Replies

1. 
   dvaut | Oct 12, 2014 05:40pm | #1

   Are you worried about Hydrogen being an air born health hazard or fire hazard? Either way hydrogen molecules are much smaller than oxygen or nitrogen molecules which make up an overwhelming majority of the atmosphere. They are about 15 to 16 times smaller. In comparison VOC's are larger any of them. Hydrogen molecules being this small will escape your house at a much faster rate than the other gas molecules, even if your house is passive house tight. I would also imagine that your house has an E or HRV that also aids in ventilation. I have now idea how much off gas you have, at work we have a very large collection of lead acid batteries and the rooms they are kept in are ventilated. Maybee try to passively vent the box with one intake pipe low and one that exits high. Even without a temperature difference the lighter hydrogen will rise.

2. 
   Kopper37 | Oct 12, 2014 09:09pm | #2

   Thanks, Dillon.

   Based on my research I don't think hydrogen gas is a health hazard---certainly not in this application.

   The PV subcontractor originally proposed installing a single vent pipe in an unsealed battery box. That's the way they normally install these systems.

   I explained that since the battery box was going to be installed in the basement, that the single vent was more likely to become an inlet than an outlet (because of the stack effect).

   So I'm concerned about an uncontrolled leak in the building. Long time readers of GBA will understand the comment that there are no "smart arrows" to direct the airflow in such a system.

   Battery banks need to be vented to avoid a build-up of H2 gas, which becomes an explosion hazard at a concentration of only 4%. So it's also a safety and due diligence issue.

3. GBA Editor
   Martin Holladay | Oct 13, 2014 04:13am | #3

   Daniel,
   Most battery boxes use passive venting (based on the stack effect) using 2-inch PVC pipe. Including a small DC vent fan (controlled to energize when a high voltage set point is reached, since high voltage indicates charging, and that's when hydrogen gas is produced) is optional.

   For more information on this topic, see Battery Box Design.

4. 
   charlie_sullivan | Oct 14, 2014 05:23pm | #4

   I suggest dedicated inlet/outlet venting, with both routed to outdoors. The light weight of H2 will drive the flow in that by something like the stack effect. But you can't make them concentric--if there is no difference is the elevation of where the connect to the outside air, their will be no net height difference to drive the stack-like effect. You'd need the inlet to exit the building near grade level and the outlet at some height, e.g. 6' above the inlet.

   Absent temperature differences, the stack-like effect would drive airflow when you have H2 present and self regulate to stop when you are done exhausting the H2. But in cold weather, you'd also have true thermally driven stack effect causing air circulation beyond what is needed, perhaps cooling the batteries excessively. A geeky overly expensive but fun solution to that would be to add a heat exchanger from a HRV system, with no fan. If positioned right, that could mostly cancel the temperature difference, leaving you with just the H2 driving the stack effect. But that's probably not a practical solution. In practice, you probably need dampers to decrease the thermal-stack air flow in the winter to keep the batteries from getting too cold. The problem would be figuring out where to set the dampers.

5. 
   Kopper37 | Oct 14, 2014 09:25pm | #5

   Thanks for the comments. The PV installer is going to use a sealed battery box, use two small diameter pipes for venting. He's going to position the outlet on top, make sure the inlet feeds the bottom of the box.