I’ve touched on my bus conversion project before. Today I will start writing about it in more detail.
The bus is a 1994 TMC 80206 “RTS”. The engine is a Detroit Diesel Series 50 rated at 275 horsepower. There are but 4 cylinders, down from 6 or 8 in most large buses. Fewer cylinders often mean better milage, just like with cars. This bus gets 11 miles per gallon on the freeway, which is outstanding for a vehicle that weighs so much and pushes so much air out of the way.
When I bought the bus in 2007, my plan was to do a quick conversion to a motorhome and then go on a six month tour of the United States with my girlfriend at the time. She hadn’t yet graduated from university, so there was time to finish a rudimentary conversion. She wanted to take a break after graduation and before starting her career, but sadly, she got offered a great job at Stanford University that she had to accept, since it came through a family introduction. She was most unhappy about not getting to take a break, and I was too, as she never had another break from full time work, and now we’re no longer together. That we never got to take the bus out for even one night really breaks my heart.
Life goes on, and there will be new girlfriends and I hope, another marriage, and, I hope, children.
My enthusiasm for the bus is still strong, and I will finish the project and use the conversion. I’ve invested too much to just sell off the partially finished project, and I would get next to nothing if I sold it today anyway.
Since this is a blog post and not a book, I will focus on just a small part of the project today, the air conditioning system.
Most bus conversions handle air conditioning while parked by installing a large diesel generator, in the 5,000 to 15,000 watt range, and use the power to run 2, 3, 4 or even 5 powerful yet not efficient air conditioners at once, to cool the entire interior simultaneously. Owners that live in hot regions have to run the generator and the air conditioners 24 hours a day for the duration of their travels, it can be that hot out. When I drove my first bus conversion across the United States from California to New York in 2002, I typically ran my 6,500 watt generator from morning until bedtime, which cost me about $20 a day in gasoline, and gas was much cheaper back then. It would simply not be cost effective to build a new conversion that uses so much fuel just to stay cool inside. So I have in mind something better, which I will write about here.
I have in mind covering about more than half the roof with 10 250 watt photovoltaic solar panels. These will cost about USD $7,000. In full sun, they will output 1,250 watts of power at 24 volts DC. Run through my Trace SW 4024 inverter, this is enough to power a 9,000 BTU mini-split air conditioner with a 20 SEER rating. This means the AC will draw about 450 watts of power at 120 volts.
9,000 BTU of cooling is not enough to cool a 40 foot bus conversion all at once, especially one with a full set of original side windows like mine has. The way I will handle this is to break up the inside into smaller zones. Right now there are three zones, the driver compartment, the living room and the bedroom. Eventually there will be one more zone, the guest bedroom, for 4 altogether.
I don’t need to occupy all the rooms at once, and I hate the noise that air conditioners make, so I have a plan to distribute the cool air from the one air conditioner to where it’s needed.
The plan borrows from the commercial air conditioning world, where cold water is used to cool rooms, as opposed to cold refrigerant, as is used in residential room air conditioners.
I plan to modify the inside portion of the mini-split air conditioner so that I submerse the evaporator plate, the part that gets freezing cold, in a large tank of water. This will cool that tank of water over time to about 35 degrees F. Then, I will use this cool water to pump through a distribution system of pipes in the ceiling of the bus to ‘heat exchangers’ in each room, or zone. Since cool air falls, these heat exchangers will be in the ceiling as well. Think of them as mini car radiators. Hot air in the room will pass over the cold fins and be cooled. The cool air will circulate down into the zone since cool air falls. The water inside the heat exchanger will pick up heat from the surrounding air. This heat will be carried back to the large water tank to be re-cooled by the mini-split evaporator that’s immersed in the tank. The mini-split will capture the heat from the water and send it outside the bus to the condensor part of the mini-split, which will be fan cooled on the exterior roof of the conversion.
The beauty of the above system, if it works, is that there won’t be any noisy fans inside the rooms. The inside part of the mini-split will probably be silent since the fan isn’t needed to push warm air over the evaporator. There may be a small noise made by the circulation water pump, but I suspect that it won’t be audible.
The heat exchangers should also be silent. It may turn out I will need a small fan near them to optimize their performance, but a quiet computer muffin fan or two should be sufficient.
Since the zones are fed by cold water in PEX plastic pipe, it should be easy to change which zone is being cooled by flipping regular water valves. No special certification is needed to install water piping, but installing copper tubing for AC refrigerant is not easy.
The final benefit of my proposed system is the sun can be used to cool the water tank during the day, but then the coldness of the tank can be ‘harvested’ at night to keep the sleeping zone cool throughout the night, without running a noisy and fossel fuel dependent generator. When I drove across the country in 2002, I hated to run the generator all night long, as even with ear plugs I could barely fall asleep it was so noisy.
When I add the 4th zone, the master bedroom will be tiny, at about 50 square feet by 4 feet high, or 200 cubic feet. I will have removable 3 inch thick insulating window panels I can press into the window openings to really insulate the compartment. The bedroom door will be weather stripped and insulated, perhaps itself 3″ thick. This will mean that the water tank is not trying to cool a particularly large area. With a 24 volt celiing fan circulating air over a heat exchanger, I think I will have found an essentially silent and free nighttime air conditioner for touring the world in hot weather.
I would love to use ceiling fans in the other zones, but sadly there is not enough ceiling height except over the bed to install a full size ceiling fan.
I can’t take credit for the idea above to dismantle an air conditioner and submerse the evaporator plate in a tank of water. This is routinely done by indoor gardeners trying to use liquid cooling to cool powerful grow lights. They take apart a $100 window air conditioners and bend the copper piping to the evaporator plate to dangle it in a tank of water. Once this water is cooled, it’s piped into the grow light fixtures themselves, which often draw a lot of power and so get very hot. The hot water from the fixtures is piped back to the water tank, where the window air conditioner again cools the water for the next cycle. i found videos online demonstrating the technique. I won’t link to them since these indoor growers appear to be growing illicit crops, which I frown on and have nothing to do with. But I do admire the ingenuity of the growers, and I thank them for helping me formulate my above plans.
While doing my research on this system, I found a venture backed company doing something similar to what I propose, but for commercial installations. They use relatively cheaper grid electricity during the night to freeze a multi hundred gallon tank of water, and during the hot day they melt that block of ice to cool the building. This is clever because grid electricity rates are much lower at night, when the grid is relatively lightly loaded. The company has products installed, and it looks good. I don’t plan to go all the way to ice because that would require something other than a cheap mini-split air conditioner, and I want to keep this system cheap and built from off the shelf commodity parts so it can be repaired anywhere should it malfunction. Also, mini-split air conditioners are available in very high efficiency models, such as 20 SEER. This is an astonishingly high number compared to the non-regulated rooftop RV air conditioners most bus conversions use, which can be around 10 SEER. Going from 10 to 20 in SEER cuts the power used in half, which is a really big deal when you have limited roof space for solar panels, and the panels cost a fortune for enough of them to run even one moderate sized air conditioner.
During the day, I would just cool the living room, of course. I have sliding shades on the windows, and silver reflective mirror on the outside of the glass, which cuts dramatically the heat infiltration through the windows. I also plan to make some 3″ thick removable panels I can press into the window opening to really insulate the windows I don’t need to be transparent. I suspect it would be much more energy efficient in fact to block all 6 windows in the living room and turn on my compact flourescent lights during the day, rather than to leave one or more windows unblocked. The living room is separate from the driver’s compartment, which is full of air leaks due to the transit bus front door, which can’t effectively be sealed. The door between the living room and the driver’s compartment will be insulated and weather stripped. I’ve often wondered why doors are so thin. The walls are often six inches thick but the doors are 1 3/4″ thick. Why not make the doors also 6 inches thick, filled with high density closed cell foam, similar to a SIP panel for house construction? This would require a new type of door knob and lock, but that is an opportunity to sell more expensive hinges and door hardware. I think thick doors should be required by code for all construction, residential and commercial, interior and exterior. Thick interior doors would cut sound transmission and make it easier to implement zones in buildings. The current practice of heating an entire home when the occupants are often in just one room needs to cease. Each room should be insulated from the others, and there should be a way to temper each room individually. How about the idea of having a switch similar to a light switch in each room? To be effective, each room will probably need two parallel systems – one quick reacting and one slow reacting but more efficient over the long time. I have such a system in my bedroom in my house. I have a 400 watt panel heater that contains no fan and thus takes hours to warm up the room. I also have a forced air wall heater that can heat up the room in a few minutes. When I enter the room and it’s freezing, I turn on both heaters, and then turn off the forced air heater after a few minutes. Then the slower acting convection panel heater takes over maintaining the temperature indefinitely. What could work perhaps would be hydronic floor water heating plus a forced air unit for rapid heating. It might even be that both heating systems could be hot water based, with a large heat exchanger mated with a powerful fan for rapid heating, and in floor piping for maintenance heating. I’m quite serious about this scenario, and I can see it being the dominant style of heating worldwide 100 years from now, with the water heated exclusively by solar hot water heating panels.
I can’t promise all my bus conversion posts will be this long, as this took some real effort to write. Please write me a comment if you like or dislike it. I would love to get a discussion going in the comments about the pros and cons of my ideas presented here.
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