Vaughan Woodruff answers student questions regarding drainback systems on the Solar Approaches to Radiant Heating discussion board…

Student 1: I have several questions on drainback systems for you…

Question #1
When the solar storage tank has satisfied and the system goes into drainback, what happens at the collector? 

Question #2
How hot does the collector get when it is empty?
Does this temperature damage the collector? 

Question #3
If a combisystem, then what solar fraction does one see when the only demand is domestic water heating during the summer months?

Question #4
How high do the collectors need to be above the drainback tank to work best? 

Question #5
Is it possible to go with an elevated ground mount drainback system if the roof is the wrong direction for a productive solar thermal system?

Question #6
At what cold weather temperatures do you recommend using propylene glycol as the solar fluid in a drainback system?

Vaughn: Great questions!

1) The collectors remain empty. Any fluid remaining in the collector will steam out.

2) The stagnation temperatures of evacuated tubes can exceed 400F, and the stagnation temperature of flat plates may reach 350F. Flat plate collectors tend to be less affected by stagnation. Evacuated tubes are rarely used in drainback systems.

3) Solar fraction depends on system size, demand, and location. It is possible to have the combisystems satisfy the hot water demand in he home during the no heating season if the system is large enough in comparison to the hot water demand.

4) The collectors need to be high enough above the crank to allow the pipe to pitch to drain. One of the most effective systems we’ve installed was for a car wash with a short run from tank to roof.

5) This is possible, yes, but rare. Realistically, the array would need to be directly adjacent to the structure, and the storage would need to be in a basement. I’ve only seen one system like this.

6) For me, the outdoor temperature is not as critical as the location and configuration of the piping to/from the collectors. A system mounted at the same angle as the roof in Alaska would make me less nervous than one in Illinois that has a long run of piping in an unconditioned attic.

Student 2: What is meant by the term ‘pressurized drainback’? Is that like putting a tank, such as a Zilmet vsg 5, between the pump and the inlet to the collector?

Vaughan: It means pressurizing the air in a closed drainback system. Typically, these systems are installed at atmospheric pressure, but you can pressurize the system with a compressor after filling it to the static fill level. The benefit of doing this is that it reduces the potential for cavitation by putting positive pressure on the suction side of the pump prior to system startup.


Enroll in Solar Approaches to Radiant Heating today! This course offers 1 IAPMO CEUs and 1 RPA CEUs. Serious professionals with relevant experience can spend six weeks learning the latest, most advanced concepts for integrating solar with radiant heating systems. 

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About Instructor Vaughan Woodruff – Owner, Insource Renewables

Vaughan Woodruff owns Insource Renewables, a solar consulting firm in Pittsfield, ME. He is a NABCEP Certified Solar Heating Installer and serves on committees for NABCEP, IAPMO and IREC. Woodruff is a co-author of the NABCEP Solar Heating Installer Resource Guide and the author of NYSERDA’s Field Inspection Guidelines for Solar Heating Systems.

Vaughan is teaching: