Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Tankless water heater pipe size vs performance

I've posted here before and sorry for all the questions, but I'm renovating my house so there are always more. In the past I've had great responses on this forum so I figure I'll come back for more!

I'm about to re-plumb my bathroom and kitchen hot water feeds to a Uponor EP valved manifold - essentially making a central distribution point with a per fixture hot water feed instead of a branched model.

I currently have a Burnham KV74 with side mounted DHW coil. The manual claims the inlets are 3/4" NPT, however, whomever plumbed this setup decided to use 1/2" copper about 4 feet above the boiler. Up until that point there is 3/4" copper supply & return.

I'm wondering if the restriction in flow from 3/4" to 1/2" would be reducing my hot water performance at all?

I know this boiler needs to be replaced for my purposes, but I am trying to work with it for now and prep the home for a boiler/tank swap by running pex in the ceiling before I close it back up with drywall.


  • Jamie Hall
    Jamie Hall Member Posts: 23,272
    You would be losing pressure, and quite possibly some flow because of that, but that's all.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,512
    edited January 2017
    It probably won't make any difference. All tankless heaters are 3/4 connections

    1/2 " pipe may improve the performance and prevent you from overdrawing the tankless which have questionable capacity