@lazypup: Thank you very much. Two questions(if you wouldn't mind):

1- When you say "damage the pump", are you reffering to the well pump or the pw pump, and why could this happen?

2- I would rather use my city water, but I don't have a pw hose long enough to get onto and move around the roof if I left the pw on ground level, and I thought about bringing the pw up onto the roof, but if I bring the garden hose up onto the roof to attach to the pw, obviously there would be a minimal supply of water because of the need for the water to rise. I know I could literally buy extra pw hoses, but I would be wasting money as I would hardly use them. Any other possibility's?

Many Thanks:
Don

Personally, I would be very hesitant to put the pressure washer up on the roof. That would be a lot of weight and vibration concentrated in a small area and I would worry about damaging the tiles its sitting on.

On the other hand, the VSHL (vertical static head loss) is equal to 0.434psi/ft vertical so if the position where you place the PW is 15ft higher than the hose bibb where your getting the water supply the vertical pressure loss would be 0.434 x 15 = 6.51psi.

The bigger concern would be the FHL(Friction Head Loss) casued by the friction of the water passing through the supply hose. FHL is very elusive because it changes exponentially as the velocity of flow increases through a pipe or hose. To minimize FHL I would recommend a good quality 3/4" hose. With a demand load of 2.7gpm the velocity of flow through a 3/4" hose or pipe is about 2.5ft/sec and the FHL would be in the order of 2 to 3psi, allowing of course that there are no sharp bends or kinks in the hose run.

You could make a rudimentary test by taking the hose and a 5gal bucket up on the roof to the point where you plan to set the PW. If you can get 3.5gpm or more it should be fine.

Now in regards to how your idea might damage your irrigation pump. First of all, we must understand that an electric motor is simply a form of an energy converter. A motor converts electrical energy into mechanical energy.

Although the actual formula for AC voltage is a bit more complex, in simple laymens terms 746watts of electrical energy equals one horsepower of mechanical energy thus if your irrigation pump has a 1/2HP electric motor it draws approximately 373watts of electrical energy.

WATTS equal voltage x amperage so a 1/2HP motor operated on a 120v supply will draw approximately 3.1Amps. (373watts / 120v).

The actual amp draw of the motor is determined by the design of the motor windings and the motor will continue to draw approximately the full designed amperage even if the mechanical load on the motor is zero.

1HP of mechanical energy is equal to 550ft/lbs so a 1/2hp motor produces 275ft/lbs if mechanical energy. Now if we attach a load to that motor that uses the full 275ft/lbs of energy your motor will sit there and happly run, but if the load is less then the motor produces more energy than what is consumed by the load, and their is a firm rule that energy can neither be created nor destroyed, only changed in form. If you attach a 1/2hp motor to a 1/4hp load, the motor will still continue to draw enough electrical current to produce the 1/2hp, but since only 1/2 of that energy is now being converted to mechanical energy, the remaining electrical energy will be dissipated as heat and your motor will soon burn out.

lazypup is an acknowledged expert on plumbing. But this about electric motors is not correct. "If you attach a 1/2hp motor to a 1/4hp load, the motor will still continue to draw enough electrical current to produce the 1/2hp, but since only 1/2 of that energy is now being converted to mechanical energy, the remaining electrical energy will be dissipated as heat and your motor will soon burn out." An electric motor loaded to less than it's full load will draw less current than the nameplate states. This is due to back EMF, which is an induced reverse voltage in every electric motor resulting from inductance. Independent and additional study is suggested to better understand that concept. It is true that an electric motor is most efficient when fully loaded (not overloaded) to the nameplate amperage. So a 1/2 hp motor loaded to 1/4 hp is not as efficient as a 1/4 hp motor fully loaded ( the 1/2 hp at 1/4 hp load will draw more amperage than will the fully loaded 1/4 hp motor), the 1/2 hp motor will draw less amperes when delivering 1/4 hp than it will when delivering 1/2 hp. No added danger of burnout.

busdriver,,you know even less about motor function than you do plumbing.

Hi, I used steam cleaners and pressure washers and they have thier place. They make high pressure noozeles for garden hoses that will do the job. The create pressure by creating velocity at the nozzel The look like some fire hose nozzels. Narrow neck, you can find them at a big box store or Harbor Frieght. This prinicple was used for hydraulic mining. It brought down mountains. Also my wife got me one for side walks. It has a wide fan of water and works great.
Good Luck Woodbutcher

brickeyee frequents these forums and has an earned PHD in Electrical Engineering. Hopefully he will comment on this subject.

Hi, I worked in a sawmill on large motors AC & Dc upto a 100 Hp So I know a little about motors. I try to be basic. Horse power rating is potential full load.
A 1/4 Hp motor at 115 Volts is rated for 5.8 amps full load
if the motor is connected do a device that will be a full load for that motor a clamp on meter will read 5.8 amps.The motor has reached its potental load.
Now hook the same load to a 1/2 Hp motor which has a full load rating of 9.8 amps @ 115V a clamp on meter will read about 5.8 amps the same as the 1/4 hp motor. The 1/2 hp motor has NOT reached its full potental. The wire for the coils in the 1/2 hp motor is larger than the 1/4 hp motor. Therfore the 1/2 hp motor will run a little cooler with this load.
Woodbutcher

"Now if we attach a load to that motor that uses the full 275ft/lbs of energy your motor will sit there and happly run, but if the load is less then the motor produces more energy than what is consumed by the load, and their is a firm rule that energy can neither be created nor destroyed, only changed in form. If you attach a 1/2hp motor to a 1/4hp load, the motor will still continue to draw enough electrical current to produce the 1/2hp, but since only 1/2 of that energy is now being converted to mechanical energy, the remaining electrical energy will be dissipated as heat and your motor will soon burn out."
What is posted says that lightly loaded motors burn out quicker than fully loaded motors. Absurd.

Ok then, it's settled, i'm going on the roof and using my toothbrush and garden hose. That should solve this dilemma about city water vs well pump.

Being serious here, all of you have been really really helpful to me(aside from making my head explode with all this number crunching)and it all came down to a simple recomendation by lazypup: take a 5g bucket up on the roof and time how long it takes to fill it. Presto, I filled about 6.5-7g's in a minute. So, yesterday I had brought a 50ft exten hose, slapped it on, and started to clean. But troubled reared it's ugly head. I had to break to do a few things in the house, so I shut the pw off and closed the fuel cutoff with the swith. Came back out an hour later and opened the fuel switch and gas started leaking out from under the carb. Now I don't know whats wrong and all I got done was above the garage. RATS!!!

There was a time when I would have stood toe to toe with all of you guys and argue that reducing the load would only reduce the amp draw, however I learned the hard way.

In addition to plumbing we do HVAC and on all of our HVAC service trucks we were carrying four AC condenser fan motors which were Marathon Motor co 230v 1075rpm CW & CCW Rot NEMA type 42 frames in 1/6, 1/4, 1/3 & 1/2HP and which we were paying exactly the same price for all of them on our wholesale.

One day we were discussing ways to reduce our inventory and someone suggested that if you reduce the mechanical load on a motor it would reduce the amp draw so theoretically instead of carrying 4 motors on the truck we could just carry the 1/2hp motors and use them universally.

Wet tried that for four months and in that four months we had 5 of the 1/2hp motors burn out even though they had been put in condensers that only required a 1/3hp. At first we thought we were getting bad motors from Marathon and we were considering going to another brand, but after we sent the 5 in for warranty we got a report back from Marathon stating that they all showed evidence of internally overheating, and they went on to say that the type of damage noted was indictative of installing a motor with a higher HP rating than the load. They went on to say that when a motor is put on a lighter load the amp draw does drop down, but the drop in amp draw is not directly proportional to the reduction in load. By example, if a 1/2hp motor is put on a 1/4hp load the load drops 50% but the amp draw only drops by 35 to 40% and the additional amp draw above the load demand is dissipated as heat.

We then went back to carrying all four motors and insuring we installed the correct HP rating and had no further problems with the Marathon motors.

I suspect that more is involved than has been posted. Read up on "Fan Laws", "Affinity Laws".
I always check the amperes draw with my meter immediately after replacing or installing any motor.

http://pontyak.com/fans/fanaffinitylaws.html

Here is a link that might be useful: Affinity L:aws