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Adjusting bladder tank pressure while the system is hot
Everything I have always read or videos I have seen about bladder tanks discusses adjusting the cold fill pressure. I have a situation where I want to make an adjustment while hot. It would seem that there is nothing fundamentally wrong with this concept, but I just wanted to get some thoughts. I have never adjusted one while hot.
The situation is that a client was popping reliefs due to lack of expansion tank volume. The highest points in the system were actually in vacuum due to the low pressure. Although all of the components in the system were rated much higher, the boilers had 30 psig relief valves. I calculated that changing the reliefs to 50 psig would do the trick, but the owner was reluctant. So instead, we installed an additional bladder tank in parallel to the existing. This was over the summer, but they still ran the boiler for reheat.
The system is 50% propylene, which I have argued is too high, but it is what it is. The client was going to change the system glycol in the fall, and then my company (I am an engineer working for a contractor) would adjust pressures and start up the new tank. Unbeknownst to us, the client activated the system after changing the glycol, and now we are of course in the heating season.
When I ran the calculations, I recommended an 18 psig cold fill pressure, and figured that the system would reach around 25 psig when fully expanded at 180 deg. I was in the building the other day for another reason and asked about the system. They are having issues in the same remote location where they have always had a lack of flow. I will be taking a GPM reading, but I have my suspicions. The expansion tank pressure was only 20 psig while hot - not the 25 psig I calculated. The pipes that feed this remote unit are the highest in the building. I am wondering if the lines are air bound, etc.
I can't get a straight answer on the fill pressure they used, so I am wondering if I should add some air pressure to the tanks while hot. i will see if I can get a gauge pressure reading at the highest point also.