can a clever plumber explain this

Discussion in 'Plumbers' Talk' started by jonob, Jun 6, 2009.

  1. jonob

    jonob Member

    Got a ufh system downstairs and a rad system upstairs , all with motorised valves , all working correctly.

    In winter with ufh and rads on (ufh on stats ) , Rads on timer and stats, the pressure in the boiler constantly drops ,and needs topping up .

    Its a potterton powermax 150l

    in summer as its warmer ive turned rads off on ufh is only on if stat calls.

    the pressure has been constant for 3 weeks now , never needed to be topped up.

    there are no leaks visible anywhere.
  2. tomplum

    tomplum Active Member

    the air is being 'sucked' in throu the rad system, either a rad valve, pump valve or air vent, check all compession fittings, and if any auto air vents are on the return, take um off,
  3. mudhut

    mudhut New Member

    drawing air in will not cause pressure drop, air or water coming out will. have you got ample expansion vessel capacity? what pressure does it all go up to when maxed out? ie, is the overpressure valve opening?
  4. imran_

    imran_ New Member

    Either water is going out of the pressure relief (pressure/exp vessel problem) OR the water is going out in the rad circuit (there could be a NR valve on the return).

    My guess is the exp vessel is knacked or undersized. £80 please.
  5. Switcher

    Switcher New Member

    tomplum are you timeserved?
    Can you read?
    Looks like you have got it wrong this time.
  6. fit*like*min

    fit*like*min New Member

    imran is spot on there like, have u noticed water trickling outside relief pipe. wall greenish,damp, floor wet. hae u noticed pressure risng to 3+ on pressure gauge after heating on for hour or so.
  7. fit*like*min

    fit*like*min New Member

    do u mean imi range powermax 150 or potterton promax hedging bet at imi powermax 150.
  8. fit*like*min

    fit*like*min New Member

    stupid me wiz awa in the cloods wi ma coos on the ferm in the broch forget last comment please
  9. mudhut

    mudhut New Member

    imran may be right, but i got there first - other than i didn`t actually suggest the diaphragm was shot. heads up guys :)
  10. mantor

    mantor New Member

    imran may be right, but i got there first - other than i didn`t actually suggest the diaphragm was shot. heads up guys

    Oh ***!
  11. jonob

    jonob Member

    cheers for all replys

    when ufh is on only there is no probs , pressure about 2 bar all ok, when ufh and rads on pressure is random between 3 bar and 0.7 bar , it eventually drops enough for a lock out. it goes out through blow off at 3 bar , why is it a prob on rad circuit not on ufh ?

    so it seem the rad circuit is to blame ???
  12. mudhut

    mudhut New Member

    3bar is way high, high enough to open the prv. is that 3bar hot then 0.7 or lower when cold? is the low pressure cut out only triggered when cold? basically when the rads are on as well as the ufh the volume is greater and consequently the expansion volume is greater. either your expansion vessel is too small or the diaphragm is split, as already stated.
  13. mudhut

    mudhut New Member

    oh, and mantor - fair enough, i did get a bit handbaggy there, good call :eek:
  14. tomplum

    tomplum Active Member

    or like i said before, air is being sucked in on the rad circuit, this builds up, compresses then blows off at the prv,if its the pressure vessel, then the problem would still be there when ufh only is on, as the circuit is shorter and therefore the pressure greater,
    i am assuming the vessell is not on the rad circuit and is in or near the boiler,
  15. mudhut

    mudhut New Member

    how does the shorter circuit equate to greater pressure tom? do you not size expansion vessels based on the volume of a system? also drawing air in is more of a problem in a vented system, no? not gonna draw air in at 2bar, even on the return, even if all convectors have trvs and it`s hot as hell, and there`s no bypass loop. ufh pipework has much smaller volume than typical equivalent convector system and some pipes can expand fairly, compensating for expansion. not much give in rads but plenty of volume.

    i have a little knowledge (often said to be a dangerous thing), so am perhaps missing something. so, tomplum, can you explain how a sealed system can suck in enough air through automatic vents or whatever to blow a prv at 3 bar??

    and to the o/p, can you give us an idea of the system size (eg no/size(ish) of rads) and the type/size of boiler and expansion vessel, and if the vessel is mounted loosely on a flexible pipe can you weigh it when the system pressure is too low/locked out? also observe the build up of pressure when you turn on the whole system, if it goes from 2.5bar as it heats then suspect the expansion vessel.
  16. imran_

    imran_ New Member

    Well done mudhut, both for talking sense and figuring anything over 1 ish bar can't possibly suck in air!

    As for "One swallow doesn't make a summer", very true. But it does make a good night out!!

    You can buy it on the internet now, I've seen it:
    "Swallows on Amazon"!

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