I have a condensing gas boiler (tank-less) at one end of my house. I have a bathroom at the the other end of my house, at a higher level.

In the morning if I open the hot water tap it takes about 15L (~4 gallons) of water, and 60-70 seconds to get hot water because the water line is very long and the cold water needs to be displaced before the actual warm water "arrives". The gas boiler is powerful and produces hot water very fast and very hot. But the cold-water-displacement part is the problem.

Obviously this is annoying and I was thinking to install a (classic) electric boiler (small, 30L / 8 gallons) to use as a buffer MOUNTED ON THE WARM WATER LINE AND NOT ON THE COLD ONE. This would hold a buffer of warm water by using electrical energy. In the morning when I open the hot water tap I would get warm water from the electric boiler which, in turn, would refill from the warm water line which for the first 60-70 seconds would get quite cold water.

This would be, IMO, an acceptable trade-off of space, electric bill etc. in exchange for the convenience of having warm water anytime.

My CONCERN is if I keep using it after the cold water "finished" and fresh, hot water is fed in the electric boiler, would it hurt it? Or would it hurt me (by serving too hot water) or some other disadvantage that I cannot think of?

  • Think most people go with pipe insulation and a small circulating pump to keep warm water near where it is needed.
    – crip659
    Jan 21, 2023 at 19:18
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    I have read of the re-circulation method but as far as I could understand this would not work with a tank-less boiler... Jan 21, 2023 at 19:19
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    I have a Navian on demand HW heater and it does support re-circ. But the OP should weigh the costs of keeping a hot water supply up to temp constantly vs. the cost of running the water until the hot water "arrives".....there is also the convenience factor to be considered... who wants to wait for up to a minute to get hot water??? Everything is a tradeoff. Jan 21, 2023 at 20:59
  • @GeorgeAnderson: Wow, I thought re-circ is not suitable for on-demand/instant/tank-less heaters! On the other hand, it would keep the flame going and consuming potentially lots of gas. Oh, the 60-70 seconds of waiting for the water to reach the faucet (sorry for using "arriving" - English is not my native language) is only the first usage in the morning, the pipes get "primed" and subsequent usages are far more decent from a waiting time point of view. Jan 21, 2023 at 21:06
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    @AndreiRînea The on demand WH w/re-circ does not keep the flame going all the time, it only fires up when the temp in the re-circ line drops to a certain set point, and then only for a minute or two. But and here's the big BUT, unless the WH re-circ can be put on a timer, like "don't do this a night" it will do this 24/7 and will consume additional fuel. Re-circs are very nice for large houses with spread out bathrooms, etc. but they come with the price of keeping the hot water lines hot all time as well as running the re-circ pump. Comes down to personal preference. Jan 21, 2023 at 21:52

3 Answers 3


Point of use water heaters are a common solution to the problem you are trying to solve. Most operate on 120 Volt and can plug into a standard outlet and take about 12 amps. You can set the temperature a degree or two lower on it so when the water from the main heater gets to it, the smaller unit will turn off. This will not hurt your smaller unit. Just a safety note, both heaters should be set so the water won't seriously burn you.

  • that is a good idea, I would set the main heater to 104F (40C) degres and the small electric one to 107F (42C)! Jan 21, 2023 at 19:49
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    Thanks @George Anderson
    – JACK
    Jan 21, 2023 at 20:56
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    @Andrei Setting a storage heater at much less than 60C (140F) is a plan to breed very nasty bacteria that will send you to the hospital. This was exposed after Flint, Michigan made changes to their water supply and had a rash of these cases at once. This has been happening all along, but as random isolated incidents, so a pattern was never observed until Flint. Of course this advice contradicts "don't use scalding temperatures", and the cure for that is a thermostatic mixing valve. Jan 22, 2023 at 0:36
  • @Harper-ReinstateMonica It seems what happened in Flint was much more complicated than people not heating their water hot enough. ncbi.nlm.nih.gov/pmc/articles/PMC5353852 If it weren't for all the other problems there would have been enough chlorine in the water to avoid the bacterical contamination ncbi.nlm.nih.gov/pmc/articles/PMC5828617
    – D Duck
    Jan 22, 2023 at 12:47
  • @DDuck Well that would be great if your goal was to rationalize away any need to deal with the issue. "Just don't live in Flint! Solved!" But I would follow the advice of my national health department, I think. Jan 22, 2023 at 21:51

First, I would actually measure your numbers. Because I find 15l of water in the pipe to be non-believable. That's 100m (300'+) if the pipe is 3/8" trade size, 60m (200') if 1/2" pipe, and 35m (100+') of 3/4" pipe, but 3/4" would be a dumb choice because it has a huge inventory of water, which means a ridiculous wait for hot water.

If your house really is that big, a single water heater for the whole house is impracticable.

Your tank needs to be pretty big, though.

Here's the thing. As your tank refills, it's going to be filling with cold water. Hot water tanks are specifically designed to separate flow. So for the first 2/3 (20 lit34s) of water, you'll take that off the top of the tank and it'll be tank temperature. But with only 10 liters of old water left, what's in the tank? 15 liters of cold water from the pipe, and 5 liters of actually hot water that has finally made it through. Let's say the pipe lives at 10C and you want the water at 40C. Half the water in the tank will be 40C, and the other half 10C, now mixed together because the separation is no longer working. What temperature will that water be? That's right, 25C. Barely tepid.

And it will be like that for the next 20 liters of usage. Then for the following 30 litres or so, it will slowly blend to warm up bit by bit.

This will result in an annoying "mid-shower cold lump" that will probably just motivate you to take Navy showers lol.

But let's do it again with a 120 liter tank. Now your first 80 liters are fine, and the first 15 liters of cold water will have plenty of time to diffuse into the last 40 liters remaining.

Temperature will also help.

You seem to want to run the hot water system at very close to desired final shower temperature. That is dumb. You want to mix significantly hotter water with cold water, so you have some headroom for variances in hot water temperature. (This might be a big part of why you need to run 15 liters of water to be comfortable).

Ideally, you want an automatic mixing valve, which will automatically adjust out these hot water temp variances, and also provide anti-scald protection. Anti-scald protection in turn allows you to run the tank hotter still, which further stabilizes spigot temperature.

But much more importantly, hotter tank water allows the tank to stop dangerous bacteria growth - a phenomenon only recently understood, because fatalities from it happened sporadically and randomly. When Flint, Michigan made abrupt changes to their water supply, among other things it triggered a large enough cluster of cases to become detectable! Investigation ensued and found low-temperature water tanks festooned with legionella and other bacteria. Before that, no one knew!

Now if you don't want a clunky "hotel style" valve, I hear you. They make much better thermostatic mixing valves which allow 2-dimensional control of flow and temperature - like a joystick. Sometimes flow is an ungainly push/pull, but temp is a fine control knob that can be simply left in the desired position.

But if you really like two valves, thermostatic mixing valves can be plumbed in right before the hot water tap. This will mix in cold water as necessary to keep the the hot water tap right on target temperature. (and prevent scalding, of course).

  • The pipe is 1 inch in diameter. Even thicker than the 3/4 you mentioned.. Jan 24, 2023 at 14:27
  • Outer diameter, that is.. Polypropylene : leroymerlin.ro/produse/teava/662/… Jan 24, 2023 at 14:37
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    As for pipe distance between WH and faucet it is about 25-30m indeed. Jan 24, 2023 at 17:34
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    @AndreiRînea That seems roughly equivalent to 3/4" NPT, so yeah. Obviously someone thought bigger pipes were more deluxe... true on cold, but hot causes this. Jan 24, 2023 at 20:34
  • @Harper according to the instruction manual of my Bosch tankless WH operating it at a relatively low output temperature reduces the formation of boiler scale in the heat exchanger. It makes sense to operate the WH at the lowest temperature that will meet the demand. We set the gas flow to the burner to the lowest that will provide hot enough water for showering. So at the shower 90 % of the water is from the hot line and 10 % from the cold line. This means that the water at the kitchen sink is not hot enough to kill bacteria on flatware so we mix in scalding water from a kettle. Jan 24, 2023 at 21:51

One method is to run the hot water supply as a "loop" and each faucet is a small tee off the loop.

Then a small pump that drives the flow from and back to the tank will have water to each faucet quickly.

This is done in larger houses and buildings due to the waste of water that you describe.


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