I'm in the process of purchasing a tankless water heater and have been looking at units with an integral circulation pump. I don't have a hot water recirculation return line and plan on using a crossover valve at the furthest point of use. The integral recirculation operation of the tankless unit is poorly defined and near to impossible to get detailed information on. I fear that it will not perform in a manner that best suits my needs.
The lightbulb finally came on and I thought why don't I just get a basic tankless heater without a pump, purchase an external pump and control it with my Hubitat C7 that I currently use for some simple outdoor lighting tasks.
The concern with the crossover valve recirculation is to be able to turn off the pump at or about the same time that the hot water reaches the crossover valve, causing it to close, which in turn blocks the flow of hot water to the cold water line.
I'm looking for a sensor that I could use to monitor the hot water temperature at the crossover valve. I'd prefer it to be Z-Wave and not battery powered. Any suggestions?
The Fibaro Smart Implant is one option that seems to meet your requirements. An external temp sensor would be required.
Another option would be an Ecowitt gateway and water temp sensor. It's a WiFi system but works locally. Might cost $10-$20 more at first, but opens up a lot of options for weather and environmental sensors. Most sensors communicate with 900Mhz wireless.
I've not used either, but both Fibaro and Ecowitt have lots of happy users in this community.
Let me get this straight... You want to recirculate water back to a tankless water heater. Most of these are flow-controlled (the very act of flowing water through them turns them on). So isn't the device going to just run constantly, albeit heating somewhat warm water?
To be frank, I've never understood how any hot water recirculator is energy efficient, but this just seems like a huge energy hog. Tankless water heaters are designed to work ONLY when hot water is needed, so I must be missing something here.
I have been using a recirculating pump for 15 or so years. I had an external one installed on my original electric tank water heaters and now have an integrated one in my Rannai tankless water heater. Like you I don’t have a dedicated return line and have used the crossover valve for this entire time. I have had zero issues with the valve and it works well.
One of my primary reasons for getting into HA in 2004 and my purchase of SmartThings was to automate the recirculating pump since my wife and I never consistently took a shower at the same time each day. All showers/tubs have a separate light so I automate mine by turning on this switch. We all know to turn on this light a few minutes before a shower and we will have quick hot water. I also have a virtual switch setup in Alexa and HomeKit to turn it in as well.
The old external pump was easy to automate where I set it to run when power was turned on and used a smart outlet. My Rannai was a bit more complex to automate since it has a display keypad on the wall in my master bathroom and I had to solder a wire to the recirculate button so I could “press” the button with a dry contact relay.
My Rinnai is in the crawlspace directly below my master shower, by design. So hot water to my master shower is nearly instant. My kitchen is near the other end of the house. But I wanted "always hot" water at the kitchen sink. So I use the Grundfos.
It is set at 15 minute increments, 24/7 to circulate hot water until the Rinnai reaches the set temp at 130°F. I work day and night shifts throughout the month, so having that hot water circulate all of the time is certainly not efficient, but quite desirable, and a good WAF.
I've seen you post this before.
It just sounds too complicated for me, lol.
I know, you posted once you get over the learning curve hump, it's easy.
Maybe it's just psychological, lol.
(PS: Does it do thermocouples? I need something high temp.)
I too have had a bad experience with the SI.
I used it to monitor the temps at three levels in my 200 gal pellet/oil boiler buffer tank.
The sensors had to be buried between the insulation and tank-not without some hassle.
After a relatively short while, one of the (cheap, crappy), sensors, I believe, stopped working, which knocked them all out.
I had to figure out which one was bad, access the sensor and the SI, exclude/include, blah blah blah.
Then, there are the little wires,
I removed the setup-it was telling me just obvious things about temperature gradient, etc.
Not worth the hassle.
Even though, for this purpose, it can kick out pretty fast readings...when it works.
Plus, it has built in relays as well.
Although I get my dhw from a 200 gallon super-insulated buffer tank with a stainless 17 gallon coil inside. It still takes a little while for the hot water to get up from the basement.
Even in winter, when the heat might not be wasted, you're heating the crawl space.
I wonder if a separate little tank electric water heater under the sink would do the trick cheaper?
I wouldn't imagine the sink would use that much hot water, and the dishwasher has its own heater.
Of course, you'd have to power it, etc, but do some little one have 120v supply?
I'm interested in this because I've wondered how my setup might work with tankless. I have a dedicated Grundofos recirc pump that sips electricity and an insulated recirc return line that runs from our distant master bath. The hot water feed goes past the branches to the kitchen and laundry room. I'm willing to pay for the convenience of hot water on demand -well, so no water bill. My solution to 24/7 recirc is motion activated recirc running long enough to keep the tap temp "close enough."
I'll chime in here too . Just a few weeks ago I installed a circulation pump and did a fair amount of research first.
I do remember reading that a retrofit style circulation pump meant for a conventional water heater is not a good fit for a tankless. Something to do with the tankless coming on at full blast when there is any water flow, which would incur a large cost. In contrast, pushing some cool water back into a conventional tank isn't necessarily going to be enough to cool the whole tank down enough to where the element(s) are going to come on.
I probably would go with the tankless model that has the integral pump. Presumably it has been optimized to work with that style of heater.
I have my pump on a z-wave outlet that shuts the pump off if we're sleeping or not at home. According to the power metering report, it would cost roughly $50/year to run the pump 24/7, so between away/nighttime and vacations, I doubt it will even cost us $25/year and no doubt we'll save at least that by not running cold water down the drain.
I would keep things simple and just run the pump on a schedule. Monitor your energy costs for a month or two, then decide if you really need to control the pump more precisely.
Edit: You may find you need (or want) more than one crossover valve. I needed one in our kitchen and another in our patio half bath because they were the two most distant points from the water heater.
ya i just got a new rinnai with the intgral pump has it had the highest efficiency and most rebate.
but didnt enable the recirculation or put in a valve.. for one it only seems to really work from what i read based on knowing the time you take showers (ie usage monitoring) and working then..
anyway it seems a waste to me to waste all that water just to have hot 50 secs sooner. and we are away from the house half the year.. Maybe ill rethink it.. but i am sure the wife would rather wait than waste the water which is quite expensive in our town.. (i had a 1k water bill one summer due to irrigation)
A pump is pretty much a pump, and if the objective is to get hot water to the point of use, the tankless is going to have to fire up. I don't see any way around that.
If I get a unit with an integral pump, I will have pretty much lost the flexibility to control it as I see fit. My concern is in regard to the Rheem EcoNet recirculation schedule. I'd like to be able to only run the pump at certain times of the day, but it looks like you can only schedule when it turns on, not when it turns off. Reading between the lines, it looks like it may turn off after 5 minutes or when the tankless inlet temperature rises to SP-15°F. This information is apparently top secret, because I haven't been able to find anyone at Rheem that answer any questions about recirculation. I've installed the EcoNet app on my phone, but without a unit to connect to, you can't get a look at how the scheduling works.
In order to minimize the amount of hot water pumped into the cold water line I'd prefer to have the pump run for less than 5 minutes. In my application I suspect the pump will be deadheaded against the crossover valve long before 5 minutes. And if the crossover valve closes, the inlet temperature may never reach SP-15°F, which would cause the pump to run for the full 5 minute period.
My plan would be to schedule the recirculation pump to start periodically during the prime usage periods and to use the hot water temperature reading at the crossover valve to turn off the pump. I could also use that hot water temperature value as an additional condition to start the pump.
For some unknown reason the Rheem Ikonic unit I'm looking at warns against having more than one crossover valve.
Unless you like cold showers, you're turning the water on and waiting for it to get hot before getting in. All that cold water and the money you're paying for it goes down the drain. In our particular case, a fair amount of water due to the large distance between our master bath and the hot water heater.
A recirculating pump does exactly that, recirculates the water through your system. No water is lost.
My entire crawlspace is encapsulated and conditioned. I discussed an under-the-sink water heater with my builder, but I was not that happy with those options.
Well, you already own one recirculating pump option and if you don't have the crossover valve you're going to need it either way. I suspect the crossover value is the same for tank and tankless heaters. This is the one I used:
So why not install the crossover value and an energy monitoring outlet on the built-in pump, and run it for a month or two to see what your actual costs are? Then you'll have some hard data on which to base a decision. You may find that your cost is minimal and save the cost of an external pump, plus temp sensors, etc.
I'm not an engineer, but if I were designing this I'd measure the pressure on the cold water side rather than temperature and turn the circ pump off when the pressure on the cold and hot water sides are the same. That would occur when the crossover valve closes. But who knows. Given the issues around recirculation for a tankless unit, it's really not surprising that Rheem doesn't want to divulge exactly how they do it.
I have not done a cost/energy analysis, but I bet the cost of running water down my kitchen sink drain waiting on hot water would be very close to the cost of my current recirculation system. Besides, who wants to wait 20 seconds at the kitchen sink for hot water to wash your hands or a few dishes? YMMV.
As far as Hubitat control for any of this.....I do not. Our schedules are so variable every day of the month that it would be nearly impossible to write code for practical daily use. The only Hubitat code that I use for my water system is smart switches to turn off everything when on vacation and such.
It still takes a little while for the hot water to get up from the basement. 
. Just a few weeks ago I installed a circulation pump and did a fair amount of research first.
