Any boilers have output larger than 15mm ?

Discussion in 'Engineers' Talk' started by ChriatopherB, Apr 3, 2018.

  1. ChriatopherB

    ChriatopherB New Member

    I am installing a new boiler and my property has really good flow 18l/min.

    I need to run the water about 12m and then split to two bathrooms. I would like to run the 12m in 22mm and then split to 15mm for each bathroom ... but all boilers seem to have just a 15mm outlet.

    Do any Bosch/Valliant/other(?) boilers have 22mm outlet? (Ie more than the standard 15mm)

    If not should I just run in 15mm and then split to 2x15mm?

    Thanks in advance :)
  2. KIAB

    KIAB Well-Known Member

    Having 22mm for hot water achieves nothing, stick to 15mm with a combi.

    But, if your system was gravity fed, then you would use 22mm due to low pressure in pipes.
  3. ChriatopherB

    ChriatopherB New Member

    I agree adding 22mm to a Combi with an output of 15mm adds nothing (except the minimal pipe friction reduction). System was gravity fed before but I will be removing that so yup, seems that 15mm is the only option.
  4. rogerk101

    rogerk101 Well-Known Member

    12 years ago, I made the silly mistake of using 22mm pipes for the domestic hot water from my 36kW Grant Vortex Combi oil-fired boiler. It has 22mm cold water feed and hot water output. I had good water pressure and good water flow rate, but in my ignorance, I thought it would help with shower temperature changes if someone flushed the toilet or turned on a tap somewhere else in the house.

    In fact it wasn't just pointless ... it has the significant disadvantage that you have to run twice the amount of hot water through the pipes before the hot water comes through. Yes, the volume of cold water that has to move through a 15mm pipe is half the volume that I have to move through a 22mm pipe before the hot water arrives!

    All three bathrooms in my house are a long way from the boiler, and although the pipes are all well-insulated, that insulation only really helps with heat retention during a shower. It's as good as useless in retaining heat between showers if they're more than 15 minutes apart from each other.

    Stick with 15mm ... it'll work, and you'll waste far less water and heat than if you went for the 22mm initial runs.
    KIAB likes this.
  5. ChriatopherB

    ChriatopherB New Member


    Thanks for the brilliant answer.
    I have been on a quest for the most power for my showers and in doing so I have thought that 15mm would be better if it was bigger - i.e. 22mm. I thought that if only I could get 22mm then splitting that (in combination with a powerful Combi) I would get loads of power even when splitting the 22mm to 2 x 15mm. What I had not considered was the speed to get the water from the Combi to my tap ... and hence how long I would need to run the tap to get the water hot.

    Simply washing my hands would necessitate a period of time running water and wasting water.

    Currently I have a 22mm gravity cold feed and (I assume) a 22mm gravite fed hot water feed from my hot water tank. The tank is a couple of meters away and it takes a little while for the 15mm tap to run hot. The Combi will be several meters away (i.e. 12 ish meters) and your argument has convinced me to stop the search and go for 15mm all the way.

    Thanks a lot :)))))

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