DHW boiler pump cavitation

steveyared

My DHW pump creates cavitation and then the air vent releases the air. I am thinking the pump is over sized. I have lowered the pump speed but the condition still occurs. Is the current pump oversized? Grundfos UPS26-99FC, 3-Speed Circulator Pump, 1/6 HP, 115 volt,

should I try a Grundfos UPS15-58FC, 3-Speed Circulator Pump, 1/25 HP, 115 volt

The pump is installed on a 1 inch copper loop from a boiler 5 feet from the storage tank, Lochinvar high efficiency boiler is approx 250,000 BTU

Jamie Hall

Air entrainment and cavitation are two entirely different things, and the ways to attack them are completely different.

I doubt that the specific pump is the problem, but it's quite possible that something about the piping or location of constant pressure may well be.

You mention 5 feet from a "storage tank". Would this be a pneumatic tank? And what pressure is it set at and what pressure does the system maintain?

hot_rod

A pic would help.

Is this a DHW boiler circulating to a storage tank?

Or a heating boiler piped to an indirect tank? It matters for the pump sizing.

steveyared

Here's the system, thanks.

Alan (California Radiant) Forbes

If it's an indirect water heater, that is, a tank with some kind of heat exchanger inside, the pressure drop through the heat exchanger and associated piping will dictate the pump size. What is the make and model of the indirect tank, if that is what it is?

hot_rod

What I can't see in the pic is an air separator in the piping? Looks like the black circulator is a heating circuit, the red pump supplies the indirect tank.

No, the circ for the tank is not oversized, but you may have air trapped in the piping loop.

On trick, carefully remove the large screw in that Grundfos circ. You will get some water drip out and it may be very hot, even steam if that circulator is air locked. Allow any air out until a solid stream of water comes out. It may purge enough air to quiet it down.

Sometimes on an indirect loop like that it is good to have an air separator.

Alan (California Radiant) Forbes

If what you have there is a Lochinvar Squire or Lochinvar glass-lined indirect water heater, the pump is too large. The pressure drop through the heat exchanger and piping is only about 5' and unless your boiler is over 130,000 BTU, you would only need a Grundfos 15-58 on speed 2 or 3.

Edit: I just noticed your boiler size: 250K BTU. The pump looks like it's sized correctly.

steveyared

Every time the dhw pump operates, soon after running i can hear that air is in the system, the air eventually escapes out an air vent at the top of the boiler output pipe.. The boiler has been installed for 10 years.

This seems to occur when the boiler output temp reaches 180 degrees and higher

hot_rod

The higher the boiler temperature the more air is driven out of solution, so the higher temperature condition is when the air separators do the finishing off of air removal. It is possible that not all the air in the coils never makes it to the boiler vent.

That boiler vent is a high point air vent, not an air purger or separator. Highest temperature and lowest pressure are the best places to grab air.

If you can burp air out of that pump screw occasionally, that is an indication that the system still has some entrained air circulating.

Now, if for some reason you have a partially closed valve, collapsed pipe, plugged Y strainer, obstructed check valve in pump, etc, that can induce cavitation, which is often confused for an air problem