Taco 007e & High altitude

[djdavenport]

I just ordered in the new Taco 007 ECM pump (mostly for the cool LED), but in reviewing the installation instructions, I came across this:

CAUTION: Installations at elevations over 5000 feet must have higher fill pressure of 20 psi minimum to prevent pump cavitation and flashing. Premature failure may result. Adjust expansion tank pressure to equal fill pressure. A larger size expansion tank may be required.

We're at 7,100'. I've been using a regular 007 for years with no apparent problems. Boiler pressure has always been set about 12 psi.

Am I understanding this correctly? Do I need to boost the pressure to 20 psi?

Tnx.

 

[Dana]

At sea level the atmospheric air pressure is ~15 psi, and charging the system to 12 psi above the ambient air pressure means it's really ~27 psi in absolute terms.

At 5000' the air pressure is about 12 psi, which it 3 psi lower than at sea level. At 7000' the air pressure is ~11 psi, 4 psi lower than at sea level.

I suspect they got lazy and used the 20 psi to cover the full range of altitudes where humans live. At 17,000' the air pressure is about 7 psi, and running the system at 20 psi would deliver the same 27 psi total that would happen if pre-charging to 12 psi at sea level. Very few people live at 17,000', yet quite a few live above 16,000' (just not in the US.)

At a mere 7000' bumping the system pressure to 16 psi would be the same as setting it to 12 psi at sea level.

There is no harm in raising the pressure as long as the expansion tank is properly sized and properly pre-charged, but I doubt it has to be anything like 20 psi to prevent cavitation and flash-boil in your location.

 

[Sponsor]

 

[djdavenport]

Thanks, Dana. Things are so nicely dialed in at the moment, I'm hesitant to mess with it too much. When I swap out the circulator, I'm might nudge the pressure up a little, since I'm going to need to refill the system anyway, but going up 40% seemed a little much.

 

[Dana]

Snooping around the web a bit, some hydronic system designers use a linear pressure rule of thumb model, specifying 12-15 psi at sea level (depending on system particulars- higher psi for boilers with higher head heat exchangers), then adding 0.5 psi per 1000' of elevation to minimize flash-boil and cavitation issues. Using that rule of thumb, at 7000' that would mean bumping it up to 12 psi + (7K x 0.5 psi)= 15.5 psi as the "reasonable" pressure at which most system pressure problems are well-controlled. Setting it to 15-16 psi should be good enough, and prudent even though you've proven that your system works fine at 12 psi.

The problem is the same with all pumps, all vendors- the laws of physics are self-enforcing. Seems Taco has been at least acknowledging the physics for at least a half-dozen years, even if their prescriptive 20 psi is a bit ridiculous at altitudes below 15,000'. Clearly physics doesn't take a sharp excursion at 5000'. Even if the pressure change with altitude isn't exactly linear, is still a pretty smooth curve, and close enough to linear that the 0.5 psi per 1000 rule of thumb is practical.

 

[Jadnashua]

Keep in mind that the amount of height above the boiler you need to pump the water also determines the pressure you need to prevent problems. That 12psi is based on a typical 2-story home so that the pressure doesn't get too low at the top. As you know, water boils at a lower temperature as you go up, and at the top of the run, that pressure must be high enough to keep it from flashing to steam there. Typically, the pump isn't up there, it's at the bottom, but that could also be a factor with the boiler pressure...it must be a positive value at the top. MOst modern residential boilers tend to not use super high water temperatures, so that's not as big of a deal as with some older installs. Pressure drops at 0.43#/foot elevation gain.

 

[djdavenport]

Good info, Jim. Thanks. We're single story, in-slab radiant, and the supply side never gets above 120*F, so getting into a situation of flashing to steam is less a problem. I just was surprised since I had never realized that there was any engineering considerations having to do with the pumps tied to elevation. And, I tend to believe that hydronic pressure--like blood pressure--should be kept as low as possible while still doing the job. But, bumping it up a few psi is easily done. I'll give it a try.