The Wall
Forum / THE MAIN WALL / Boiler/monoflow question
  • Post a Reply to this Thread

    Boiler/monoflow question (17 Posts)

  • Glenn Harrison Glenn Harrison @ 2:47 PM
    Contact this user

    Here's a couple of links for you.

    The first one is for a rule of thumb sizing chart for hydronic expansion tanks. If you look, at 155,000 BTU's you should have a #90 expansion tank. The second one is a spec chart for the different size tanks. If you look a #90 tank has a 15 gallon capacity, but a #60 and #30 tank put together has a 10.5 gallon capacity, and 2 #60 tanks is only 12 gallons capacity. So your tank is still to small by the numbers. Now you also said the old system had a #30 tank, but also the relief valve was seized. I'm thinking the relief blew a long time ago, seized up, and the old system was running dangerously high pressures. After that, I would have to assume the feeder is trickling thru, and time will tell shortly.
  • jrc2905 jrc2905 @ 3:40 PM
    Contact this user

    Well today I found the problem. The boiler pressure still went up with the supply to the boiler and Super Store shut down. Started thinking maybe I was looking at the wrong side. Turns out the Super Store does have a leak in the coil. I put refrigeration fitting between the shut off valves and pressurized it with nitrogen, to confirm leak. The boiler pressure is going up because the cold water at the sink is leaking through to the hot water side through the 3 way faucet even with the supply to the Super Store shut off causing boiler pressure to rise. I did another test to confirm this. Finally problem solved. Thanks for all the comments.
    Contact this user

    Your cushion/expansion tank is too small. C.I. bsbd or free standing c.i. radiators would require at least a #90 tank. Short term solution/proof would be to drain off some of the water at boiler to get an air pocket in the tallest/highest radiator. This added cushion will help keep the pressure in check. I doubt the reduced water volume @ radiator(s) will affect heat output significantly. If you want to add the extra tank capacity, another #60 tee'd w/your present one would do the job nicely.
  • jrc2905 jrc2905 @ 3:06 PM
    Contact this user

    Boiler/monoflow question

    I have a Burnham HW boiler 156mbtu, on a one pipe monoflow system; this boiler has chronic problems with over pressurizing. All air elimination components and #60 expansion tank have been checked and double checked. When the boiler gets up to temperature of 140/150 pressure rises steady, from 15psi. It has blown the relief valve in the past. Steam radiators converted to HW have all been bled. Taco 007 is pumping away after the expansion tank. Is it possible that the 007 circulator is too small? This is not a big house.
  • brucewo1b brucewo1b @ 4:23 PM
    Contact this user


    Any pictures of the piping at the boiler and is the water feed at the extrol?? B.S.
  • jrc2905 jrc2905 @ 4:40 PM
    Contact this user

    The boiler is piped pumping away, water feed at point of no pressure change. Circulator after expansion tank. This is a replacement boiler 2 weeks old. The other boiler was leaking at 7 years. I now suspect there is a relation ship between the boiler failure and this pressure problem. I have never seen a problem like this, could there be a blockage in part of the monoflow system. All the usual culprits in an over pressurizing problem have been checked and double checked. I am thinking of changing the 007 to a 0010 .
  • brucewo1b brucewo1b @ 5:58 PM
    Contact this user


    everything is heating I can't imagin its a blockage, but if you are creating low enough pressure at the feed you could be adding water everycycle, I would suspect still not enough expansion tank though since it lasted longer with the extra tank. Try a broken union test, not just shut off valves that way you know no extra water is coming in fron tank or feed. B.S.
  • brucewo1b brucewo1b @ 6:05 PM
    Contact this user


    to cover all the basses this was never ran as a gravity system with a expansion tank waterfeed in the attic??? B.S.
  • jrc2905 jrc2905 @ 3:16 PM
    Contact this user

    Boiler has been isolated from water supply and indirect water heat and still pressure still rises. Exp tank is new and charge checked 12psi even played with charge and does not help problem. Safety valve is also new and does seat
  • clammy clammy @ 3:10 PM
    Contact this user

    expansion tank

    Sounds like either you have a bad expansion tank or a bypassing pressure reducing valve .This boiler does not have a tankless coil on it .Are you leaving the water line open after the raditors are bleed ?Does the safty valve seat closed after blowing off ?
  • Jim Dawson Jim Dawson @ 4:39 PM
    Contact this user

    Blowing relief valve.

    I had a similar problem.Changed tank also
  • jrc2905 jrc2905 @ 2:10 PM
    Contact this user

    I added a #30 tank I had to the system and now the unit runs a steady 17psi, thanks for the help.
  • jrc2905 jrc2905 @ 10:08 AM
    Contact this user

    I called the customer today and was told the pressure was back up to 25 psi. I am going to give the customer a list of things to do such as shutting water off to parts of the system and record the results, even though this has been done once. Customer says this did not happen with the old slant fin boiler that had a #30 expansion tank on it. It has the equivalent of a #90 expansion tank and still is not at the right pressure.
  • bructer bructer @ 6:36 PM
    Contact this user


    there are only three reasons that i can think of that would cause overpressure ,domestic water pressure bypassing,inadequate expansion,over heating,[what is the boiler temp.set at]..bructer
  • jrc2905 jrc2905 @ 6:51 PM
    Contact this user

    This is a cold start boiler. I thought maybe bad high limit. Hi temp limit is set at 180. I put my temperature probe on the supply leaving boiler, never got over 150. Customer tells me today that the highest loop on second floor only gets marginally warm.
  • bructer bructer @ 8:02 PM
    Contact this user


    i'm probably not telling you anything water systems are simple the checklist on the hydronic side is short you already checked everything that could overpressure that part...circulator pumps circulate they don't ad pressure.i have been installing burnham boilers 35yrs love them .i have had some that shutoff on highlimit with the gage reading 180 but the temp. kept rising to over sometimes you have to play with the limits hi/lo and dif...question did you baby sit through complete heating cycles...the sections that are not heating properly,check that valves are open and air is completely out..being a monoflo system could make it a pain to blead...check that the water flows through in right direction...hopefully the customer is't screwing with you looking for a give back...bructer
  • jrc2905 jrc2905 @ 11:13 PM
    Contact this user

    This job started out with me replacing a blown expansion tank. Then I found that the aquastat on the Super Store indirect was not opening on set point because the home owner had packed it with dielectric grease and it had seized, he was told this was good for it. Then had to replace a seized pressure relief valve, then found that the water feed was clogged and with sediment and replaced that. Then I find the boiler is leaking water, boiler is replaced now this problem. The homeowner who is retired has been by my side watching this circus and is more worried that I will give up and not find the problem. He keeps asking me if I want money. I plan to go back and go through the entire house look at every pipe and radiator. I have installed a steam pressure gauge that I have been using to monitor the change in pressure. The system is now off the water feed with the homeowner monitoring the pressure, I will go back Monday.
Post a Reply to this Thread