Solar Pre Heater for a Hot Water Heater

[Tim Stuart]

Ok folks I am new to the solar world so be easy on me. I was thinking about adding a solar Pre- water heater system to my 40 gallon electric hot water heater. I was thinking this may save me a little money each month on my electric bill. I was also thinking this would be something I could build myself. I am think I can build a square box 10'X10'X8" out of treated wood painted black and mount it outside my roof above my hot water heater. Than insulated inside the box with foam board inside. Than run copper pipe or cpvc pipe back and forth inside the box to absorb the sun's heat. Than cover the box with a piece of glass. My thoughts are that if my ground water average temp of 72 degrees going into the Hot water pre-heater it may come out of the pre-heater at about 92 degrees plus ( just a guess) That would raise the temp of the water going into my 40 gallon electric hot water by 20 plus degrees there for not working my hot water heater as hard to get to the 120 degree setting it's on. Do any of you have a system like this? What kind of return could I expect on my investment. Like I said, I am new to the solar world so any information or guidance you can give me would be appreciated

 

[MikeD74T]

Don't use plastic pipe in a covered collector especially if flow can stop on a sunny day. In Texas you'll easily make steam with that setup, I've done so in New Hampshire. Run your system with a circulator pump, circulating from the bottom of your tank thru the collector and back to the top. You may even want to upgrade to an 80 gallon tank. I guarantee you will get more hot water than you want. The only thing I'd change in your drawing is to run the pipes vertically from a horizontal header across the bottom to another on top. This allows the water to move slower through the collector and promotes natural circulation. There are inexpensive pump controllers that sense tank & collector temperatures and only circulate when there's heat availible. This keeps the system from sending heat back to the night sky. If your system makes too much heat just cover part of the collector with greenhouse shade cloth. I think solar hot water is the best bang for the buck of any green technology. MikeD74T

 

[SnowRidge]

The National Solar Tour was held last weekend in this area. We looked at an older home that had been retrofitted for solar hot water heating and PV co-generation.

The owners had installed a solar hot water system that used evacuated tube collectors on the roof, a large "water heater" tank that included a heat exchanger, and a 10 gallon drain back tank in the attic. The drain back tank held the circulating heating fluid at night. There was a circulating pump and an automatic controller.

According to the owners, the system provides more hot water than their family of four can use. It was pretty impressive. I think the said pay back was about seven years at their electric rate of just under 8 cents per KWH.

 

[randydupree]

tim,i have a system like this on my home,we averaged a $50 per month savings on our power bill for the last 5 years.

 

[RPK]

Tim,

Check out Solar Components Corporation - window quilts - window insulation You may want to consider a "solar batch heater". It should be a lot easier to make. The company sells all the materials for any solar project you may want to tackle.

 

[Davetech]

You might be interested in my project page where I have built a preheater like you described using inexpensive readily available materials. I use mirrors to reflect the sun on my collection box and it gets quite hot.

Right now I have to manually aim the mirrors, and the system works well, but I am working on automating control of the mirrors.
See the project here: Davetech's projects

It is a non-commercial site, no pop ups or other garbage. I won't try to sell you anything. I'd just like to see more people waste less energy and quit making the energy corporations fat.

 

[Luremaker]

Tim, check out this site for lots of details on building your own solar DHW system of $1000. Gary gives details on building HW solar collectors in copper and pex tubing. He also includes details on building a heat exchanger.

The $1000 Soalr Water Heating System

 

[ampsucker]

i would also add a pressure tank (bladder) for expansion and pressure and temp gauge. i was surprised when i added a cheap pressure gauge from home depot to our new hot water heater. the pressure of our normally 60 psi city water goes well over 100 psi at the water heater just from thermal expansion.

you should also make a provision for draining and bypassing the solar collector if there is any chance of subfreezing temps in your area.

amp

 

[GaryGary]

Hi,
I think that you may have trouble picking up enough heat in the water with just a single pass through the collector.

In full sun, you have about 9 sq meters of area, which will get you 9000 watts of solar in -- if you convert half of that to heating the water (which would be a good efficiency), then with a flow of 3 gpm, the water would be warmed up by about 11F in one pass through the collector. If you have the warm the water up from (say) 60F up to 110F, then the 11F gives you a solar heating fraction of about 20% -- worthwhile, but you could do better.

The 100 sqft of collector you have is a lot, and you could probably get nearly 100% solar fraction if you could incorporate some storage in the system so that the water is being heated whenever the sun is shining on the collector, not just in a single pass through the collector.

You could think about a batch system like these:
Solar Water Heating Projects and Plans
They put the storage right in the collector.

Or, you could try a system like the $1K one mentioned above (this is the system I use on my house, and I like it).

I agree with the other comment that the plastic tube inside a solar collector will quickly become toast when there is no flow through the collector. I've done that experiment

Gary (from BuildItSolar)

 

[Duffster]

Hot Water Heater

If the water is already hot why would you need a heater?