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So what should the boiler pressure be??

dbsoccer
dbsoccer Member Posts: 28
I've have completed the majority of some long over-due maintenance on my 1985 Burham boiler. It is one of their power vent series and has worked pretty much without issue for 35 years. This past winter I discovered a defective expansion tank but not until I had created a bit of mess and drained water from the system. This caused noise in the registers as the house was being heated. During my assessment of the issue, beside needing to replace the expansion tank ASAP, I also need to replace the boiler drain valve and I needed to replace the purge valve. The tridicator was defective and I figured while I'm at it I'll upgrade the pressure gauge to my Watts RBFF.

The tank was replaced immediately so I could heat the house, albeit with noisy pipes, with the plan to shut things down this summer when the boiler only runs to heat the hot water. And today was the day. And all the replacements went as planned - two valves and two gauges. The purging went as expected.

After the boiler fired and brought the Hot Water Maker up to temp things shut down as expected. And it will stay this way until someone takes a shower or runs the dishwasher.

So my question is in these periods of downtime what should I expect the system pressure to be? I upgraded the gauge on the RBFF with a quality Ashcroft replacement (0-30psi) . And when I just looked, the gauge was reading 10psi. The pressure on the new tridicator is also 10psi but it is more difficult to read accurately due to its location and because the scale is 0-80psi. (When the boiler was firing the pressure peaked at about 22psi.)

I am thinking the cold pressure should be a bit higher than 10psi - specifically 14psi. If this is the correct value what determines this - the pressure in the expansion tank or the pressure regulator? Or both? As noted I did replace the expansion tank about 6-7 months ago and set the pressure at 14psi. I plan to check its pressure again as it's easy to do with the RBFF. Should the tank be 14psi?

It seems to me the pressure regulator should be set at 14psi and the expansion tank pressure should be the same. And this should result in a system pressure of this same 14psi (this is when the boiler has been shut down for awhile). If need be I can adjust the regulator pressure using the RBFF as well. Over the years the regulator may have been tweaked from its factory setting.

Any wisdom in this area would be greatly appreciated.

Comments

  • HVACNUT
    HVACNUT Member Posts: 4,209
    The tank should be the same pressure as the system (cold). Residential pressure reducing valves are factory set to 12 psi but obviously should be checked.
  • Derheatmeister
    Derheatmeister Member Posts: 1,140
    We set our pressure on the fill 2 psi higher that the expansion tank which helps maintain that the Bladder/diaphragm is not rubbing against the inlet..
    One formular to pay attension to is that 1PSI will give you 2.31 feet of column/Static...Then add 5 psi to maintain pressure on the highest point of the system.
    On most of our 2 level residential systems we have our feed/fill valve set to 18 psi and our expansion tank to 16 psi.
  • Zman
    Zman Member Posts: 7,001
    Does the pressure rise as soon as the boiler circ turns on or does it rise after the boiler heats up?
    How are the circ, expansion tank, and gauge oriented? Air problems are often caused by improper orientation of the expansion tank and circ.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    CBRob
  • hot_rod
    hot_rod Member Posts: 15,055
    The required pressure is determined by the height of the heat emitters above the boiler. For ease of math, figure 1/2 psi for every foot of distance above. If it is 28', the fill pressure should be 14 psi.
    Ideally you want some positive pressure at the highest point in the loop so add couple psi to the calculated number. For a typical 2 story home, 12 psi is adequate.

    Sounds like you need a good power purge of the system.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • dbsoccer
    dbsoccer Member Posts: 28
    I have thin wall radiant emitters that are on the floor. The highest emitter is on the second floor 6-7 feet above the boiler. (The boiler is on the main floor and it's a two story house.) In the picture below you can see the tank, scoop and prv are of a pretty standard orientation. I can use the RBFF to check/reset the pressures on both the prv and tank. From what I've gathered from your comments 12-14 psi cold should work.

    Not sure what a power purge is but I used the Fast Fill feature on the prv to purge the zones.




  • hot_rod
    hot_rod Member Posts: 15,055
    If you have 10 psi holding in the off season you are fine. It could be that brass float vent on the air purger is not functioning properly. If all the radiators are purged and flow is moving everywhere, that sir purger should handle any residual air.

    increasing boiler temperature for a day can help to get problematic air eliminated, also raising the fill pressure a couple psi helps remove air, basically squeezing the bubbles smaller so they make their way back to the purger. 
    Ideally your pump would be just downstream from the air purger to aid in air removal, but it is very common to see pumps on the return in those older cast boilers.
    As you have discovered, air removal is a tougher go with pumps on the return and tanks on the supply.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • dbsoccer
    dbsoccer Member Posts: 28
    So today, since it was such a slow day (not), I reset my tank pressure to 14psi. And I tried to set the prv pressure to match. And then I ran into a problem. Independent of which way I turned the adjustment screw the pressure stayed the same - 9-10psi. And, after further messing around, the fast fill feature on my 1156 prv does not work. The stem is stuck so the fff won't depress it any further. And, on top of that Watts discontinued the rebuild kit for this prv in 2015. So I ordered a new prv and will probably just pull the parts out of it and replace the ones in my current. Replacing the prv might require a major house remodel. :smiley:

    I've wondered about the vent as well. I recall it used to hiss a bit when the system ran. I might a well replace it as well.

    Thanks Bob. I'll keep messing around and get this baby purring like a kitten.
  • dbsoccer
    dbsoccer Member Posts: 28
    I did a google search on how to tell if the vent was working and guess what? I found a YouTube where Bob "hot rod" Rohr was the host. Small world.

    So Bob, you mention above that the vent may not be working properly. Is there a way to determine this or is it simpler to just replace it? I believe the one I have is a Caleffi. I can't see any stamping that's on it without looking on back side (relative to the picture above). But in the grand scheme of things they aren't that expensive to replace. If replacing it is the way to go, what would the proper part number for my system be assuming you can give out such information on this forum.
  • dopey27177
    dopey27177 Member Posts: 759
    Measure the height from the top of the top floor convector to the top of the boiler. multiply that by.434 for the pressure add 2 PSI to that number.

    Example: height 20 feet times .434 = 8.68 psi + 2= 10.868

    This a cold fill temperature. Water pressure increases as it is heated.

    You can set the expansion tank at 15 psi. Make final adjustment when boiler is hot.

    A cold o water temp at 60 degrees with 12 psi pressure when heated to 180 can have a pressure of 28 psig.

    Jake
  • hot_rod
    hot_rod Member Posts: 15,055
    That is not my favorite type of air purger and it should have 18" or so of straight pipe upstream. But, it probably is doing some removal. You would need to isolate the vent to remove and inspect. Or just replace it. The top is easily removed to see if some debris is stuck that would keep the float from moving freely.

    If you loosen the vent slightly you will see either air bubble coming out, or water. If water, then that would indicate the vent has removed the air.

    Some installers will do a flying, wet replacement, get the new one ready, unscrew and quickly replace. The system would need to be cold, turn off the fill valve. It's a risky move if you have not done a switch out like this :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • dbsoccer
    dbsoccer Member Posts: 28
    Appreciate all the input and comments. During all this discussion I have set the expansion tank at 14psi. I thought my prv was defective but as it turns out it is fine but over the years the pressure had dropped from the factory setting. I adjusted the prv to match the tank at 14psi. Based on Jake's input this may be a few pounds high but I also received input that having system pressure a couple of pounds higher may help drive air into the water. I've watched it during the heating cycles and the max pressure seems to be about 25. I'll keep an eye on it this winter as the boiler will stay hotter longer than it does during the non-heating months (Amtrol Hot Water Maker). Also during all this thrash, besides replacing a couple of gauges and a couple of drain valves, I purged all my zones. I followed this with a trial heating of all zones to assure water would circulate. Things seem fine and it appears I'm ready for winter. I'm going to leave the air vent for now. I have a new one at the ready in case it appears a replacement is required.

    Thanks to all.