Hydronic system

palombardi22

I was working on a set of boilers that were piped into the same system on a 5 story building. It is a hydronic system and both boilers run together and are gas. A tenant at the end of the riser is having an issue with one of his fan heating/cooling units. Air keeps getting into the system because the super and tenants “bleed the lines” and the super tells them they are responsible for doing this... BAD IDEA... I’ve managed to hopefully fix this issue. 

But I now have a question. So I shut both boilers and the system cooled. I bled the air out of the feed and return line from the tenants unit on the 5th floor (end of run). 

As the system started to heat up the pressure increased to more than the capacity of the boilers capability. So I opened the relief valve several times to lessen the pressure. I then realized that it may have been better to fill a bucket of water, and put a hose on the hose cock for the return, and stick the hose in the bucket of water and open the hose cock intermittently to bleed the excess pressure out of the system. 

Does this ultimately matter or I guess what I am asking is, will the safety popping reintroduce air back into the system resulting in me needing to bleed the system again.

If anyone has any helpful answers please let me know. Thank you!

EdTheHeaterMan

This sounds like you have insufficient room for the water to expand. This happens when the conventional expansion tank becomes waterlogged. If you have a bladder-type expansion tank then the air pressure is too low or the bladder has failed. From a cold start, the system on a 5 story building with the boiler in the basement or ground floor should have a static (cold) pressure of about 18 PSI on a working pressure gauge. Once the boiler is filled to that pressure the air can be removed from the radiators or hear emitters all the way up to the 5th floor.

When the water is heated the pressure should rise to about 22 PSI. If that is not the case you need ot figure out why the expansion tank is not accepting the expansion. If your have a standard boiler that is rated to 30 PSI (the rating is on the boiler rating plate along with the relief valve specification and the BTU ratings) then it is very important to have this pressure regulated properly. Some commercial boilers have a 50 PSI maximum operating pressure rating. This allows for a little more leeway in adjusting the static fill pressure and the operating pressure.

What pressure is your boiler operating with and what is the BTU input, BTU output, or Net I=B=R rating in BTUs? This will help to determine your situation.

Yours Truly,
Mr.Ed

EBEBRATT-Ed

@palombardi22

In addition to the above comments. I have seen buildings with multiple floors suck air in if you have automatic air vents at the top floor IF THE circulators are not pumping away from the expansion tank.

Also what type of expansion tank do you have. If it's a bladder tank you can use auto air vents. If it's a plain steel compression tank you cant

palombardi22

So you’re saying from a completely cold start, the system itself should be able to handle the pressure increase and if not then there is a faulty expansion tank? Because I plucked the expansion tank and it was all filled with air or seemed hollow from what I could tell. Which typically means the bladder is functioning. But I totally see what you’re saying. The pressure I had brought the system to on cold was 40psi expecting the expansion to increase to approximately 55psi which would put me at the pressure needed to get me to the unit, 1psi = 2.31 feet of water. I mean it sounds like I was completely wrong. With what you guys are saying. It seems as though maybe I needed less pressure or is 40psi correct?

palombardi22

Also I don’t recall what expansion tank it was, but it looks like it was sized for the system. I’ll see if I can double check that or return at a later date and get back to you. There are no vents on the line at all except for one atop the boiler.

Jamie Hall

Roughly 50 feet up from the boiler to the highest radiator? That would be 22 psi cold as a minimum. This is going to increase as the temperature of the water rises and it expands, and you don't have a whole lot of leeway there with a 30 psi boiler and 30 psi relief valve.

So my first question is -- what is the rated maximum operating pressure of the boilers? It should be marked on them. Likewise, what is the rated cracking pressure of the relief valves? If the boilers are rated at 30 psi, you are going to have to do two things.

First, the expansion tank will have to be big enough. There are sizing guides for this, but I believe that Amtrol -- among others -- has assistance for professionals in sizing their tanks for unusual situations. It is very likely, from your description, that the tank simply isn't big enough, even if it's operating properly.

Second -- and this is really important -- the circulating pumps will have to be installed so that they are pumping away from the expansion tank. If they are not installed that way, the pressure throughout the system will drop when the pumps run, and I can almost guarantee that you will be pulling a vacuum on the highest radiators -- and getting air in.

If your boilers and relief valves are rated for 50 psi instead, you have more headroom to play with. You still want to pump away, and you still want your cold pressure around 25 psi, but the expansion tank can be a little smaller and still serve.

JimP

It would be a good idea to eliminate any automatic air vents that might be on the upper floor(s). I've encountered this situation where any air in the system makes it's way to those radiators that are highest and farthest away. I've been told that a system like this would work better with the boilers and pumps on the top floor.