An Inexpensive Solar Drainback Outbuilding Heating System

Dave Congour - Colorado Clean Energy Systems, LLC

Revised 4/7/09

I own a small solar and radiant heating business in Western Colorado. I recently converted my garage into an insulated workspace for the business, but was getting tired of working in below and near freezing temperatures in the winter. I had a couple of panels left over from an orphaned system, and a new 280 gallon solar storage tank that I had fabricated for testing as a prototype for my business. I decided to put in a solar heating system to take the edge off of the cold. Though I knew that there would be days when the solar would simply not keep up (cold, cloudy days), I thought that it would work well enough for my needs.

With the onset of winter, as well as to save money, I decided to opt for a simple tank top radiator (actually a convector, difference discussed later in this paper) instead of a radiant floor heating system.

One rule of thumb that is used in solar collection is to use approximately 1-1/2 to 2 gallons of water heat storage per square foot of collector aperture. Though this is a good rule, it's perfectly fine to bend it under certain circumstances. I only had room for (and only had) two 4 x8 panels, or 64 sft of aperture. I also had a 280 gallon tank, leading to a ratio of about 4-1/2 gallons per sft of aperture! However, I decided to go ahead and try it anyway. First of all, I wasn't going to use the tank to heat domestic hot water, but only to heat the garage space, so the temperature requirement wasn't as high (DHW realistically requires at least 130 degrees). Also, I knew that the efficiency of solar heat collection was a function of both the ambient outdoor temperature (warmer the better), and the temperature of the fluid that you circulate through the collector (cooler the better). Obviously, I have no control of the former, but I could control the latter by oversizing the storage tank water volume!

Sending cooler water up to the collectors increases efficiency in two ways: first, there is less heat to “re-radiate” from the supply/return tubing (even with insulation), and also from the panel itself. Second, the cooler water is able to absorb more heat per unit area from the panel absorber plate because of the larger heat differential.

Having a large storage volume implied that the storage tank temperature would not be as high at the end of the day; but it also implied that I would increase collection efficiency by circulating lower temperature water through the collectors. Also, though the temperature of water I could deliver to the garage space wouldn't be as high, there would be a much greater fluid volume, resulting in a much greater total heat carrying capacity. In other words, the temperature would be lower, but the total amount of BTU's collected and stored in the tank would be higher than if I had used a smaller tank. The smaller tank would allow a shorter “blast” of higher heat to the convector, but the larger tank would have more BTUs in storage, as well as a greater “thermal flywheel” effect. The flywheel effect would allow (I reasoned) the tank to carry over an extra day or so when the sun didn't shine.

For solar collection, I used a drainback system with the large “atmospheric” (non-pressurized, open to outside air) tank. By the way, an “open” system means that the internal plumbing components (such as the circulator, tubing, tank, etc) are in constant contact with air, which would corrode any ferrous components. Consequently, the circulator had to be a more expensive bronze version. A “closed”system, by contrast, is a pressurized system from which nearly all air has been either expelled, or has been used up (passivated) in initial minor corrosion of ferrous components. Glycol based systems are usually “closed” pressurized systems, and can thereby use ferrous components (such as cast iron circulators), which are much cheaper.

Anyway, in the drainback approach to solar heat collection, pure water is pumped up to the panels whenever they are hotter than the storage tank. The water is heated about 10-20 degrees on each pass through the panels, and then falls back into the bulk storage in the tank. Though I live at 6000', and we experience below zero temperatures frequently, I was not worried about freezing, as I'm am careful about keeping adequate downward slope on the supply/return tubing. I have seen my other (DHW) drainback system come on and collect heat on a sunny morning when the outdoor temperature was 8 degrees F! The only freeze danger occurs if a sensor/control failure causes the solar pump to turn on at night when temps are well below freezing. A separate circuit with a snap freeze sensor can be used to disable the pump when internal panel temperature is below freezing, but I didn't bother in this case, as my panels were already old, I had a reliable controller, and I was on a budget.

As mentioned earlier, I wanted to build a convector to draw heat slowly from the tank to heat the shop. As the name implies, a convector heats up the surrounding air, causing it to rise and spread through the space being heated. Most people are more familiar with the term “radiator” which implies heating by radiating infrared energy through the air until it impinges on the object to be heated. Actually, standard building radiators (the old, clunky, but beautiful cast iron ones that you may remember from childhood) also work mainly through convection, but anyway, on with the story...

In order to make a simpler, cheaper convector, I did not want to use a circulator. Instead, I opted to use convective thermosiphon flow to pull heat out of the tank. If you look at the picture, you can see that there is a triangular shaped “loop”. The loop pulls hot water out of the topmost (hottest, least dense) part of the tank on the left side. The hot water rises through the insulated vertical section to the top of the loop. When it reaches the top, it starts to release heat as it cools. As it cools, the water becomes more dense, slides down the angled section of “fin tube” type heating elements, releasing heat as it goes, then slips back into the tank on the right side. The right hand tube leads to the bottom (cooler, denser) part of the tank.

I put cutoff valves on left and right sides of the loop to aid in “charging” the loop with water; along with a hose bib (foreground) for charging/expelling air. A coin vent, invisible in the picture, but on the highest point of the loop, is used to expel air when charging the loop. Note that the loop is actually a siphon loop, and so needs to have all air expelled from it. Either of the valves can also be used to stop the thermosiphon flow, thus “turning off” the convector. I insulated the pipe on the “hot riser” side, in order to enhance the thermosiphon flow rate. I didn't want the fluid cooling off until it reached the top of the riser, or it would become denser too quickly, thus slowing the flow. Indeed, when I first opened the valve to flow (with tank top water about 125 degrees) it only took ten seconds for the tubing at the top of the convector to get hot to the touch! By the way, I also insulated the cool return riser tube inside the tank, to aid thermosiphon flow, but that was more of a technical challenge, since the insulation is immersed in hot water.

In constructing the siphon loop, I had to be absolutely certain that there were NO paths for air to leak into the loop, as an “air break” in the loop would stop thermosiphon flow! As such, I thought that it would be necessary every so often to recharge the system, and drive air out the coin vent. Being an open system, air is entrained in the water, and may collect in the lowest pressure point at the top of the siphon loop, which is at negative pressure with respect to atmospheric. It should be easy enough to know if this happens, however, as the unit will no longer feel hot to the touch. (Note from a few weeks later- The loop did have to be flushed of air. I had the loop intake positioned too close to the drainback return outlet from the panels, which splashed small bubbles which were then sucked into the siphon loop, eventually stopping the thermosiphon flow. I extended the siphon input 6” lower, and put a baffle around the intake, which solved the problem).

By the way, there is another siphon loop in the system; the one on the drainback tank. In this loop, cooler tank bottom water is pumped from the tank bottom, up and over the top edge of the tank, and down through the circulator located on the side of the tank. In the case of this loop, the siphon is “flushed out” every time the circulator is turned off, and water “drains back” in a rush from the intake side of the collectors. As the water rushes back down, it flushes out any air bubbles that may have become trapped at the high point of the siphon loop. This is not the case in the convector thermosiphon loop; any large air bubbles that get stuck there will remain there until flushed out with the garden hose!

Finally, with respect to the convector, note that it is “too small”! I wanted to test the concept before building something larger. As noted earlier, when the thermosiphon is “turned on” by opening the valves, it only takes 10 seconds for the loop to heat up. This shows that thermosiphon action (whether planned for or not) can be quite significant!

Now, on to system performance. I finished the system in mid February, and took twice daily readings of the outdoor temp, indoor shop temp, collector, and upper/lower storage tank temperatures. The range of end-of-day tank top temperatures during this period was 102 – 155 degrees F (depending on sunshine strength). These temps were higher than I had expected. Had I installed an “active” (pumped) heat draw system, such as a radiant floor, rather than the “passive” thermosiphon loop, the tank temperatures at any given time would have been lower due to more aggressive heat drawdown.

Early morning outdoor temperatures ranged from the 0’s to the low twenties, but my shop temperature never went below the mid 50’s to the upper 60’s from mid February thru the end of March. Note that there was some (but very little) heat drawn from the nearby mechanical room, through the crack under the wooden door. The warmth in the shop was a welcome change from last winter, when the temperature was almost always below freezing during the same period!

Finally, I used a tank that I built for my business; it is very hard to build “shop built” tanks, though possible if you can work with concrete... A better alternative may be to find a surplus stainless steel agricultural or restaurant tank. They range from very expensive to downright affordable if you search hard enough; google “used stainless steel tanks”. In the case of such a tank, you would not even need to build the convector; just leave a “window” in the insulation to “leak” heat out of the tank and into the space to be heated! It’s amazing how much heat can be gained from a large, hot tank which is just sitting in a room. Now there’s a recipe for a simple heating system!

Components used in solar shop heater:

two salvaged 4x8' Sun Earth collectors

CCE 280 gallon atmospheric (non-pressurized) solar storage tank

Caleffi iSolar controller

Blue-White variable area flowmeter

Wilo Star bronze wet rotor circulator

home fabricated channel strut panel racking

3/4” type “M” hard copper tubing

Armacell 7/8” (for 3/4” tubing) x 1” (thick) pipe insulation

shop fabricated convective radiator