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atw58 10-06-2011 12:02 PM

Water Pressure Switch Selection
 
Hi all,

Need your help in selecting the cut in/out value for a pressure switch. Here is my situation. On city water with 44 psi. Wish to pressurize hot water from a solar tank and feed the hot water supply with a jet pump. Not sure if it matters but the solar hot water tank is on the roof 35 ft above the pump in the basement. This provides 15 psi at the input of the jet pump.

Which pressure switch should be selected 20/40 or 30/50 psi. And should it have a low pressure protection (doesn't turn on unless there is a min or 10-12 psi).


Thanks,

ddawg16 10-07-2011 12:18 AM

If I understand this correctly....

You have a solar heater that dumps water into a holding tank on top of the roof?

The 15 PSI of water head I understand....

So you want to pressurize that solar water so you can use it with your cold water?

Oh boy.....I see lots of possible issues.....

On the pressure switch side...30/50....I think you need to be thinking in terms of abosolute....

BUT...in order to get proper mixing.....you really need something that will work based on the actual pressure of the cold side. I know you said it was 44 PSI....but I really doubt it stays at that pressure 24/7.

Without spending a lot of money....I don't see a simple solution for just the hot side.....unless the pressures are the same, one or the other will back fill into the other line unless you have a check valve....and even then, taking showers would not be fun.

My suggestion would be to add a second cold water tank up by the solar tank....dump your city water into it....that way the cold and hot have the same head pressure....

Or....use city water to pressurize the hot water and make it a sealed system. You would then use a smaller circulation pump to heat the holding tank...but it would all be at the same pressure as the city water.

atw58 10-07-2011 09:40 AM

Hi Ddawg16,

You are correct and raised some very good points.

Unfortunately the solar system is non-pressurized. There is a controller which senses the water level in the tank and opens the solenoid to fill with cold then shutoff after reaching full, 3/4 or 1/2 full. The hot water side is gravity feed. The tank will leak if 44 psi is applied.

The valves around the old hot water tank gives me the flexibiity to feed solar hot water directly into the house or use the solar hot water as a pre-heater for the old natural gas tank or bypass the solar tank entirely.

Hope this drawing makes thing clearer.

http://dl.dropbox.com/u/7779238/2011-10-07_092302.png

Alan 10-07-2011 10:19 AM

I would say that in order to make that work, you'd need to eliminate the cold feeding the backup water heater, and eliminate the bypass valve also.

I'm not sure how that will affect your gas usage, but if you're feeding that heater with hot from the solar tank it should help.

Grundfos makes a great little booster pump, real simple to install, but i'm not sure if it's rated for hot water or not.

As far as the pressure issue mentioned above, you can always put a regulator on both systems and adjust them until they are equal.

Again, finding something rated for hot water might not work, so you might just have to adjust the city side to whatever your hot side ends up being.

atw58 10-07-2011 11:20 AM

Forgot to mention there is an adjustable anti-scald valve which mixes hot and cold to keep the temp below 90-120 degrees before feeding the home's hot water supply. Not sure if this will comphensate for pressure differences.

ddawg16 10-07-2011 04:09 PM

I think your best bet would be to do a hot water circulation system.

Basically, put a heat exchanger in the solar tank (just a big coil of copper tubing). Use a real small pump with a thermostatic switch. As long as the water in the solar tank is hotter (or above a certain point), it runs. When the solar water is colder than then hot water tank....it stops.

The advantage of this is that you don't run the risk of unequal pressures and you can use treated water in the solar heater which will give you longer life on it's components.


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