QA-spotlightheader image
0 Helpful?

Keeping Cool in Detroit

Will a single minisplit head be enough to cool the entire second floor of a 1962 colonial?

Posted on Jun 26 2017 by Scott Gibson

Like many houses built in the 1960s, Nathan Efrusy's 2,000-square-foot colonial in Detroit has baseboard heat but no central air. A single wall-mounted air conditioner keeps the first floor of the house comfortable, but Efrusy would like to extend AC to the second floor — the question is now to do that effectively.

In a Q&A post, Efrusy says he's been given several options for cooling on the second floor, but he's leaning towards a ductless minisplit.

"The main challenge is the layout," he writes. "It's essentially a 16-foot hallway with four bedrooms off of it. Originally, we were quoted for a four-zone system, with a head in each bedroom. However, after further reading, it seems that design would be overkill... especially after getting a cooling load calculation of roughly 10,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h for the entire upstairs!"

Efrusy assumes he would be running the minisplit continuously in order to keep the second floor dehumidified and between 72°F and 76°F. Bedroom doors could be left open during the day.

"So the question is, how well would a single 9,000 Btu/h minisplit be able to cover the upstairs of our house?" he asks. "My hope is that we wouldn't need additional fans to push the air into bedrooms. Would there be enough cooling through convection and the internal walls to combat the heat entering through the external walls and windows? Each bedroom has two small double-paned windows, and we get enough shade to prevent much direct sunlight, except for early morning and late evening."

Those questions are the basis of this Q&A Spotlight.

Flexibility will be key

Whether a minisplit of that size will be adequate depends in part on how flexible Efrusy's family proves to be, says senior editor Martin Holladay.

"This is a tough question to answer," Holladay writes. "I think that the main variable is the expectation of your family members. If you are all flexible, and are willing to put up with some room-to-room temperature variations, I think that a ductless minisplit in the hall (with open doors during the day) would work fine."

If that's too much to ask, he adds, Efrusy might look for a second-floor closet where a ducted minisplit could be installed. The basic mechanics of the system would be the same as a ductless, except that short lengths of duct would direct cool air to each of the bedrooms. With the minisplit installed near the ceiling, ducts running to the bedrooms could be boxed in, assuming Efrusy has the headroom for that kind of design.

Dana Dorsett also thinks the plan might work, and he has some other options for Efrusy to consider:

"If it's the standard wall coil type, think about which direction it blows relative to the door openings or you may end up over-cooling some rooms, and under-cooling others," he says. "Also consider where the stair well is located, since there will also be convective air motion happening there... In some cases you'll be better off with a floor-mounted version rather than a wall-coil, if hallway width or ceiling heights interfere with a wall-mount."

Distribution is the issue

With an upstairs ceiling height of about 7'6", Efrusy says, boxing ductwork for a ducted minisplit probably isn't going to work.

That leaves two minisplit options: either a single-head or a multi-head design. With a single head, the question is: What kind of room-to-room temperature variation is likely? The rooms are equipped with ceiling fans, he says, adding, "Would they help in any way to mix or pull some of the cool air from the hallway?"

A multi-head ductless system raises different concerns. The cooling load for the master bedroom is 2,200 Btu/h, or 3,000 Btu/h if the attached bath is included. The other two bedrooms have cooling loads of 1,500 Btu/h and 1,800 Btu/h respectively.

"Even if all doors are open and the units can each cool one-third of the upstairs, at most each head is only responsible for 3,000 Btu/h of cooling (on the absolute hottest days), but the smallest heads are designed for 6,000 Btu/h," Efrusy says.

Holladay wouldn't advise the three-head approach. It's too hard to make an accurate prediction about temperatures (even sunlight on a window could change everything), but in any event he sees nothing wrong with a system that keeps the hallway a little cool in order to make bedrooms comfortable.

More reasons to stick with a one-head system

Dorsett suspects that temperatures and indoor humidity will be relatively even on the second floor as long as the bedroom doors stay open until a few hours after sunset. Even when the temperature reaches the upper 80s, "the cooling load attributed to conducted heat through the walls is pretty small, and even a 5°F difference in temperature drives quite a bit of open-door convection."

The second floor should remain comfortable, unless bedroom doors are closed during the day when outdoor temperatures are above 85°F.

There's another reason to avoid the multi-head minisplit system, he says: with those cooling loads, it won't operate efficiently.

"A room with a design load under 3,000 Btu/h can't be efficiently served by its own ductless head, since the the minimum modulated output of the head is above the average load, and it will do a lot of on/off cycling at minimum speed rather than modulating with load," Dorsett says. "When operating in that mode its latent cooling falls off, too, leaving the humidity higher than with a single head in the hall serving up the whole-house load.

"In my opinion, you will probably be happier with the overall comfort range with the hall-mounted head than with individual room heads, as long as the doors remain open until after dark, when the direct wall heating and window gains go away."

Minisplits would be money savers

Dorsett had suggested that no matter what type of heating system the house has, it would be worth paying a little extra for a minisplit capable of heating as well as cooling, particularly in the shoulder seasons. That, says Dorsett, is when minisplits shine: "not just shine, but really, really shine."

Referring Efrusy to a Department of Energy report on minisplit heat pumps, Dorsett compares operating costs of a condensing gas boiler to a 1-ton heat pump. The heat pump would have a COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. (coefficient of performance) greater than 5 when the outdoor temperatures is 47°F and likely better than 6 when the temperature is 55°F and the unit is running at a low speed.

Plugging in electricity rates and gas prices in Michigan, Dorsett believes the minisplit would be cheaper. "If those are roughly your rates, it means heating with the condensing boiler runs a few % more expensive during the shoulder seasons than heating with the minisplits," he writes. "But that isn't discounting for the distribution heat losses (which minisplits don't have) or the electrical power used by the heating system (which is already included in the minisplit's COP.) In reality it's going to be more than 10% cheaper at those utility rates."

During the coldest months of the year, it's a different story. The condensing boiler would be substantially cheaper to run, he says, adding, "So, that's why it's worth the upcharge for a heat pump version, and why paying attention to HSPF [heating seasonal performance factor] may be as important as the SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. numbers."

Second thoughts about a ducted system

Efrusy's original question, and the comments that followed, were posted in May. By early June, Efrusy was back with a follow-up, suggesting that maybe a ducted minisplit could work after all. He's identified a closet where the unit night be located (see the drawing below), but must still determine how to run ducts, the coolant line, and drain lines for the air handler and evaporator.

"In order to reach that closet, we'd either need to run the coolant and drain lines through the unconditioned attic (bad!) or along the ceiling and frame it in," he says. "I am not opposed to either option, but I'm wondering how much of an efficiency hit the system would take in the attic; I've read that it's roughly 20%-25% by having ducts up there, but not sure if it'd be as terrible for the supply... especially if it can be dropped between joists for the whole run, and buried in the loose cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection.."

It's OK to run refrigerant lines in the attic, Dorsett replies, but not the drains or ducts. It would be a better option to build out soffits below the ceiling for ducts, providing there's enough headroom.

Our expert's opinion

Here's what GBA technical director Peter Yost has to say:

This may at first seem like an off-topic response, but bear with me.

I just wanted to share my own experience with trying to maintain “coolth” comfort on the second floor of our home in southern Vermont.

In the late spring and early fall, we have a very efficient ceiling fan/light unit in the master bedroom, which keeps us cool with our bedroom door closed and windows open.

During much of the summer, we stay cool in all three of our upstairs bedrooms with a powerful window fan unit, pulling air throughout the upstairs with all bedroom windows (and doors) open. Privacy is pretty much achieved by leaving doors slightly ajar and held in place with doorstops.

But when it turns hot and muggy, we have tried innumerable combinations of AC window units and fans in an effort to distribute that cool air. It has never worked; those fans trying to move air between rooms and down the hall simply don’t cut it.

Interestingly, it’s easy to rely on diffusion and air movement to equalize relative humidity upstairs, but not temperature. Those small and speedy water molecules can overcome the room and hall geometries, but the big and slow air molecules can’t.

Lessons learned? When air is cool and dry inside the second-floor spaces, efficient ceiling fans can provide adequate air movement and thermal comfort. The Hampton Bay Gossamer Windward II ceiling fan (Image #3, below) has three speeds and a dimmable fluorescent light. (In 2001, I wrote an article about the fan for BuildingGreen). Our second-floor ceilings are just under eight feet, and we have never found the ceiling fan/light unit to be in our way, height-wise. It has worked flawlessly — both fan and light — for more than 15 years now.

A powerful window or attic fan can move enough air from outside to inside for comfort, when the outside air is cool and dry. We have had the window fan shown in Image #4 below for at least 15 years. It works perfectly, although this particular model is apparently no longer available.

Across second-floor bedroom geometries, when you need active cooling, you need dedicated distribution. You can pull lots of air through complex geometries when there are ventilation pathways (like open windows). But you can’t push air into complex geometries from a single source of cooling. The bottom line: A single ductless minisplit head probably won't solve your second floor AC issue.

Closing technical note: Some of you may be shaking your head at my idea of using my Omega anemometer to approximate air movement of my window fan (see Image #5). I checked in with a close friend and colleague who is a mechanical engineer and he told me that as a rough approximation, you conduct a fan “traverse” with the anemometer. This means moving the anemometer across the face of the fan, and then doing an area-weighted averaging of the readings to more closely approximate true air flow. But he also cautioned me that outside of the confines of ducts, it’s pretty hard to measure air flow for a setup like a window fan.

Having said that, know that while I did not do area-weighted calculations to input to the engineering online calculator, I did stick with my measurement right at the center of the fan, which for all three settings, was the lowest air flow measurement, and was therefore conservative.

Tags: ,

Image Credits:

  1. Images #1 and #2: Nathan Efrusy
  2. Image #3: Hampton Bay
  3. Images #4 and #5: Peter Yost

Jun 26, 2017 7:48 AM ET

Cooling the second floor of a two-story house
by Martin Holladay

As longtime readers of GBA know, many owners of compact superinsulated homes cool their second floor with a single ductless minisplit in the hallway. These owners report that this approach works, as long as (a) the footprint of the building is fairly compact, without big ells or bonus rooms, (b) the envelope is fairly tight and very well insulated, (c) the windows have above-average specifications and aren't oversized, and (d) the bedroom doors are left open during the day.

This approach didn't work for Peter Yost. My guess is that Peter Yost's house differs in significant ways from the homes built by Carter Scott, Brian Post, Stephen Sheehy, and others who have had better success with the "minisplit in the upstairs hallway" approach.

Jun 26, 2017 8:11 AM ET

Minor correction
by stephen sheehy

My house is a single level. But I've been finding that by leaving doors open, I get pretty good heating and cooling in rooms with no minisplit.

Jun 26, 2017 8:55 AM ET

Response to Stephen Sheehy
by Martin Holladay

Thanks for the correction. I appreciate it.

Jun 26, 2017 11:30 AM ET

Flexibility IS key
by Antonio Oliver

My 4 ton R22 central unit went out 2 summers ago, and I decided at that point not to replace it, considering the complete overhaul to the house envelope we have planned. We have been cooling with two 12k BTUH window units (for a 2200 s.f. one level L-shaped house that is very poorly insulated). The original 1954 structure (of 1400 s.f.) was masonry built with zero insulation in the walls, only a thin one inch air gap between the masonry and plaster due to furring strips. The 1994 "L" addition has more window area than wall area on exterior walls. But a window unit in each of the two sections keep us mostly comfortable (in Zone 4a) if all doors are kept open. Here is the caveat: even before the central unit went out, we only cooled to 78 F. That is not cool enough for most, but works for us.

Open windows will not work for many (like me) who sneeze endlessly when exposed to allergens and the amount of additional dust that a window fan would pull in.

Jun 26, 2017 10:10 PM ET

A floor fan in an open
by Jon R

A floor fan in an open doorway has worked well for me to increase cooling airflow into an unconditioned room.

In some cases, it might be possible to blow air from a bedroom closet to the first floor - causing a circular flow pattern, with air returning up the stairwell and then through the open bedroom door.

Jun 29, 2017 2:15 PM ET

Transfer Fans?
by Harris Woodward

I'm surprised that a discussion of interconnected transfer fans mounted above the bedroom doors wasn't discussed. We've installed Mitsubishi wall heads that when operating, trigger the hardwired transfer fan to come on and blow conditioned air into the bedrooms. Then again, our houses are very well insulated, and tight...

Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!