Tiny House Chronicles: A Plumbing Polemic

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Published on the Doomstead Diner on July15, 2016

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The ongoing tiny house chronicles: A venturi exhaust system for the shower stall and solar hot water arrays incorporating a direct HWC

Geoffrey Chia, July 2016

DIAGRAM 1  DIAGRAM 2  DIAGRAM 3  DIAGRAM 4 HeatpipeCoreEvacTubes

DIAGRAM 5 HollowCoreEvacTubes  DIAGRAM 6 HollowCoreEvacTubes  DIAGRAM 7 Thermosiphoning HeatpipeCoreEvacTubes  DIAGRAM 8 Thermosiphoning HollowCoreEvacTubes

VENTURI EXHAUST SYSTEM FOR THE SHOWER STALL:

I could not find any "off the shelf" exhaust fan models specifically designed to be located in a shower stall which operated on 12V or 24V DC.
A simple computer-type fan can certainly do the job, at least in the short term, and there are many of suitable size eg 12cm diameter, which can run on 24V DC diagram 1. However even if rated as "moisture resistant" they may not cope with sauna type humidity and are likely to fail prematurely. Even though they are quite cheap, the idea is to seek durable items and not be forced to keep replacing them. Furthermore heat loss from this open vent will be substantial, especially at night. A removable internal insulated cover can be added, however that will be another finicky add-on.

24V DC blower type ducted fans are easily available but are probably not suitable to be used "in-line" for air extraction. If embedded in the wall, due to the longer profile than a computer fan, it will stick out awkwardly. Even if mounted exteriorly and extracting air via a duct, it will be directly exposed to the sauna type moisture and may also fail prematurely, being made to do a job it was not designed to do. One way to ensure it has no contact with such moist air is to position the blower upstream of the shower exhaust port, to create a venturi effect to suck air out of the shower stall diagram 2. This will admittedly be an experimental system however I received no practical objections to the design from the intellectual resource folks I consulted (Doone W: scientist, engineer, geologist and homesteader, Lara N: architect, designer & builder, and Lara J: mathematics and physics expert). Any failure in execution of this system will of course be the fault of the author. This system will sacrifice some efficiency for better durability and longevity. If the external "inverted U" pipe or duct is well insulated, this configuration will help prevent heat loss because warm air from within the tiny house will tend to sit static in the top bend of the inverted U at night when the external atmosphere is cool (unless there are strong external breezes).
 

ADDITIONAL HOT WATER SYSTEM OPTIONS:

Roof mounted solar tube arrays utilising direct hot water cylinder (HWC) on loft floor

Designing a system in theory is one thing, but in practice we must always modify the design according to whatever components are available to us in the real world. I was offered false hope about obtaining a small indirect copper hot water cylinder by the false advertising of a UK vendor on eBay. Having now found the option of an affordable, small 50 litre simple direct HWC from an Australian vendor (diagram 3), this can be incorporated into the system and placed in the loft to increase the volume of hot water available and reduce concerns about inadequate pressure head for the taps. Unfortunately the roof mounted options require an additional small electric water pump and a small solar PV panel, however these are simple robust devices with good longevity. All these solar hot water system options remain free of dependence on microprocessors.

 

"Heat pipe core" type tubes (manifold on top of array) diagram 4

Caveats of this system: the manifold does NOT use glycol for heat exchange through internal copper coils in the HWC, water is circulated through a direct HWC. Hence it is NOT suitable for locations prone to substantial frost and subzero temperatures. However occasional ground frost should not be a problem as this should not affect the roof mounted system.

Pipes and Circulation:

All pipes are insulated except pipe 6, the overflow pipe from the header tank (which drains externally)

  1. Each morning, water is actively pumped from the rainwater tank at ground level into the header tank (this inlet pipe is not shown in the diagram for simplicity). Water from the header tank passively flows down through wide calibre (DN32) pipe 1, through the low resistance valve, filling the loft HWC to the brim and also filling pipe 5 up to the same level as the water level in the header tank.

  2. Water passively fills the electric pump which hence becomes primed. During the day when there is sunlight striking the PV panel, the pump drives water up pipe 2 and through the heated manifold into pipe 3. Note this manifold circuit will therefore automatically bleed air out by itself during initiation.

  3. Pipe 3 joins pipe 1 but cannot backfill into the header tank due to the presence of the upstream valve in pipe 1. Water in pipe 3 is hence forced into the HWC. Water circulates continuously through this HWC/manifold circuit in the day, progressively heating up the water in the HWC but this flow ceases at night when there is insufficient light to power the solar PV panel.

  4. Pipe 4 supplies the hot water taps. As water is extracted from this pipe it is replaced at the base of the HWC by cooler water from the header tank. At night there will be thermoseparation between the top hot layer of water and bottom cooler layer.

  5. If the sunlight is too intense and the pump is working too fast, causing the loft HWC to overfill and overflow via pipe 5 into the header tank, then the flow rate from the pump must be dialed back with the potentiometer. The expectation is that the potentiometer will be set for the brightest summer day and thereafter be fixed in that setting and not need attending. The loft HWC cannot overpressurise or boil over because such pressurised water will spill over from pipe 5 into the header tank and be replaced by cold water via pipe 1. Water in the header tank does not overheat due to the large volume of water here. as well as the heat being radiated out of the steel walls of this uninsulated matt black header tank.

  6. Over filling of the header tank in the morning is seen through the kitchen window as external spillage via pipe 6

 

"Hollow core" type tubes (small HWC on top of array) diagram 5

Pipes and Circulation:

All pipes are insulated except pipe 6, the overflow pipe from the header tank (which drains externally)

  1. Each morning, water is actively pumped from the rainwater tank at ground level into the header tank (this inlet pipe is not shown in the diagram for simplicity). Water from the header tank passively flows down through pipe 1, through the low resistance valve, filling the 50 litre loft HWC to the brim.

  2. Water passively fills the electric pump which hence becomes primed. During the day when there is sunlight striking the PV panel, the pump drives water up pipe 2 into the rooftop 30 litre HWC, filling it eventually to the outlet of pipe 3.

  3. Pipe 3 joins pipe 1 and although the pressure head in pipe 3 is higher than pipe 1, water cannot backfill into the header tank due to the presence of the valve in pipe 1. Water in pipe 3 is hence forced into the 50 litre loft HWC. Water circulates continuously through this loft/rooftop circuit in the day, progressively heating up the water in both HWCs but this flow ceases at night when there is insufficient light to power the solar PV panel.

  4. Pipe 4 supplies the hot water taps. As water is extracted from this pipe it is replaced at the base of the HWC by cooler water from the header tank via pipe 1. At night there will be thermoseparation in the loft HWC between the top hot layer of water and bottom cooler layer. There is no flow down pipe 3 at night.

  5. There is normally free movement of air to and fro within pipe 5. The HWCs cannot overpressurise or boil over, because overpressurised vapour will exit pipe 5 (and will ultimately vent to the external air via pipe 6). If the sunlight is too intense and the pump is working too fast in the day, causing the rooftop HWC to overfill and overflow via pipe 5 into the header tank, then the flow rate from the pump must be dialed back with the potentiometer. The expectation is that the potentiometer will be set for the brightest, longest summer day and thereafter be fixed in that setting and not need attending. This overflow scenario is very unlikely if pipe 2 is narrow in calibre, thus limiting the inflow rate into the rooftop HWC, and pipe 3 is wide in calibre thus enhancing the outflow rate.

  6. Over filling of the header tank in the morning is seen through the kitchen window as external spillage via pipe 6.

 

NOTES: in this system, the rooftop 30 litre HWC is treated as no different from a simple manifold. Hence the water in the rooftop HWC is "dead water" being generally unavailable for use*. Total available hot water remains 50 litres from the loft HWC only. The system will NOT work properly (for reasons too complicated to get into here) if water is pumped into the rooftop HWC at the level of pipe 3 connection and is drained from the base of the rooftop HWC at the level of pipe 2 connection.

 

*It will be possible however to manually harvest the hot water from the rooftop HWC at night from pipe 7 by opening the manual tap as indicated in the diagram. This tap in pipe 7 must be kept closed in the day for the system to function (otherwise water will merely circulate between pipe 2 and the loft HWC without going up through the solar array)

 

**Alternatively the connection in diagram 6 with an additional valve in pipe 4 should work well without the need for manual input. In this system, there will be an abrupt reduction of flow rate from a hot water tap when the rooftop HWC empties and the hot water then derives from the loft HWC. The disadvantage of this arrangement is that having a extra valve in pipe 4 can further reduce the forward flow rate from the loft HWC compared with diagram 5. To minimise this, it will be necessary for pipes 1 and 4 to be as wide as possible.

 

Thermosiphoning solar tube arrays utilising direct hot water cylinder (HWC) on loft floor

Whether or not the arrangements in diagrams 7 and 8 will work is unclear, because the basic direct HWC is not designed for thermosiphoning. Proper thermosiphoning cylinders have a convex top, in the centre of which is located the hot water outlet. Furthermore the connection ports are larger.

In diagram 8, air at the top of the 30 litre HWC may be an issue, however it should be possible to manually bleed most of this air out using the pressure release port at the top. I intend to give them a try.

 

Potential issue with all these systems: If the level of water in the header tank drops below the top level of the loft HWC, there will no longer be sufficient driving pressure to expel water from the latter. This can easily be resolved by refilling the header tank, a matter of turning on a switch.

 

G. Chia, July 2016

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