Ok, I know everyone says not to do drainback in cold climates, but it seems to me if you have a temp sensor on the array,
and your controller knows not to turn the pump on until the sensor says it's hot, then there should be enough thermal mass
in your room/basement temp water to not flash freeze until after it gets to the array and starts picking up heat. 

Am I wrong here?

The $1000 Soalr Water Heating System This guy is a retired engineer that has a drainback system that has worked flawlessly for several years now.  And he lives in Montana...not exactly warm country. Check it out...I'm going to copy it soon as I get back to my place in NW Mt. 

We have done them for over 30 years in Maine.
Insulate the lines, slope back toward the tank, use PEX or copper, vent the return line and let it go.
Have had very few problems.

Tom
www.americansolartechnics.com

Do you use a separate vacuum breaker on the return? Or is it sufficient to have the return into the tank be above the level of the water in the tank? I've added a vacuum breaker, but it seem excessive and it's noisy because it opens frequently.

We have used a vacuum breaker on the return line, but usually installed a tee with a piece of pipe that was plumbed with a vertical stub about 12" up as an air vent. The water did not escape from this since it is headed downhill back into the tank. It looks a little crude, but you can hear the return water gurgle a little and immediately know it is working. And there is no vacuum breaker to scale up and fail.

I like the noise. It is usually quieter than the vacuum breaker.
Tom
www.americansolartechnics.com

The guy in Montana says that he has had the system up for one winter. That particular winter was quite mild, with above average temperatures. It only takes ONE incident to damage a system and I have personally dealt with flash freezing in collectors that were configured with a drain back delivery. This happens during sunny, windy, below zero weather. We think that the sensors, located to maximize heat collection, were too well insulated from prevailing outside conditions. By the time that the sensor felt the temp change, the furthest collector has begun to freeze up in the outside riser tube.
This was an extreme weather event, but those of us that have spent our lives in Montana understand that these events will happen. Look at redundancy in controls or include propylene glycol in your heat transfer fluid.

I'm thinking this is a good application for a redundant sensor.

Redundancy is a good thing a good thing. Secondary protection (glycol) will also prevent those unlikely but possible disasters. Not every location is a candidate for this approach, but you can look at your climates history and find the possibility of extreme weather events. Florida solar water heating was in a hey day during the early 1900's and a lack of freeze protection caused irreparable damage to many thousands of roof top solar water heaters. "What ever it is, it just has to happen once."

Unfortunately, glycol brings with it a bunch of other issues to solve too, it's not the holy grail cure-all.

It is not intended as a 'cure all', but a mixture that lowered the transfer fluid freezing temperature to 20 F., combined with careful sensor placement, was effective in this climate (Southwest Montana). Be sure to test your collector loop transfer fluid on a regular basis if you are using glycol based solution.

The two systems are not interchangeable.  If you want/need to use glycol then you need a closed loop through the collectors.  It must be a closed loop because glycol will degrade rapidly in the presence of oxygen and produce corrosive acids.  Just adding a little glycol to an open system will eat though the copper of the collectors.  You can do a drainback with glycol, but it still has to be closed.  You could now either have a completely closed system, including the all hydronic piping, or, more typically, you need a heat exchanger to heat the water in a storage tank, that is, the water that you will ultimately run through the hydronic system.

You can design a drainback system without glycol as an open system and use the same water to run through the hydronic piping.  This makes the system much simpler, less pumps, no heat exchanger, etc. BUT, as has been pointed out you need to drain the system before it freezes.

Gary Reysa the "BuildItSolar" guy, has a drainback solar heating system that's been running for couple of years now in Montana.  Full details here:

Solar Shed

There's no right/wrong, but the systems are fundamentally different and adding glycol adds complexity as well as security.

Just because a system is a drainback system does not mean that the solar collector side loop is an open loop, or even exposed to the atmosphere. A drainback solar loop with a heat exchanger transfer to the storage tank could contain, say, 15 gallons of liquid and 10 gallons of air. It strikes me that the oxygen in those 10 gallons of air in that closed loop is not going to eat through or degrade the 5 gallons of glycol in the 33% glycol mixture.

Jeff

You're describing a closed system. I've long since forgotten just about everything I knew about chemistry. The question is what's the final pH of the solution and what damage that would do to the copper. My recollection is that copper is not very reactive in an acid solution and I've seen others recommend the drainback system with Glycol that you propose, so I would suspect that there isn't a problem.

However, the point still stands that you need a closed system and a heat exchanger. An open water drainback doesn't need these and can work with a single loop. That doesn't make it better since now it can freeze if the vacuum breaker fails (I do like the idea of a vertical chimney vent which avoids this problem) or for other reasons.

I have seen the system you suggest proposed as a pure water system. It is supposedly more reliable than an open system, no vacuum breaker to fail, no problems caused by the collection system, etc. You could also use iron/ferrous components in either of the closed systems.

Avoiding the extra heat exchanger and extra pumps is worth it in my opinion. Every exchanger is a loss in efficiency. Every pump adds to electric cost which lowers system efficiency. Smart controls are a must.

What you save in energy costs you may sacrifice in lower collector efficiency when the liquid returning to the panels is not as cool as it might be with "the extra heat exchanger" or the double-pumped system. For example, if your "extra pump" is on 5 hours a day, and uses 1 amp of power, it'll cost you about 8 cents / day, or about $30 per year. Is that more or less than the value of the extra heat that you'd collect?

I think that's backwards. The inefficiencies introduced by the heat exchanger will mean that the water returning to the panels will be warmer that it would be with an open drainback system.

In the open drainback, you take the coldest water from the bottom of the storage tank and run it through the panels. This is the most efficient way to run the panels. With the heat exchanger, you take the coldest water from the storage tank and run it through the heat exchanger. Since the heat exchanger is not 100% efficient, the water leaving the heat exchanger to return to the panels will be warmer than the water in the bottom of the tank (you cannot cool it to the same temperature or lower than the temperature of the incoming water). Thus, the water returning to the panels will be warmer than the water in the bottom of the tank and so the panels are working less efficiently.

So, with the heat exchanger you have the extra energy used to run the second loop AND a less efficient system.

I'm not sure that in any practical way these inefficiencies really matter. Design the system according to your particular needs.

I have considered running the pump on solar energy. While the amount of energy used by the pump is not significant, it would provide a freeze failsafe system. Since one of the causes of panels freezing is a failure of the control system or the sensors, if the pump itself is solar powered it could absolutely not be running at any time when there is insufficient sunlight to heat the panels. It might also provide more flow when there is more sunlight which would improve the operation of the panels and it would avoid the stop/start cycles at the beginning of the day when the water first enters the panels and cools them enough to shut off the pump.

You are correct; I misspoke.

I am in Michigan and have installed them here but the one BIG mistake I see self installers make is they don't slope the piping enough. Also the array needs to be sloped enough so as to be sure they drain completely when the pumps turn off.

I am interested in where the stonecaveman is located. Montana has seen some of the most extreme temperature change events in the lower 48 states. Mr. Reysa may have a couple of winters of use, but we have not had an EVENT for several years. At Great Falls, in 1980, the temperature rose 47 degrees in seven minutes(-32 to15). A temperature of -70 was recorded north of Helena in 1954. My point is that freeze control is a very big issue in some locations. If normal conditions are used when designing systems and controls, it is possible to be caught with slush in your riser tubes.
Repairing or replacing collectors can eliminate any cost savings that you may accumulated by going solar. IF you live in an area that can encounter these extreme weather events, it pays to add the redundant freeze sensor and glycol. By the way, the drainback systems that used the glycol mixture did not corrode away. Slope and venting are primary concerns for drainback and drain down systems. Be sure to check your response times and perform regular maintainence on these systems, as you would any other mechanical system.

I don't think that there's any dispute here. I stand by my earlier comment:

If you live in an area that freezes, design your system for the amount and type of freeze that you expect.  For example, if you're expecting -70°F temperatures, you'd better have a system that drains even if you add glycol (60% propylene glycol freezes at -60°F).

I still wouldn't recommend adding propylene glycol to an open system.  Again from Wikipedia

Propylene glycol oxidizes when exposed to air and heat. When this occurs lactic acid is formed.^[2][3] If not properly inhibited, this fluid can be very corrosive.^{[citation needed]} Protodin is added to propylene glycol to act as a buffer, preventing low pH attack on the system metals. It forms a protective skin inside the tank and pipelines which helps to prevent acid attack that cause corrosion.

Besides cooling system breakdown, biological fouling also occurs. Once bacterial slime starts, the corrosion rate of the system increases. In systems where a glycol solution is maintained on a continuous basis, regular monitoring of freeze protection, pH, specific gravity, inhibitor level, color and biological contamination should be checked routinely.

Propylene glycol should be replaced when it turns reddish in color.

I don't install/maintain systems (other than my own) so experience may triumph, but I'd suggest from the chemistry that you only use glycol in a closed system.

I live high (7200ft) in the mountains in New Mexico.  We do get major temperature swings, although not as dramatic as the one Dteltech describes and it's not as consistently cold as Montana. 

New Mexico is a great state for solar.  At our elevation, we routinely get lows in the low teens (the HVAC design temperature here is 6°F) however we rarely get days that are much below freezing because it's sunny.  (Average low in Jan. is ~19°F and the high ~40°F).  This means that we can get a significant percentage of our heating needs from Solar and it's relatively inexpensive compared with, say PV.  We get ~10,000 BTU/sq. ft/day -  ~5 KWh/m2/day.

We're just bringing the solar panels back on line because we had problems last year not with quick-freeze but because of inadequate venting.  I wish I'd caught that earlier in the design cycle..