Ex 2 pipe steam FHW tankless system blowing off

Dan_17

I'm a heat novice, Engineer with tools, still fighting my old system.
Believed to be 2 pipe steam at one time, now FHW with tankless boiler circa 1970 (looks to be original gun, too)
Has large cylindrical steel expansion tank, no-diaphragm type.
Last year it started blowing off about a half gallon or so of water every time the system heats up and the circulator went on (single zone...). It did not do this previously. Here is what has happened / been observed:
Cold, it fills to around 12-15 psi. As it heats up, the pressure builds to nearly 30 psi. Then when the circulator goes on it goes up to 30 and blows off a little water. (blowoff working properly, right?)
If I shut off the water infeed, it will eventually go down below 5 psi and level off, then never blow off when the circulator goes on.
I believe the infeed valve is working properly.
I have drained the expansion tank (with the valve at the top of the boiler closed) twice, and got about a gallon of water out each time. (**shouldn't this be around half full??) Then when I open the valve again I hear a rush of water.
Here is what most likely started this whole mess:
We took out a small loop to 1 first floor radiator and just put 1" ball valves about 3 feet from the main F&R for future in-floor or something. (We took out the wall to the kitchen, it's a long story) This loop looks to have been added on, since it is 1" copper and the rest of the house is black pipe. There are 4 other radiators on this floor. The system was drained down to do this, then we bled the system starting on the first floor (of 3). Oh yeah, except for the one in the bathroom on the second floor that I can't get to the bleeder (an even longer story) and I just loosened the top connection near the valve to let the air out.
Since then I have bled the system several times and have gotten much air out of one radiator on the 3rd floor every time.
So....
If my infeed valve is working properly, and my sizeable expansion tank is not overfull, what is causing the pressure buildup? Do I still have much trapped air? Could the ball valves be letting air into the system? (read of this somewhere, makes no sense to me) Is my expansion tank really draining fully, or does it airlock or something and not let all the water out without an inlet for air on the top?
I just can't imagine that taking off a loop that was obviously added within recent history compared to the rest of the system could cause this much trouble.
Or is my infeed valve really not working properly? Could it be opening up as it heats up? Probably not, since the blowoff only happens the first time the circulator comes on when it cycles on several times in a row.
Please help. I have had bad experiences with just about every person I have hired to fix anything of mine in the last 10 years so I am determined to fix this myself. If I can figure out my wife's 97 Passat sh!tbox lemon electrical issues I can certainly get through this.
Thanks for your help, and letting me 'vent'...
-Dan in Boston.

brucewo1b

Dan

If you are only getting a galllon out of the compression tank then I will bet it is not draining properly. Is there a way to let air it while you drain usually a small nut around the drain that lets air in while water comes out, sometmes is just the drain and a long process of air in water out over and over until its drained. Also any air in rasdiators will work as an compression tank also so if the tank is full and you let more air out the worse you make it on yourself. Try draining that tank again close it off to the system and drain.

Timco

Under 'shop' you will see 'Pumping Away' by Dan Holohan. This book describes in detail how to deal with all aspects of non-diaphram tanks...how to drain, fill and check the tank. These tanks require a special fitting so heat does not migrate the air out and flood it.

Tim