This is the fourth post delving into the various systems in a tiny house. For the others, please see the links at the bottom of the introductory post, here.
Humans prefer to live within a certain range of temperatures. this is known as the human comfort zone, or alternatively, the thermal comfort zone. It is defined as the range of temperature, air movement, and humidity conditions that most people will find comfortable the majority of the time. Mechanical Engineers that design heating, ventilation, and air conditioning (HVAC) systems consider this so important to being able to do their work that they have developed ANSI/ASHRAE Standard 55 to be able quantify and specify what is required to ensure human comfort. The below chart is known as a psychometric chart, or human thermal comfort chart. This chart actually was developed through observations of conditions and surveys of actual people as they experienced them.
Luckily, tiny home denizens don’t really need to know how to read the above chart, as all it does is show the typical comfort zone in relation to both air temperature and humidity, although the chart on the right does show some ways that you can extend the comfortable zone by using moving air, or soaking up heat from the sun.
Which leads us to the two types of cooling, passive vs. active.
Passive cooling is cooling which does not require a fan or pump to cool you down. In other words, open a window! Now, this seems simple and it is! But there are some things to consider that will improve the efficacy as well as overall comfort when you crack them open.
One of the first is to ensure that you have cross ventilation. What this means, is that you have operable windows on both sides of your house, in locations where air that comes in one side can come out the other. In a full size house, many times interior walls get in the way of this, but in a tiny house, there typically are not as many walls to get in the way. The below diagram shows the typical ventilation schemes from good to best.
If we refer back to the psychometric chart at the top, you will see that increasing ventilation allows you to still be comfortable at higher temps and humidity. And what better way to increase ventilation than through the use of fans!
Ceiling fans seem to be the bane of modern interior decorators everywhere, and are the first thing torn out on all the design shows on HGTV and the DIY network. But a ceiling fan, when used properly, can save you up to 40% in the summer and up to 15% in the winter.
In the summer, it does this by creating a draft, cooling you through wind chill. To maximize the effectiveness, you should use the faster speeds of your ceiling fan to move more air across your skin.
In the winter, you want to slow the speed of the fan down, while at the same time reversing the direction. There are a couple of factors that make this the best option. First, heat rises, and in a still house, the air will stratify, or layer itself, with the hottest air being at the ceiling of a space and the coldest at the floor, which happens to be closest to the inhabitants.
Turning on a ceiling fan will de-stratify the air, pushing the heat at the ceiling down into the room to where the people are. However, the same wind chill effect that is put to good use in the summer is detrimental in winter, as you want to stay warm, not cool off. This is why you use the lowest speed of your fan in winter. You merely want to circulate the air, not create a cooling draft. This is also why you reverse the direction of the fan, because by pushing the air up against the ceiling, it pushes the hot air down the sides of the room, while at the same time diminishing the air speed of the moving air so that when it gets to you, it does not cool you.
There are many, many styles and configurations of ceiling fans, able to match the decor of just about any taste, from tropical, to modern, to industrial. The primary considerations are the features of the fan in question. Do you want a light or not? Do you want a remote control? (They are actually, very handy to have as, when you think about it, the fan is mounted way up there, and you are way down here, and the on-off chains are unsightly.) Also, the better brands are quieter and will last longer. Good brands include Hunter, MinkaAire, Emerson, and Casablanca.
Window / Floor / Desk Fans
Window fans are also a time honored means of ventilation. When the breezes outside aren’t quite cutting it, make your own! There is the traditional box fan, whirring away in the window, providing a handy “robot voice” for the imaginative child. These work best when set up as exhaust fans (pointing out). They pull the air out of the house pulling fresh air in. If they are set up in a window that is higher up in the structure, with a window or vent closer to the floor open, then it exhausts the hot air out while pulling cooler air in, especially if used in the cooler, night time hours.
Pedestal fans are also useful, in that they are larger than desk fans, and therefore can move as much air as a similarly sized box fan, while having the rotating function of a desk fan. Unlike a box fan however, they are designed to provide direct cooling by blowing air across the occupants. The rotating allows the breeze to be shared.
Desk fans are like Pedestal fans, only sized to fit on a desk or counter. Being smaller, they don’t move as much air, and therefore are typically used for one person. They range from the cheap plastic types that you can pick up at Wal-mart, to high quality, well built ones designed to last a lifetime. You can find old ones from the early part of the century that are still running today from GE, Westinghouse, and Robbins & Myers.
A decent version is made by Hunter. But you can find similar ones from many manufacturers.
Ultimately, a fan is worth making room for in a tiny house, as its usefulness outweighs the space that it takes up. However, it is up to you to decide what style best matches your decor.
RV-style Roof Vent Fans
The RV Style roof vent fans work on the same principle as attic or whole house fans in a regular house. Capitalizing on the concept of hot air rising, a attic fan pushes air from the inhabited space into the attic, forcing the hot air in the attic out through the eave vents. And as the optimum time to use one is at night when it is cooler outside, the forcing of air into the attic draws the cool, nighttime air into the living space through open windows.
An RV vent fan is designed to be mounted in the roof, through a square hole. They can be used in a tiny house, but as with all penetrations in the roof, you need to be extremely careful with sealing and flashing. I recommend checking out the website of your roofing material’s manufacturer for their recommended flashing and sealing details for skylights, as RV vent fans are technically the same, only smaller. Popular options for these fans are multi-speed motors, the ability to work in reverse, thermostats to control on/off functions, and even remote controls. The fan will have a hinged cover of typically smoked plastic that that is weather tight when closed.
Of course increasing ventilation is only effective to a point, as sometimes, it is just too humid, or too hot to matter. In those cases, most people consider an air conditioner a must.
However, there is an alternative stream of thought on the matter, in that some lay the increased rates of asthma and other respiratory ailments at the base of the prevalence of air conditioning in western society. For a thought provoking exploration of the topic, I highly recommend “Losing our Cool” by Stan Cox. As the blurb from Amazon states: Losing Our Cool shows how indoor climate control is colliding with an out-of-control outdoor climate. In America, energy consumed by home air-conditioning, and the resulting greenhouse emissions, have doubled in just over a decade. it also makes a much more positive argument that loosening our attachment to refrigerated air could bring benefits to humans and the planet that go well beyond averting a climate crisis. Though it saves lives in heat waves, air-conditioning may also be altering our bodies’ sensitivity to heat; our rates of infection, allergy, asthma, and obesity; and even our sex drive. Air-conditioning has eroded social bonds and thwarted childhood adventure; it has transformed the ways we eat, sleep, travel, work, buy, relax, vote, and make both love and war. The final chapter surveys the many alternatives to conventional central air-conditioning. By reintroducing some traditional cooling methods, putting newly emerging technologies into practice, and getting beyond industrial definitions of comfort, we can make ourselves comfortable and keep the planet comfortable, too.
Mr. Cox makes some very valid points in the book, However, even with the points brought up in the book, there are some discrepancies in Mr. Cox’s arguments as well, especially from an engineering standpoint. With his agricultural geneticist background, Mr. Cox is very well qualified to discuss the pitfalls of corporate food and medicine, but his knowledge of basic energy theory is a bit lacking.
He makes the very common error of confusing power and energy; A kilowatt is a measure of power (the rate at which energy is produced or used); a kilowatt-hour is a measure of energy (a means of measuring the work performed through the consumption of energy) and you cannot interchange the two. A common analogy for watts and watt-hours is speed and distance. Speed is a rate of how fast you drive at an instant in time; distance is the length, or amount that you drive over a period of time. For example, if you drive at a constant rate of 60 miles per hour for one hour, then you will have traveled 60 miles. Similarly, if a 60 W light bulb is on for one hour, then that light bulb will have used 60 Wh of energy. If left on for two hours, then the 60 W light bulb will have used 120Wh of energy.
He also does not understand how the laws of thermodynamics as they relate to the refrigerant cycle of an air conditioner works, or the way that the electrical system in the U.S. works, confusing the efficiency of power generation at a central power plant as having anything to do with the efficiency of the refrigeration cycle itself.
One point that he does raise is very valid however, and that is that if you must condition the air in a space, it is better to condition a small volume (say the entirety of a tiny house), than to condition a large volume (such as a McMansion). So with that in mind, I will delve into some of the options available for conditioning the air through the use of active systems. (Which is a fancy way of saying, lets talk about air conditioners!)
Air conditioners are rated in British Thermal Units or BTUs. A BTU is defined as the amount of heat energy needed to raise the temperature of 1 pound of water 1 degree Fahrenheit. For air conditioners, obviously, we are looking at how much heat can be removed by the unit, but the amount of heat energy is the same.
Many people buy an air conditioner that is too large, thinking it will provide better cooling because bigger is better! However, an over-sized air conditioner is actually less effective — and wastes energy at the same time. Air conditioners remove both heat and humidity from the air. If the unit is too large, it will cool the room quickly, but only remove some of the humidity. This leaves the room with a damp, clammy feeling. A properly sized unit will remove humidity effectively as it cools. Home Depot actually has a very good explanation of how to choose the right size for your room (or tiny house), which I recommend consulting prior to deciding what you need.
As an example, an 8 x 16 foot tiny house is 128 square feet, however as most tiny houses have a loft area in them, I recommend adding 20% bringing it up to 152 square feet. Using the rules of thumb presented at the link above, this points to us needing 5,000 BTUs, but as our example tiny house has a kitchen in it, we need to add 4,000 BTUs, and for each person other than ourselves, we will need to add another 400 BTUs. So, for our example, if it is inhabited by 2 people, we will want a unit that has 9,400 BTUs in capacity. However, if you plan to have your tiny house in a shaded location (highly recommended) you will need to deduct 10% from the total, reducing it to 8,460 BTUs. You can round up if need be, but I would caution not to get too far from the calculated amount, and would actually recommend going down slightly to 8,000 BTUs for our example.
The cheapest option for air conditioning is a window style unit. It is also the most popular, as it is the easiest way to retro-fit a house or building to add air conditioning when the house or built was built without it. As such, there are many styles and options available, and many levels of efficiency. As we all want to save money, I highly suggest that you get a unit that is Energy-Star Qualified. You can get them with digital controls and remote controls, and of course multi-speed fans. Of course, you will need to plan for a place to put the unit, so it will help to consider this in the planning stages.
Through wall, Hotel-Style Air Conditioners aka. Packaged Terminal Air Conditioners (PTAC)
Another option available to the tiny house inhabitant is that of a Packaged Terminal Air Conditioner or PTAC. You may have never hear of the name or acronym, but if you have stayed in a budget hotel over the past decade or two, you have undoubtedly used one. Designed to be a self contained unit that is mounted through the wall of the room typically beneath a window, these have the benefit of being a single packaged unit that includes a heat function. Although they were traditionally energy hogs, PTACs manufacturers have kept with the times and made them more efficient. The linked model from Amana also has the benefit of being made in America in a plant in Tennessee. PTACs require a wall sleeve to properly install them, so be sure to account for that if you go this route.
Roof Mounted, RV-style Air Conditioners
The RV market has come up with their solution, which is to mount the AC unit on the roof. Like a PTAC, they are self contained and can contain a heat function in addition to the cooling. Because of the design of the compressor, they can only be mounted on a flat or mostly flat roof. If mounted at too much of an angle (greater than about 15 degrees), the compressor inside the unit will quickly burn out. Also, being designed for a RV application, they are designed for a roof cavity of 2.0 – 5.5 inches. They can be used with a small ductwork system to allow for air distribution, however the ductwork would need to be designed by a HVAC mechanical engineer for proper operation.
The final air conditioning solution I will cover is a solution that the U.S. is behind the rest of the world in adopting, that of the Mini-Split system. These are similar to a normal house sized split system, in that the mechanical workings of an air conditioner are split into two pieces, one designed to be installed inside the conditioned space (the Evaporator) while the other is to be mounted outside (Condenser), connected by refrigerant piping. The mini-splits are just, well, miniature versions of the full size, intended to cool (or heat) a single room rather than an entire house.
For tiny houses, some consideration must be made for mounting of the condenser on the outside of the tiny house, as well as a wall surface on the inside on which to mount the evaporator. Luckily, being smaller capacity, the two units are fairly small and compact, and the various manufacturers have designed their condensers to be wall mountable. As always, paying for a higher efficiency unit will be worth it, when you consider the energy savings over the life of the unit. The linked Pioneer unit is a good, well rated unit, typical of the category.
As you can see there are many cooling options available for tiny houses, and the best solution for one person may not be the best for another. Although some of the units mentioned have heating capabilities, the focus has been on cooling. I will expand upon heating options in next Friday’s post.