I have a Mitsu WP205Q water pump that needs replacing because the tank has rusted out. A neighbour has just installed a Hitachi water pump that has no tank, which I assume just draws from the underground reservoir, same as mine does, difference being water is held in the pressurised tank with the Mitsu. Any advantages of one over the other? Price wise they are about the same.

Tank pump will cycle and pressure vary.  Constant pressure pump will have to operate for any water use and stay on until water completely stops.  As you seem to be pulling from underground check specs as have no idea how high the new pumps and pull and what output pressure will be.  My neighbor here in Bangkok has one and have had to brick up windows on that side of house - they can be extremely noisy if have solenoid valve slamming on and off (don't believe they are all this bad).

I can only help with we have a govt water supply sometimes there's no water and not always good for  showering so a plastic type storage tank from Global Hse at 2000 baht is good to go for us. 

45 minutes ago, giddyup said:

which I assume just draws from the underground reservoir, same as mine does

Is your pump drawing from a bore hole?  Must not be very far down in any case.  And, you are referring to the tank which is part of the pump, yah?  A tank will be better for keeping pressure and reducing the hammer affect.  Does your neighbor's pump maintain flow to the house?

Edited September 2, 20206 yr by bankruatsteve

37 minutes ago, bankruatsteve said:

Is your pump drawing from a bore hole?  Must not be very far down in any case.  And, you are referring to the tank which is part of the pump, yah?  A tank will be better for keeping pressure and reducing the hammer affect.  Does your neighbor's pump maintain flow to the house?

Yes, the tank is part of the pump, and the pump draws from an underground tank when town water pressure is either low or non existent. Haven't asked the neighbour yet, but I assume that a pump without a tank runs continuously as long as a tap is open?

Edited September 2, 20206 yr by giddyup

6 minutes ago, giddyup said:

Yes, the tank is part of the pump, and the pump draws from an underground tank when town water pressure is either low or non existent. Haven't asked the neighbour yet, but I assume that a pump without a tank runs continuously as long as a tap is open?

Ah OK. All pumps will run continuously with tap full open. The tank, or bladder, is to help maintain pressure if it does cycle.  A popular pump is the "constant pressure" type between 150 to 250 Watts. 

Pumps that suck directly from below can be a pain to prime, with over ground tank just open the tap. Any small air leak in the intake will cause it to get air in and cause problems. 

 

I lately had to replace the entire intake system to the pumps in the house because the previous somchai had screwed up the threaded joints that were leaking air in, as well as making it a piece of art instead of a functional system. Underground tank. 

Edited September 2, 20206 yr by DrTuner
Removed the mixup with pre and post tank

1 hour ago, giddyup said:

Haven't asked the neighbour yet, but I assume that a pump without a tank runs continuously as long as a tap is open?

I have a tankless Grundfos 900W ( CMB5-46 PM1 ). It doesn't run 100% unless multiple taps are open, but cycles. There's about 40m of pipe in the house that acts as the pressure reservoir. Running at 2bar.

Edited September 2, 20206 yr by DrTuner

5 minutes ago, DrTuner said:

I have a tankless Grundfos 900W ( CMB5-46 PM1 ). It doesn't run 100% unless multiple taps are open, but cycles. There's about 40m of pipe in the house that acts as the pressure reservoir. Running at 2bar.

That is really low pressure for a Grundfos pump (unless current pipes can not take much) - mine is running 3.5 bar and having good pressure was a prime reason for spending more for a Grundfos system the last several decades (redid the house water pipes to take higher pressure).

13 minutes ago, lopburi3 said:

That is really low pressure for a Grundfos pump (unless current pipes can not take much) - mine is running 3.5 bar and having good pressure was a prime reason for spending more for a Grundfos system the last several decades (redid the house water pipes to take higher pressure).

Forgot to tell I measured at the highest point in the house, a shower about 8m above the pump itself. I haven't checked at the pump outlet.

Edited September 2, 20206 yr by DrTuner

2 hours ago, bankruatsteve said:

Ah OK. All pumps will run continuously with tap full open. The tank, or bladder, is to help maintain pressure if it does cycle.  A popular pump is the "constant pressure" type between 150 to 250 Watts. 

The Mitsu doesn't. it cycles on and off when the taps are open.

If the pump is to be used to fill a tank that has a ballcock type valve, then the tank type pump is better as the air bladder in the tank will keep the pump from cycling on and off while the ballcock is closing.

 

If the pump is to be used to draw from the tank to points of use, the non-tank type pump is better because it will provide more constant flow/pressure at the point of use, and will only cycle on when a valve is opened.

 

In most applications the quality of a pump is not determined by the pressure it generates, but rather by the volume it delivers at the point of use.