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For a new build, I'll be running pipe from the hot-water tank to the two bathrooms, which are about 40' away on the other side of the house. One aim is to avoid the situation where it's necessary to run the bathroom hot-water taps for several minutes just to get the first few drops of hot water.

I see two options:

  1. run a 3/4" trunk-line from the hot-water tank across the house, and then t-off to each bathroom with 1/2" pipe.
  2. run two 3/8" pipes from the hot-water tank directly to each of the two bathrooms.

The first option seems to maximize overall flow, but means having to empty out a long length of 3/4" pipe before hot water reaches the taps. The second option would help minimize the lag, but I'm concerned about anemic hot-water flow, particularly to the showers.

I wish to avoid a recirculating pump.

What's the better option?

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    Insulating the pipes with pipe insulation or go with a re-circulation pump system.
    – crip659
    Oct 1, 2021 at 16:25
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    If its new construction, consider a second water heater near that bathroom or at least point of use heaters for the sinks so you get almost instant hot water. If a second heater isn't an option, you can install a second pipe to support a recirculation pump.
    – JPhi1618
    Oct 1, 2021 at 16:35
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    Now I'm curious why you don't want a recirc pump? I've always thought they were a good option if you can plumb in a loop for it, but have never had one.
    – JPhi1618
    Oct 1, 2021 at 16:52
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    Would go option one and insulate the pipes as much as possible, pipe insulation plus batt insulation. Larger pipe usually loses heat slower compared to smaller pipes. Maximizing insulation will keep water hot longer, should get a few hours useful heat retention.
    – crip659
    Oct 1, 2021 at 17:29
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    Move the hot water tank closer to the point of use.
    – Ecnerwal
    Oct 1, 2021 at 22:12

3 Answers 3

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I don't see much difference in the two options you've given. Either way you've got 40' of pipe of cold to tepid water to clear out before you get hot water from the tank.

The only advantage to option 1 is if you expect both bathrooms to be in use simultaneously or nearly so - then there's one long line to be cleared by the sucker first user, and the other gets hot water much more quickly.

You'd do a bit better with insulation, but that still won't help for that first use in the morning, it will have lost whatever amount of heat overnight, it'll have just done it more slowly.

Your best best are either:

  • Recirculation pump to ensure that the line is always full of hot water. This is somewhat inefficient as you've got a pump running regularly and you're pulling hot water from the tank, allowing it to cool to a set point, then pumping it back in to replace it with fresh hot water. Besides, you've ruled it out.
  • Point of use instant hot water heaters.
    • If you run one line, then Tee it, you could have the heater just before the Tee. You'd still wait a few moments for the remaining line to empty of cold water, but it would be a far shorter wait.
    • AIUI, you can put it on the hot line and it'll only heat if the incoming water is below the cutoff temp. This way, it gives you hot water instantly, but cuts off once you've got hot water from the (gas powered, right? :D) main tank heater.
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    "Either way you've got 40' of pipe of cold to tepid water" but it takes longer in pipes of larger diameter, since for a given flow at the fixture, replacing the total volume of water in the pipe depends on the pipe diameter x length. But yes, either way you still have to move water over 40'.
    – P2000
    Oct 1, 2021 at 20:41
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    The difference between the two options is that option 1 needs to move a gallon of water before hot water reaches the bathroom, while option 2 only needs to move a quarter-gallon.
    – Mark
    Oct 2, 2021 at 3:02
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One solution is to run a loop from the tank to each tap and back to tank, well insulated and with a small pump controlled by flow demand - as soon as there is demand then the hot water gets pumped around so fully hot is almost instant.

Less waste and do consider temperature control if needed.

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  • The OP excluded this in an edit to the question, but it's still a good idea.
    – FreeMan
    Oct 1, 2021 at 17:41
  • @FreeMan yes, the OP made the edit after I pisted this answer, perhaps it will be one of those forever changing questions.
    – Solar Mike
    Oct 1, 2021 at 17:49
  • Yeah, the after was implied in my comment. It wasn't clear, sorry about that. It's still a good idea!
    – FreeMan
    Oct 1, 2021 at 17:53
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A variation on the recirc pump:

  • You will need the recirc pipe, also control wires
  • Pressing a button at the point of use (maybe additional buttons elsewhere?) triggers a timer, which runs the recirc pump for a few minutes

Hot water, no wasted water and don't have the same level of heat loss from the pipes when not in use.

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