Air in OWB line?

is it solder? i had that happen on my hot tub loop. temp gauge fitting was kicking my but when i solder it together. i pulled the pump apart and little pieces of solder came out.

it could depend on how much head pressure there is on the pump, depending on the height of the pump in relation to the rest of the run, and any changes is pipe size up or downstream you could get cavitation inside the pump (air), the water in the boiler may not be enough. In my CB manual they wanted the pump located at the lowest part of the system to avoid this issue.
cavitation in this case would be from the water boiling inside the pump, as the boiling point of water is lower with reduced pressure (the reason for different high altitude cooking times on cake mix etc.), instead of 220F it could boil at your system temp under 200F, restricting the out flow from the pump increased the pressure and increased the boiling point to get rid of the air. you need more head pressure.

From my experience this sounds like cavitation as rowerwet says. The marble sound you probably happens when the boiler is on the hot side of things and the water is flashing to steam. If you could accept less flow or lower boiler temp it would probably solve it. I don't see moving the pump to the other end is going to help since you are going to create less suction pressure in the pump unless the head gained is more than the pipe friction loss you've added. I would look at keeping the pipe loss before the pump to a minimum such as keeping a straight run and removing any strainer first.

Mike

MyOutdoors said:

I do have shut off valves on each end of the pump in order to isolate/ change it out if needed. Could I restrict the flow with one of those valves without hurting the pump?

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I have never done that on a circulator set up before though I have restricted the discharge valve on plenty of of other centrifugal pumps to maintain suction. I presume you have ball valves installed which are not the best for throttling applications but you could try slowly closing in on the discharge valve to see if that determines that cavitation is the problem. I don't know where you are on the pump curve vs flow rate so just don't close it all the way, i.e. keep some flow going.

Mike

MyOutdoors said:

I do have shut off valves on each end of the pump in order to isolate/ change it out if needed. Could I restrict the flow with one of those valves without hurting the pump?

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In a word......no You never want to restrict the intake of a circ on an open system in any way. Keep the piping as large as possible right up to the flange on the circ to avoid cavitation. If you restrict the discharge all you are doing is cutting down the GPM the circ is delivering to the system. It may help the cavitation issue but will probably leave you with less heat than you need in the house.

It sounds like you have a classic case of undersized lines going to your house and the 0014 is not happy about it. That particular circ wants to see about 14 gpm (+/-2) at around 13 feet of head. If you have a total of 350' round trip (did I see something about 175 ft one way?) and the line is 1" your going to develop about 20-24 feet of head at only 7-8 gpm. At 8GPM the 0014 will only do about 18 ft of head.

restricting the flow out of the pump would be a quick solution, it would defeat the purpose of the larger flow pump at the same time.
moving the pump to the lowest part of the run possible would increase the head and discharge pressures, reducing the possibility of cavitation. depends on how much work you want to do. the pipe sizes would have some bearing on flow also, but if you already have your runs buried and paid for I wouldn't want to dig them up or add more (parallel runs to double flow) unless changing pump locations didn't work.

rowerwet said:

restricting the flow out of the pump would be a quick solution, it would defeat the purpose of the larger flow pump at the same time.
moving the pump to the lowest part of the run possible would increase the head and discharge pressures, reducing the possibility of cavitation. depends on how much work you want to do. the pipe sizes would have some bearing on flow also, but if you already have your runs buried and paid for I wouldn't want to dig them up or add more (parallel runs to double flow) unless changing pump locations didn't work.

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Moving the pump would probably not help in this case because one would be adding all the flow restriction of the preceding 175'. That would likely reduce the inlet pressure available at the circ.

Here's the way it works out with 3 different circs. You can figure that under normal circumstances you can get about 10,000 btu per every gpm of flow. That's only if the heat emitters will suck 20* of temp out from supply to return.

Using the 350' round trip with 1" pex, 4 elbows and 2 ball valves and no other additions for HX or other piping here's how the numbers crunch out. (Your head will likely be higher and the GPM actually lower than shown.)


Taco 009 = 5.8gpm @ 16.01' head & 6.75 PSI differential

Taco 0011 = 7.5gpm @ 24.99' head & 10.58 PSI differential

Taco 0014 = 6.46gpm @19.02' head & 8.02 PSI differential

Now..what the heck does that mean for you? Let's use the 0014 that you currently have in place as an example.
First bear this equation in mind. 1PSI = 2.37 feet of head.

On an open system where you have only the static height of the water column above the circ inlet to make your available inlet pressure, feet of head can be looked at literally. For example, assuming you are able to mount the circ 4 feet below the top of the water column in the boiler you have about 1.7PSI available at the suction inlet of the circ. (4 ft of head / 2.37 = 1.7) Looking at the 0014 you can see that it develops about 8PSI differential. That's measured from the suction inlet to the discharge flange of the circ. The "eye" of the impeller being the "halfway" point in that 8 PSI difference you can divide it by 2 and arrive at the fact that you need about 4PSI at the circ inlet. 4PSI x 2.37 = 9.48 ft of head. At present you have only the 1.7PSI noted above which means you are pulling a negative pressure (below atmospheric) at the circ inlet. Dropping the pressure below atmospheric allows the water to boiler at less than 212* which is exactly what is happening in your system.

Now, why is this a bad thing? All those little circs, be they Taco, B&G;, Grundfos, Wilo or whatever are actually cooled by the water in them. When that water "boils" it allows air bubbles to form in the circ, find their way into the can/rotor area, and effectively eliminate the water cooling effect. When that happens long enough, all the wet rotor design circs will die. My brothers have a hardware store and they sell Taco and Grundfos both for OWB use. They have a pallet size shipping crate about half full of dead circs in the back room. You are not alone because all the OWB sellers I have run into here in Michigan seem to think that 1" pex is the only size of tube there is. Sadly, this has lead to the early demise of countless circs all over the country. Most guys selling OWB's don't have a clue about any of this stuff so I wouldnt say they lie to their customers, more like they just don't understand basic pump laws.

Now just for kicks I inserted 1.25" pex in the calculation and ran it using a Taco 007 and a Grundfos 15-58 on speed 3. Here's how it comes out with the larger tube and bear in mind that both of these circs will pull roughly half the watts that their larger brothers do.

Taco 007= 7.13gpm @ 9.06' head & 3.82 PSI
Grundfos 15-58 = 8.26gpm @ 11.72' head and 4.94PSI

Using the facts above that 1PSI = 2.31 ft of head and the pressure available at the impeller "eye' is 1/2 the differential, we find that the 007 will need 4.41 actual feet of head above it to stay at atmospheric and the Grunt would need slightly more. Both of those circs will live longer, use less electricity and provide more flow with 1.25" tube than their larger counterparts will with 1".

I'm sorry man but them's the facts. If there's one thing I've learned about hydronics it's that there is no way to cheat. A circ isn't going to flow a given or desired amount of water just because some salesman claims that it will. You, me and everyone else in the world are subject to the laws of physics and they don't bend for anyone.

In your case there are 3 viable options

1. find a way to elevate the boiler or lower the circ to provide more head
2. Start putting aside a few hundred $$.00 a year for burned out circ replacement
3. Dig up the 1" and install 1.25" tube to your house.
So much for Saturday evening's class titled Circulators 101

or one more option (almost as painful) buy and bury another run of 1" theromopex, "T" it in at each and it will be equal to a 2" run (or something close)

I still think putting the pump at the lowest spot would help (CB does show it that way for just that reason), I am not sure that the flow restriction would be much of an issue if the in size matched the out size of pipe.
This also leads me to wild ideas about adding a bypass (1/2" dia) if flow restriction is the issue, to recirculate and reduce the 125' effect. Or a small storage tank, or... stuff I know nothing about so take with a grain of salt.

I am running a 007 pushing through 85' of 1" thermopex, I couldn't see spending money for the 014 even though CB wanted me to.

What would happen if he added a second pump? So he would have one pump pushing water from the boiler 175' to his house, and another pump in the house pushing it 175' back to the boiler? Would that help the situation at all?

Rick Stanley said:

What would happen if he added a second pump? So he would have one pump pushing water from the boiler 175' to his house, and another pump in the house pushing it 175' back to the boiler? Would that help the situation at all?

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Not really, the problem is not enough suction head at operating temperature for a certain flow rate. Slowing the flow may help with the cavitation but then you get reduced output from the boiler. I would almost be tempted to install a recirc line from the pump discharge back to the suction throttled in. Depending on the flow loss to the house it may work. Just thinking.

What is the height difference between the OWB pump as it is now and the place where it could go in the house?

Mike

Sorry-I see where you stated you could get another 4 ft of static head. Anyone calculate what the line loss is at what the flow rate is?

Mike

Rick Stanley said:

What would happen if he added a second pump? So he would have one pump pushing water from the boiler 175' to his house, and another pump in the house pushing it 175' back to the boiler? Would that help the situation at all?

Click to expand...

You might try that with a couple smaller circs ....007 or 15-58 on speed 2 maybe.

Thanks for that info heaterman, very well written and informative. However, according to that data my pump is cavitating. I can't hear any marble-like gurgaling, in fact my pump sounds smooth as can be. Is it possible that my pump is cavitating and I can't tell?

sorethumbs said:

Thanks for that info heaterman, very well written and informative. However, according to that data my pump is cavitating. I can't hear any marble-like gurgaling, in fact my pump sounds smooth as can be. Is it possible that my pump is cavitating and I can't tell?

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Sometimes that's the case. Those wet rotors make little noise in the first place, especially the small ones. Take a screwdriver and put the "pointy" end on the circ casting while holding your ear to the handle end. You should hear it if it's going on. One thing to bear in mind is that all those numbers I had listed were calculated at a fluid temp of 180*. At temps below that you will be able to get away with less suction pressure than I listed before you get cavitation. The opposite is true for higher temps.

You can look at the pump curves yourself at

http://www.taco-hvac.com/en/products/Wet Rotor Circulators/products.html?current_category=39

You want to pick a pump that puts you near the center of the performance curve. There is another guide on this site that tells you how to calculate flow rate and head pressure and BTU delivery for each flow rate and what pipe size you should use, but I can't find that one right now but if you look long enough you will find it.

Here is a simple dynamic head calculator (it doesn't add your fittings, but at your distance that's a very small percentage).

Wisbro 1" PEX has an ID of 0.863 (you need to know that to use this calculator). It varies a bit by manufacturer so if you are using Zurn or other you really need to look that up to know exactly what the wall thickness and ID is.

http://www.csgnetwork.com/csgdynamichead.html

The bad news is that replacing your PEX with bigger lines it really expensive, which is why I haven't done it. Maybe next fall.

The 0014 pump is designed for high flow / medium head. But at any reasonable GPM flow you are going to have high head pressure at your distance, so it just doesn't look like the 0014 is right for you. I think the 009 pump is for high head / medium flow applications like yours.

Maybe your dealer (like mine) didn't bother to figure this out ahead of time....

Found that Taco pump-selection guide here:

http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

It's really helpful and informative!