Seeking design input for solar dhw

[Dune]

Scenario: I have an existing electric hot water heater in new condition. I also have a 50 gallon Superstor booster tank with a sidearm off the wood boiler. Next I want to install flat panels on the roof of my own construction. I have two 120 gallon superstor tanks with built in heat exchangers.
I am wondering if I can plumb this for gravity flow, or if I should use a drain back system of some type with a pump.
I think I like the idea of gravity (if possible) with manual drain valves for winter.
I understand that one component of a gravity system is oversized piping.
Any input apreciated.

 

[mikeyny]

For a gravity system the tanks must be higher than the collectors. This works great if you have ground mount panels. gravity systems take longer to heat up so you should use more panels than you would if you were using a pump.

 

[peakbagger]

For most systems you need a pump, unless as previously stated, the tanks are at a higher elevation than the panels. Drain back systems require a much bigger pump and somewhat more sophisticated controls than a pressurized system. A DC pump and solar panel works pretty well plus is a great plus if the power goes out when the sun is shining but the front end cost is a lot higher than a AC pump. If you go with a DC setup make sure you still get a pump controller Art tech in Maine makes one. THe other trick with a DC set up is double the solar panel wattage that the maufacturer recomends as they tend to size them for Florida (usually 20 watts) versus 40 watts up north.

My system is valved up so that I either use the solar for preheating water upstream of a Amtrol hot water maker during the winter, early spring and late fall and direct from the tank during the later spring, summer and early fall. Evacuated tubes tend to make smaller volumes of hotter water so they dont need as much supplemental heating but when its get cloudy for a few days in a row you still need backup.

Make sure you put a "dump loop" to get rid of excess heat in the summer, the panels are a lot more efficent in the summer and boiling the collector fluid is not a good thing. Its extremely important to have with evacuated tubes. One way to cut down on this is to tilt the panels for optimum winter angle but the trade of is that at my latitude (46 degrees) optimum anlge is 61 degrees and that doesnt blend in with most roof pitches.

Invest in a good check valve and have a way of inspecting its function on a yearly basis if you go with a pressurized system . Check valve failure is one of the primary modes of failure on these systems. If the check valve fails open, on a cold night, the fluid will start circulating by thermosiphoning and the end result can be a frozen heat exchanger.

 

[Nofossil]

For those of us with wood boilers (maybe most people her, you might think) a viable option is to use plain water in cheap (not evacuated tube) panels and just drain them when it's frost season. You're probably running your boiler by then anyway. I have really inexpensive panels set up for gravity flow and they work fine. My brother made his own out of 1/2" copper and flashing. They work fine also. You can make them out of PEX and flashing too. There's enough sun in the summer so that a few extra square feet of cheap panel gives you more than enough extra heat to make up for less than perfect efficiency.

If you want solar hot water in the winter, then you need more expensive panels - either glycol or evacuated tubes. Me - I just use my boiler in the winter. It would take a REALLY expensive solar panel to give me as many BTUs on a January day in Vermont as I get from a couple of logs in the boiler.

 

[Dune]

Thanks for the info so far.
To clarify, I am building my own panels, they will NOT be evacuated tubes.
To me; the simplest system is the best. This (other than funtionality) is my highest priority.
The panels are going on the roof, the tanks in the basement, so gravity is off the table.
My wood boiler will supply hot water for half the year. The solar is just for the hot months.

Further questions:

1. Since I will be using a pump, what pump is recomended? I have a family of four (two teenagers).

2. Can I use pex tubing between the collectors and the tanks? I happen to have enough 1/2" water pex to do the job.

3. Other than a check valve and ball valves for isolating the system and drain valves, what other components might I need? I already have a large mixing valve to temper the water from the wood boiler.

4. Will I still need a dump zone and if yes, what should that entail? Can I use the existing heat dump from my wood boiler ( a small modine unit with a fan)?

Thanks again.

 

[Dune]

Heading out of town till friday, apreciate any help.

 

[in hot water]

How many collectors and how much total storage? For larger arrays, that you intend to use for heating loads, I prefer drainback. With a drainback system you get overheat and freeze protection with just plain water in the collectors. no need to use expensive, messy glycols.

Drainback may or may not need a larger pump than a closed loop glycol. It depends on the lift head from the top of the tank to the top of collector. Often a Grundfos 15-58 has enough lift for single story homes.

You can use a dual pump system, drop one pump off after the system fills. I use a 15-58, piped in series with a Grundfos Alpha on mine. After filling the Alpha runs 4- 4X8 collectors with 17W of power!

Most of the new solar controllers have drainback functions. You chose either a dual pump system, with one dropping off, or one larger pump, sized to the lift, running on variable speed function. The speed changes as the delta T between the collector and tank increases. If your lift is 20- 32 feet the Grundfos 15-100 is an ideal pump.

You don't need a separate tank to do a drainback, and the drainback fluid can also be the heating fluid. I hope to have a 300 gallon LP tank mounted vertically before this heating season to cover my DHW and radiant with solar.

Look at examples on pages 26-38 in this Idronics 6.


www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_6_us.pdf

Go with a pressurized drainback and a sealed tank.

hr

 

[benjamin]

First, read through Nofossil's site, there's a lot of good information there.

I use a grundfos 25-64 with 1" pex run up and back from about 260sf of old and abused flat plate panels, it provides space heating and heat storage until I add a bigger drainback tank with a 3/4" copper coil inside for DHW. This drainback system with plain water works fine through the winter, but with a boiler it might as well be shut off, there's just not that much solar energy available in winter.

I'll take a stab at the questions

1 the pump depends on the head and flow you want to achieve, which depends on whether you go with glycol, drainback (pump shuts off and water drains back into tank), or draindown (freeze detector goes off, valve opens, water drains out of system). Glycol is thicker, harder to pump, and a drainback system has a higher initial head to overcome.

2 I say go with pex for a water system, maybe not if you're useing glycol, if the water overheats it turns to steam and vents, if glycol overheats it turns acid, eats your copper and freezes, bursting your copper if the high temp didn't cause the pex to leak first. The pros wouldn't recommend pex. 1/2" is most likely too small to move the amount of water you need, as the collectors will work best with a low temp difference. 1/2" is what doitsolar.com uses in some collector designs.

3 the components you need depends again on what system you go with, a differential temp controller is nice, but my drainback pump runs on an old honeywell aquastat set at about 90degrees with the bulb in the top of one panel, no isolation valves, one drain valve.

4 if you use water no dump zone is needed. most likely the panels will only overheat if there's no circulation, so there's no reason to have a dump zone unless your two 120 gallon tanks get hot enough to to vent and that causes a problem, if that happens let us know how you built your panels please!


If you're building the panels from the ground up I'd use 1/2" pex for the collector piping, 1" pex for the supply and return, a small pump Taco 007 or so, and sleep well enough that even if you forget to drain it in September, the chance of any damage to frozen pex is pretty slight.

If you're using copper collectors, then I'd have to recommend using a drainback system that is very carefully designed not to allow water to remain in the collectors, or anywhere a freeze-leak would have bad consequences.

Let us know what your collector and other system plans are.