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.

wait time and water use: tankless DHW & pipe length

It takes a few minutes to get hot water at the tap with my new Bosch Greenstar combi boiler. I have turned the "eco mode" off because the water comes out cold, then briefly warms up a bit, then runs cold again until the whole thing has reached the DHW temp.
I just measured 21.5 feet of "extra" water pipe between my furnace and the taps. This run has two old couplers, an extra old blow off valve (that drips), mostly galvanized pipe, a convoluted square of pipe: down, across, up and across, that are from the old DHW tank remnants, and a couple heavy steel gate valves.

Would going all copper, removing 4 fittings and couplers, while shortening the DHW run by 21.5 feet (galvanized pipe) lessen my DHW wait times? Or is a long wait native to the tankless boiler?

I feel bad paying by the gallon for city water to let it go down the drain, and to use the gas for heating up all that "extra" pipe. Convince me I am being a silly (uninformed) perfectionist. Caveat: I am a simple 1st time home owner, with an engineering degree (I barely passed Thermodynamics). Thank you with respect.


  • Larry Weingarten
    Larry Weingarten Member Posts: 3,289
    edited June 2021
    Hi @condenseit , Two points... First is to see how long it takes for usable hot water to be leaving the heater. That waiting time is built in. Next is to figure out how many gallons are in the piping between heater and tap. Understand that if there are two gallons in the lines, you'll need to run about four gallons out before it's hot. If you have enough pressure, I'd replace that old piping with the smallest diameter PEX or copper that you can without limiting flow. The B&G System Syzer, (available as an app) would be a useful tool here for figuring out how small a pipe you can put in. I'll just guess that with the right sized piping you can easily cut the wait and waste in half, maybe better.

    Yours, Larry

    ps, A plan "B" would be to add a small electric tank type heater as close to the tap as you can, and feed it with the hot line.
  • condenseit
    condenseit Member Posts: 20
    that is great (heating) help. Thank you Larry
    Larry Weingarten
  • Robert_25
    Robert_25 Member Posts: 527
    Welcome. I had a similar situation in my house and last year I ripped it all out, relocated the water heater, and installed new pex.  Not only is the wait time for hot water less, the flow rate is improved (there was a lot of corrosion inside the old galvanized pipe and fittings).

    Definitely remove that old piping and make it as short as possible.  Copper is great for longevity but pex and CPVC get hot water to the tap faster.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,828
    edited June 2021
    It is a costly project at retail pricing, but I'm in the business so I purchased a Manabloc system at wholesale and one of my employees re-piped my home during a slow work time. Every run is as short as it can be from the water heater to the tap. This reduced my wait time by half, and on some sinks as much as a 75% reduction in time.

    You don't need to go that extreme but pick the one or two faucets that bother you the most and follow Larry's suggestion.


    I use a 40 gallon indirect for DHW and don't have the added start-up time of a tankless. So I can only imagine what a tankless would have added in wait time on my old piping system. I would be spending all my spare time waiting for DHW and not have time to go to HeatingHelp.com.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    Larry Weingarten
  • Dave H_2
    Dave H_2 Member Posts: 554
    Why not add a recirculation system. A small circ moving a small amount of water. It sounds like you have a large amount of water that cools down. A small circ could handle it, it sends back the waterthat cooled down back to the water heater to be heated again rather than dumping it down the drain.

    There are lots of different ways to accomplish this task between the installation method used and desires met.
    Just remember one thing about the circ size, its all about moving temperature, not gallons per minute.

    Dave H.
    Dave H