serious overheating in the winter and terrible humidity in the summer. After dozens of service calls and meetings with the installer, the regional distributor and Mitsubishi of America, everyone stepped up to the plate and agreed that the entire system should be replaced. Amazing customer service by all parties! Now...about the agreed upon fix...

    We are replacing the single 48k btu HyperHeat outdoor unit with a single 18k for my 1st floor feeding an 18k wall unit, and a multi 36k compressor for a 12k wall unit in the basement and the new Mitsubishi 18k multiposition air handler to duct to my three upstairs 100sq ft. bedrooms and bathroom. I was really excited for this solution. And then I started reading the endless web pages talking about how you should NEVER put ductwork in an unconditioned attic!

    My house is small. My attic is about 20 x 30 and insulated on the floor, but wide open to the outside temps. Given that the basement and first floor will be wall units, should I not worry about the loss of efficiency for the upstairs given that this should be an otherwise very workable solution? I know I will lose efficiency, but if I'm OK with that $300-500/year loss if there are no other negatives in doing this. But I don't want to increase the possibility of ice dams, lower the longevity of the equipment, etc.

    Any thoughts are appreciated. Once I agree to this solution, there is no going back. I only get one bite at this very fortunate apple.

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    Where is then load calculation?

    If my math is correct, they have increased capacity...are they freaking nuts? Does your house have holes in the exterior walls to the outside and no insulation. 18000 btu for 500 square feet?

    The single airhandler ducted to the bedrooms and bathrooms is likely what should have been done from the beginning. This is not a problem to put in the attic provided it is installed and insulated correctly.

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    Quote Originally Posted by BALloyd View Post
    Where is then load calculation?

    If my math is correct, they have increased capacity...are they freaking nuts?

    The single airhandler ducted to the bedrooms and bathrooms is likely what should have been done from the beginning. This is not a problem to put in the attic provided it is installed and insulated correctly.

    Sent from my SM-N910W8 using Tapatalk
    They did a load calc. 18k is what is needed in the upstairs. We go from a single 48k outdoor unit to one 18k and one 36k. Same capacity, but the single, 1:1 18k can ramp down very low, unlike the multi-zones that can only get down to 6k. (I'm starting to wonder why we're not using a 30k btu for the 18k air handler upstairs and 12k basement wall unit. Will ask about that on Monday.)

    Glad to hear you think the attic situation is OK. Then why am I reading so many horrors about ducts in the attic and how it should NEVER be done? Is it just if the install is poor? It does make sense to me that ducts in a New England attic are inefficient. But if that's what I have to do, and it's not horribly detrimental, then I want to go with it.

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    As long as the duct is SEALED and insulated well then the duct in the attic shouldn't be a problem. It could have condensation problems that if the blower is not set to run at all times which could result in having uncomfortable "cool drafts" in the winter. You won't see 300-500/year in inefficiency though maybe 20-30/year more if sealed and insulated properly.

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    It is best to have ducts in conditioned space but that is not always possible. That is why there is duct insulation. At a minimum the ducts should be R-6. Better still is R-8.
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    Did they show you the load calculation? Can you post it here? There are a couple guys on here that are really knowledgeable about load calculations.

    A couple quick rule of thumb calculators I found (which are really basic and not a proper load calculation) are both showing approximately 10000 btu for your 500 square foot first floor.

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    I should've mentioned this before, my house was built in 1871. It has very poor insulation. It is stucco on the outside, along with a few layers of sheet rock and strange framing. The stucco helps, but it is a poorly performing house, energy wise. And we're sizing for the coldest days.

    Along with a gas fireplace on the first floor, this is my sole source of heat. I don't have the heat loss/load calculations. But everybody wants this to work. No one has any interest in providing the wrong equipment. No one is making any money anymore! A lot of people have been involved at this point. The 18K is on the high side, but not crazy at all...according to the people involved.

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    Quote Originally Posted by jtrammel View Post
    As long as the duct is SEALED and insulated well then the duct in the attic shouldn't be a problem. It could have condensation problems that if the blower is not set to run at all times which could result in having uncomfortable "cool drafts" in the winter. You won't see 300-500/year in inefficiency though maybe 20-30/year more if sealed and insulated properly.
    jtrammel, thanks for this info. So you are saying the blower SHOULD be set to run at all times, or should NOT be set to run at all times? And you think the loss of efficiency, if installed well, will barely be noticeable? What I'm not understanding are all of these dire warnings, such as this one entitled, "Case Closed: Get Those Air Conditioning Ducts out of the Attic". It cites, and links to, a comprehensive study showing attic ducts are very bad under all circumstances. I'm just outside of Boston and we get hot summers and cold winters. The ducts will be run close to the eaves, at the edges of the attic to save space within the main parts of the attic. Thanks again for any info.

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    Quote Originally Posted by dansminisplits View Post
    jtrammel, thanks for this info. So you are saying the blower SHOULD be set to run at all times, or should NOT be set to run at all times? And you think the loss of efficiency, if installed well, will barely be noticeable? What I'm not understanding are all of these dire warnings, such as this one entitled, "Case Closed: Get Those Air Conditioning Ducts out of the Attic". It cites, and links to, a comprehensive study showing attic ducts are very bad under all circumstances. I'm just outside of Boston and we get hot summers and cold winters. The ducts will be run close to the eaves, at the edges of the attic to save space within the main parts of the attic. Thanks again for any info.
    ya, everyone says "don't put duct in an attic" ~~~~ well sometimes you don't have no option. I have done a few attic installations with metal duct. I wrapped the supply and return duct in foil faced 'duct wrap' and then the customer pumped 24" 'blown-in' on top. It's never been an issue.

    The important thing, THE MOST important thing is do the installation right and seal the joints really, really well so there is NO infiltration from attic air.

    you will be fine as long as the installing contractor pay attention to what he's doing

    btw: you anywhere near North Billerica, my parents use to live there...
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    Quote Originally Posted by jmac00 View Post
    ya, everyone says "don't put duct in an attic" ~~~~ well sometimes you don't have no option. I have done a few attic installations with metal duct. I wrapped the supply and return duct in foil faced 'duct wrap' and then the customer pumped 24" 'blown-in' on top. It's never been an issue.

    The important thing, THE MOST important thing is do the installation right and seal the joints really, really well so there is NO infiltration from attic air.

    you will be fine as long as the installing contractor pay attention to what he's doing

    btw: you anywhere near North Billerica, my parents use to live there...
    Funny, both the installer and the regional distributor are just minutes from Billerica. I'm about 45 minutes south, right next to Boston.

    As far as sealing the joints, I spoke with the owner of the install company and he said they use mastic tape. I asked why not the mastic that you spread on with a brush and he said it's too messy. He assured me it will be well sealed. But should I be worried about mastic tape?

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    Quote Originally Posted by dansminisplits View Post
    Funny, both the installer and the regional distributor are just minutes from Billerica. I'm about 45 minutes south, right next to Boston.

    As far as sealing the joints, I spoke with the owner of the install company and he said they use mastic tape. I asked why not the mastic that you spread on with a brush and he said it's too messy. He assured me it will be well sealed. But should I be worried about mastic tape?
    Mastic tape is very effective as long as it's applied correctly and carefully.

    Correctly applied foil faced insulation is also very important. I've seen countless installs that looked great at initial install but a few years down the road, the taped seems pull apart, insulation falling off, etc.
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    Quote Originally Posted by Brian8383 View Post
    Mastic tape is very effective as long as it's applied correctly and carefully.

    Correctly applied foil faced insulation is also very important. I've seen countless installs that looked great at initial install but a few years down the road, the taped seems pull apart, insulation falling off, etc.
    OK, so the tape is acceptable and basically we just need to be sure the install is good. I have confidence in these guys, so I think I'm good there. Last question I have relates to the central return. To recap, there are three small bedrooms and a small bathroom, all getting a supply near a window. The return is to be central in a common hallway. The whole floor is about 525 sq. ft. The return is slated to be in the ceiling. I have read that the return should be low or striation will exist. With the current wall units, the temps a couple feet from the ceiling jump way up. I don't remember this when I had baseboard heat, but I never really got up near the ceiling to check. My boys share a bedroom and the top of their new high bunk bed is really hot. We use a small fan aimed at the ceiling to make the air more uniform. Once we move to the ducted system, will the striations be lessened if I can get them to duct down for the return? It will not be so easy to do this. If it would make a big impact, I could consider it. Otherwise, the ceiling return will be far less costly and easier to install.

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    Quote Originally Posted by dansminisplits View Post
    OK, so the tape is acceptable and basically we just need to be sure the install is good. I have confidence in these guys, so I think I'm good there. Last question I have relates to the central return. To recap, there are three small bedrooms and a small bathroom, all getting a supply near a window. The return is to be central in a common hallway. The whole floor is about 525 sq. ft. The return is slated to be in the ceiling. I have read that the return should be low or striation will exist. With the current wall units, the temps a couple feet from the ceiling jump way up. I don't remember this when I had baseboard heat, but I never really got up near the ceiling to check. My boys share a bedroom and the top of their new high bunk bed is really hot. We use a small fan aimed at the ceiling to make the air more uniform. Once we move to the ducted system, will the striations be lessened if I can get them to duct down for the return? It will not be so easy to do this. If it would make a big impact, I could consider it. Otherwise, the ceiling return will be far less costly and easier to install.
    stick with the ceiling return, it would be cost prohibitive to open up the walls to put the returns in the walls.

    The air velocity should be enough to prevent stratification. If that turns into an issue, install ceiling fans

    also, make sure that central return has a built in filter. so you don't have to get in the attic to change the filter
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    Insist on a permit & inspection & duct testing.
    Looks like it’s require in your area. We’ve had it a long time here.
    http://homeenergypros.lbl.gov/profil...-testing-in-ma

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    Quote Originally Posted by dansminisplits View Post
    They did a load calc. 18k is what is needed in the upstairs. We go from a single 48k outdoor unit to one 18k and one 36k. Same capacity, but the single, 1:1 18k can ramp down very low, unlike the multi-zones that can only get down to 6k. (I'm starting to wonder why we're not using a 30k btu for the 18k air handler upstairs and 12k basement wall unit. Will ask about that on Monday.)

    Glad to hear you think the attic situation is OK. Then why am I reading so many horrors about ducts in the attic and how it should NEVER be done? Is it just if the install is poor? It does make sense to me that ducts in a New England attic are inefficient. But if that's what I have to do, and it's not horribly detrimental, then I want to go with it.
    36 plus 18 is not 48... its 54...... so... bigger.

    I am going out on a limb and call your problems will be worse.
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    Quote Originally Posted by vstech View Post
    36 plus 18 is not 48... its 54...... so... bigger.

    I am going out on a limb and call your problems will be worse.
    I checked on this and they were wrong about the part numbers. It's a 30k btu and the 18k. 30+18=48kbtu. The 18k is 1:1 and my understanding is that those things are MUCH better and can ramp down, according to the submittal, to about 6500btu in AC and 5k in heat. Since they are not factoring in other indoor units, the 1:1 systems are the best. Then the upstairs and basement will share the 30k which will only have two indoor units, and the disparity between them is small with an 18k and a 12k. The 18k AH unit feeds the entire upstairs, which is appropriate for heat in that space. On the cooling side the 18k is too much, but the inverter can slow down (right???) and can put out as little as 6k.

    The current system includes what is likely a malfunctioning 48k btu compressor with 5 heads ranging from 6k to 18k. Apparently, according to mitsubishi, that is not ideal (maybe even unworkable.) The more heads you have, of disparate btus, the more problems you will experience with overheating and under cooling. I'm having both problems. So this solution, they say, will solve all these issues.

    You still think the problems will be worse? This solution has the stamp of approval from Mitsubishi of America, the regional sales rep and the installer. No one wants this to not work because everyone knows what the next step would be and no one wants to go there. That said, I wouldn't be shocked if I have problems and that's why I'm coming to you guys! Thanks for your opinions.

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    Quote Originally Posted by precision hvac View Post
    Insist on a permit & inspection & duct testing.
    Looks like it’s require in your area. We’ve had it a long time here.
    http://homeenergypros.lbl.gov/profil...-testing-in-ma
    Are all installers trained in duct testing? Is this a difficult process? Will they be offended or ticked off if I tell them I want it? I'm walking on eggshells here! But I'll do what I have to do, of course.

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    Quote Originally Posted by dansminisplits View Post
    Are all installers trained in duct testing? Is this a difficult process? Will they be offended or ticked off if I tell them I want it? I'm walking on eggshells here! But I'll do what I have to do, of course.
    Pull a permit & it will be required. 3rd party, you pay for test. Worth every penny. Search for a HERS rater in your area.

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    Quote Originally Posted by dansminisplits View Post
    jtrammel, thanks for this info. So you are saying the blower SHOULD be set to run at all times, or should NOT be set to run at all times? And you think the loss of efficiency, if installed well, will barely be noticeable? What I'm not understanding are all of these dire warnings, such as this one entitled, "Case Closed: Get Those Air Conditioning Ducts out of the Attic". It cites, and links to, a comprehensive study showing attic ducts are very bad under all circumstances. I'm just outside of Boston and we get hot summers and cold winters. The ducts will be run close to the eaves, at the edges of the attic to save space within the main parts of the attic. Thanks again for any info.
    Well if possible it's best to not have equipment or ducts in the attic but that's not always possible. I'm sure those studies were don't on average older duct systems that leak and are insulated with r4.2 at best. Duct Leakage destroys the effectiveness and capacity of a heat pump.

    The fan MAY need to run at all times IF you have condensation problems in the off cycle in heat mode in your cold climate.

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    Quote Originally Posted by jtrammel View Post
    The fan MAY need to run at all times IF you have condensation problems in the off cycle in heat mode in your cold climate.
    This...
    The TRUE highest cost system is the system not installed properly...

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