off grid

[Colonel]

What booster pump do you recommend for an off grid inverter system, 220v/110/, 5500 gal storage tank 18 psi gravity pressure, 2500 sq.ft house?  power consumption is a factor.

[Cary Austin]

With 18 PSI from gravity, if you use large enough pipe and drill out shower heads and such, you should not even need a pump.  That would be the best thing for an off grid system, no energy use at all.  However, if you want more pressure you can use as small as a 1/2 HP jet pump.  This will add about 40 PSI to the 18 you have coming.  I would use a couple of big bladder pressure tanks and no Cycle Stop Valve.  Anytime the pump is running you want max flow for the KW being used.  Letting the pump run at max flow and storing the excess in a pressure tank is the best way to do that.

[wrekinwanderer]

Thanks for your reply Cary. I'm in a similar situation to the OP, in that I have an off-grid system where watts is important, but with zero head. I have looked into your excellent suggestion and just want to ask you a question based on one of the findings...

If you take a look at fig.1 in the specifications for a grundfos pressure tank: http://www.segamuhendislik.com.tr/urunler/grundfos/pdf/GENLEsME_TANKI.pdf it seems to suggest that for a pump with 40l/min flow the biggest tank advisable would be the 100l pressure tank.

Is this something you would agree with for a 370W jet pump, with a nominal head of 37m and a flow of 37l/min? Is that why you recommend two tanks? Or do you think a single larger tank could take a lower than recommended flow? If so, how large would you go?

Thanks, Mike

[Cary Austin]

it looks like that chart is recommending a 170L tank for 40 LPM flow.  If the pump makes 40lpm, then you want a tank large enough to hold 40L of water, which would be about 170L tank.  That is the minimum size tank you want to make the pump run for at least 1 minute at a time.  However, two minutes of run time is better for the pump and the more water stored in a pressure tank, the less you have to count on batteries.

[wrekinwanderer]

Thanks Cary, So I don't need to worry about getting a bigger tank than the graph recommends, like the 240l, the 1/2hp jet pump will pressurise it okay with the appropriate switch...

[Cary Austin]

If I were running on solar and batteries, I would install a couple of the largest tanks I could find and make sure they were topped off everyday before the sunlight faded.  But a few extra batteries can also store that same energy if you had rather.

[Longley]

With 18 PSI from gravity, if you use large enough pipe and drill out shower heads and such, you should not even need a pump.

What would be the minimum psi to not need a pump and what sized pipes are recommended?

[Cary Austin]

I would think it would take at least 30 PSI to make washing machines fill correctly and to get the soap rinsed off in the shower.  But there are systems in many countries that get by with much less than that.  To get 30 PSI without a pump you would need the water coming in from a height of 69’.

Pipe size is figure by volume and length.  The more volume you use and the longer the run, the larger the pipe must be to keep from losing all your pressure to friction loss.