Drainback or closed loop..........

[WoodMann]

. system? I understand with a solar DHW system there are losses with a heat exchanger, and that it's size is crucial to handing off heat the best. However- with a drainback system, my big reservation is containing the water that drains back and if the system turns on and of repatedly on a day then that water will add up. Is there some way to pipe it back into the intake side of the water heater so I don't conciously have to go empty out the water, or something similair? And just how big of a water heater can I get away with on 2 collectors on a drainback system? I'm pretty sure that 30 gal. is tops with a closed/ pressurized system................

 

[WoodMann]

Thanks Car. All great ideas, but reclaiming the drainback water is what I'm after.............

 

[Rhone]

Well, there is no limit to how much water you heat with either system, and no one is dumping water usually unless they want to.

A drainback system usually has a little drainback tank on top of the actual tank itself to hold the fluid it sends to the panels, maybe in New Mexico they will let you pump the actual water you use through the system (which is more efficient but you better make sure pitch is maintained). They require a high-head pump (or multiple pumps in series). With the mini-tank on top for the solar loop fluid it can't overfill. If you want to send the actual water you plan on using, you have a fill shut-off valve to prevent overflowing in the tank. For example, get a 120 gallon tank and put a float in there to prevent the tank from filling beyond 110 gallons. When your solar loop kicks on (lets say the loop has 5 gallons) your tank will replenish back to 110 but then won't add anymore cause your float will close. It will keep cycling the water of the tank up to the panels and back (your water level remains constant). At night, the 5 gallons drain back filling the tank to 115 gallons... you're good since it can hold 120. It won't add any water unless the level drops below 110 gallons. In either case, neither should overflow, and both can heat an unlimited amount of water.

A closed loop system is more reliable but takes a slight hit on efficiency. It too heats any amount of water, you can either get a tank with a built in heat exchanger or get an external one. I have three 4x8 FP's in a closed system at 20psi and a 120 gallon tank with a wrap around heat exchanger and included a picture of the temps I'm getting in Northern New England in February. In New England it's recommend to get bigger storage and more panels cause half the days here are cloudy and winter is pretty cold so when the sun shines you need to quickly heat a lot of water cause you may not see it for a few more days. In dead winter my setup handles a 1:1 ratio (one sunny day I collect enough energy to buffer me through 1 cloudy), in spring/fall one sunny day will heat the water enough for the next 2 cloudy days) 2 sunny days in a row in spring/fall will heat the water enough to buffer the next 3 cloudy days. Summer I have to sometimes waste hot water, my system has safety measures to prevent overheating but sounds like a jet engine when it does it so I try to prevent that myself. Here's a picture I took of my temps back in February (end of 2 sunny days/row in Northern New England), the solar fluid was 151.3F and the bottom number is the temperature of the water inside my 120 gallon tank at the very bottom so the whole tank is at least 140.1F). Enough to buffer hot water needs for 3 cloudy days. Last week my solar loop was 180F and heated my tank to 158F... just 22F short of it's max and it's not even Spring yet. So, be careful in New Mexico (or get yourself a lot of storage).

If anything, I wish I had more than 120 gallons of storage.

 

[WoodMann]

THat's one of my other reservations with the open loop system, freeze protection. Had this happen on 6 of my dad's 20 collectors for his 10,000 gallon pool. Evidently the sun came out for a breif period- I was too young to care at this point; and shut off again and the system wasn't evacuated completely and well, some of the panels tubing burst......................

Edit; BTW I currently have a 60gal water heater, pretty new, too................

 

[Rhone]

60 gallons is pretty small, I haven't lived in NM but figure in summer you'd just cover one panel and in winter open it up. You can link multiple tanks together for storage but in NM I'm guessing you don't need that.

I just don't know how much you're thinking about paying, prices vary all over. If you want it done right & permanent you'd use copper piping with high temp insulation and drinkable glycol designed to be used for heat transfer (which elminates the $5/gallon RV glycol) with an external heat exchanger and get a solar controller. Much of that can be purchased off e-bay used or new.

Or, you can simply get a black garden hose coiled on your roof and a pump you turn on/off yourself. Take it in when it's going to freeze, and even if it does... cut out the damaged section and splice back together.