Minisplit rough in

Thinking ahead and taking a little gamble sometimes pays off.

Despite having not yet decided on the cooling system back in 2012, we decided to rough-in the cooling lines for the minisplit. Back then, I was about to install drywall, and convinced myself that I should rough-in the minisplit infrastructure before I closed up the walls.

I had to decide on the location for the outdoor and indoor unit. I was advised to place the indoor unit in a central location, yet fairly close to the ERV return. The south wall of the dining room met those requirements and had the wall space to accommodate the indoor evaporator.


I was now looking for a fairly short run for the cooling supply and return lines that would connect to the outdoor unit. I opted to mount it on the east facing wall of our building and ran the supply and return lines across the corridor ceiling, the pantry ceiling, and through the exterior brick wall.


The minisplit was the only cooling option in the picture at that time, and I am glad I opted for the rough-in. It would have been a shame to open up the drywall after everything was finished.

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Wait and see

Mini what?

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Wait and see

When it came to cooling options, some ideas were left in the dust, such as:

The viable options that remained were:

Cathy and I wanted to see what our cooling needs on the 1st floor really were. So we decided on a wait-and-see approach: “Let’s live a summer or two in our deep energy retrofit without system air conditioning and see how bad it gets.” That also bought me extra time to come up with some more cool ideas.

Our wait-and-see experiment over the past two years did not yield any surprises. There are summer days where air conditioning is essential, despite our thorough deep energy retrofit. And with climate change barreling down on us, it would be wise to assume that the number of those hot days will increase.

But we also found out that it doesn’t take much air conditioning to keep reasonably cool. We had used a portable 8,000 Btu Energy Star AC when it became unbearable, and it did a decent job, even in keeping the relative humidity down.


I used this time to follow the development on the “Magic Boxes” – energy or heat recovery ventilators (ERV or HRV) with an integrated air-to-air heat pump. I also put this question on the Q&A secion: Would the Magic Box be a good alternative to the minisplit in combination with our ERV?

The answer was an unambiguous no!

Dana Dorsett, a frequent contributor to the forums pointed out that “…ERV/HRV air volumes are too low, with temperature differences too small to make them useful for distributing heat/cool.”

Furthermore, Dana pointed to the difference in cooling efficiency. Where a Magic Box may deliver a mere Seasonal Energy Efficiency Ratio (SEER) of 11, an efficient minisplit could have an impressive SEER 33.

This finally pushed me over the edge and I felt comfortable going with the most frequent recommendation: Using a minisplit in combination with our ERV to provide the needed summer cooling and dehumidification.

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Mini what?

We had options, but that didn’t make the decision about how to stay cool any easier.

I could eliminate an air-to-water heat pump because of cost. Martin Holladay at said it well: “The cost of these appliances is so high that it’s more economical to buy three separate appliances: an HRV, a water heater, and a ductless minisplit heat-pump.”

An ERV (Energy Recovery Ventilator) with a right sized integrated air-to-air heat pump could work, though.

One should ask for help when it’s needed, so I turned to some green building experts to assist me in the decision making process. The recommendation was to use a minisplit in combination with our ERV to meet our cooling and ventilation needs. And I have come across the same recommendation over and over again, implicitly or explicitly, like in the quote above by Martin Holladay.

What is a minisplit

It is an air-source heat pump that is split into an outdoor condenser and indoor evaporator – thus the split in the minisplit. It is very similar to a conventional AC system, which also has an outdoor condenser and indoor evaporator, but with two important differences.

The minisplit does not require duct work but instead has a wall mounted cassette, which accommodates the indoor evaporator and blower fan.


The minisplit has a expansion and check valve set up that allows for cooling during the summer months. But those valves can be switched, in which case the unit provides heating during the cold season.

How can that be?

The evaporator can be turned into a condenser and vise versa. This requires a reversing valve in the outdoor unit along with the option to switch between a bypass and expansion valve in both the indoor and outdoor units.

During cooling mode, a compressor in the outdoor unit takes the hot, low pressure refrigerant vapor and converts it into hot, high pressure refrigerant vapor. That hot, high pressure vapor is routed to the coil of the outside unit, which acts as a condenser. The coil is cooled by a fan blowing outside air over it. This cooling removes heat from the refrigerant and allows for a phase change – from vapor to liquid. In other words, the hot high pressure vapor condenses into a cooler, high pressure liquid refrigerant, which is now pumped to the indoor unit.

The indoor unit has an expansion valve and heat exchanger coil with a fan. Here the coil acts as a evaporator: the expansion valve allows the cooler, high pressure liquid refrigerant to evaporate into low pressure vapor. This phase change is the reverse from that of the outside condenser. The phase change (from liquid to vapor) requires heat. That heat is extracted from the indoor air, which is blown over the coil (heat exchanger). Once the air passed over the coil, it is cooled and dehumidified, because excess humidity condenses on the cold coil.

The now hot, low pressure refrigerant vapor is traveling back to the compressor in the outdoor unit where the cycle repeats itself.

In heating mode a reversing valve turns the refrigerant flow around. The expansion valve in the outdoor unit is activated, turning the coil into an evaporator, and in the process absorbing heat from the outside air. The hot refrigerant is pumped to the indoor unit, where it bypasses the expansion valve and turns the indoor coil into a condenser, where heat from the phase change is used to heat the building.

Still confused? I don’t blame you! Here is a link to a Youtube video that I found. It shows how a minisplit heat pump works.

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Cool ideas?

Sometimes I get these seemingly insane ideas, only to find out later that they were somewhat rational. The subject matter of those ideas is typically not my strong suit. That’s why I tend to be infuriatingly quick to dismiss them.

Air conditioning is on my list of subjects that are not my strong suit. I grew up in Germany, where air conditioning is virtually non-existent. I ran into active cooling for the first time while visiting the Caribbean, and again years later when I moved to Chicago.

I regard summer air conditioning in the Midwest as a necessary evil and have a pronounced dislike for the temperature extremes we have to tolerate between the outdoors and conditioned indoors. I dread stepping into a grocery store or office building that feels like an ice box.

Since we started our deep energy retrofit, I’ve had to wrestle with the question on how we will keep cool during the dog days of summer without creating an ice box. I quickly learned that “cool” is somewhat secondary. The primary problem to tackle is how to keep the relative humidity at a comfortable level, i.e. under 60% (preferably at 50%).

We already have an Energy Recovery Ventilator (ERV) that supplies fresh air into the house, yet keeps the outside heat and mugginess at bay. My seemingly insane idea was to install an air-to-water heat exchanger in the fresh air supply duct to remove humidity through condensation, and deliver pleasantly dry air into the house through the ERV duct system.

Simply put, I wanted to run the muggy outside air across a cold air conditioning coil to dry it out. But I was told that there was no such air conditioning device that could be combined with an ERV. Yet, I clearly wasn’t alone with this idea.

Just in the past couple of years I started reading about ‘Magic Boxes’ that basically combine the function of an ERV, or HRV (Heat Recovery Ventilator), with that of a small air-to-air heat pump, i.e. a small air conditioner.



Unlike our ERV, which uses and enthalpy wheel as a heat exchanger, a Magic Box uses an integrated air-source heat pump to transfer thermal energy between air streams. But they also can provide some additional limited conditioning – additional cooling (and drying) as well as heating.

One company (Build Equinox), located two and a half hours south of us, brought the CERV (Conditioning Energy Recovery Ventilator) to market just a few years ago. The CERV has an advertised heating capacity of 3,850 Btu/h, and cooling capacity of 2,400 Btu/h. These numbers vary depending on outdoor temperatures. One article listed a price of $4,500 for the CERV.

A competing product is the Boreal 12000 by Minotair out of Quebec, Canada. This Magic Box is more compact than the CERV and, in heat pump mode, has a listed heating and cooling capacity of 9,400 Btu/h and 8,700 Btu/h respectively. I read one article that pointed to a price of around $3,200 for the Boreal 12000.

Either the CERV or Boreal 12000 could be used instead of an ERV or HRV.

Going back to my original idea, combining an air-to-water heat exchanger with our existing ERV, I came across what looked like a promising option. An article on described a variety of air-to-water heat pumps that could provide chilled water to the air-to-water heat exchanger.

In this function, the air-to-water heat pump would basically function as a chiller. But it could also reverse its cycle and produce hot water for domestic hot water consumption or a hydronic heating system.

The challenge would be to find a right sized unit that I could combine with our ERV, and that would be affordable. If you thought the CERV or Boreal 12000 were expensive, prepare yourself for a sticker shock while shopping for air-to-water heat pumps.

Nevertheless, I feel vindicated that my idea wasn’t that insane after all. But it has only brought me somewhat closer to a solution that would provide cooling and a comfortably low relative humidity during the dog days of summer in our deep energy retrofit.

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Un-freaking believable!

This is not a proud episode, because I unintentionally exercised cruel and unusual punishment.

Last fall, one of my clients let me salvage a small hickory from their yard that otherwise would have been cut down. But the salvaging quickly turned into butchering.

I am not sure if there is a word for upside-down decapitating. If there is, that is what I did to the hickory: I cut off its vital two inch tap root in the transplanting process. Not only that, but I also hauled it for 40 miles in the back of my truck during that warm and dry fall day.


Back at my house, what was left of any leaves looked like dried tobacco and crumbled upon touch. I still planted the poor fellow, yet had no expectation that it would survive.

And that seemed to be its fate. By late May, everything around us had leafed out, except that dry stick in our park way, which once was a proud hickory. Even my neighbors noted and inquired if I would remove it.

Before going down that route, I cut off the very tip of a lower branch – and detected some green cambium behind the bark. Was it really there or was it something that I wanted to be there? In any case, it was a good excuse to delay the removal for a little longer.

By mid June, I got confirmation that the green cambium wasn’t wishful seeing on my part. This resilient son of a bitch developed buds, which kept growing and eventually exploded into leaves.

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Un-freaking believable!

I should also disclose that I poured a five gallon pail of water at the tree base pretty much every day since late April. And thanks to my new rain barrel, I continue to do so.

Yet, this is still a very tenuous situation!

While we had days with dry and hot wind in late June, some of the leaves started to dry up again. I kept watering and it looks like I was able to stop that trend.

Have no doubt. This fellow will take a few years to recover from my butchering. I am just glad I didn’t kill it after all – yet.

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