PENULTIMATE AND ULTIMATE PASSIVE SOLAR AND PLUMBING DESIGNS FOR THE TINY HOUSE
The latest internal configurations shown here represent what I describe as the penultimate and ultimate passive solar and plumbing designs for a tiny house on wheels. If in the future I am able to tweak things to achieve additional improvements, I reserve the right to describe later revisions as the "super ultimate version" or "superduper ultimate version" etc, etc, outrageous ironic hubris intended.
In all versions I have always located the LPG stove and wood stove side by side (and the sink next to the LPG stove) so that a single rangehood can extract vapour from either stove, and to facilitate quick transfer of hot pots and pans from either stove to the nearby sink.
As mentioned in previous articles, passive solar heating requires broadside orientation of the dwelling to the sun, expansive double (or triple) glazed glass windows/doors on the sun-facing aspect, thick insulation of floor/walls/roof and a source of thermal mass. For a tiny house on wheels, concrete is not appropriate thermal mass, it is dead weight on the chassis. Water however has a high specific heat capacity and, weight for weight, offers superior thermal mass to concrete. Water tanks can be emptied when the tiny house is transported.
My initial intent was to locate internal water tanks(s) under the lounge seats to provide thermal mass, however solar heat transfer would be inefficient in that configuration and it would represent a great deal of dead weight on the chassis. I subsequently decided, for reasons previously explained, that a header tank for the cold water system will be an important component, however combined with the hot water cylinder and under-seat tank(s), the all up weight just for these water filled components will be excessive, around 700kg. It would represent an adverse long term load on the chassis. Hence I have now decided to eliminate the under-seat water tank(s) which alone would contain 400 to 500 litres of water and thus weigh more than 400 to 500kg.
THE PENULTIMATE PLUMBING DESIGN
The steel header tank, if uninsulated and painted matt black, despite its smaller size (150 litres), could still confer good thermal mass, in addition to its primary function of providing the pressure head. However, is there a way to make this thermal mass even more efficient? My advisers from the Tiny House Company http://www.tinyhousecompany.com.au/ (Lara Nobel, Andrew Carter and Greg Thornton) suggested that a mid point stair configuration will be more space efficient than my original design with stairs at the west end. This new configuration in fact offers a number of improvements. As the header tank can now be located almost directly above the wood stove, it makes sense to take advantage of this arrangement to design a gravity/thermosiphoning circuit between the backboiler tank of the mini wood stove and the header tank.
This will effectively harvest heat from the wood stove, for later slow release of heat from the header tank after the fire is out. One danger of this arrangement may be overheating of the water in the header tank, however this can be avoided by always ensuring the tank is full of cold water before firing up the stove and by not running the stove for extended periods eg more than two hours. The intent here is to utilise the header tank water as thermal mass, and NOT to turn the (uninsulated) header tank into a hot water cylinder (the header tank will not and cannot replace a proper, dedicated hot water cylinder supplying the taps).
THE ULTIMATE PLUMBING DESIGN
The next natural question is whether it may be feasible to harvest heat from the wood stove to supply the hot water cylinder, while also using the same cylinder to gather heat from the solar thermal array, all by means of gravity thermosiphoning which, not requiring pumps or sensors, will be the most reliable and robust system possible. The answer is yes, however it will require particular design of the hotwater cylinder according to custom specifications. If you live in an area prone to frost, the heat transfer from the external solar thermal array into the cylinder must be indirect, via copper coils containing a glycol solution. Heat transfer from the backboiler tank of the wood stove to the cylinder can however be direct. Hence the hot water cylinder design should be as shown in Figure 2
For adequate thermolayer separation within the cylinder, a tall vertical cylinder is best (rather than a squat horizontal cylinder).
It is essential to consider the requirements for effective gravity thermosiphoning which are:
To drive a circulating convection current, the heat source(s) must be below the hot water storage cylinder, and the hot pipe must be relatively higher than the cold(er) pipe.
Adequate flow requires minimum resistance within the circuit, which requires that the calibre of pipes be large (at least 28mm), that there are few or no right angle bends (gentle curves/bends in the pipes are allowable) and that the pipes should be relatively short (which requires close proximity between heat source(s) and hot water cylinder). Short pipes also minimise heat loss in transit (which is inevitable even with good pipe insulation).
The higher the temperature gradient between hot and cold pipes, the stronger the convection current. Hence the intense heat from the backboiler of the wood stove will still enable effective thermosiphoning through a long circuit, whereas the less hot solar thermal array should have a shorter circuit to function effectively.
As such, my ultimate iteration based on these plumbing considerations is as shown:
Solar heating of water
In the "ultimate" version, there is no connection between the (cold water) header tank and the wood stove. For thermal mass purposes, the most efficient way to transfer solar heat to the header tank will be for this uninsulated matt black steel tank to sit directly against a double glazed window on the sun-facing aspect of the house. Obviously this surface area for solar heat gathering will be tiny compared to the volume of water in the header tank. Nevertheless, a modest five degree rise in water temperature within the header tank (eg 15degC to 20degC) will be excellent for thermal mass purposes. However 20degC will be completely inadequate as a source of hot water for the taps. The uninsulated header tank cannot and will not replace an insulated hot water cylinder as the supply for the hot water taps.
A dedicated solar thermal array feeding a dedicated insulated hot water cylinder is necessary for the latter purpose. This outdoor solar thermal array, if located directly in front of the hot water cylinder, could potentially suffer from shadowing from the timber deck (and its overhead awning) in the morning, hence the array may be better located toward the west end, despite the slightly longer pipes required (which of course must be heavily insulated).
Passive Solar Heating of the composting toilet:
Proper composting of faeces to kill pathogens requires high temperatures and adequate duration of composting. High temperatures can be naturally achieved by exothermic reactions within a large mass of decomposing waste, however the volume within the bin of the composting toilet is way too small to achieve this. Hence to speed up the initial decomposition of such a small volume, it makes sense to enlist passive solar heating. It is therefore important to locate the composting toilet on the sun facing aspect of the house, immediately adjacent to double glazed frosted windows. Indeed, now being located at the north western corner, the toilet will receive heat from the evening sun as well. Obviously when one is using the toilet, the frosted windows will be blocked off by pull down modesty screens.
Full time use of the Nature’s Head composting toilet by a couple may require that it be emptied once per month. Minimal composting will have taken place by then (indeed no decomposition of freshly deposited waste will have occurred). Odour is actually eliminated during active use of the toilet primarily by means of dehydration (continuous ventilation) and coverage with sawdust/wood ash. Fresh compost within the full bin (now removed from the toilet) will be sprinkled with fresh water, because additional moisture will be required for further aerobic decomposition. The bin will then be transferred to an outdoor "solar storage" greenhouse chamber where it will undergo further passive solar heating for a month, by which time the waste will be truly innocuous. Further composting will need to be conducted by emptying this bin onto a larger composting mass the size of, say, a rubbish skip (with a rainproof lid). The month old compost will deposited on the top of the larger, older mass of compost. The oldest compost (perhaps two years old) can be harvested from a cutaway opening at the very bottom of the skip. This biologically safe compost can now be scattered at the base of trees.
Uneven weight distribution in tiny house: there is significantly more weight from the header tank and HWC on the sun-facing side of the dwelling despite the fridge and washing machine being on the opposite side. This should not be an issue if the weight (when parked) is not borne by the tires but by jackstands or footings (important to locate jackstands or footings not only at the corners but also under the mid point of the chassis, thus directly bearing the weight of the header tank). This uneven weight will not be an issue during transportation when the water tanks are empty.
Standard mains pressure in domestic taps is around 3 metres of water height. The same pressure can be achieved by locating the header tank outdoors, on top of the roof, which will however eliminate the possibility of greenhouse heating of this header tank. The pressure head will be lower if the header tank is located indoors on the loft floor, but lower flow rates can be overcome by using wider calibre pipes to the taps (perhaps twice the standard bore).
Water overflow from the header tank should be directed to the exterior, proud and clear of the wall of the house, by means of a gargoyle poised above the kitchen window. Figure 4. This overflow will be visible through the window from inside the house, signifying that the header tank is full and that pumping of water up to the header tank must cease.
Comments on the "penultimate" design:
In this configuration where there is connection between the woodstove backboiler tank and the stainless steel header tank via copper pipes, electrolytic corrosion of both tanks can be prevented by the simple application of a magnesium anode in the header tank. The advantage of this configuration is that it confers residual heating to the house after the fire from the wood stove is out. The disadvantage is that it does not heat water for the hot water system. However it will be easy enough to just boil a kettle and mix that with cold water in a bucket to obtain tepid washwater.
If the header tank has a loose lid (and hence the water in the header tank is connected with the atmosphere of the house), and if this water is warmed excessively, it could result in copious condensation on the inside walls and windows of the house overnight. The way to prevent this is to construct the header tank to be airtight, however it will need to have a wide bore venting/overflow port near the top, which must be vented to the exterior, which will also allow the moist air to escape outside, via the gargoyle.
Comments on the "ultimate" design
The vendor I sought who makes custom hot water cylinders, Trevor, specialises in copper cylinders. I am uncertain if the copper hot water cylinder and copper pipes connected to the steel backboiler tank will in the long term lead to electrolytic corrosion of the backboiler tank. Mark of Salamander stoves was unable to advise me about this, apart from saying that after five years of use (with copper pipes) he has personally not detected any problem in his backboiler tank.
When the wood stove is fired up, water in the HW cylinder can easily become scalding hot, hence adequate care must be taken to dilute hot with cold water when operating the taps. Rather than depend on complex electronic sensors (which regulate the water temperatures in modern domestic systems), the philosophy in this design is to depend on simple common sense.
In view of the above considerations, I am actually partial to the "penultimate" design rather than "ultimate" design at this time.
Please note the last diagram in the addendum of this article https://www.resilience.org/stories/2016-02-26/tiny-house-electrics was incorrect and the correct diagram should be this one:.