Tankless Water Heater for Solar Off the Grid system

Old 12-22-09, 12:14 PM
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Red face Tankless Water Heater for Solar Off the Grid system

I live off the grid and make about 2.5 KW a day in 12 volt electricity which is keeping my batteries (1110 amps) nicely charged. I am setting up a solar collector on my south facing roof. It is made of 50 evacuated tubes which will conduct hot gylcol to a 80 gallon storage tank that has two heat exchangers in it. One heat exchangers will be hitched to the solar collector, to heat the 80 gallons of water. The other heat exchanger will be hooked to my radiant floor gylcol tank ( currently a 29,000 BTU 30 gallon LP water heater) I am thinking of buying a tankless hot water heater to turn on when necessary if my 80 gallon tank is not hot enough for domestic use... BOSCH makes a unit - 1600PS - supposedly designed for this very operation. Has anyone used it? It has a standing polot, and being off the grid, that is better than electricity, but some one said this is equivalent of wasting 700 showers. (Is this true?) The larger BOSCH have the advantage of being eligible for a tax credit, but have 120 volt plug ins. Are they used for just ignition? Would thye use a lot of electricity? My inverter provides me with the 120 but I have it set on pulse so that if it does not sense a load, it won't stay on. It check every second. Would that electrical interruption cause a problem? In my limited understanding, any tankless water heater would take my preheated water up to the desired temp, would it not? Is there any advantage to the BOSCH 1600 PS? I am also going to have to do something ot help my 29K BTU water heater as it is not warming my radiant floor system up enough now that it is 10 below zero F... brr. Maybe another tankless water heater?
Any other suggestions for my in my off the grid home as I set up my system? Thanks
Old 12-25-09, 09:09 AM
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tank less heating, insulation

You might like to reconsider your project?
Tank less heaters are perfectly OK for one person, they will work for two people, where you know what the other person is doing all the time but, they are not good for families, where you can end up being frozen, or cooked when someone uses hot water in another room.

Now as far as your project goes, insulation is all important.
If you have effective insulation, then a small boiler will cope at low running cost., that is to say,
it you have a property where all the holes have been sealed and there is at least five inches of polystyrene all round.
Insulation on the inside equals quick heating up time, insulation on the outside equals slower heating times and perhaps the ability to be warm on autumn evenings without the heating coming on.
Insulation, if you do it yourself is cheap.
The saving to be made in heat cost and the improvement in comfort is well worth the effort.
Remember, insulation pays all year round every year.
Heating input is money spent and thrown away, year after year.
Old 12-25-09, 03:59 PM
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From your description, I will assume you are talking about a gas operated tankless.

The ones that need 120v use it for the electronics that control the burner and output temp, these units would require power at all times, even though the draw would be low, no doubt they reset to defaults if power is lost. Some use it for the ignition as well, some have a built in spark generator (runs off the water flowing through it).

That being said, these units are designed based on an incoming water temp, desired output temp and flow rate. Change one of those variables and the others change.

If your incoming water is 40f, and you want 100f out, that's 60 degrees of rise. The unit you look at will have a chart that will show the rise it can handle at a given flow rate.

A small unit may only be able to do that at 1 gpm, a larger unit could do it at 6gpm. Clearly that's a big difference, so unit sizing is crucial.

Next is fuel flow, these units use zero fuel when not in use, but they use a LOT when running, you will need to make sure your fuel source can handle the draw in cold temps without freezing (aka proper vaporization rate).

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