Bob Stuart

I got an electric food warming tray at a garage sale for a footrest.  I added a dimmer switch to get the temperature down, a light to remind me it is on, and tapered supports at each short side to give it a bit of angle and make it easy to slide around with my feet.  

I've been prototyping radically efficient, benign technology for most of my 75 years.  I'm very frustrated by not having a single student.  

For art's sake, I improve or re-manufacture brassware.  I only buy it for less than scrap value.  

Thanks, David.  I have been using roughly-carved styrofoam blocks set in a "mortar" of spray foam, trying to avoid all air gaps behind and between, but I know the job can't be perfect.  I've tried to arrange for drainage, but again, it can't be perfect.  If I were doing it again, I'd use all spray foam.  Considering all the fuss I've had, doing a perfect vapor barrier with fiberglass might have been easier.  

Thanks for the terminology, Terrell.  What I'm finding is too big, too expensive, and unsuitable.  I don't want any humidity recovery, and maximum heat recovery.  For now, I'm  doing some insulation because it has to go in first, but I'll just finish the unit for occasional, above-freezing use for now.  If I do need it for continued and/or colder use, I'll add a wood stove first, and see if I need enough ventilation to make the heat recovery job worth it.  One nice thing about wood heat is that I can extract charcoal for biochar to be a good citizen, not just a fuel waster.  If I expect to be driving 20% of the time, I'll go to the wax tank to store engine heat instead.  

Robert Ray wrote:With such a tiny house storing 20 gallons of heated wax is a pretty big volumetric bite out of available space. My problem with propane heat is the volume of condensate that comes with their use. I'm with Johnathan on the small wood stove, that could be used as your cooking appliance as well. Venting the humid inside air to the outside will probably be key to whatever design you are looking into. Just exchanging heat while retaining interior moisture from sweating, breathing and propane usage might not address the condensation issue. Ventilation sounds like an important piece of the puzzle.

The whole point of the heat exchanger is to have high rates of ventilation without wasting much heat.  Moist air goes out and dry air comes in, but pre-warmed for free.  Agreed that a wax tank would need significant volume, but so does a wood burner plus fuel.  

That's an interesting point about the possibility of the heat exchanger itself icing up, and one I'd have to keep in mind.  However, I should have probably stated that the purpose of the heat exchanger is to just expel considerable quantities of moist indoor air while recovering the heat for a corresponding intake of fresh, dry air.  

Hi Durrell,
It is great to have your experience for this.  My recollection of Texas is both hot and humid.  For my northern climate, what would you think of using an air-to-air heat exchanger instead of a dehumidifier?  If there's a significant difference in temperature it puts out fairly warm, dry air similarly to a dehumidifier, for less energy drain.  It could keep the absolute humidity inside almost the same as outside, making condensation very unlikely.  The commercial ones are pricy, but a custom-fit DIY version could be cheap.  

After thinking about sail boats, which have more problems with water than just my condensation issue, I realized just how wonderful a bilge is.  A dry bilge is pretty rare, but a wet one is not a problem until there's enough water to make it worth pumping.  
In the car, I can just use the gutters on each side that extend from the box sections under the doors as collectors for any running condensation from farther up, with small drain holes.  
I'll insulate much of the bodywork above that, Where there's good enough access that I'm confident about getting a good seal from the foam to the metal to create a vapour barrier.  Places where gaps are needed for various reasons can just drip into the gutters, with a bit of planning.  I plan to install chunks of styrofoam that fit loosely together by injecting foam through a hole through the foam, so it will displace any air behind the chunk, and then expand into the gaps like mortar.  Unless someone here can think of a snag, I'll probably get to work tomorrow.  

Robert Ray wrote:Whether fluid or air exchanger, during the night what medium and where is the heat being stored for the temp exchange? Is your design going to use the heated wax for the exchange? If you will be using the wax, what do you see as the volume of wax and anticipated temperature of the wax in its storage vessel. What length of time do you anticipate the heat reservoir will be able to keep the exchange up to inhibit condensation? Interesting concept but the available cubic inches will make it a tricky solution. Looking forward to see version 2.0.

Rough estimates are encouraging with a 20 gallon tank, but exact numbers depend on the level of insulation achieved, and the local weather.  The heat exchanger would conserve most of the heat usually lost to a vent.
A gasoline engine puts about 25% of the potential energy in the fuel out the shaft, 25% into the coolant, and 50% out the exhaust pipe.  So, the general plan would be to use a water muffler to recoup exhaust heat, and circulate that, and the coolant through a heat storage tank.  The tank would be mostly filled with closed tubes of paraffin.  It has a heat of fusion close to that of glauber's salt, without the problems.  It does shrink a lot when freezing, so that has to be allowed for.  That temperature can be adjusted up with stearic acid to improve heat release and increase capacity slightly.  I expect that just moving a blanket would suffice for regulating car temperature, but it might need a fan, etc.
However, I don't have an inter-city courier business or much motivation to travel, and if I decide to look for a new home again, I can probably do it in the summer.  So, perfecting this rig is probably more trouble than it would be worth.  Now, I'm thinking of just adding foam where it is easy to get a perfect seal to the metal, and making sure that any condensation returns to the air the next day.