Expansion tank pre-charge pressure

[ulooknatme]

I'm getting real close to adding water to the tanks. With all the pipe plus the tanks I guess I'll be real close to 2000 gal of water. What I have is 3 (52 gal exp tanks) They come pre charged to 38 psi. My system will be 12 psi. Is this pre charge OK or should I fine tune it?

 

[jebatty]

My thought is that system pressure in a pressurized system should be about 2-4 psi higher than the pressure of water at the maximum system height. 12 psi will raise water to about 30 feet, plenty for most two story houses. If you have only one story, 6 psi should be fine. Then, your pressure tank should be set at the system pressure point. If you have higher pressure in your pressure tank, it will not start accepting expansion water until the system reaches the pressure tank setting. As most home boilers have 30 psi maximum pressure, you now should see your problem with your tanks.

BTW, with your high pressure tank setting, are these cold water, well pressure tanks, not hot water expansion tanks? If so, make sure your tanks are rated for high temp boiler water. Then reduce the pressure appropriately.

 

[stee6043]

Per the above, I think you're currently WAY high. I charged both of my Elbi tanks to 12psi per the factory spec. I run a cold system pressure of 14psi. I typically max out at 20-22psi when up to full temp.

Are your tanks hard-piped in the middle of your garage? Seems like a damage-prone area to have your tanks. If I were you I'd build a shelf near your storage tanks for your expansion tanks. Get them out of the line of fire...and I agree with the above...those tanks look awful similar to my well pressure tank...

 

[ulooknatme]

Well, I have the tanks in the basement and there are two floors above that, so are we talking maybe 18psi? And yes, these tanks are your standard well tanks but are rated up to 120 degrees. My thought here was to keep the tanks away from the heat storage enough that the temps should be well below the 120. I have almost 40' of copper running to them and hope that since the water slowly expands the water to the tanks, that the water temp will drop significantly before arriving to them. The shelves I picked up at Costco and are rated at 2000# each shelf.
(talking about the tanks, I must admit I'm doing a little tap dancing because when I bought them, I didn't know there was a difference until I read 120 max temp in the manual) although I've been reading this web site fathfully for two years! Maybe there should be some kind of test you are required to take here before attempting to install it yourself. :lol:

 

[stee6043]

Ohhhhhhh......I don't know about this plan, Ulook. My storage temps fluctuate between 180 and 140 between firings. I guarantee you the hot water pushes into these expansion tanks when your system bumps from low temp to high temp. You can feel it in the lines. And it's not "luke warm" by any means, at least on my system.

I have approx. 15' of pex between my "system" and my expansion tanks. My whole expansion tank warms up when I reach the end of a burn cycle. I also know the feed-end of the pex going to my expansion tanks reads 180 when my boiler is cranking. Unfortunately, I don't measure temp at the expansion tank since there is really no reason to typically.

I think you may be asking for trouble here. Failed bladders will be a pain in the butt to deal with mid-heating season. You may be flirting with disaster...or you may be fine given the distance of your copper run....tough call. But I like your willingness to give it a shot! Never give up!

 

[in hot water]

You calculate from the point at which the expansion tank connects into the system to the highest point in the piping. Here is an example that may fit your job, with 850 gallons of fluid.

As for the temperature, risky at best. Those tanks will see some hot fluid when the fluid expands.

Most hydronic expansion tanks are rated to 210F max. temperature on the diaphragm.

We have a similar situation with solar, where it is possible to get a slug of 350F or higher fluid against the tank. Zilmet the folks that build our tanks recommend a surge tank between the piping and the expansion tank itself.

This link explains a bit about that. You may just buy a steel compression, expansion tank, maybe 12 gallons or so to install in the piping between the boiler and tanks.

(broken link removed to http://www.zilmet.com/cataloghi/14solarplus_ingl.pdf)

Ideally you would buy the correct expansion tank of course. :0

hr

 

[pybyr]

Interesting info and link HR, thanks-- it's terrific how you provide info on products and techniques that even pros and vendors by and large may not be familiar with in the US.

Regarding the extra tank that you mention between the main system and ulooknatme's well-type tanks could be handled by way of a non-bladder old style domestic water storage tank- as you basically just use that in-between tank to mix its own cool water in order to reduce the temperature going to the expansion tanks

 

[in hot water]

yes, a basic off the shelf hydronic compression tank. under 200 bucks at heating suppliers.

Or use copper fin tube from HW baseboard to pipe from the boiler to the tanks.

The Gray one is a blank hydronic tank, I added some connections for a solar drainback tank.

Th other one is an air compressor tank, again modified for drain back.

Or buy a scratch and dent 10 or 20 gallon water heater and strip the insulation.

An old gas fired HW tank with the hole in the center for more heat dissipation. Plenty of ways to build a tank/ emitter.

 

[deerefanatic]

Why not use a small air coil plumbed inline with a small fan.... That will cool the water and act as a small space heater......

 

[ulooknatme]

You guys have me running scared. Talked to my brother who works at a plumbing supply store and he says he can get me a used hot water heater for nothing. Seems the plumbers like to use their dumpster to drop off old tanks when picking up new ones. I guess I'll try running this tank before the expansion tanks and keep an eye on the temps.
Finally got my cir pumps in yesterday and will install today, pressurize and hopefully not find too many leaks in the remaining system.

 

[deerefanatic]

Yah, a water heater tank should work dandy.... Plumb the hot water from the system into the top, and the bottom to your bladder tanks....

Or do a small air handler like I suggested....

 

[in hot water]

In the solar industry we use these heat dump radiators to handle excess heat. They get mounted outside the building. A fairly simple convector, a piece of commercial fin tube would work as well.

Yes, a fan convector would work, but it would cost $$ to power the fan.

I'd try the blank steel water heater, pipe from the boiler to the top, and pipe from the bottom to the expansion, let the higher temperature stratify in the tank.

hr