open loop solar water heating in SF bay area

jcm55November 6, 2007

Hi all,

I'm considering a solar water heating system for my home in Oakland, CA. I'd like to do the install myself, and I'm pretty handy with plumbing tasks, so I don't think that should be a problem.

I do want to get the design nailed down though before I begin. Right now I'm considering an open loop system (where the potable water is circulating through the solar collectors). For freeze protection I will use a drip valve and/or recirculation.

My question is, will that provide enough freeze protection for the climate here? Usually during the winter we get a handful of nights with frosty windshields, but no temps below 32 F. Last winter was an exception, and we got a few nights down to 29 F during the cold snap in January that damaged the citrus crop in southern California.

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I wouldn't rely solely on the drip valve. I'd also have the system set up for recirculation to circulate a bit of warm water from the tank through the collectors when the temp got close to freezing.

I've thought about a drainback system, but there are two reasons why I'd prefer not to go that way:

1) A drainback system requires a bigger, costlier pump. Higher up-front cost, and it uses a lot more electricity which negates some of the energy savings of the system overall.

2) It seems to me that a drainback system requires a heat exchanger of some sort -- to keep the unpressurized solar loop separated from the pressurized potable water.

If I'm wrong on either of those counts, then maybe a drainback system is worth another look.

    Bookmark   November 6, 2007 at 10:22PM
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I take it then that you're not interested in solar tubes?

I've been "kinda" looking at such a system here (Montana) but it uses glycol and depends on heat exchange into an electric hot water tank (which is obstensibly just for backup). Even though it's not cheap there are some credits available.

Good luck,


    Bookmark   November 7, 2007 at 10:32AM
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Yes, the drain backs for domestic water heating do need a heat exchanger. Its often a coil of copper pipe in the drain back tank. You also need the drain back tank itself.

The pump depends on how high the collectors are above the drain back tank. It also should be a bronze pump since the system is open to air. $200 buys a nice bronze Taco or Grundfos.
When you are collecting sun, you are pulling about 160 BTU/sf-hr of collector, or abut 8000 BTU/hr if you have 50 sf of collector. This is about 2400 watts being collected. An extra 50 watts to drive a bigger pump is pretty small potatoes compared to that?

I can see the appeal of the open loop system for you. I suppose that if you made a point of checking that the differential controllers circulate when close to freezing feature and the drip valve were working at the start of each season, it might work out OK. I would guess most failures come from owner inattention.

The collectors are looking at the cold night sky, and lose a lot of heat to sky radiation -- this will make them run colder than the air temperature on cold nights -- so if the air is near freezing, your collector panels are likely below freezing. So, you would really need to stay on top of maintaining your freeze protection features.


    Bookmark   November 7, 2007 at 10:57AM
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