HVAC is a challenge – even in a normal house it’s a challenge because of all the air plenums, and in general it’s just not space efficient.  It is ten times harder inside a rig because space is a huge constraint.  Jake probably spent about 60 plus hours researching and talking to people in the HVAC industry, and what follows is the summary of what decisions we made and why, based on that research. 

We went with a mini split unit.  Jake looked at all kinds of different evaporators (there is a lot of variation out there!), but considering all of the sizes, layout options, air plenums, air flow, efficiencies, brands (LG, Mitsubishi, Panasonic, Samsung), the DC versions that are starting to come out now: 12/24/48 volt units, DC marine ones,  the conclusion that he came to was a mini split – more specifically: the Mitsubishi 15K M-Series.  

Anything that we even considered had to be an inverter driven unit; having fixed speed, or even dual speed compressors is a very bad idea from an energy efficiency standpoint.  By inverter driven, I am referring to variable speed, which means variable capacity.  If you look at the power consumption curve on an appliance that is just on or off (like one with a fixed speed compressor), vs an inverter driven unit - the power consumption over time is less on an inverter driven unit.   For example, a 12K BTU rated unit might be able to cool from as little as 1,500 BTU all the way up to 12K or maybe a little more.  Appliances in general are at their highest efficiency when they are always on – so – always on, matched at the capacity that it needs to be is what you want, but it isn’t achievable.  Startup current on motors is a huge waste of energy, so units that are always going on and off are constantly wasting energy, but always on at exactly the capacity that you need is impossible – there needs to be draw down capacity – opening the fridge door, putting warm food in, sunny hot days vs shady days, you can never exactly match the capacity with the need, so your appliance will cycle on and off.  Inverter driven units offer that variable capacity that is necessary for the variable loads that occur in everyday life.

Marine Units

The way that the DC marine units that we have seen work is the indoor part that has the evaporator coil is a module that also has the compressor on it.  This allows you to take the condenser coil and remotely mount that elsewhere.  So, on the surface that is really neat, because you have a lot more options for where to put the coil; it is smaller, and you could even put it under your chassis on a rig.  The problem is, now you have the noise and heat generation from the compressor inside the area you are trying to cool, so you actually lose some capacity that way.  They are DC powered, expensive and don’t have heating capabilities.  And that’s all we have to say about that.

Direct Current Units

Until you get to at least 48 volts DC, you are wasting energy and efficiency.  It is a bad idea to go below 48 volts for appliances, air compressors and air conditioning units (winches don’t matter quite as much given how little they are used).  If you are running a motor, 12 volts DC is really inefficient.  This is what you really need to know: for motors - high voltage, low current equals more efficiency. 

Another reason that we aren’t going with a rooftop AC unit is that our roof is going to be full of solar panels – there won’t be any room!  Also, all of the rooftop DC non-variable speed units are not very high capacity – they max out at around 12K – totally maxed out, and we want more capacity than that. 

Mini Splits

What are mini splits?  Basically, a mini split is a heat pump that has two separate parts – one on the inside of the habitat (house, rig, etc.) and one on the outside.  The condenser part of the unit (usually with the compressor) is located outside, the evaporator is inside.  They also can usually heat as well as cool – something that we are definitely taking advantage of. 

We put the condenser into one of our underbody storage boxes for a couple of reasons.  One, the condensers are not pretty, so we want to hide it.  Two, we have a large window in the back and so the space on the back wall is limited and we wanted to preserve it for various storage boxes for bikes (that is what is on there now) or ski equipment.  Fitting the condenser into a storage box is mainly a height limitation problem.  Jake looked at all the brands mentioned above and all their product models, especially the dimensions on the inside and outside parts, - usually the condenser on a 12K BTU rated unit is as short as you are going to get, - so a 12K, 10K and 9K would all be about the same height. As soon as you get up to 18K rated units the condenser is a few inches taller.  To be fair, most mini split condensers are not designed to be small, because they are just meant to be stuck on the outside of a house and no one cares if they are super compact, plus no one even really sees them. 

We selected Mitsubishi because they have a 15K model that has the same physical size condenser as a 12K. Plus, the turbo mode on the 15K model goes all the way up to 18K BTUs, so you are kind of getting a 18K BTU capacity in a 12K BTU (physical) size condenser. A 15K isn’t oversized for our rig because of the variable speed compressor – it can run at a low enough capacity, so it doesn’t matter that it is bigger than we generally need, and it is very energy efficient. Our goal was to get maximum capacity to take care of really hot days, while having the smallest possible condenser unit (physical size). Ideally it will be running at half capacity or thereabouts.We have never had any issues with cooling the rig, and it cools quickly too. We can have it run all day and night without a problem. The power consumption is pretty low and we are really happy with it.   

Mitsubishi and LG are the only brands that Jake found that can go above the rated capacity a bit. This was important to us because of our space limitations -  you can’t just put an oversize unit in to solve the draw down problem. Not sure why those brands were the only two that have that feature, and it was usually only on their higher efficiency product lines. The turbo mode overpowers the unit for a certain amount of time. That means that a 12K BTU rated unit might go all the way up to 13K in turbo mode – when you are first trying to draw down a hot rig for example. That is what really matters - if you come into a hot rig, you don’t want to wait two hours for it to come down to temperature! 

After looking at all the evaporator units, by far the best value of BTUs per cubic inches of space that the evaporator occupies are the wall hung (ductless) models. Unfortunately, the wall hung units are not awesome for our rig because they need to be set down a certain amount from the ceiling and should be located on the back wall for the best air flow. We have a large window, our dinette, and our bed that drops down from the ceiling in the back, so there is not a place to put it there. So, it went above the galley. This really limits air flow, especially into the cabover – which is a challenging place to get air flow in the first place. We considered all kinds of places, but eventually decided on this less than ideal spot - it is the best we could do given our limitations. It was a fairly short path to run lines from there to the outdoor unit, and a decent spot for drain lines, etc.  We had to add some circulation fans, but rig design is full of compromises.   

Air Flow

A cabover is a super difficult area to get airflow in no matter what – especially ours since it has walls on both sides and just an entry opening in the middle.  We did it that way because the bathroom is on one side and floor to ceiling cabinets on the other.  Plus, it gives each of the girls a place to sit and lean against the wall – which is their preferred position for studying or reading. We could have added a second AC unit, but that would be rather crazy and not space efficient at all. We ended up putting a plenum with a variable speed fan behind the fridge to get enough airflow in there – both for heating and cooling. Unfortunately, the cabover is just its own little area, but the plenum worked out well. 

We had a different problem with air flow for the exterior unit.  Since we put it in a storage box, we needed to provide it with adequate air flow or it won’t work. We solved this by putting a hole in the back of the storage box and louvers on the side; it worked better than we had hoped and we could even run the AC while driving. If you want to mount yours to the back of your rig– you definitely have more options – and you can put a 18K one back there  - it won’t matter. Also the airflow of the vehicle – it wouldn’t matter if you were driving – you could have it on, if it was mounted on the back. 

Heating

Most of the units have pretty good heating performance these days – they are fairly efficient even down into the teens (Fahrenheit).  Once you get below that, you aren’t really getting the benefit of the heat pump, you are just basically heating it – one to one.  That’s just like a normal resistance heater.  There are hyper heating units – especially from Mitsubishi, that will heat down into the single digits, but the compressors start to get larger again, which doesn’t work for us.

Humidity

It's an AC unit – it is going to take humidity out by default.  Most units also have dehumidification modes that will take more moisture out.  If you aren’t - you really should be worried about moisture in your rig, mold can be a huge issue if you haven’t planned for moisture removal.  Rigs generally have tight, highly insulated boxes with people breathing, showering and cooking in them, all of which create a lot of moisture.  Water drips off the evaporator coil into a drip tray when it condenses and the drip tray is typically drained by gravity through a hose.   

Brand Choice

Mitsubishi is the Cadillac of AC units – they are the best in the industry and are pushing the envelope on technology quite a bit.  It is bewildering to go and look at the offerings because they have scores of mini splits available and even different combinations of condensers and compressors that you can pair up.  It is an extensive list. 

It is also interesting that LG and Mitsubishi have product lines that are standard, high and ultra-efficiency, and there is not a lot of cost difference between them.  In some cases, it is the size of the internal components that might be a little bit larger, but there are scenarios where for a 5% increase in cost you can get a much more efficient unit.  Why are they even selling the low efficiency ones!?   Probably because they need to have an offering that is the lowest cost possible because that is how people are shopping.  Of course, we are shopping based on the physical size of the compressor – so who are we to judge? 

Multi Zone Units

As a side note – if you have a massive rig or a house, you can get these mini splits with multi zone units.  These have one outside condenser that can feed up to 3-6 indoor units.  This means that you wouldn’t need to have AC ducting around your house.  And each one can cool the room to a different temperature, so you wouldn’t even need to cool down your whole house.  You could have just the bedrooms cool at night.  Mitsubishi is even advanced enough where they can heat some rooms and cool others at the same time.  They are all connected and have WIFI. It is truly amazing how far the mini split technology has advanced over the last several years.

Power Usage

BTUs/hour can be converted to watts/hour in order to figure out how much power the AC unit is going to consume from your precious battery bank.  On variable speed units, the specs will tell you the maximum power draw.  So - if it is on full blast for several hours – the calculation is pretty simple to figure out how much it is going to take out of your battery.  Once it has cooled the space down and is in maintenance mode, just like a refrigerator – if you aren’t opening the door and putting more load on it - it will run at a much lower rate.  So, the power draw is really completely situational.  Do you have sun shining in the windows, are you opening the door, are you cooking, do you have people in there - all of these things are heat loads that will impact how much capacity is being used and how much power is being drained from your battery bank.  The only way to figure out how much power it is actually going to take is to start using it and track your consumption, and that will at least give you an idea.  We’re honestly not that worried about it - we have plenty of battery capacity – that is one of the reasons that we ended up with a huge battery bank.  We basically have one energy source (electricity) for all of our needs – heat, hot water, cooking, cooling, etc.  - a large bank gives us a lot of flexibility.  For example, if we used propane for cooking and electricity for cooling, if we ran out of propane, we couldn’t use electricity to light our stove.  There are other reasons we aren’t carrying propane, but that is another subject. 

 A side note about power: the Mitsubishi that we chose is a 220 volt unit.  This isn’t a big deal for us as we already planned to have two inverters for split phase power, but if you are only going to have one inverter - LG is one of the few companies that makes a 12K unit that is 115 volt instead of 220 – so that will probably be your best bet. 

I hope that this helps with your HVAC decisions!