Wood Boiler Circulation Pump

Here's the deal, I don't know!! At the expense of the pumps he has answered some questions. If it was water quality that might eat the brass impeller and if was over temp that might ruin a plastic impeller. I would be flushing the system out because it sounds like he has dirt in the water and then add a Y strainer before the pump. Maybe others will have different input. Keep us posted & add some pictures always like homemade.

The city water in Ann Arbor Mi. will eat up a SS indirect water heater and about anything else in 6 months. "My point is check the water" I am sure this isn't the issue but make sure every thing has a good ground. Re piped a 100 lot trailer park back in the 80's because galvanized pipe was used and it rotted from the inside out. We were told by electrolysis.

sounds like the pumps are cavitating or suction boiling. Here's what happens, and it is probably the main cause of pump failure in wood boiler systems by far.

Suction boiling happens when the pressure at the impeller inlet (center) drops enough to allow the water to actually boil. How does this happen? At sea level water boils at 212*. The pump moves water through your system by creating a pressure differential from the impeller inlet to the outlet of the vanes, going from low at the inlet to high at the outer edge. If you get into a situation where the differential pressure is great enough (think high head, long lengths of small diameter tube) the water will boil at only 200-190 or even less if conditions/pressure are right. In other words, assume for example you have a circ creating a pressure differential of 8PSI. That mean you are dropping the pressure 4 PSI below static at the inlet and raising it 4 PSI above at the outlet of the impeller. Now, 4 PSI doesn't sound like much but in the case of wood boilers, many of which are open systems, it's about impossible to supply any positive pressure at the inlet.
So if you lower the pressure far enough the water boils and forms bubbles. As the system fluid, which no contains gas also, moves through the impeller the bubbles collapse once the pressure reaches a certain point and you have what amounts to hundreds of tiny hammers hitting the impeller. Plastic or reinforced plastic will actually last longer under these conditions from what I have observed.

So what do you do? Lower the head in your system by using the correct tubing size. Raise the static pressure in the system is possible. Use a pump that is sized to the pipe or tube not what GPM you have to achieve. If your tube is undersized and causing the head pressure in the system to be elevated beyond the rating of the circ, you're pretty much screwed. You can't stuff 10# into a 5# sack.

If you really want to get into the nuts and bolts of what happens, here's a good synopsis of what is going on.

http://www.engineeringtoolbox.com/npsh-net-positive-suction-head-d_634.html
http://en.wikipedia.org/wiki/NPSH

Think you are correct -- Heaterman. The Wikipedia was a good read. I was aware of this potential issue but just never ran into it. [suction boiling] Splitting up the pressure drop across two pumps might help, easier than sitting the wood boiler in the loft of the barn.

olf20 / Bob said:

A friend of mine made a homemade wood boiler. He has basically two
loops, one for the house and one for his shop. The shop loop consist
of a copper coil in the boiler and in floor Pex tubing.
He has been using taco pumps with plastic impellers.
He has gone threw two or three of these. The housing around the
bearings seem to start leaking.
So he thought it was the brand of pump and purchased a Bell & Gossett
pump with a brass impeller. Everything was fine for a while and then
he noticed he could not get enough heat in the shop. Checked this and
that, then decided to pull the pump. The brass impeller was almost gone.
He described it as it looked like feathers.

Thanks
olf20 / Bob

Click to expand...

Where are the pumps located?

On or near boiler? Or in the house and shop?

bigburner said:

Think you are correct -- Heaterman. The Wikipedia was a good read. I was aware of this potential issue but just never ran into it. [suction boiling] Splitting up the pressure drop across two pumps might help, easier than sitting the wood boiler in the loft of the barn.

Click to expand...

The thing to remember is to use two smaller size pumps, at least in most applications. Otherwise you just end up with the same problem with two pumps instead of one. What a person wants to do is reduce that internal pressure differential in the pump somehow.

my point was that the suction boiling is caused by a large [enough] pressure drop a cross a larger pump and by installing two smaller pumps that either pump will ever see that pressure drop but still develop the required head. In the non pressurized system this would make the most sense to me.

Just guessin' here, but I wonder if this gentlemen has his pumps at a half way point in his loops.

These pumps can only push so much head but if they can't get the water to them without a big pressure drop the impellers are going to get eatin' up. This is what I'm thinking. He's trying to "breathe through a straw", if you will, with these pumps.

My suggest would be to move the pumps as close to the bulk water supply and use at least 1" to feed them. Shutting a discharge valve on "smaller centrifugal" doesn't cavitate the pump but start closing the suction and it will. This is what I picture.

This same phenomenon (cavitation) can happen not just in a pipe.

Put the wrong propeller on a boat and you can watch a heavy piece of the toughest marine alloy get eaten up the same as those flimsy little plastic numbers in a circulator.