Ok I'll give that a shot. I'll drop the circ too the floor and increase the piping. I'll simply cut out the existing 1" port and weld in a new 1.5". Then I'll neck it down to 1.25" just before the circ inlet. I'll get a new isolation valve as well. I found this one on supply house.
Cavitation issues
- Thread starter warno
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Ok I'll give that a shot. I'll drop the circ too the floor and increase the piping. I'll simply cut out the existing 1" port and weld in a new 1.5". Then I'll neck it down to 1.25" just before the circ inlet. I'll get a new isolation valve as well. I found this one on supply house.
brenndatomu
Minister of Fire
Lowering the pump by itself will make no difference...the suction line must be moved to the bottom port to raise the head pressure on the pump.
Think about it like this, fill a 5 gallon bucket with water, drill a hole 1/2" from the top, weak stream right? Then drill a hole at the bottom...strong stream. A big difference in flow with only a ~15" difference in height...about a 0.544 PSI increase.
So moving the suction line to the lower port (even if its only a couple feet) can make a big difference.
(BTW, I said .485 PSI per foot water column height earlier...I meant .435 PSI per ft...oops
)
Think about it like this, fill a 5 gallon bucket with water, drill a hole 1/2" from the top, weak stream right? Then drill a hole at the bottom...strong stream. A big difference in flow with only a ~15" difference in height...about a 0.544 PSI increase.
So moving the suction line to the lower port (even if its only a couple feet) can make a big difference.
(BTW, I said .485 PSI per foot water column height earlier...I meant .435 PSI per ft...oops
)
hondaracer2oo4
Feeling the Heat
Where is the thermal well for the aquastat mounted? Have you tried take a temp measurement of the top of the tank vs the bottom? With a 1 inch line you are probably pushibg 10 gpm in the stove. If the probe is mounted lower than the currently supply port you could have 200 * water r higher entering the pump putting you even closer to cavitation with very little drop in Npsh.
Lowering the pump by itself will make no difference...the suction line must be moved to the bottom port to raise the head pressure on the pump.
Think about it like this, fill a 5 gallon bucket with water, drill a hole 1/2" from the top, weak stream right? Then drill a hole at the bottom...strong stream. A big difference in flow with only a ~15" difference in height...about a 0.544 PSI increase.
So moving the suction line to the lower port (even if its only a couple feet) can make a big difference.
(BTW, I said .485 PSI per foot water column height earlier...I meant .435 PSI per ft...oops
Would the lower port need to be bigger than 1"? Because right now it's also a 1" line.
The aquastat well is about 1" above the supply port.
brenndatomu
Minister of Fire
Larger certainly wouldn't hurt...but simply moving the line to the existing bottom port will be an improvement...will it be enough? Dunno...but I would think it would be pretty easy to try...with a helping hand I think you could do it without losing much water at all.
maple1
Minister of Fire
Lowering the pump by itself will make no difference...the suction line must be moved to the bottom port to raise the head pressure on the pump.
Think about it like this, fill a 5 gallon bucket with water, drill a hole 1/2" from the top, weak stream right? Then drill a hole at the bottom...strong stream. A big difference in flow with only a ~15" difference in height...about a 0.544 PSI increase.
So moving the suction line to the lower port (even if its only a couple feet) can make a big difference.
(BTW, I said .485 PSI per foot water column height earlier...I meant .435 PSI per ft...oops
I'm not so sure about that. The overall thing that matters, is the height difference, between the circ inlet, and the top of the system water in the boiler. If you leave the pump where it is but tap in lower on the boiler, that won't change that height difference. It won't raise the head pressure.
It would likely lower the inlet temperature, which would improve cavitation issues - but that is a separate & different factor.
Your bucket analogy would apply, if the circ was up at the top hole. But it's not - it is connected to the top hole, by a pipe. That pipe height also adds head pressure.
In order for that to work he would need to seal the vent from the boiler jacket, construct the pipe such that it comes out of the boiler jacket and raises as high as physically limited, then have a small tank (2.5 gal might be plenty) that is open to the atmosphere for venting, then pipe down to the suction of the pump.
Lowering the pump by itself will make no difference...the suction line must be moved to the bottom port to raise the head pressure on the pump.
Think about it like this, fill a 5 gallon bucket with water, drill a hole 1/2" from the top, weak stream right? Then drill a hole at the bottom...strong stream. A big difference in flow with only a ~15" difference in height...about a 0.544 PSI increase.
So moving the suction line to the lower port (even if its only a couple feet) can make a big difference.
(BTW, I said .485 PSI per foot water column height earlier...I meant .435 PSI per ft...oops
Your analog regarding the bucket is totally incorrect sir. So is your assumption that NPSH will increase if you relocate the suction port lower and leave the pump where it is.
Only way to increase NPSH is to increase the distance from the pump to the water level that is exposed to atmosphere. Simply changing the suction port without lowering the pump does nothing to the NPSH. I know this because I have studied fluid mechanics.
brenndatomu
Minister of Fire
OK, easy to test this...stick a hose in the hole long enough to reach the ground...compare the top hole vs the bottom with the same length hose, both discharging at ground level...I think you'll find the lower hole flows much stronger (at least for a while
)Yes, correct. It's the total height from the pump to the water line exposed to atmosphere. Thst is why you would have make the boiler "solid" ie pipe tight and raise the point where the system is exposed to atmosphere.
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All do respect, your analogy is poor and not applicable to this. I can cite references from my text books if you would like a course in NPSH. It will take me an hour to type out a correct analogy to explain it. The OP would probably dislike the derail to his thread. Plain and simple, the available NPSH is the height difference between pump and water level exposed to atmosphere. Doesn't matter to the pump NPSH if the water jacket port is 8" or 20" below water level if the height between the pump and water surface exposed to atmosphere remains unchanged.OK, easy to test this...stick a hose in the hole long enough to reach the ground...compare the top hole vs the bottom with the same length hose, both discharging at ground level...I think you'll find the lower hole flows much stronger (at least for a while
terrible drawing I know but what about this. I change my upper suction port to 1.5" and run it to the floor and then somewhere in that line above the circ tie in my lower 1" port. I'll put a valve on the lower port to throttle the suction of that port. Then tie it all into my circ suction side.
Red - new plumbing
Blue - ball and isolation valves
Yellow - circ
![[Hearth.com] Cavitation issues](https://www.hearth.com/talk/attachments/screenshot_20180113-095627-webp.220593/ "[Hearth.com] Cavitation issues")
Red - new plumbing
Blue - ball and isolation valves
Yellow - circ
Teeing into the 1.5" will do no good. And improvements will come from the increase in the suction pipe size and moving the pump lower. You might get some mixing of cooler water from the lower port though, it's uncertain how thst will really help you if you are trying to supply a higher temp to your htx. It might be counter productive to the temp being sent. Could try it. Worse case is you spend money on the fitting and end up leaving the 1" lower port closed.
Btw, if you are going to 1.5" to the pump, get a 1.5" flanged valve to the pump, don't step back to 1.25".
brenndatomu
Minister of Fire
No need...I'm not here to stir anything up...and I've read the same text books as you...work on pumps for a living...not an engineer, but have corrected more than a few engineering mistakes...gonna watch this one play out from the stands...
maple1
Minister of Fire
The T'ing in the lower port will help cavitation, but only in that it would allow you to pull in cooler water. Which would be counter to your real goal, which is to get hotter water to storage.
I think most of us are on the same page in all this stuff, just have different ways of saying things.
I think most of us are on the same page in all this stuff, just have different ways of saying things.
Ok I was thinking about the idea of the lower port adding to the pressure form the higher water level on the lowered pump location. basically the valve would be cracked open. But if it's not with it then i won't add it.
I couldn't find a 1.5" flanged ioslation valve. I'll check again.
maple1
Minister of Fire
Ok throwing something sideways in. After looking at your hx area pic. Have you tried the dump circuit? I'm just wondering how good it would flow or not, because it looks like it goes up a lot higher than the boiler water level. Making for a good place for air to wander to and lock up. Maybe?
The dump zone does work and there's an auto air vent on the water/air HX up there. But that's irrelevant because I'm changing my dump zone. I'm moving it down to right next to the plumbing in the cabinet.
Here's the dump coil
maple1
Minister of Fire
Ok - sideways, and not relevant, will forget that. Just something that came to mind when I saw the pic.
But hearing that you are going to be replumbing that area some, and stepping back a bit - how ugly a job would it be to move your HX, and put the boiler circ in there with it? Like, move the HX down, to close to the level of where the undergound comes in. And put the circ on the return side of the HX? Per Marshys earlier suggestion? At first, after seeing that pic, I was going to just suggest moving the circ there, to right before where the return lines goes underground. Vertically. That being the lowest point between the HX and where it goes back underground. But it might not be easy to mount it right there. So if you also move the HX down to that level, you might create a piece of solid horizontal piping you could mount the circ into more easily. And you would get everything below your boiler water level. Said without knowing exactly the height difference between your HX & boiler water level. But I think getting the circ where the water is coolest (after the HX) will do the most for the cavitation issues in your situation of wanting the most heat to storage you can get & not having much leeway in increasing your circ inlet pressure.
Not sure I said all that the right way, but hopefully with everything everybody is throwing at you, you can find a tidbit or two to help.
But hearing that you are going to be replumbing that area some, and stepping back a bit - how ugly a job would it be to move your HX, and put the boiler circ in there with it? Like, move the HX down, to close to the level of where the undergound comes in. And put the circ on the return side of the HX? Per Marshys earlier suggestion? At first, after seeing that pic, I was going to just suggest moving the circ there, to right before where the return lines goes underground. Vertically. That being the lowest point between the HX and where it goes back underground. But it might not be easy to mount it right there. So if you also move the HX down to that level, you might create a piece of solid horizontal piping you could mount the circ into more easily. And you would get everything below your boiler water level. Said without knowing exactly the height difference between your HX & boiler water level. But I think getting the circ where the water is coolest (after the HX) will do the most for the cavitation issues in your situation of wanting the most heat to storage you can get & not having much leeway in increasing your circ inlet pressure.
Not sure I said all that the right way, but hopefully with everything everybody is throwing at you, you can find a tidbit or two to help.
My hesitation for that suggestion is you end up with 20' of pipe in the suction side that is 1" and the restriction of the htx.
@warno, what's the elevation difference between the lowest point you can mount the pump in the boiler cabinet vs the lowest point you can mount it in the house plumbing room?
If it was moved to the plumbing closet in the house it might make the most sense to put the pump on the floor where the pex comes up through the floor. Again, you still have the restriction of 20' of 1" pex which may or may not be beneficial depending on the elevation difference.
It may work it might not, might be worse idk. I'd start with the easiest thing and try moving it lower in the boiler cabinet first so he doesn't have to rerun the electrical.
maple1
Minister of Fire
Just also kind of realized there is also a catch-22 in the talk of raising the height of the boiler water, and expansion tank adding. That would also likely raise the height of your hottest boiler water, further away from your circ inlet. May be good for cavitation but not the end goal of hottest to storage. Depending on mixing factors within the boiler.
My boiler shed and the garage floor are basically on the same level. So I think with what has been discussed here it wouldn't matter a whole lot if I put the circ at floor level in the boiler shed or in the garage with the other plumbing. Since it would be basically the same distance from top of the water level.
I think the easiest thing I could do is replumb the suction side with bigger piping and move the circ to the floor level in a horizontal position.
I think the easiest thing I could do is replumb the suction side with bigger piping and move the circ to the floor level in a horizontal position.
Yes.
I'll double check my text on pump speed vs NPSH. From memory, lower speed should lower NPSH and mitigate cavitation. If you don't need as much flow a smaller pump or 3 speed pump would help you. I'll double check though and confirm.
Watching a fire burn right now.
Reading right on the pipe going into the pump is 199°F. With the pump on speed 2, I get cavitation. With the pump on speed 1, there's nothing.
I'm reading a 25° delta T at my plate HX on the boiler side.
The pump is a bell and gossett NRF-36.
Should I still go through with the replumbing in the boiler shed or just run on speed 1?
Reading right on the pipe going into the pump is 199°F. With the pump on speed 2, I get cavitation. With the pump on speed 1, there's nothing.
I'm reading a 25° delta T at my plate HX on the boiler side.
The pump is a bell and gossett NRF-36.
Should I still go through with the replumbing in the boiler shed or just run on speed 1?
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