Expansion Tank Capacity/Pressure

What is the air pressure in the expansion tanks now? It would preferably be measured with the wet side having no pressure - but with system cold & showing 10psi, air psi should be around 11, I think. I installed a new B&G tank a couple months ago here, it was supposed to be precharged to 12psi. I checked it & got around 16. Thought I had a wonky guage, but let air out to get it to 12-13 (with system cold & at 8), and it didn't seem to take a whole lot of heating before my system was rising more than I was comfy with. So I let another couple pounds out - much better. So seems it came charged high from the factory.

Should be an air valve on them - looks like tire valve stem.

One way to fudge a little more expansion capacity out of a given tank is to mount it up above the level of the boiler.

My system is about 1100 gallons total and I use an 80gallon 304 stainless tank I got from an industrial surplus place. It's mounted in a 2nd floor closet about 17 ft. above the bottom of the boiler. At the start of the heating season (system temp about 68F) the pressure in the tank is only about 2 or 3PSI with only an inch or so water in it. At that point the pressure at the bottom of the boiler is about 10PSI. When the whole storage tank is about 190F (top to bottom) the pressure at the boiler is about mid 20s PSI (I forget exactly).

This size tank might be below optimum for a conventional installation, but by cheating with a little elevation it works well within the normal range.

By starting out with lower pressure in it, the expansion tank can accept more water before it gets up to the same max pressure.

There are sizing tools around - but yes sometimes there's some trial & error. In my world at least. You want precharge to be a pound or so more than cold system pressure, I think. I also had a time finding a low pressure guage - still leery of it.

After I plumbed in my new bladder tank I found I was getting more rise than I wanted, so I added yet another one. It's an old-school bladderless cushion tank (but pretty new - kijiji find), 15 gallons. I could get it into a space way up high under the top of our stairway to second floor. So it's almost at the very top of my system. Now I'm happy again.

Before all those off-season changes, I had a 110 gallon LP tank for expansion, on the floor beside my storage. I was very happy with my system pressures with it, but I was getting a bit of air in my system at times from it. I would have kept using it if I could have found another place for it up high, but 3 years of thinking about it went nowhere. So now I've got a slightly used fully equipped 110 gallon LP expansion tank sitting in the middle of my basement that I'm always tripping over. It would likely make a really nice air compressor tank.

If you're re-plumbing now, it might not hurt to plumb in an extra stub with a valve on it so you could add in more later if you think it might be a possibility.

Mainly I saw my relief valve weeping last season as the pressure crept to 25 psi. The relief valve being replaced, but see no need for more than 15 psi in the system with storage fully charged.

My thoughts as well, exactly. 15 is what I like to see. The relief my boiler came with is I think only something like 22psi, which is what the design pressure of the boiler is also. So when it was getting above 18, I was getting a bit squirmy.

I am curious about this topic.

Marc originally installed with my BioWin 210 a SX-20V, but I had him up it to SX-60V instead. I am still seeing pressure from ~13 psi to ~25 psi when going from 70°F to 180°F. I have about 130 gallons of water in the heating loop, with about 120 gallons of that in a thermal storage tank.

I didn't actually measure the psi of the SX-60V, or the SX-20V before that. The SX-20V is gone, but I can measure the SX-60V this weekend.

Galroc said:

I am curious about this topic.

Marc originally installed with my BioWin 210 a SX-20V, but I had him up it to SX-60V instead. I am still seeing pressure from ~13 psi to ~25 psi when going from 70°F to 180°F. I have about 130 gallons of water in the heating loop, with about 120 gallons of that in a thermal storage tank.

I didn't actually measure the psi of the SX-60V, or the SX-20V before that. The SX-20V is gone, but I can measure the SX-60V this weekend.

Click to expand...

Google up the Amtrol hydronic expansion sizer & play with it. I input what you're seeing & it said a 20V tank would do that. I had to lower the upper PSI from 25 all the way down to 17 before it gave a 60v solution. So something seems off - could be the precharge. You will have to cool your system all the way down to your lower temp (70) to get a decent read on air precharge. Or, if it's plumbed for it, valve off the expansion tank & drain all the pressure out of the line between the valve & the expansion tank. Not sure how high (elevation) your system is but you might be able to drop a couple pounds on your cold system PSI.

Are you sure about your volume? That would only leave a very few gallons for the water in your boiler - but no idea what your boiler capacity is.

I played with that calculator as well and I got either 20V or 40V. I opted for the 60V because it doesn't hurt to go bigger (other than my wallet) and it was a small price difference considering the total cost of the system.

The pellet boiler and the thermal storage/baseboard are separate. They have to be in Massachusetts. Marc uses a heat exchanger to move heat from the boiler to thermal storage/baseboard.

I will have to check the precharge, but it might have to wait until after winter. I do have a pellet insert for back up, but it is so nice to have the pellet boiler running. It takes a day to cool down the thermal storage, assuming I dump heat into the house.

Marc just did install a valve on the line going to the expansion tank, so I could actually check it while the boiler remains on I suppose.