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Discuss Impossible to fix flow problem? 4 grand down so far... in the Central Heating Forum area at

  1. Darryl Pullen

    Darryl Pullen Member


    Was hoping to get some advice... When I moved in the central heating was not working (they didn't tell us), paid quite a bit of cash to get it looked at, everything pointed at flow problems as the "Ideal" boiler would display flow error code. In the end, the plumber let me down, so went with another plumber and decided to install brand new boiler, a Viessmann Vitodens 100-w.

    Basically, 2 years on, still having the same problems. It was open vented, but has been converted to sealed system, doesn't lose pressure. Had new pumps (currently got Grundfos Alpha2L 15-60 (130)), and still not getting any heat into the rads. We have 12 rads, 3 large ones, all doubles. Most have TRVs.

    The boiler starts up, shoots up to 50 degrees quite quickly and cuts out, then repeats. Oddly it does this even on hot water, a much much shorter circuit. Eventually the hot water does heat up a little bit. Today I started it up to see if it was working (as it was sort of "working" over the winter, but badly), and it just short cycles as above, no heat into any rads after 45 mins of running. The heat/burner display has never ever been above 1 (out of 4 or 5 levels).

    It has been power flushed (although not sure how well), and various other bits of work and has eaten a ton of money. have tried balancing but something more fundamental is wrong I thing, or this boiler is just terrible. Also, doesn't seem to be air in the system as it is sealed now, and never any air in rads at all, immediate water trickles out (very clear water).

    I am a bit desperate now as I honestly don't know what to do next, can't keep throwing money at it... Any advice really is much appreciated. Hoping someone might be able to help.
  2. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    Sounds like you need a engy to look over the system could be a pump or a blockage as it sounds like its not circulating
  3. Karl

    Karl Member

    Really sounds as if the system is badly blocked, or something stupid like an isolator out of view turned off maybe. If it was me I would take a radiator off make sure all other rads are isolated put a hose on radiator pipe and see if you get flow when filling system (as long as filling loop is on boiler) test both pipes on radiator this way, if no luck try on a couple other radiators at opposite ends of system. If pipes are that badly blocked then it might be cheaper to replace pipe work and is it any chance micro-bore pipes? Can’t believe someone has fitted you a new boiler and walked away without the system working!
  4. AMGasServices

    AMGasServices GSR

    Do your flow and return pipes get warm under the boiler ?

    How far can you trace these pipes ? Check if they’re heating up.

    Do you have a hot water tank in the house ? If so Please post pictures of this.
  5. rpm

    rpm Trusted Plumber Top Contributor!!

    Microbore pipe anyone?
  6. Darryl Pullen

    Darryl Pullen Member

    Hi all, no microbore in the system. It does have a hot water tank (will try and grab some pics tomorrow). The ground floor pipes are mostly under screed. There are three returns, hot water, upstairs and downstairs rads. The system has been working, and have had heat into all radiators, but always been slow and rubbish. The more rads I turn off, the better the boiler performs (as in, it cuts out less).

    When it was powerflushed (with the boiler on) heat got into every rad within seconds. And when the hot water turns on for the first time, the boiler heats up for a bit, heat comes in to the return, and yet the boiler still cuts out!

    I have never ever had a "hot" return pipe coming up from the floor (the downstairs return), the hot water return has been hot, and the upstairs return has been hot too. But at the moment it just cuts out and heat never gets into the return. Flow gets hot while heating, then quickly goes cold when cut-out.
  7. Darryl Pullen

    Darryl Pullen Member

    All three return pipes meet in the boiler cupboard near the boiler. Boiler is downstairs in garage, pump is upstairs in airing cupboard. Pump is on full, anything less and the system is even worse.

    Was wondering if it is possible to put a secondary pump on the return. Or if there is any clever way to actually determine if there really is a flow problem, like flow meters strategically placed??
  8. rpm

    rpm Trusted Plumber Top Contributor!!

    Well that didn't help so would a thermo camera help?
  9. chris watkins

    chris watkins Plumber

    There are flow meters available, Caleffi do a good one.
    It does sound like you may benefit from hydraulic separation of the boiler & the system but a good heating man would need to have a look at the system layout as these 3 returns don't sound quite right
    Balancing valve with flow meter
    Last edited: Aug 12, 2018
  10. chris watkins

    chris watkins Plumber

    Nothing wrong with the Viessmann Vitodens, well at least there wasn't until someone was power flushing through it. :confused:
    • Agree Agree x 1
  11. townfanjon

    townfanjon Trusted Plumber GSR Top Contributor!!

    You need to check if there is a temp difference back at the boiler before spending anymore money .
  12. chris watkins

    chris watkins Plumber

    Just a thought but when the system was converted to a sealed one was the old pipework around the tee's to the open vent / cold feed removed or were they left in & capped / adapted?
    It sounds like there is a large restriction somewhere & power flushing does not always clear them.
    • Agree Agree x 1
  13. Best

    Best Trusted Plumber Top Contributor!!

    I had one job that had similar problem which I eventually discovered had a seized motorised valve below a floor.
    Not saying that is possible on this occasion, but just putting out that point.
  14. Darryl Pullen

    Darryl Pullen Member

    Thanks for taking the time to reply. When converted to sealed, the old stuff was removed (F/E tank) and capped off.

    Does it not sound normal to have 3 returns converging at the boiler, one comes up out of the screed, and two from upstairs. One other thing the return on most of the radiators (or all) is always cold. So def is a problem with return. But none of it makes sense when you add all the symptoms together.

    It's hard to comment on the temperature difference between flow and return at the boiler as it doesn't stay on long enough to get good heat into the system, but when it has been working the flow is always hot, return lukewarm to warm (after hours of running). The boiler always "kettles" on starting up too.

    Everything keeps telling me it needs a bigger pump, or secondary pump on the return, or possible another more thorough powerflush (or a powderflush?!). Don't mind spending a bit more on it, but just have no idea what should be the next move.
  15. Harvest Fields

    Harvest Fields S. Mod Trusted Plumber GSR Top Contributor!!

    I would Scrap the old flow and return and start again.
    • Agree Agree x 1
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