Pressurized Shower Heater

[sanramon]

Following on from my initial post "Introduction before questions", here is the question!

I'm designing a simple low cost solar water heater for a low income Costa Rica home. Here, they shower in cold water. Other options here are 'suicide showers' (6 KW electric heaters built into the shower head), or instant tankless heaters under a sink, also about 6 KW power consumption. But electricity is pricey!

So far the plan to use PET (drink) bottles with black 1/2" CVPC pipe inside as collectors. (A common practice invented in Brazil.) Plan is to use 1.5 meter pipe rise, which I hope will give a good temperature difference bottom to top, creating good thermo-syphon effect. May use 8 vertical pipes to get sufficient volume of heated water. This will give an area of 1.5 sq meters collector. (Of course area is relative, depending on the density of collector tubes.)

Does that sound good so far?

Now for the storage tank - planned to mount as high as possible on the roof, above the collector. However, due to roof height, I suspect a gravity fed shower will not be effective. This would also require a ball valve in a supply tank mounted above the storage tank - more cost and work. So the plan is to use street mains pressure, which here varies from 30 to about 60 psi. An adjustable pressure regulator could be fitted.

Here in Costa Rica we can get plastic 150 liter transport barrels with pressure proof lids. I suspect they can withstand 100 PSI or more. I plan to leave an air space above the water level in the storage tank to allow for water expansion. Cold water would enter the bottom level of the tank, hot water exit at the top, just below the air space. And of course collector in and out pipes fitted at appropriate heights on the tank. Tank would then be insulated.

I see references to needing a pressure relief valve on similar plans. Here, we do not have hot water storage tanks, so I do not see pressure/temperature relief valves for sale.

Anyone have any further suggestion or requirements for such a plan? All input will be greatly appreciated.

[sensij]

Here in Costa Rica we can get plastic 150 liter transport barrels with pressure proof lids. I suspect they can withstand 100 PSI or more.

Ah... how sure are you about this? The drum should be marked with a rating... something like

UN 1H1/Y1.9/100/07 USA M-4152

The 100 in there refers to the hydrostatic test rating, in kPa.. 100 PSI would be 690 kPa, a very high value for a drum that size. Even 30 psi would require more 200 kPa rating, hard to do with a removable lid and several pipe fittings on the tank.

[Logan005]

How are you getting 100psi? I have good water pressure and it's about half of that. Here in the US, we can get 500' of black pex 1/2" tubing for about 40 bucks, can be spiralled under glass for hot to nice warm water most of the day.

[sanramon]

Thanks for the info guys and explanation of the barrel markings. My mention of 100 psi was an off-the-top-of-my-head guess at what the barrel would stand. It was used to transport liquid soap, I believe from Europe. No where was the 100 psi in reference to our mains pressure. Here it can vary from about 30 - 60 psi.

So I checked the barrel. 1H2/Y/100/14/ and 1H2/Y204/5/ and D/BAM6260-M and MIPI and PE-HD and GUTEZEICHEN RAL RG 72611. I know this last refers to German quality standards marks. I don't know what the rest means, but assume 100 kPa or 204kPa.

From my reading of gravity fed showers I doubt if the height will provide enough pressure to be satisfactory. If 10 feet (3m) from the storage tank outlet (not bottom of tank) to shower head, pressure would be just 5 psi. ? Not enough ?

[inetdog]

Thanks for the info guys and explanation of the barrel markings. My mention of 100 psi was an off-the-top-of-my-head guess at what the barrel would stand. It was used to transport liquid soap, I believe from Europe. No where was the 100 psi in reference to our mains pressure. Here it can vary from about 30 - 60 psi.

So I checked the barrel. 1H2/Y/100/14/ and 1H2/Y204/5/ and D/BAM6260-M and MIPI and PE-HD and GUTEZEICHEN RAL RG 72611. I know this last refers to German quality standards marks. I don't know what the rest means, but assume 100 kPa or 204kPa.

From my reading of gravity fed showers I doubt if the height will provide enough pressure to be satisfactory. If 10 feet (3m) from the storage tank outlet (not bottom of tank) to shower head, pressure would be just 5 psi. ? Not enough ?

If you have large holes in the shower head (like the popular luxury "rain" shower head which requires an internal pressure regulator to work properly off main pressure) you do not need high pressure. Most off the shelf shower heads are designed with small openings to provide a strong spray and need high pressure, but that is not necessary.
I really do not like the idea of operating a non-certified plastic container full of hot water at line pressure, and a pressure regulator would probably cost more than the rest of the system.

A camp shower is designed for use with gravity feed only, and a small volume (maybe 2 gallons) storage bag, and it works well enough for some people.

For safety with a closed system (no backflow into the water mains, especially if a shutoff valve gets closed without draining or bleeding the system) you absolutely must have both a temperature and a pressure relief valve system.

If you Google "water heater pressure temperature relief valve" you will see a lot of very similar products made for the US market to screw into a threaded hole in the top of the tank and with a probe rod to better sense an excessive water temperature by measuring the water temp rather than the tank wall temp.
You should be able to order them even if local plumbing supply places do not stock them. The price, $US 13.00 and up, could be a problem but it is a serious safely issue.

[sanramon]

Here are 3 versions of the barrel storage tank planned. 2 are for gravity fed shower, one for pressurized. A 4th option using a non pressurized tank and a copper coil heat exchanger for the shower water cannot be considered due to the cost of copper tube here. For 1/2" coiled copper - 10 - 12 $US per meter (about $3-$4 per foot!)

[LucMan]

If the choice was a cold pressurized shower or a gravity warm shower I would pick the gravity fed warm shower. I have used the shower bags on camping trips instead of 50 F lake water and they are a real hit.
I think the gravity system with the ballcock is and the way to go in your situation. Very simple.
Don't forget to use UV resistant barrels and piping in your collector.

[sanramon]

Well, looks like we're going with plan B - that is the Gravity Fed Shower #2 in previous post.

I did a little hydrostatic test of my own, putting just one fitting into the barrel, clamping the lid and connecting it to the garden hose. By the time the barrel got half full under mains pressure the top bulged and the the built-in pressure relief valve in the top bung started venting air. So looks like the UN data on the side is true. In my defense, I said earlier that "I THOUGHT it looked like it could stand 100 psi." No, it can't!

Live and learn, that is what this is all about, isn't it?

[Logan005]

depending on the valve it will release pressure until the tank is full. and then again in the event of over pressure due to high temp.

[sensij]

In my defense, I said earlier that "I THOUGHT it looked like it could stand 100 psi." No, it can't!

Live and learn, that is what this is all about, isn't it?

That you even allowed your thinking to be questioned and reacted by performing an appropriate test, are evidence of qualities I do not commonly see. I sincerely hope this project is successful for you.

[sanramon]

Sensij, thank you very much for your sentiments. I do appreciate using civil, helpful forums (fora?).

For LucMan - the blue PE-HD storage tank will be covered with insulation and inside a painted metal drum - so no UV problems there. Collector tubing will be CVPC, with probably black PVC between collector and tank - again pretty safe I feel. Collector "glazing" will actually be PET drink bottles, and I see they can last up to 10 years in sunlight. (This is the design invented in Brazil around 2002.)

Perhaps I could move on to "plan B pt 2" - using a pressurized heat exchanger . Since copper tube is $12/meter, but black flexible PVC tubing is readily available here, it might serve as an affordable heat exchanger coil inside the storage tank. In most DIY systems I see, they use copper tube. My own commercial system uses 28 meters inside a 200 liter tank. It always exchanges heat faster than we can use with showers, dishwashers, etc. But copper is just too expensive.

So, now some thoughts. This storage tank will hold about 125 liters (33 USgal) below the float valve. Temperature is hoped to be around 50*C (120*F). Mains water temp here is about 24*C (70*F). For a low flow shower, perhaps about 15 meters of 3/8" tubing would give rapid enough heat transfer into the exchanger coil? I don't know what the water speed (feet/min) or thermal characteristics of the PVC are or how to calculate the heat transfer rate.

All ideas and suggestions appreciated.

[J.P.M.]

Late to this party. See builditsolar.com for a lot of ideas. For a lot of reasons, I'm not a fan of plastic for DHW or potable H2O at elevated temps.or where access is limited, but understand the costs and possible fab. limitations associated w/copper. The world is full of tradeoffs. I'd stick with copper were access/visibility/inspection is difficult as much as possible.

[sanramon]

Thanks J.P.M. I have read all of builditsolar.com, and even saw a simpler version built here in Costa Rica.

I'm not worried about using plastic as this project is for shower only. Costa Ricans do not use hot water in kitchens, or even bathroom sinks. I'd sure love to use copper, but unless I bring a roll down in my suitcase next visit - it won't happen.

Work is progressing!

[J.P.M.]

Thanks J.P.M. I have read all of builditsolar.com, and even saw a simpler version built here in Costa Rica.

I'm not worried about using plastic as this project is for shower only. Costa Ricans do not use hot water in kitchens, or even bathroom sinks. I'd sure love to use copper, but unless I bring a roll down in my suitcase next visit - it won't happen.

Work is progressing!

You're welcome. If you can get/use copper at all, it'll be better overall for serviceability if used where you can't see or get at the fluid contact surfaces.

[sanramon]

Work is continuing. The heading should now be "UN-pressurized shower heater. Tank has all the necessary holes in it and fittings installed. The ball valve has been built into the highest point in the storage tank to avoid having to build a separate filler tank. It means a storage of about 120 litres instead of 145 or 150.

Collector tubes are almost completed. I would not have thought it so difficult to round up 48 PET soft drink bottles of the same size and shape! Especially here in CR where there is no deposit and recycle program. It has taken weeks to get 'almost' enough. Tank stand, made of steel and collector frame, made of wood are complete.
Photos will follow soon. Then it is on to testing.