Stocker Steve
Well-known member
I have an outdoor hot water wood boiler that uses up a lot of recirculating pumps. I bought a better quality pump and they suggested a filter or screen. Any suggestions on this, or water additives, or?
Boiler Recirculating Pump ?
- Thread starter Stocker Steve
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bigbluegrass
Well-known member
What temperature do you run your boiler? What is the pipe size from the boiler? Is the pump located above or below the elevation of the boiler? What was the make and model of the old pumps vs the new pump?
I have had the same pump on mine for 10 years. Last fall it was sounding a bit rough, so I am looking for a replacement. It actually sounds fine now, so I might buy one and have it on the shelf just in case. Mine is a Bell and Gossett PL 45. I do not have a filter or screen. The water is usually slightly discolored, but no chunks or flakes. If you are getting flakes or chunks, you need to work on your water treatment. I add the boiler treatment recommended for the boiler.
I have had the same pump on mine for 10 years. Last fall it was sounding a bit rough, so I am looking for a replacement. It actually sounds fine now, so I might buy one and have it on the shelf just in case. Mine is a Bell and Gossett PL 45. I do not have a filter or screen. The water is usually slightly discolored, but no chunks or flakes. If you are getting flakes or chunks, you need to work on your water treatment. I add the boiler treatment recommended for the boiler.
Silver
Well-known member
I've never had anything to do with these, but a friend was showing me his boiler that he had built from scratch and I do remember him saying that all his pump problems went away when he moved his pump from the hot side to the return side. Maybe that's how all the manufactured boilers work, I don't know.
simme
Old Dumb Guy
Does it produce steam or hot water? My comments are assuming it heats the water, but does not make steam. Pumps have a minimum suction (inlet) pressure requirement. That number goes up with temperature. If the suction pressure is lower than that min suction pressure number, the pump will cavitate and won't last long. Cavitation is when the liquid (water in this case) goes below the vapor pressure. Vapor pressure depends on temperature. The pump suction "sucks" the water into the pump - causing a lower pressure. Hot water wants to boil at low pressure. Might be getting cavitation in the pump impeller. As soon as the water move to the discharge, pressure goes up, vapor bubbles collapse and release energy that beats the metal up. This fits with the @Silver comment above that moving the pump to the return (colder) side might be the solution. Cooler water much less likely to cavitate. Severe cavitation will make a sound. Is the water temperature from the pump within the system specs? Also look at how the system is pressurized. Do you have a makeup water line with a pressure regulator and expansion tank that keeps the closed system at a certain pressure? Keeping the system pressure higher may be a solution as well.
bigbluegrass
Well-known member
Most of these outdoor wood boilers are open systems, so no expansion tank or pressure regulator. On mine, make-up water is manual. I have to check it every month of so and top it off. I imagine some of the nicer ones might have a fill valve.
Stocker Steve
Well-known member
Open system set to 185 F. Pump is in the basement below the boiler on the hot side.
bigbluegrass
Well-known member
Having it in the basement is a good thing.
Do you use your boiler to heat hot water?
The 185 F is pretty close to boiling so your pump is probably wearing out from cavitation as mentioned by @simme above. The easiest fix is to set your boiler temperature lower (I set mine at 140F as long as I can, until it gets really cold). But if you use it to heat hot water that might not work without creating other problems. Really, that depends on the type of heat exchanger used to heat hot water. If it is the type that goes on the cold water supply, you are probably fine at a lower temperature. If it is the type that goes on the side, then you won't have as much hot water.
Another fix is to move the pump to the return side, as mentioned by @Silver above.
Most likely issue is the water is too hot going into the pump.
Or maybe you just got some bad pumps...
Do you use your boiler to heat hot water?
The 185 F is pretty close to boiling so your pump is probably wearing out from cavitation as mentioned by @simme above. The easiest fix is to set your boiler temperature lower (I set mine at 140F as long as I can, until it gets really cold). But if you use it to heat hot water that might not work without creating other problems. Really, that depends on the type of heat exchanger used to heat hot water. If it is the type that goes on the cold water supply, you are probably fine at a lower temperature. If it is the type that goes on the side, then you won't have as much hot water.
Another fix is to move the pump to the return side, as mentioned by @Silver above.
Most likely issue is the water is too hot going into the pump.
Or maybe you just got some bad pumps...
Stocker Steve
Well-known member
I do heat my hot water with the boiler, and I think there is some room for adjustment. I will try lowering the water sp
Will the pump efficiency change if it is on the return side?
Will the pump efficiency change if it is on the return side?
bigbluegrass
Well-known member
No. Theoretical pump efficiency should be the same regardless of if the pump is on the supply or return side. In reality, if the pump is cavitating, the efficiency will go up if it is moved to a location it is not cavitating.
JKCattle
Well-known member
Make sure the lines are bled very well because they vapor lock fairly easy and those pumps don't last long if they are dry. My water jacket wood stove is a closed system with a chemical I replace every two years that reduces corrosion and lime in the water. The way the outflow and inflow lines are set up, the sediment falls to the bottom. I clean that out every two years and there is usually 1-2 gallons of sediment out of 450 gal. of water.
Stocker Steve
Well-known member
You have a drain on the bottom of the boiler?
simme
Old Dumb Guy
I don't have any experience with wood burning water heaters, but do have experience with industrial pump systems and pipe systems. As mentioned, a centrifugal pump has a minimum suction pressure required in order to operate properly without cavitation damage. With an open system, the suction pressure at the pump in the basement is due to the elevation difference from the water tank in the boiler to the pump in the basement (the "head" of the water in the pipe). That is with the pump not running. When the pump runs, you have pressure drop in the suction pipe due to friction. That pressure drop lowers the pressure at the pump suction. If you move the pump to the return side, then there is also the pressure drop through the heat exchanger which lowers the suction pressure at the pump even more. But the temperature is lower, so the required minimum pressure is less. The term is NPSHr (net positive suction head required). Moving the pump to the cool side gives you less suction head (pressure), but also the pump required suction head is less due to cooler water. If you have the pump model and size, you should be able to find/calculate the suction pressure required.
Here is some technical explanation.
blog.craneengineering.net
Larger suction pipe will increase the suction pressure (less pressure drop in the pipe) and will help if the existing pipe is undersized. Also, any valves in the suction pipe should be full open and high capacity (like a full port ball valve for less pressure drop).
Here is some technical explanation.
How Does Fluid Temperature Affect NPSH, and Why is it Important?
Many factors go into calculating NPSHa and selecting the right pump for your process, but your fluid temperature might be playing a larger role than you think.
blog.craneengineering.net
Larger suction pipe will increase the suction pressure (less pressure drop in the pipe) and will help if the existing pipe is undersized. Also, any valves in the suction pipe should be full open and high capacity (like a full port ball valve for less pressure drop).
We have a Hardy outdoor unit and if I'm remembering correctly, the water temp is set at 165-170F. It has a Taco brand recirculating pump and it's the original one that came on it 6 or 8 years ago. I bought a new pump last year just to keep on the shelf, it's a carbon steel pump assembly whereas the original one is stainless steel.
Stocker Steve
Well-known member
Menards no longer stocks the replacement cartridge to repair a Taco 007 pump... They are on internet, but they cost more than a new pump in the store.
I am treating my boiler water and lowering the max temp.. I think high temp did me in. Moving the pump to return side is the last resort.
I am treating my boiler water and lowering the max temp.. I think high temp did me in. Moving the pump to return side is the last resort.
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JKCattle
Well-known member
We have a Taylor water stove. Water temperature in the stove is set at 180 degrees. Used to go through a pump every year until I learned how important it was to keep water above certain level in the stove. If air ever got in the lines the pump would vapor lock and overheat. Since I figured that out, haven't replaced a pump in 7-8 years.
What do you treat your water for?
Our Hardy is 300 series stainless, and doesn't need the water changed each year per mfg. The man I bought it from tested my water and said we didn't need to treat it per Hardy's spec. My water level float bit the dust this year, and it had a good 1/8"+ buildup of calcium on the moving parts - kind of concerns me.
Stocker Steve
Well-known member
There are a huge number of water treatments out there to deal with corrosion.
The actual test kit that came with our boiler is a nitrate test to indicate the amount of corrosion inhibitor in the water.
The actual test kit that came with our boiler is a nitrate test to indicate the amount of corrosion inhibitor in the water.
