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PRV over feeding when circ is started

TechTok Member Posts: 9
I was sent out to help this customer start this boiler up for the year. They have never had one and we're nervous.
 When I first arrive I notice the cold pressure is at psi. Definitely higher than need. Maybe a 700 sq ft single story ranch house. All pipes run into the crawl lower than the boiler. I did the basic maintenance on it and before I started it I drained the system down to 14 psi and started the system. The second the circ pump starts the PRV started feeding until the pressure was back up to 20 psi... Once hot system was teetering 30. So I drained the system down again and left the iso valve shut for the PRV and started the system again. Problem solved. 

I'm saying it's the PRV being pipes in so far from the pressure tank and immediately before the circ pump on the return.


  • TechTok
    TechTok Member Posts: 9

  • Jamie Hall
    Jamie Hall Member Posts: 18,958
    You're diagnosis is quite right -- the PRV and the expansion tank should both be hooked into the system about ten pipe diameters upstream from the inlet side of the main system pump. That location will always be the lowest pressure in the system, hot or cold, and with the expansion tank there will also be the point of constant pressure ("point of no pressure change" -- PONPC). The concept is called "pumping away", and there's a lot of information on it here on the Wall and in some of Dan's books and Caleffi's literature.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 16,326
    Also, those fill valves can creep up after time. Turn off the isolation valve after you adjust the pressure confirms that. But if air burps out after you worked on it, the valve cannot keep, pressure maintained.

    Could also be an under-pressurized or flooded exp tank.
    Tap the tank or push in the air valve to see if water comes out.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Big Ed_4
    Big Ed_4 Member Posts: 1,887
    Yes that's a normal mistake on pressure reducing valve instal . Piping on the low pressure side of circulator . Another fix would be backing off on the valve .. The pressure setting is needed for the rise of the water column to the highest radiator not the square footage of the building . 12# is need for a three story building or home . Three foot rise per pound , add 2# for safety . .
    I have enough experience to know , that I dont know it all
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,203
    edited October 2020
    Here is a slide I used when teaching a Hydronics class.
    Credit to many parts pirated from others over the years

    I would actually find this piping arrangement a lot. you have it in your problem system.

    If the system is cold, and you get everything to 12 PSI, then the air pressure in the expansion tank is 12 psi, the feed valve is set at 12 PSI, the water pressure in the boiler is at 12 PSI and all the gauges in the diagram are at 12 PSI and all in the world is good. Book of EdTheHeaterMan 21:3-5

    Then the thermostat said "Let there be heat"
    The pump activates and there is a pressure change between the inlet of the pump and the discharge of the pump. Since the "point of no pressure change" is at the expansion tank.( See Book of Ed Carlson 12:4-22), the pump only raises the pressure on the gauge at the discharge of the pump by less than 1 PSI (the pressure drop from of the boiler and piping to the expansion tank)
    If the pressure difference developed by the pump is say, maybe, 6 PSI, then the outlet pressure of (maybe) 12.5 means the inlet pressure must drop to 6.5 PSI

    Now look at the pressure gauge on the inlet side of the pump (6.5 PSI) and compare it to the setting of the PRV (12 PSI) and do the math. More water is added.

    Its a poor design... but "how come it never gave me trouble before" Book of Customers 1: 1-2. Ahh says the wise technician, As long as the pressure never reaches 30 PSI, you will never know the error of your ways.

    @DanHolohan wrote a whole book on this subject and includes it in many of his other teachings from on high. (I learned this from him while he was on a platform 2 steps higher than the floor where we were sitting)

    I hope my words of wisdom will shed some light on this dilemma

    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
  • TechTok
    TechTok Member Posts: 9
    Thanks for the reply everyone. I've read our.oing away and primary and secondary pumping. That was 2 years ago when I was getting my feet wet. I do boilers a lot more now so I just read classic hydronics, now I'm re reading pumping away and I'm also going to buy some steam books soon.
     The timing of this find was funny because I just was telling my service manager that I've come across several units that were installed incorrectly and causing high pressure situations. He said he has never heard of this issue. 2 days later I find this and video recorded it turning on as the circ pump came on. Now he is a believer. 
    EBEBRATT-Ed Member Posts: 12,010

    The bottom line is always weather your circulator pumps toward the expansion tank or away from the expansion tank the expansion tank and the make-up water need to be on the same side of the circ pump. Pumping away is better and is required if the piping system has high resistance