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otherRecirc pump for tankless water heater: Take 2 after more thought and research (self.DIY)

submitted by GeekX2

After reading the responses to my previous post (thanks to all who replied), I realize that I didn't make clear what I am thinking of doing or why I think it's worth looking into.

Review: I have a 2600 sq ft house completely re-plumbed about 12 years ago. We have a gas tankless water heater mounted in our basement. All plumbing is accessible in the crawlspace (except what goes into the walls to get to the fixtures, obviously). We chose tankless primarily for the idea of unlimited hot water with savings over keeping a tank heated secondary. It is my wife and me in the house and we are both retired so home pretty much all day.

The problem: It takes a some time to get hot water to the fixtures. In the kitchen particularly, the wait is over 30 seconds. Perhaps more importantly, we're running cold water down the drain while we wait for the hot at any fixture. Our dishwasher manual recommends ensuring there is hot water at the fixture before starting the dishwasher. So any time we want to wash dishes in the dishwasher or by hand we will waste .5 to 1 gallon of water just to get hot water. It's a similar situation--to differing degrees--with all of the fixtures.

To remedy this, I've been researching adding a recirculation scheme to the hot water system. I am not considering continuously recirculating the water. I expect that would kill the heater in short order. Here's the plan:

  • A dedicated return loop (probably with spurs to all of the fixtures)
  • All hot water pipes heavily insulated
  • A pump at (or close to) the heater
  • The pump runs on a timer or a thermostat/aquastat
    • If timer, the pump runs (as an example) 2 minutes every 30 minutes. This would require some experimentation and experience to fine tune. This would be limited to our normal waking hours.
    • If stat, the pump starts when the water at the stat is below some temperature (90F ?) and runs until the water at the stat reaches a given high temp (100F ?). Again this would require experience and experimentation based on the available equipment

The idea here is to (1) maximize comfort (after all, I have to decide whether to wash my hands in cold water or wait for the hot water to arrive), (2) conserve water lost from waiting on hot water. I realize that this will come at the cost of a likely higher gas bill. I'm still trying to work out the potential cost there.

A question that just occurred to me. How will the water heater react to having hot water as an input. I know that it will only heat water to the set temp. If the input water is at that temp, will the heater just quit heating? Or could it damage the heater?

It's not my intention to argue my idea here. I'm just trying to clarify what I feel like I left out of my previous post. Thanks for reading.

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[–]faucet_part 4 points5 points  (1 child)

i was you a few years ago. sounds like you’re still in the research phase, great!

you know there are a lot of off-the-shelf solutions here, right? my recommendation is not to overthink this.

maybe you have a Rinnai hot water heater (maybe you don’t). they are pretty solid, standard tankless units.

play around with google and a set of these keywords and you will - eventually - find some technical bulletins and manuals that tell you exactly how the manufacturer recommends doing this:

rinnai. tankless. recirculation.

happy hunting.

note: we ended up just swapping around some jumpers to allow it to take the circulated water (the issue is not the temperature, it’s the unit sensing water flowing through) and a pump on a timer to have it going during peak usage times.

you can also go all in and have various expansion tanks or serialized heaters or whatnot. you can spend a little money or an awful lot.

search around for the manuals/literature and you can get an idea for which approach you want to take.

[–]faucet_part 0 points1 point  (0 children)

looking at the previous post, there’s just some flat out wrong responses there.

[–]BigSkyMountains 1 point2 points  (3 children)

For the love of god, do not do an open loop recirculating system. I have one of these in my house and it’s a nightmare. Some of the problems I’ve had:

  1. The check valve in the open loop wore out before I understood what was going on. This created a thermosiphon that I was unaware of. Hot water was running through the system 24x7, even when the pump was off. This was costing me about $500/yr on the gas bill. It also likely explains the short life of that water heater.
  2. I eventually replaced the water heater. Something in the new unit triggered a thermosiphon flowing in the opposite direction of the check-valve. After three attempts at cutting pipes and re-plumbing things, I eventually fixed it by routing the return line into the tank drain. I dunno why it worked, but nothing else did.
  3. I did eventually found a solution to the energy-hog problem, as these things do use a ton of energy. I set up some Flic buttons in each bathroom to turn on the recirulator for ten minutes each time you push the button. It’s a pretty good solution, but occasionally the recirulator fails to turn off. I rely on a home energy monitor to spot when this happened.

My recommendation is to find some type of closed loop on-demand solution. I believe the Taco pumps do this, but I haven’t looked at it in too much detail.

[–]GeekX2[S] 0 points1 point  (2 children)

Just so I'm clear. What do you mean by closed and open loop?

[–]BigSkyMountains 0 points1 point  (1 child)

Open loop is where you have the full loop that circles back to your water heater.

I believe closed loop systems push the water back into the cold water pipes, but I only briefly looked into them.

I can only say the headache of my system has been my largest homeownership headache. And while instant water is nice, the energy bill is quite large unless you really lock down the amount it runs.

Another potential alternative would be one of those water pre-heaters that sits near your faucet/shower. I haven’t used them, but it might be worth the research.

[–]GeekX2[S] 0 points1 point  (0 children)

Thanks. Open loop may be my only choice. I recall that we used different types of pipe for the cold and hot systems. I'm not sure the cold pipe is rated for hot water. I have been looking at flow sensors and planning to put in the ability to close off the loop (and turn off the pump) if there is some sort of failure.