Bob Ramlow answers student questions about solar thermal. He tackles hard water, PV powered solar thermal systems, and best practices for air vent installations.

  1. Is hard water an issue with closed loop solar thermal systems?
  2. Are solar PV powered solar thermal systems cheaper and easier to operate then traditional powered ones?
  3. What are the best practices for placement of automatic air vents in solar thermal systems?

Question 1: Hard Water with Drainback Systems

In a closed loop drainback system, is the quantity of minerals that would be present in hard water an issue?–I would think not. If it is though, I guess you would need to charge the system with distilled water.

Bob Ramlow:

In a closed loop drainback system, if tap water is used as the solar fluid, having hard water may pose no problem. It may pose a problem if the water is corrosive. You are certainly correct that there is a fixed amount of solar fluid in a drainback system and that water does not get changed, so if there were non-corrosive minerals in that water those minerals will precipitate out but there is not that much in there, a very small amount whose buildup would be insignificant. The use of distilled water is recommended in water-only drainback systems because that fluid will absolutely be fine.

Most professional installers I know who install drainback systems use either distilled water or (most) a mixture of 30# propylene glycol – 70% distilled water. Extensive testing has been done to examine if this mixture is stable over the long run and whether there are any harmful effects inside the closed loop from using this mixture. These tests have shown that the glycol mix should last the life of the system with absolutely no harmful effects whatsoever. The reduction in efficiency with this dilute glycol mix is almost unmeasurable. This practice has been used successfully for over 25 years in Michigan, Wisconsin and Illinois by the top installers in each state. All cold climates. This is cheap insurance in case the system does not drain properly during extreme cold episodes.

Question 2: PV Power Solar Thermal System

I did a rough search to find the price of a pv panel that would power a water heating system and turned up empty. Does anyone know approximate pricing for a pv panel that could power a 2-4 flat plate pressurized indirect system in the Boston area?
Are there significant savings in pv powered pumps over the electronic method (differential controller, pumps and sensors)? Any recommendations Bob?

During  a power outage, will a solar water heating system continue to feed hot water to the house? I didnt think gas heaters were tied into the main electric system of a house, however the news reported hundreds of people were without hot water when Irene hit and after the October snow storm.

Do you generally recommend copper over corrugated stainless steel?

My parents have an in-ground pool and I’d like to install a solar pool heater. Is there a particular company that you can suggest?
Forgive me in advance if this question is foolish: Hypothetically, could a combination solar water/space heating system use the outdoor pool as a shunt load thus converting it to a year round pool?

I was interested in building a homemade solar hot water heater as a side project. Im doing it for fun and as a learning experience and the system will most likely be dismantled afterwards.  Can you recommend an inexpensive way to do this?

Bob Ramlow:

You can get small PV panels at AEE Solar ( The cost of the panel would depend on the size of the panel you need for a particular job. You would need to calculate the size of pump needed and then match a PV panel to the pump. For a 2-4 panel system you probably will need around a 50 watt panel, but that is just an average.

A PV powered system probably would cost slightly more than an AC powered system because the cost of a PV panel and DC pump would probably cost slightly more than AC pump and a differential temperature controller. I also recommend using a DC controller with a PV powered system to at least get the digital readout. The big advantages of a PV powered system are no operating costs to run the system and the system would continue to operate during power outages. During a power outage the pumps would still run and the system would not stagnate if it was sunny. Also, if you still had running water during the power outage you may get some hot water. My personal preference is a PV powered system. On my new house I have a combination water heating and space heating system (sand-bed system) that is PV powered. We had a 1 day power outage recently during an ice storm/wet heavy snow. My system worked and I had space heat during the outage, which was a big bonus.

Regarding gas water heaters working during power outages, it all depends on the type of gas water heater you have. Most modern gas water heaters are power vented and if the power vent does not work, the unit will not operate. Also many modern gas water heaters do not have a pilot light and instead use electronic ignition which also would not work without power.

Copper pipe is less expensive than corrugated SS pipe. Copper pipe has significantly less friction head than corrugated pipe so a smaller pump would be needed with copper. Corrugated pipe is a lot easier to install, saving labor costs on the installation. Both types of pipe have their place in the market. SS pipe is popular because of the labor savings aspect, plus no joints except where the pipe connects to the collector and pump station.

For outdoor pool heating systems, if the pool is seasonal I suggest using a seasonal plastic solar pool heating collector/system. They are very affordable and work great. I have many customers who have a solar space heating system use their seasonal pool as their heat dump/shunt during the summer, thereby utilizing their solar energy system all year round which gives them a great rate of return because the solar energy system would then operate at 100% capacity all year round. Regarding the hypothetical combi system you asked about, if you are doing space heating you will need all of that heat for space heating during the winter, so there would be no extra heat to dump into the pool during the heating season.

It is very difficult to make a solar water heating collector absorber plate. I have seen several people try it with no luck. The way companies make absorber plates is to either weld the absorber to the riser tubes or they solder them. Welding copper to copper takes a special machine. When they solder they do it in an oven so the whole absorber plate is at the same temperature. If you try to solder it with a torch the plate gets heated unevenly and the thing warps into a pretzel. I have made my own collectors but I always purchased absorber plates and just made the box they fit into (the box has to be metal, no wood). You may manage to solder a very small absorber plate without excessive warping if you are just looking for a demo.


Question 3: What are best practices for automatic air venting placement and other product specifications.

  1. In video lesson 5 you indicated the use of an automatic air vent at the high point portion of the piping on an Active Indirect Pressurized Antifreeze system. I believe you said that after a certain mount of time for air venting, the ball valve located upstream of the air vent would then be shut off. If the system goes into a mode of overheating (or stagnation) during the time the ball valve is open to the automatic air vent, isn’t there risk of steam being released out the vent and causing a necessary adjustment to the anitfreeze solution?? If yes, wouldn’t it make more sense to have a vertical dead leg of piping with a manual vent valve located at the high point so that the manual vent could be opened after the system was allowed to operate for a few days. The thought here is that any entrained air in the system would find its way to the high point system dead leg where it could be manually purged out.
  2. In reference to the SRCC System Design Criteria Section″Tank and Heat Exchanger Design Requirements”, It identifies that storage tanks shall comply to ASME Section VIII Construction unless exempted in Part U-1c. I don’t have a copy of that ASME code in our office, but I believe its the code requirement for unfired pressure vessels. Do you know if the exempted reference section pertains to storage tanks below 120 gals in size??  In other works storage tanks 120 gals and up have to be ASME code stamped and tanks under that do not?
  3. In video lesson 6 you cover the construction and “Transpired Collectors”. I deal with a lot of make-up air handling unit system that could benefit from this type of collector but wondered how they would hold up against insects, and the cotton wood fuzzies. In the spring around the Burlington, Vt area, the cotton wood trees spill out such debris you would think it is snowing. Could this possibly plug up the air openings?
  4. Questions on Quiz for week two. I reviewed the ones I had wrong. Maybe you can set me straight on the following:
    1. For question # 5: can’t active systems also be powered by a PV panel in some types of systems and therefore considered to operate without external power??? Or is the PV panel source considered to be external power??
    2. For question # 9 can you explain why active direct systems would not also require the installation of a  mixing valve after the backup water heater?? If a controller fails that causes the pump to continue to run, wouldn’t the storage tank potential get as hot as any other system? For what it’s worth, In the State of VT, our plumbing code dictates that all water heaters require a mixing valve to be installed.

Bob Ramlow:

Question 1: Personally, I do not like to use automatic air vents on pressurized systems. I always use manual bleeder valve, if any vent at all, and still use a ball valve to isolate the bleeder valve. On smaller systems, like most domestic water heating systems, I do not put one in at all. On large systems that have multiple collectors or very long pipe runs or pipe runs with multiple traps, having some type of method to vent entrapped air is a good idea. The whole reason for having this type of vent is because it is hard to get all the air out of a system with the charging pump. From my experience, the use of these vents are needed because the installer does not take the required time to purge the system when the system is filled. It takes a little extra time and patience to do the job right the first time and get all the air out then. It can be done, but the installer has to be patient.

In any case, I like the manual vent on a vertical dead leg at the highest point. The vertical pipe should hold any un-purged air that can then be released later. In a related matter, I always put a couple of extra pounds of pressure in a system when I am charging it so I can vent any accumulated air and still have my needed pressure in the system.

So I agree with your point of view. Good thinking.

Question 2: I do not know the answer to this question. I know that very strict code inspectors have never had a problem with using an electric water heater as a solar storage tank. I do not know if domestic water heater tanks must conform to ASME standards. As far as I know, virtually all the companies that make solar water storage tanks also make traditional water heaters and essentially use the same tanks except that they may have an additional port of two in them. It is pointed out in this weeks lesson that any storage tank that has a heat source must have a pressure relief valve.

Question 3: I doubt the fuzzies would be a problem. I have inspected many transpires systems and never saw a plugging or insect problem. I do not doubt that under certain circumstances, problems could arise, but they should be able to be taken care of with a garden hose or power washer.

Question 4a: A PV panes is an external power source. It happens to be a renewable, solar power source, but a power source none-the-less. This differentiates the system from a truly passive system like a batch heater. Good question.

Question 4b: The correct answer to this question should be all the options (a, b, c & d). I will check with Duncan to make sure that is what we have. So you are correct, all types of systems require this valve. Just a note; it is virtually impossible for a pump to run if the controller fails. In every model I know of, if there is any problem, the pump goes off. This would be a feature any controller should have and it would be prudent to see if the controller you will be using has this feature. Also, you want a controller that turns off if there is any problem with a sensor or if a sensor wire gets shorted out.

Check out Bob Ramlow’s Solar Hot Water Design & Installation course for an in-depth look at designing and installing SHW systems AND prepare for the NABCEP exam.