Cabin water system

ramloui · June 2013

Hello all,
It's been a while since my last post. I am building my off-grid solar system at a slower pace than what I would have wished (my wife has other priorities... go figure...) but I think I'm on the right path. This forum is a gold mine.

So, right now, I have my battery bank which consists of 4 x 6V CR430 wired as 2 strings of 12V. I have a cheap 3000W MSW inverter (175$ at CT!!!) which will do the job until I finalize the system. I recharge the batteries with my generator and Iota charger. Got to keep those batteries full and happy, right?

My question right now is about pumping water through plumbing to the cabin.

Just a little bit of background info: I pump water from the lake to a 250G tank on a small hill (about 20ft elevation so not high enough tomeet the pressure requirements on the instant-on water heater). From that tank, water is gravity fed to a 12V Shurflo pump that feeds the cabin. I also installed a pressure tank (pre-charged bladder type) between the pump and the cabin with the intent of minimizing pump cycling.

So here it is: the pump does not come on very often, as intended. However, when it comes on, it stays on for quite a while until the pressure switch on the pump turns it off. So much so that I'm wondering if I'm really saving anything. If I did not have the tank, the pump would cycle more often at a lower pressure (faucet is open), hence less amps, and shut down soon after the faucet is closed because there is no room for expansion. What I'm worried about is that the pump amps go up as the back pressure increases. Now, when the pump comes on, it stays on until the pressure in the tank is restored to its max value. So its pumping for a longer time at a higher pressure, consuming more amps?

I have just received the Trimetric and I will install it next week when I'm at cabin. I should have good hard numbers with the current installation. However, I thought I would ask you guys what you think. Did anyone of you go down this path before? Are my worries justified? Should I just yank out the tank?

Thanks for your feedback!!

Cariboocoot · June 2013

Re: Cabin water system

You have a point. Running a pump longer to get another 10 psi into the tank tends to be a waste of energy. What you need to achieve is a balance between # of cycles and length of cycles. Going above 40 psi, for instance, is usually a lot of power consumed for a small amount of extra stored water.

Using a PT with the Shurflo is not quite as advantageous as using one with a standard pump due to the lack of a big start-up demand. You didn't say how big the PT is on your system. A pump that flows 3 GPM is going to take a lot longer to fill a tank than one that flows 9 GPM.

ramloui · June 2013

Re: Cabin water system

The PT is 20G and the pump is model 2088. I actually have 2 pumps in parallel.

I have not yet adjusted the pressure in the PT. From what I have read, the bladder presure (no load) should be adjusted at 1-2 psig below the pump cut-in pressure, in this case around 20psig.

westbranch · June 2013

Re: Cabin water system

why not just have the pump on the supply line to the OD water heater and just have Gravity feed cold water to the rest of the Cabin?

Cariboocoot · June 2013

Re: Cabin water system

ramloui wrote: »

The PT is 20G and the pump is model 2088. I actually have 2 pumps in parallel.

I have not yet adjusted the pressure in the PT. From what I have read, the bladder presure (no load) should be adjusted at 1-2 psig below the pump cut-in pressure, in this case around 20psig.

This is correct: 2 psi below pump on pressure. The Shurflo is factory set for 25 psi +/- 5 psi so it might be a good idea to put a pressure gauge on it and have a look. Also its shut off point is 45.

A 20 gallon tank and two parallel pumps should work fine. Just how long does it take for the pumps to refill the tank? You should be getting around 6 GPM with two of them, so it ought not run for more than 3 minutes normally. If it is running 6 minutes or more check that both pumps are running and that there isn't something wrong with one of them (crack in valve head allowing water to flow back from outlet to inlet on one of the pumps for example).

Disconnect one pump and see if the system performs differently. Then try it on the other. If there's no change something is wrong as doing so should remove 1/2 the pumping capacity and be highly noticeable.

ramloui · June 2013

Re: Cabin water system

westbranch wrote: »

why not just have the pump on the supply line to the OD water heater and just have Gravity feed cold water to the rest of the Cabin?

That's how the previous owner had it setup. But the whole thing was a mess of piping and a nighmare to winterize. So I tore everything out this spring and replumbed the whole cabin (shower, toilet and 2 sinks). It also caused some inbalance between the hot and the cold at the faucets and it was difficult to dial in the right temperature (especially for the shower). Now that it is replumbed, it will be very easy to drain everything for the winter by gravity. And I may actually use the pumps to push RV antifreeze throughout the whole system.

The new setup is perfect as far as supplying water to the cabin. I'm just concerned that the pumps are using more energy than needed for that purpose.

ramloui · June 2013

Re: Cabin water system

Thanks Cariboocoot, those are great ideas. I will try them next week. and I will take pictures to post.

I have not timed the cycles but they seem to be more like 6 minutes rather than 3...

zoneblue · June 2013

Re: Cabin water system

I seem to recall reading a paper about parallel pumps. It said that you dont get twice as much flow for two pumps, and that you should fit backflow valves to each pump. But i want to say that 2088 have backlflow valves in them, not sure.

Anyhow my thoughts would be that all that starting and stopping cant be good for a wee pump like that so the less starts the better.

Cariboocoot · June 2013

Re: Cabin water system

zoneblue wrote: »

I seem to recall reading a paper about parallel pumps. It said that you dont get twice as much flow for two pumps, and that you should fit backflow valves to each pump. But i want to say that 2088 have backlflow valves in them, not sure.

Anyhow my thoughts would be that all that starting and stopping cant be good for a wee pump like that so the less starts the better.

The Shurflo 2088's are positive displacement pumps: they have intake and outlet valves built in. Two of these or any kind of pumps properly installed (centrifugal pumps need check valves added) will give you double the volume. This includes having the piping sized correctly; you can't flow twice the volume through the same size pump without doubling the pressure and any given pump will only operate at its maximum pressure. So if you put two together on pipe sized for one it will simply reach maximum operating pressure more quickly, and above that will not flow any more water.

Blackcherry04 · June 2013

Re: Cabin water system

Why don't you just buy a 5 Diaphragm, 5.5 GPM Aquatec pump that can be adjusted with a set screw to give you up to 70 PSI. 3 Diaphragm pumps are a thing of the past.

http://www.theenergyconscious.com/aquatec-sw001.html

BB. · June 2013

Re: Cabin water system

Note that pumps/piping are very much like electrical systems (why so much of the performance/math/issues are similar and why "water analogies" work so well with electrical systems).

Positive displacement pumps will parallel nicely (assuming single pressure switch/controller). Sort of like solar panels paralleling because they are "current sources" (analogy to positive displacement pumps).

Centrifugal pumps (output volume is dependent on system pressure) do not parallel well if you are operating them near their maximum pressure--You can have one pump that is newer/slightly more efficient, pushing water back through the weaker pump. Similar with Batteries (Voltage Source). Add in diodes/check valves, water hammering/inductance, etc...

-Bill

Cabin water system

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