water not circulating

[uglydukwling]

My heat exchanger on the first floor of the house is 4 or 5 feet above the top of the boiler. When I installed it, I wondered if the pump would be able to keep it full, but it's worked perfectly for 5 winters. Sometimes I haven't even had to bleed the air out of it after the pump has been shut off all summer. Until this week, that is.

I had an underground leak and the system lost most of its water. I fixed the leak and now I can't get the heat exchanger to fill. The only way I can get water up to the bleed valve at the top of the exchanger is to shut off the valve on the outlet, with the inlet valve open and the pump running. Otherwise nothing comes out. Obviously, the pump is moving water, but not enough to keep the exchanger full.

The pump is in the basement at the low point of the system, lower than the leak, so I doubt if it ran dry. Even if it did, I believe it has sealed bearings, so it shouldn't depend on water for lubrication.

The next thing I'm going to do is take the filter apart and see if it's obstructed.

If that's not the problem, the only other thing I can think of is to pull the pump and look for damage. If the vanes are damaged, I've found at least part of the problem. If they look ok, it's going to be harder to tell if the pump is turning fast enough (it's turning enough to move at least some water).

Any suggestions of what else I should look for?

 

[wborum]

More than likely your impeller is toast.

 

[Encore]

Agreed. My guess would be the impeller fried when the water ran down.

Either that or you have an air lock in the heat exchanger.

 

[trophyhunter]

All the symptoms of a worn or shredded impeller, bet you find your problem there. Air is easy to get bled out of the system when the pump is moving a sufficient volume of fluid.

 

[uglydukwling]

I cleaned the filter and that seemed to do the job. I'm not sure whether the impeller is perfect, but it's working, so I didn't bother to open the pump to examine it. Maybe next summer when I'm not in so much of a hurry to get it back into service. My guess is that when the water level dropped, the water boiled and the boiling stirred up the crud (corrosion, hard water scale, welding slag, mill scale) that had been stuck to the inside of the boiler or sitting harmlessly at the botttom.