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Do magic boxes distribute loads for ductless units?

For my pretty good home I am building( 2150 sq.ft. Zone 5 northern ct. near Sturbridge mass) I have recently got a quote back for mini splits. 5 heads 2 outdoor units Mitzi. Hyper heat. The contractor is at 17k installed. Diamond dealer. He has one head in each bedroom and 2 on the open floor plan. I am curious if there is a way to quantify how much a ducted ventilation system mixes the air around the home? Would a Minotair or cerv balance out the heating and cooling requiring more like 2 or 3 heads?

Asked by scott mangold
Posted Mar 20, 2017 9:59 PM ET
Edited Mar 21, 2017 8:38 AM ET


11 Answers

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Neither a ducted ventilation system nor a "magic box" is designed to even out heat differences from one room to another (although that may be a side benefit of a Minotair or CERV). The number of cfm required for ventilation is too low for that to happen. Moreover, with an ordinary HRV or ERV, incoming ventilation air is always colder than indoor temperatures during the winter and warmer than indoor temperatures during the summer.

A magic box can introduce warm air in winter to rooms where the air is directed, so it's a better solution to the issue of distribution than an ordinary HRV or ERV. That said, the number of cfm or BTU/h provided by the magic box may not be enough to accomplish your goals.

It sounds like you have way too many indoor minisplit heads, by the way. You won't have a cold bedroom with a head in every bedroom.

For more information on this issue, see these articles:

Rules of Thumb for Ductless Minisplits

Minisplit Heat Pumps and Zero-Net-Energy Homes

Practical Design Advice for Zero-Net-Energy Homes

Just Two Minisplits Heat and Cool the Whole House

Your first order of business is to find a heating system designer who recommends fewer indoor minisplit heads.

-- Martin Holladay

Answered by Martin Holladay
Posted Mar 21, 2017 5:29 AM ET
Edited Mar 21, 2017 5:31 AM ET.


The head in every room approach leads to gross oversizing of the equipment. In some cases if you take the installed cost of the head (and it's fraction of compressor) and apply that money to higher-R and better windows sufficient to bring the room's load down to where the head isn't needed will work.

Dry air has a thermal mass by volume of about 0.018 BTU per cubic foot per degree-F. If you are supplying 73F ventilation air into a (code min) 68F room at 20 cfm (1200 cubic feet per hour), the ventilation air is delivering

0.018 x 1200 x 5F= 108 BTU/hr

To put that in perspective, a sleeping adult human is delivering more than twice that much heat into the room

Answered by D Dorsett
Posted Mar 21, 2017 8:54 AM ET


If you exhaust the air from the room, then that 20cfm should be able to exhaust all heat from the sleeping person + some. Since heat and humidity rises and the intake from under the door is going to be extra cold and dry.

Answered by Anon3
Posted Mar 21, 2017 9:40 AM ET
Edited Mar 21, 2017 9:44 AM ET.


If that extra cold & dry air is entering a fully conditioned space in a high R house, the heating for that room will make up the difference. If it's heated with ductless mini-splits the colder it is outside, the higher the blower speed, and the less stratification there is in the rooms.

Exactly this done in an 1850 square foot Deep Energy Retrofit full-gut rehab I was involved with in Worcester, MA a few years ago (Iess than an hour's drive from Scott's place). The doored-off bedrooms and bathrooms had only exhaust registers (at the ceiling level) for the balanced HRV systems, and the ventilation supply to those rooms was a door cut into the common space that was heated with a mini-split head, with the primary HRV supply registers located near the ductless head.

That 3 story 3 apartment building was insulated to roughly Pretty Good House levels, and is heated & cooled by one ductless head per floor, with the head located in the kitchen/living room areas, with NO heads in the bedrooms or bathrooms. The only comfort problem was a third floor bedroom with an unshaded east facing window- it could get uncomfortably warm by 9AM in early July unless the bedroom door was left open. When its sub-zero outside they bump up the temps on the mini-splits, but it has no problem keeping occupied bedrooms warm enough, due to the very low loads and the self-heating provided by mammalian occupant(s).

Answered by Dana Dorsett
Posted Mar 21, 2017 11:38 AM ET


Dana, What you are saying is what I had hoped for. ( only a half hour from Worcester,btw)
Back a couple of months ago when I hired a hers rater I felt that he was competent and thorough. When I asked him about doing the manual j calcs he replied that the HVAC guys do this and recommended a one. The HVAC contractor stuck me as more though than others I have worked with who have often seemed to be doing mini splits as a add on to their ducted work. To be fair I have some tough south west windows as the structure of the barn I am converting was oriented this way. My gut feeling is that 3 heads would be appropriate for my layout and orientation .
Not sure what I should do next, I guess I need to get more quotes.

Answered by scott mangold
Posted Mar 21, 2017 10:38 PM ET


The method suggested by Dana Dorsett -- installing an HRV system with exhaust grilles in bedrooms and supply registers in the living room (or near a minisplit head) -- has been discussed (and argued about) on GBA for years.

Here is a link to the GBA article that started the debate: A New Way to Duct HRVs. Note the Author's Postcript at the bottom of the article, which states, “Dr. John Straube, a principal of the Building Science Corporation, has challenged the logic behind the novel method of HRV ducting advocated here by Robb Aldrich. In an article titled ‘Choosing HVAC Equipment for an Energy-Efficient Home,’ Straube is quoted as saying, ‘Ventilation air doesn’t do much to move around heat. ....Ten cfm of 72 degree air to a 65 degree bedroom won’t make any difference to the temperature in the bedroom at all. Open doors work better than HRV ducting.’ ”

-- Martin Holladay

Answered by Martin Holladay
Posted Mar 22, 2017 5:43 AM ET


"Ten cfm of 72 degree air to a 65 degree bedroom won’t make any difference to the temperature in the bedroom at all."

... as I calculated in response #2- a sleeping human makes a bigger dent in the heat load. (Though here they're talking about a 10cfm @ a 7F delta-T, which is even smaller than the 20cfm @ 5F delta-T example I used.)

I wasn't suggesting at all that the ventilation ducting plan to gain that 100BTU/hr of heat was worth considering, even though a project that I was was involved with did it that way (it wasn't my call.) It came up here only in response to the "...extra cold and dry..." ventilation air anxiety brought up in #3. Even if there were significant room air stratification, there isn't enough extra-coolth injected by using the door cut at 10 - 20cfm ventilation rates to create a comfort problem. The effect on the room temp is still less than the presence/absence of one sleeping human. In the Deep
Energy Retrofit house most of the heat transfer from the warmer space to the doored off rooms with the doors are closed is through the partition wall (and closed door), not the ventilation system.

With doors open the convective air transfer can be an order of magnitude more than the ventilation. The top floor bedroom's summer-solstice AM overheating issue wasn't a problem with the door open, only when the door was closed. (A very easy "fix" in a 1-bedroom apartment.)

Answered by Dana Dorsett
Posted Mar 22, 2017 1:30 PM ET


If you are in a situation where you are tempted to oversize the heat pump system in order to get heads in enough rooms to avoid worrying about temperature uniformity, a different solution is a Chiltrix air-to-water heat pump. Chiltrix sells fan-coil units that are like mini-split heads, but fed by heated or cooled water instead of refrigerant. The smallest fan coil units are smaller (and cheaper) than mini-split heads, and if you want even lower heat output for other rooms such as bathrooms, you can use panel radiators with even lower heat output.

The only problem is finding qualified designers and installers for that unusual type of system.

Answered by Charlie Sullivan
Posted Mar 22, 2017 7:06 PM ET


Charlie, I will check those out. Dana ,and others. My thought pattern was that if the magic boxes move just a little air but have good control of the humidity levels that it( Minotair). would serve to even out the realitive comfort of all the spaces. Typically I would be inclined to go with a more foolproof thing( exhaust only venting) my thought process was that by having less complexity with the ductless units I may get more for simaler.
My kids , when they are home, much prefer the doors closed. And as they get neared to teenage ,I can't imagine they will want their doors open...

Answered by scott mangold
Posted Mar 22, 2017 7:19 PM ET


Controlling a Minotair to reduce temperature differences between rooms may be problematic. Any heat (or cool in summer) added by the Minotair would be delivered to the rooms with supply registers, I believe that the temperature sensor that controls whether it heats or not is sensing the temperature of the rooms with exhaust registers. I suspect it would exacerbate temperature differences.

Answered by Reid Baldwin
Posted Mar 22, 2017 9:06 PM ET


I didn't mean to misrepresent your suggestion, and I'm sorry if my comment sounded as if I disagreed with you. I think that we are in agreement.

-- Martin Holladay

Answered by Martin Holladay
Posted Mar 23, 2017 6:02 AM ET

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