batch heaters

System · August 2007

i am installing a batch heater on my roof and would like to eliminate pumps. will thermosyphon move water to a level of 20' from my ground floor water heater. i would use it as a pre-heater to raise the temperture prior to my main grid powered heater. also the dc powered element is new to me, will the drain be heavy on my batteries with 2 kids and heavy use?
jam

mike95490 · August 2007

Re: batch heaters

A batch heater is not a thermosiphon. Batch heaters heat up a 15 gallon slug of water on your roof, and when your hot water is called for, it runs into the default hot system, giving it heated water, instead of ground temp water. In the AM, it's full of cold water, and you gain most benefit when you use water during the daytime, when it can feed preheated water to your tank.

Thermosiphon relies on a heat collector of just a gallon or so, heating water, that circulates back to a storage tank. The hot water ends up at the high point of the system. If you put a small solar power pump in it, you can store the hot water downstairs. Add more collectors, and you can gather more hot water.

In the past, these work really well for swimming pools, but when you use in a house, and shock the system with cold water, hot, cold, hot.... they seem to develop leaks, from the extreme temperature differentials.

My 2 cents worth

BB. · August 2007

Re: batch heaters

First, the DC heating element (for hot water, hot air heating, etc.) is, generally, a very costly way to get heat made. Making heat from solar electric on a day to day basis is just not cost effective. Making it from batteries is even less so.

Usually, the DC element is only used when you batteries are full and you need either to place a load on a wind turbine (to prevent over speed in windy conditions) or if you have excess solar PV power. There are diversion controllers that can automate running the DC heater and some solar PV charge controllers let you program a relay to turn on aux loads when the batteries are charged (I believe the MX60 from Outback can do this).

Your best bet to make hot water is by using a solar heat system designed for the job... For a rough example, a solar PV electric panel costs about $5.00 per Watt vs ~$0.50 per watt for the equivalent solar hot water collector. Also, the equivalent solar hot water collector is, at least, 4x as efficient as a solar PV panel--so for the same amount of "heat", the solar hot water panel takes only 1/4 the roof space.

Yes, you can get a solar siphon system working--but getting a small pump connected to a stand-a-lone solar panel of a few 10's of watts would probably give you a more reliable system--usually, thermal siphons are pretty prone to vapor/air locks. A pump system should have less problems (other than the maintenance/replacement every few years for the pumps themselves).

Assuming you don't have hard freezes where you live--it will make the system design much easier and less expensive (no antifreeze, don't need a secondary heat transfer loop unless you have hard water, etc.).

I have not installed a solar hot water system yet for my home--but I probably should (couple kids that like to take long hot showers).

-Bill

batch heaters

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