I have a whole house tankless water heater installed, and so far it's nice except one problem. I also have a nonelectric hot and cold toilet mounted bidet installed. Since a high pressure jet would not make the bidet suitable for it's task, it's max output is less than 0.4 GPM.

The tankless heater has an activation flow rate of 0.4 GPM and the bidet fails to pull enough water to start it heating.

The current solution is to run either the sink or tub hot water to avoid a very cold jet of water in an uncomfortable place.

Is there a common solution to this issue?

I'm considering getting a 2 gallon mini tanked water heater for the bidet. I think that should be plenty, but I'm worried that a small tank heater would have a lot of surface area and lose heat fast. It would have the advantage of being local and instant, as long as it was installed near the toilet.

Alternatively I could get point of use tankless water heaters. I have one installed in the kitchen in series with the whole house unit to boost the temperature for the faucet and dishwasher. That works good, but I'm having trouble finding one with a low enough activation flow rate for a bidet.

A third option is to use an electric bidet. This has some nice bonuses, but I don't really want to do that. They're expensive, and since they're all in one units, then each additional function is an additional point of failure. I'd rather keep the bidet and water heating separate. Further I already have the bidet, and toilet seat, and don't want to replace them if it's reasonable and possible.

So what I'm looking for is more information.

How is a low flow appliance normally handled with a tankless heater? Is the minitank a good solution energy wise? Should I look harder for low flow point of use tankless heaters? Is there something I'm not aware to solve this? Thank you.

  • 2
    Unless you have a recirc pump the bidet would spray cold water anyways until warm water reaches the fixture. By turning on the sink you're actually solving two problems: getting warm water as close to the bidet as possible and activating the tankless heater.
    – MonkeyZeus
    Mar 17, 2021 at 19:41
  • The bidet has a "cleaning mode" which works to clear the cold water out of the line without spraying it. But a recirc pump is an interesting idea. That could maybe be set up to run temporarily to trigger the water heater without wasting water. Going to research it more. Thank you for the idea
    – netsplit
    Mar 17, 2021 at 19:48
  • 1
    That is certainly an idea. If you have the means and knowledge to achieve it then that sounds quite do-able; assuming the recirc can achieve 0.5 GPM of course :)
    – MonkeyZeus
    Mar 17, 2021 at 20:02
  • 1
    How long does it take to warm up - motion sensor seems like it might give a bit more time, than fast toilet user ...
    – Mr R
    Mar 17, 2021 at 20:04
  • 1
    This recirc says it's for a tankless heater specifically so I'm going to assume it can achieve the 0.4 GPM ¯\_(ツ)_/¯ homedepot.com/p/…
    – MonkeyZeus
    Mar 17, 2021 at 20:06

1 Answer 1


Thanks to MonkeyZeus suggestion. Here's the answer I'm trying. If you look around you can find a 12VDC Circulation pumps that do 2 GPM for under $50.

Simply install an outlet in a location safe from water near the appliance or fixture that has some means to control it whether a switch or motion sensor.

Install the pump between hot and cold lines. Plug the Pump's AC adaptor into the outlet mention above. And life is good. A bonus is no water is wasted getting the water heater up to temperature.

Edit: I'll mark this as correct once I have mine installed and made sure it's working good.

Update: installed and working good!

I used these parts: 1/2" Back-Flow Preventer: https://www.amazon.com/gp/product/B000EDUTN6 Hot Water Heater Circulation Pump with DC Power Supply Adapter 2.1GPM: https://www.amazon.com/gp/product/B01G305PK0

The incoming hot water line is 1/2" pex, going to the bidet, and the out going cold line is 3/4" pex, because the shower line was easier to connect to for that. Per the comments, I added a backflow preventing valve. The pump didn't come with one.

You can hear the pump running, and once it kicks on there's hot water in the line in less than 20 seconds. Works so good I'm going to install one on the kitchen faucet.

The pump it's self is submersable, 12VDC, and low amperage. Parts including the pex connectors, crimps, and such was less than $50 and took less than an hour to do. Most of it was figuring out the in and out on the pump.

The install is less than 10 feet from the water heater, so this may not work for longer runs, or narrower pipes, or some other variables. I mention this because the pump isn't rated for longer runs. It works good for my loo, that's all I know.

Haven't yet put the motion sensor switch and outlet in yet, but I'm confident about those.

  • "Install the pump between hot and cold lines" You're planning on pumping partially to fully warmed water into the cold water line? Wouldn't it make more sense to pump it back into the hot water side?
    – FreeMan
    Mar 18, 2021 at 15:49
  • @FreeMan Pumping it into the hot water side would make it a closed loop and never activate the tankless heater; you need to pump it into the cold line albeit very close to the tankless. i.stack.imgur.com/A0Kii.png
    – MonkeyZeus
    Mar 18, 2021 at 17:35
  • That image makes sense, @MonkeyZeus. Though, TBH, I was envisioning a hot & cold line coming from the whole house tankless, not a single cold line coming into this particular heater. Therefore, it would loop in the hot water line that's in the bathroom, not all the way back to the main water inlet.
    – FreeMan
    Mar 18, 2021 at 17:45
  • @FreeMan Truth be told, I've never installed a tankless so you've force me to verify my own sanity. From everything I've found the cold line does not come out of the tankless. Was this a misunderstanding on your part or is this a chance for me to learn something new? I see things like i3.ytimg.com/vi/i4Dches4xtg/maxresdefault.jpg
    – MonkeyZeus
    Mar 18, 2021 at 18:00
  • @FreeMan My image was heavily simplified for OP's situation. Yes, in the real world there would be a kitchen somewhere along the way, outdoor spigot, and even a tub/shower :-)
    – MonkeyZeus
    Mar 18, 2021 at 18:02

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