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Installing My Solar 
Domestic Hot Water Heating System

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Installing a check valve

October 21, 2006.  Time Invested today:
0:45 Drain system, install valve, re-fill system.
I noticed that as the weather started to drop down around freezing at night that the reverse thermosiphon issue became more of a problem.  The fluid in the collectors cools so much that it causes a reverse flow that removes heat from the tank.  See the plot below for an example where the night low was 30F.  By 8:00pm there is over 8 degrees difference between the 2 pipes above the solar wand.  The temperature loss is most visible in the lower part of the tank (red line).
I had talked with Barry Butler about this issue.  While it is common in most solar heating applications to use a check valve to prevent this, there is no standard part made for 3/8" copper pipe.  So Barry, being the brilliant inventor that he is, designed his own low flow check valve.  He sent me one to beta test and I installed it -- see below.
Close-up of new check valve installedI drained the collector fluid into a bucket at around 8:00pm and cut the tube that exits the Solar Wand and inserted the new check valve.   Using a pump, I then re-filled the system, and as you can see in the plot above, once the temperatures equalized the reverse flow was blocked.  I am sure that this will prevent significant heat loss in the very cold months to come here in Maine. 
An interesting footnote.  The next morning I saw that the red light was showing on my control panel (and on my custom dashboard in the kitchen) that indicates that the fluid in the reservoir above the collectors was low.  I am able to access this from the bedroom window and verified it was dry -- and it was 3/4 full the night before.

On thinking about this I realized that I had filled the collectors with 68F fluid and over night it had cooled to 30F causing the system to draw fluid from the tank as it reduced in volume.  So I topped up the reservoir with more 50% glycol mixture and the level remained normal afterwards.

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