Combi gas working pressure

Discussion in 'Plumbers' Talk' started by Steve Case, Sep 1, 2018.

  1. Steve Case

    Steve Case New Member

    Three years ago our CH system was completely renewed by a reputable gas-safe registered local company. The job included a new Vaillant combi boiler and new gas supply pipework. When installed, the working gas pressure during HW operation was recorded as 20mb (CH operation) and 19mb (HW operation). The boiler has been working fine for he last three years, apart from a couple of minor problems which were sorted by Vaillant. Recently the Vaillant engineer who came out to do the third annual service said that the working at pressure at the boiler was only 13.5mb and, if it had been slightly lower, he would have had to switch it off and declare it unsafe. He also said the pressure at the meter was about 0.1mb below what it should be (i.e. not helping the situation, but not enough to indicate a regulator problem). My question is can a new gas pipe become partially blocked so quickly, and what are the chances of unblocking it without having to replace the pipes (which would involve lifting floors etc)?
    Last edited: Sep 1, 2018
  2. candoabitofmoststuff

    candoabitofmoststuff Active Member

    I'm DIY, not a gas expert, but...

    I don't think gas pipes, (at least in the house), can/do get blocked.
    If the pressure at the meter is low then that's an issue for the supplier, surely!?

    What can be internal is a leak. your gas safe man should have been able to establish that with no difficulty, and I assume he didn't find one, (or you would have mentioned it?)

    Of course if there is a leak internally you may have to bite the bullet with lifting floors etc... unless a complete new run is possible.

    However, having said all that I suspect the root of this problem is the low pressure from the meter.
    I repaeat I'm only DIY... I'll be interested in the input of the experts here.

  3. kiaora

    kiaora Well-Known Member

    It would appear the pipes are undersized, but it’s no good guessing.
    The standing pressure and working pressure are two completely different readings.
    Standing pressure at the Boiler will / should match the meter.
    However the working pressure, at the Boiler will drop due the pipe resistances.
    Max 1 mb.

    So if the two pressures are completely different it suggests the pipe are incorrectly sized.

    The danger is.. a gas cooker on a low simmer, can loose the flame when the Boiler fire up, possibly causing an uncontrolled gas leak!

    Get it checked by your local gas engineer, it’s not a big job to test.

    Often,on a long run, the pipes need to be 28mm at the start.

  4. Steve Case

    Steve Case New Member

    I agree with all of this, but I don't think you understood my post. When installed, the working pressure was OK so the pipe size must have been OK (unless the installer recorded a false pressure figure).
  5. kiaora

    kiaora Well-Known Member

    With plumbing and heating issues, it’s best to see the real situation, and not rely on what’s been said and written down.

    Get your u gauge, and, if you are competent, test the pressures your self,

    Standing pressure at meter and Boiler,
    Then working,

    The figures will give you the answer to your issue.

  6. kiaora

    kiaora Well-Known Member

    Also, if you do have a problem it can be fixed only when we know what it is!

    It has been known for water to get in gas pipes, but a proper test is needed before you start.

  7. The Teach

    The Teach Well-Known Member

    get them back to sort it out,or splash some cash on remedial work which may after testing just include a report saying the gas supply to the boiler is Ok.

    Although the boiler may be inlet under gassed,the service technician would have preformed other checks to ensure the appliance is safe to leave working and left paperwork.

    The boiler is 3 years into its life and you are not experiencing low heat to your hot water/heating or experiencing boiler lock out codes,If you have its worth mentioning.

    internal gas pipes can become partially restricted,its rare but can happen.

    the gas supplier can also change the inlet pressure to your gas meter,hopefully the gas meter governor (regulator) is set up correctly. its all easily checked ;).
  8. Steve Case

    Steve Case New Member

    Thanks for all your replies. As the boiler is working OK up to now, I plan to leave things as they are and, when the service us due next year, get the original installer (instead of Vaillant) in to do it (I will point out what the Vaillant engineer said). I just wondered if anybody had seen this sort of thing before and knew a good way of clearing the gas pipe if there is any debris/dust in it.
  9. kiaora

    kiaora Well-Known Member

    Before you put this on the back burner, I suggest you do a little test.

    If you have a gas hob, put the small burner on, at low flame/ simmer.

    Then turn the hot tap to fire the Boiler, keep an eye on the hob flame and see if it drops ,

  10. Steve Case

    Steve Case New Member

    Thanks for that - I did as you suggested. There was no noticeable change to the small hob burner flame after turning on the HW on full (or when the boiler extinguished). Over the last 2 years I have had 3 instances of F29/F54 faults which does tend to support the idea that there is a problem with gas pressure. About a month ago I also did a test in which I turned everything on the gas cooker on full (2 ovens and 4 hob burners giving a combined rating of about 40% that of the boiler) to see if that had any effect on the boiler starting. It didn't - the boiler started OK, and there was no significant effect on any of the hob burner flames.
  11. The Teach

    The Teach Well-Known Member

    13mb is the unsafe cut off point for your boiler,your not far from that ;) 13.5mb was the pressure recorded during the service technicians visit,hopefully it does not intermittently drop and is a higher pressure during the next service visit.

    The gas transporter usually national grid/cadent can temporarily reduce the gas network pressure,now with your existing lower boiler gas inlet pressure your boiler will still try to fire up with an incorrect gas to air forced mixture.The boiler manufacture will have tested their boiler at pressures less than 13mb so they know what happens :(


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