passive solar water heating idea

[nailbender]

I am building a house in south ms. emphasizing economy and function over fancy and this seems like a good time to take some steps toward moving off the grid.  One thing that I am considering is passive solar water heating integrated with a conventional electric hwh for backup.  I have a large south facing green metal roof on 2x4 purlins on 2x6 rafters.  Call me simple, but it seems like I could just run 1/2 inch pipe loops in the 1 1/2 inch space created by the purlins, and send that heated water to my hwh with the thermostat set at normal operating temperature and that would supply me with at least one tank full of hot water per sunny day.  I want it to be that easy, but I know that alot of times, things that appear that simple really aren't.  Here are some things that I am chewing on:

1) Will the water temp exceed the 200-210 limit on pex pipe ( cheap)
2) If so, will copper (not cheap) be worth the money I save on my power bill
3) Do I need  an additional heat collector under the pipe
4) How much 1/2 inch pipe would it take to fill a 40-60 gallon tank
5) Will I have to worry about my pipe freezing in the attic on freezing days (not many in south MS)
6) Are my pipes gonna sweat

I am not a smart man and I have little money and even less time-- actually I have equally as little time and money.  I need this to be simple and economical.  Does anybody have any ideas or answers.  Thanks in advance for your help.

[John Mc]

Home Power magazine ran a good series of articles on solar hot water over the past couple of years. You may want to check out some of their back articles before going ahead with this project. There are a lot of different ways to avoid the freezing problems... no point in reinventing the wheel. Even if you do build it all yourself, you can pick up some good ideas from this series of articles.

A couple of things I can think of off the top of my head with your plan:

You won't get good heat transfer from your roof to the tubing if the tubing is not in very good contact with what ever is exposed directly to the sun.

I've never seen a commercial solar hot water panel made with Pex tubing, Plastic is a thermal insulator, so you're going to lose something trying to drive the heat through it and into the water inside.

Building the system into the roof of your house means maintenance is going to be a big headache. What do you do if your tubing develops problems, or a connection starts leaking? Even if the Pex would take a freeze without bursting (I have no idea if it will or not), will all of the connections survive? The water damage from a frozen and burst pipe could be significant -- do you want a leak under your roof, and soaking all your insulation in the middle of winter?

There are some very well made pre-packaged systems out there that can do what you want for not a whole lot of money (especially after state and federal renewable energy credits). Here's one example that is simple and easy to install: www.gosunward.com. It basically acts as a preheater for your existing hot water sytem. or the plumbing end of the installation, you just break in to the incoming cold water to your existing hot water heater. If you have enough sun, it supplies all of your hot water needs. If you don't have enough sun to heat it all the way up, you are at least preheating the water fed into your system, and saving some energy. Here in Vermont, people who have this or similar system say it provides all of their hot water needs for roughly half the year, and helps out the rest of the year (cold outside temps, and not much sun hurt in the winter).

I have a neighbor who made his own panels from copper tubing and a metal backing plate, enclosed them in a glass faced box, and mounted them on his roof. The efficiency may not be as high as the system mentioned above, but it works for him.

John Mc

[Radar67]

What part of South MS you building in? If I haven't already, welcome to the forum.

[nailbender]

Thanks for the welcome radar 67.  We have sixteen beautiful acres down here in Perkinston.

[Radar67]

That's a nice little area. I would experiment with the pex before I went full on into a project with it. We don't have to deal with the freezing temperatures our neighbors to the north do. I have recently noticed the amount of hot water you can get out of a water hose laying on the ground. 50 feet of hose takes a little time for the water to cool down. I haven't measured it, but based on average flows, it is probably close to 10 gallons of water that comes out heated, some of it is hot enough that you can't hold your hand under it.

You can go to instructables.com and do a search for solar water heaters for some interesting ideas.

[Radar67]

I was just informed that we have met before. I'm the guy that came down and helped woodbowl saw out some lumber for you right after Katrina.







3400

[Brucer]

Many years ago a friend built a simple solar collector for his off-grid summer cottage. He basically made a couple of tightly coiled spirals of 1-1/2" black polyethylene pipe, mounted on a piece of plywood. No glass cover, no insulation, no freeze protection. It worked great.

A couple of times he left the system full of water late in the season and the water froze in the pipes. No problem -- polyethylene will easily stretch to accommodate the expanding ice.

What finally did his system in was when he left if full one spring but turned off the inlet and outlet valves. He was late getting to the place in the summer and the water had got so hot it began to boil and split the pipe. Not at wall what he was expecting ???.

One thing to remember is that while polyethylene pipe will withstand freezing water, the metal valves and fittings connected to it will not.

You can make a water system truly passive by mounting the hot water tank above the highest point on the solar collector. Water will circulate by natural convection.

[Bill]

Ditto on the natural convection .

I seem to recall a fairly recent article(s) in Mother Earth News about how some were doing " passive " solar hot water ( no pumps ) . I kinda recall most built a shed next to the south facing first floor of their building ( kinda like a greenhouse ? ) and put a storage tank inside on the second floor - where the hot water was saved till needed - then plumbed to domestic hot water or the heating system . I think some just used black ( painted ? ) copper pipe inside panels also ( painted ) black . Some may have used plastic pipe instead of copper.

Seemed slick at the time . . .

Good Luck