Evolution of a solar combisystem
How my own system has morphed over the years.
My wife, Joyce, and I built our house in 1980. It was a long process because we did much of the work ourselves. Anyone who has ever lived in a house as it is being constructed can tell you it makes for some challenging experiences.
You get used to living with dust, rain tarps, temporary lighting, and the smell of various adhesives and paint. You learn how to wash dishes in the bathroom sink, because it’s the only source of running water at the time.
Joyce envisioned our house as a nest in which to raise children, decorate with curtains and cook great meals. I saw it as my personal laboratory for experiments in home energy systems, including superinsulation, air-to-air heat recovery, wood burning, solar and wind energy harvesting and waste heat recovery.
Being the gracious and supportive wife she is, Joyce indulged me through many years of such experiments. She would call me when she heard a “funny noise” coming from the mechanical room, and occasionally reminded me to write a manual on how the system operates in case we ever sold the house. That manual still is on the to-do list.
The starting point
Although our primary means of heating the house has changed many times over the last 30 years, the fundamental objectives of energy conservation, gathering “free” energy and using hydronic heat delivery have always been part of the mix.
Our original means of heating the home in the snowy, 8,000°-day climate of upstate New York, included three subsystems:Passive solar gain through 360 sq. ft. of south-facing windows;
112 sq. ft. of Revere flat-plate solar collectors; and
- An airtight wood stove.
The last was for “supplemental” heat, just in case the sun didn’t shine as brightly as I expected it would as soon as that solar hardware was installed.
A schematic of our 1980 vintage solar “combisystem” is shown in Figure 1.
During this time, I worked as a technical support engineer for Revere Solar & Architectural Products in Rome, N.Y. It went without saying that our house would have some of the collectors Revere produced. I settled on six 35-in.-by-77-in. Sun-Aid collectors with specially sloped internal headers. They were installed in a recessed area formed using special roof trusses. I figured this mounting would help shelter the collectors from the prevailing northwesterly winds and thus reduce their thermal losses. The collectors face directly south, and are sloped 60º from the horizontal. Figure 2 shows what they look like about halfway through a typical upstate New York winter.
The collectors send heat to a 12-ft.-tall, 350-gallon storage tank. I had this tank custom-made for the project. Its height would encourage temperature stratification (e.g. hottest water collects at top, while cool water settles to bottom). This is always desirable in solar thermal systems. It allows the coolest water to be supplied to the collectors for the best possible thermal efficiency.
It quickly became apparent that a 12-ft.-tall tank is not the easiest thing to install in a house with 8-ft. ceilings. We had to build a part of the house around the tank. I’ve concluded that it’s the next generation’s responsibility to figure out how to remove this tank if ever necessary. It was definitely not the best planning on my part. Fortunately, nature has been kind so far. The tank continues to serve as the system’s anchor component without issues.
My system was designed as a closed-loop drainback design. I felt this approach held many advantages at the time, and still believe it’s the best approach for solar combisystems that provide space heating and domestic hot water. Two 1/25-horsepower circulators bolted flange-to-flange (series configuration) provide sufficient lift to push water up through the collector array. When they turn off, the water drains back to the tank in about 30 seconds. This system has been through many winter nights with temperatures of –20ºF or lower, and the drainback operation has never failed me.
The air space at the top of the tank provides a drainback reservoir and serves as the expansion volume for the entire system.
We suspended two 60-ft. coils of ½-in. copper tubing, piped in parallel, from the access plate that bolts to the opening at the top of the tank. These coils heat domestic water whenever hot water is drawn from a fixture. Over the years I’ve been very pleased with the performance of this homemade heat exchanger. I’ve never had to pull the plate off the top of the tank and don’t plan to as long as these suspended coils keep working.
When the tank is warm, space heating is provided by circulating water through copper tubing embedded in a thin masonry layer installed over the standard subfloor. At the time, we could buy 100-ft. coils of 3/8-in. type L copper water tube, which we used to fashion the floor circuits. PEX tubing was still on the other side of the pond at this time. Everything was soldered together, and still is. We installed a manually adjusted mixing valve in case the water in the tank was too warm to go directly to the floor circuits.
Living with the results
During out first winter in the house, we would wake up to 55ºF inside air temperatures following a sub-zero night. If the day looked cloudy, we kindled the wood stove and waited a couple hours for things to warm up. On clear winter days, we would just wait for the sun to work its magic. By mid-afternoon the inside temperature would climb to around 85ºF. We would start our day wearing multiple sweaters, and by afternoon be down to T-shirts.
Back then, when asked about the performance of our solar house, I was inclined to say it maintained an “average” interior temperature of 70ºF on a clear and cold winter day. This was a mathematically true statement (55ºF in morning, and 85ºF in the afternoon). However, I now admit that statement was a bit misleading in terms of inferred comfort. Today, I would describe those early winters as a ride on a temperature roller coaster that few occupants other than solar diehards would tolerate.
By the late 1980s Joyce and I were ready to add an automatic heat source to the system. At that time, our local utility was offering very enticing rebates to seed the market for geothermal heat pump systems. We took advantage of this program, and in 1987 I installed a small 18,000 Btu/hr.-rated water-to-water geothermal heat pump.
Because the heat pump involved flow of both warm and cool water, I decided it also was a good opportunity for further experimentation. The system was modified to the status shown in Figure 3.
The solar portion of the system remained essentially unchanged. The water-to-water heat pump was piped to the existing solar storage tank connection points (we didn’t have the option of punching new holes in the tank for more connections). Heated water leaving the heat pump went to the top of the tank, while cool water was drawn from the bottom. This helped preserve temperature stratification within the tank. I used a simple capillary-tube setpoint controller to turn the heat pump on and off as necessary to maintain the temperature at the top of the tank about 115ºF.
Heat recovery ventilation
Water leaving the heat pump’s evaporator would drop down to about 35ºF by late winter. That’s cold enough to suck heat out of quite a few things including an air stream on its way out of the house. To recover some heat from this air, I added a small 12,000 Btu/hr.-rated air handler with a drip pan, and shunted some of the chilled water leaving the heat pump through its coil (see Figure 3).
Air entering this air handler came from two sources: the clothes dryer and a high wall exhaust in the bathroom. Both exhaust streams contained sensible and latent heat. When run past the air handler coil, they transferred some of this heat to the chilled water headed back to the earth loop. The heat exchange was such that I never had to add antifreeze to the earth loop. We installed a polypropylene grid filter upstream of the air handler to keep lint from the dryer exhaust from collecting on the coil. It did a great job and got cleaned weekly. The heat recovery ventilation system was set to run whenever the heat pump operated or when a humidistat in the house called for ventilation.
Time to chill out
Occasionally it gets uncomfortably warm and humid in upstate New York. Where we live, this only happens five to 10 days each summer. Knowing that the heat pump could produce chilled water, I just couldn’t resist expanding the system for chilled water cooling. This time we added a small ceiling-mounted cabinet-style air handler as seen in Figure 4.
This setup did a great job of keeping us comfortable on those sticky days. With 38ºF water passing through a three-row coil, the air handler was phenomenal at reducing humidity. With a cooling capacity of about 12,000 Btu/hr., the unit ran almost continually on hot days. That’s good from the standpoint of minimal compressor cycling and lots of air recirculation through the coil for moisture removal.
About 11 years ago we finally got around to insulating and finishing our garage. Snow and ice get dragged into this garage almost every time the cars come back from a winter outing. Tired of getting into ice-cold cars covered with layers of dirty snow, we decided to partially heat the garage. I’m sure this would have been out of the question based on my 1980 philosophy of energy conservation, but age, ice choppers and ankle-deep slush as you step out of the car tend to “mellow” one’s convictions over time.
We jack-hammered out the old slab, installed floor heating circuits and poured a new slab. The only remaining issue was where the additional heat for the garage was going to come from. The heat pump only had enough capacity to heat the house.
At the time, No. 2 fuel oil cost about $1 per gallon. At that price, the heat produced by a standard oil-fired boiler would cost us less than that provided by the ground source heat pump. This tipped the scales in favor of installing an oil-fired cast-iron boiler to heat the garage, the house and provide supplemental heat for DHW. The heat pump was moved to accommodate the boiler, but it was reconnected to the original chilled water air handler and still works fine for summer cooling.
The ventilation air handler eventually succumbed to corrosion and was not designed so that the drip pan could be replaced. Without the heat pump operating during winter, the potential for heat recovery was greatly reduced, so I decided to remove this part of the system. The ventilation air and dryer exhaust now go straight outside. Watching steam from the dryer exhaust dissipate into cold winter air probably would have driven me crazy 30 years ago. Now I view it as an acceptable compromise and hope the “green police” will go easy on me when they spot it.
Figure 5 shows how our heating/cooling system was modified about 11 years ago. At the time, injection mixing and primary/secondary piping were the cutting-edge technologies, so naturally they had to be worked into the mix. You’ll notice there’s an extra set of closely spaced tees on the primary loop waiting for some future experiments.
This brings us up to the summer of 2011. Those Revere collectors had reached a ripe old age of 30 years. They lasted a full 10 years longer than their anticipated design life. Although they still were working this past summer, I could tell they just didn’t produce like they used to.
It just so happened that Caleffi North America was looking for a first installation site for its new patent-pending StarMax V drainable flat-plate collectors. Figure 6 shows how this collector is designed.
Its absorber plate has slightly sloping internal headers that form a very shallow V shape. Both ends of the upper and lower headers terminate just outside the anodized aluminum enclosure with union-type piping connections. These are the connections one would use if connecting the collectors for a standard antifreeze-type system. The unique feature is found at the bottom center of the collector. The low point of the bottom header is configured as a tee to allow all fluid to drain out of the absorber plate.
Thus, when used in a drainback system, these collectors would have an external lower header in combination with an internal upper header. It’s a simple and unique idea that eliminates the need to side-slope the collectors and perfect for the retrofit situation I had.
Knowing my interest in these collectors, my good friend Bob “Hot Rod” Rohr, Caleffi’s director of training and the inventor of this collector, made me a great offer. He would personally come out and help me install the new collectors. That was an offer I couldn’t pass up. So last June, with help from Hot Rod and Joyce, we performed a collector transplant. The trusty but tired Revere Sun-Aids came down, and those sparkling StarMax V’s took their place. They fit beautifully within the same recessed roof cavity as seen in Figure 7.
The new collectors increased the area of the overall array by 42%, and so far they have performed every bit as good as the Revere collectors did in their youth. We’ve seen storage tank temperatures near 170ºF on sunny fall days.
At present the system is still controlled by the original Independent Energy C-100 solar controller. That controller is still functioning despite a power surge that cooked its innards and forced me to grab a soldering iron for a transformer transplant. The original radiant floor is still working well. I wish I could say the same about our original refrigerator, washing machine, dryer, well pump and range. Few appliances can ever hope to outlast a quality hydronic heating system.
Would I do things differently given the chance to start from scratch? Absolutely! Was the evolution of this system a great learning experience that improves what I do on other designs? Without a doubt. In a professional sense, nothing beats “quality time” watching, feeling and listening to a system operate. This experience is guaranteed to teach you things you won’t find in any textbook or installation manual. The lessons I learned from my own system proved invaluable for future design work and even some occasional writing assignments.
Every designer of solar thermal systems should have the opportunity to live with a system he can modify over time to test new ideas and verify how they work before incorporating them into other systems. I was especially fortunate to have my experimental platform right at home. I also was fortunate to have a wife who supported me through the evolution of our system.
Now if I could just get started on that system operating manual she requested.