Water pressure tank pressure with Grundfos pump

hidrologo

I recently purchased a Grundfos MQ3-35-1 (115V) 3/4hp pump to replace a previous similar Grundfos pump that was cycling at my off-grid home. Although, the Grundfos comes with an internal bladder and check.valve, I ordered a 20 gallon water pressure tank in order to extend cycling intervals. I had a local plumbing company with Grundfos experience install the pump. They did so, but before they left, I tested the system by flushing a toilet and found that the pump started on the second flush. The installer from Petroglyph plumbing had no idea on the correct water pressure tank pressure to minimize pump cycling, snd had no adjusted the water pressure tank psi. As such I had him lower the pressure until the pump started at about 4-5 1.6 gallon flushes. I do not believe this is optimal.  I believe the water pressure tank came set at about 38 psi. Under low flow the pressure gauge at the tank was reading around 35 psi when the pump started after we reduced the water tank pressure. 

My question to the forum is  - What static psi (pump shut off and a faucet turned on)  should the water pressure tank be set at to minimize pump cycling for this pump?

mvas

It is the difference between the Turn-On pressure and Turn-Off pressure that determines how long the pump will run.
I believe mine is set at a 20 psi differential =  20 psi turn-on and 40 psi turn-off

What is the Air Pressure inside of the tank?

Estragon

It seems to me a "20 gallon" pressure tank might be a total size.  The effective capacity of the internal bladder would vary a bit with pressure settings, but likely ~7 gal.  If so, 7 ÷ 1.6 gives your 4-5 flushes.

Aguarancher

The internal bladder air pressure should be set at the cut-in pressure of the pump switch. The bladder pressure is measured with the tank empty of any water pressure.

706jim

Aguarancher said:

The internal bladder air pressure should be set at the cut-in pressure of the pump switch. The bladder pressure is measured with the tank empty of any water pressure.

So in your case set the bladder pressure (with pump turned off) to either 20 or 30 psi.

Aguarancher

"set the bladder pressure (with pump turned off) to either 20 or 30 psi"

I disagree, the correct internal bladder air pressure is determined specifically by the pumps cut-in switch pressure setting. Nothing else is close enough. And all bladder air pressure adjustments are made with the tanks empty of any water pressure.

706jim

Well unless you have an adjustable pressure switch (who ever changes the default setting?) it would either be 20 or 30 psi.

m151

The bladder pressure should be set 2 psi below the cut-in pressure to reduce "pressure collapse" as the pump cycles. My 30 gallon tank has a draw down of about 10 gallons. This will very with the cut-out pressure. An accurate air pressure gauge is a must. An upgraded quality water pressure gauge doesn't hurt either.

jonr

Set the pre-charge pressure of the tank to 26 psi.  The Grundfos recommendation of 22 to 25 psi is also fine.

Dave Angelini

To the OP, who never came back, you are using a constant pressure pump. Since you can't really adjust the pumps switch internally, this idea is not going to do very much at all as far as reducing cycle time.

I have done this many times for folks and it really does not add much capacity to the system and won't save you many pump cycles or electricity.

If you want to do this and are determined, you need to add an external pressure switch to power the pump. This will work in what you want to do.

A normal size system, with decent battery capacity really is not worth this complexity. if you have the time, go for it.

LoidLiu

Hi, I've got a little lost. I want to change some bathroom stuff and consider changing the original toilet to a low-flow one. Should there be done any changes in all this pump-pressure system to get the benefit of a low-flow toilet? 

706jim

No there is no need to alter your existing water system for the low flush toilet.

BB.

The new toilets (at least in California) are in the 1.2 (down to 0.8 gallon for liquid waste flush) to 1.8 gallons per flush... The old toilets were ~3.0-3.5-5-7 gallons per flush.

If you are buying a new toilet, look at the reviews. A fair number do not do a very good job of flushing "solids" and paper (stoppage in toilet itself).

Depending on your sewer piping, angle of drop, and septic system... The lower flow of water could hurt (clearing of waste in pipe, possibly a problem with 0.8 gpf) and probably help with septic (less water to process).

No fat/oil/grease in septic systems. No garbage disposal recommended.

-Bill