New combi install - help needed

Discussion in 'Plumbers' Talk' started by DesignGuy, Aug 28, 2021.

  1. DesignGuy

    DesignGuy New Member

    Hi All,

    So I decided to switch from a system to combi boiler, a 35kw unit rated for 16ltr/m. Stopcock has a non return and pressure limiting valve, currently 3.0 bar static.

    It's just me in the property so I was looking forward to the mains pressure shower.

    I'm getting just over 6ltr/m from the upstairs bathroom hot tap with no other draw on the system.

    Really not sure what to expect from a combi but this doesn't seem normal.

    There are other problems with the install, either trapped air or a dead leg so combi fires on cold water briefly (obviously it doesn't heat the cold water). But this indicates a pressure issue.

    Have spoken to installer, his response is to fit a non return valve on the cold feed to the boiler but I'd rather get the underlying issue resolved.

    I look forward to hearing your thoughts, help needed!
     
  2. Mike83

    Mike83 Screwfix Select

    What flow do you get at the cold tap on the kitchen?
    Or if you have an outside tap that’s even better.
     
  3. Mike83

    Mike83 Screwfix Select

    There should be no dead legs on the system these days.
    Sometimes it’s literally impossible to avoid without mega upheaval but it should at least be looked into.
     
  4. DesignGuy

    DesignGuy New Member

    I used the utility room tap which is on the same pipe that feeds the boiler.

    2ltr bottle fills in about 7 seconds
     
  5. Mike83

    Mike83 Screwfix Select

    So about 17lpm.
    Either you have restrictions on the pipework or the taps are restrictive.
     
  6. Mike83

    Mike83 Screwfix Select

    What boiler do you have.
    Make sure all isolation valves are fully open
     
    DesignGuy likes this.
  7. pppmacca43

    pppmacca43 Super Member

    Some taps these days have flow limiters built in, or it could be the aerator at the end of the spout blocked(depending on the type of tap).
     
  8. DesignGuy

    DesignGuy New Member

    That was another reason for getting the boiler done, there was a small leak from an en suite shower, behind the cylinder, behind the wall.

    That would have cost a lot of money to sort out so I thought get the cylinder out and gone, access to the plaster board, cut a hole, reflux the pipes and we're all good.

    On the day the installer wouldn't do that because 'it's not in the contract' and just capped off the h/c feed to the shower. The cold is a 22mm feed with a isolation tap on it, then capped. Could probably be bled.
     
  9. DesignGuy

    DesignGuy New Member

    These are 20+ year old standard brass taps so I don't think there's any flow restriction, it's equally bad across all basin taps, maybe a little better downstairs but not much.
     
  10. DesignGuy

    DesignGuy New Member

    I went for the viessmann 100w 35kw

    Are you talking about inlets to the boiler? No other valves, that I'm aware of anyway.
     
  11. Baxi Boy

    Baxi Boy Active Member

    Your 6 litres a minute is very low,and I can’t explain why unless a valve is restricting flow somewhere.However manufacturers claims of litres per minute are based on a low figure of 35 degrees rise above the cold mains,but in reality we aim for 40-41 degrees which would give 12.5 litres a minute with 35kw.
     
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  12. DesignGuy

    DesignGuy New Member

    Seems like all the valves are open, he's used a little plastic elbow that may be restricting the flow

    Interestingly, whilst I was down there taking the pictures, the boiler just fired up for no reason at all.
     

    Attached Files:

  13. Mike83

    Mike83 Screwfix Select

    That fitting will be slightly restrictive and I don’t understand why he’s done it like that.
    A normal 15mm bend could easily have been fitted.
    There may be an isolation valve fitted somewhere aswell and if it’s not full bore will restrict the flow.
     
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  14. pppmacca43

    pppmacca43 Super Member

    None of that should restrict the flow that much tho
     
  15. pppmacca43

    pppmacca43 Super Member

    Get the installer back. He should have ensured the incoming main was sufficient for a combi boiler
     
  16. DesignGuy

    DesignGuy New Member

    Yeah, I don't know if you saw but the flowrate on the cold water pipe (tap in the utility) is 17L/m not sure how we get down to 6L/m at the bath tap, and all other outlets.
     
  17. Mike83

    Mike83 Screwfix Select

    Is that pipe under the sink for something else maybe?
    Where is the combi in relation to the kitchen sink?
     
  18. DesignGuy

    DesignGuy New Member

    It's definitely the feed for the boiler, there's a tap at the top and the iso valve feeds the washing machine

    That white plastic elbow in an earlier photo is new so that's 100% where it's going.

    I mean, the plastic pipe will be 11mm internal, maybe down to as little as 5mm inside that corner joint. It's possible it could reduce the pressure significantly. Maybe there's an iso valve I missed somewhere but I guess we'll find out.

    Ideally I'd like full bore copper all the way to the boiler.
     

    Attached Files:

  19. Mike83

    Mike83 Screwfix Select

    55B8FEF5-217D-4687-A2E2-44EAFDBCF7BD.jpeg What’s the flow rate at this hot tap?
     
  20. DesignGuy

    DesignGuy New Member

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