# Anyone have a point-of-use tankless water heater at their home?



## thompwa (Aug 26, 2019)

I'm looking at installing one at our kitchen sink as a way to take the burden off of our whole house tankless unit. The way we use the kitchen sink just seems impractical for a whole house tankless. It's great for endless hot showers - but seems wasteful to fire up a 200,000BTU tankless unit just to wash a few baby bottles or rinse a few dishes.


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## MasterMech (Sep 24, 2017)

Following as I have been considering similar. My home is slab-on-grade with most of the downstairs fixtures plumbed through the slab. The kitchen sink is the farthest run away from the gas-fired tankless and never gets hot water in a reasonable amount of time. All of the energy used to heat the water is absorbed by the slab by the time the work is done.


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## MrMeaner (Feb 21, 2017)

Had 2 tankless Rianni's connected to small electric 5 gal water heater and constant flow pump for instant hot water to all faucets...loved it. New house has two 50gal hot water heaters with constant flow pump for instant hot water at all faucuets love it as well.

@mastermech both my last house and current house are slab on grade. no basement but all plumbing running was under slab. Might look into the additional water heater and constant flow pump for instant hot water.

At my office we installed a point-of-use heater for the break room sink....seemed like it took forever to get any hot water from that unit and it was decent sized..did not like it compared to my houses.


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## thompwa (Aug 26, 2019)

I've considered just using a small tank system right at my kitchen sink. Something like a 2 or 4 gallon - but if I use up what's ready in the tank wouldn't that create a cold water sandwich? How does that work? I've tried to find a decent plumbing diagram or visual but I haven't seen any that are specific to my application. The "right" solution is probably to install a recirculation pump for the line, but i just hate the idea of having to do that.


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## burntfire (Dec 10, 2020)

I guess I'm a bit confused here. You currently have a whole house POU but you want to add a small one for your kitchen? Are you gas or electric? Being your in Alabama I'm assuming natural gas.

At the end of the day your large POU doesn't care about turning on/off even for a short time; that's what it's designed to do. If you're worried about wear/tear you could theoretically do what you're talking about. Disconnect the hot water line being fed from your large POU from your sink and add a small POU and connect to your cold water line and add an electrical line.

I personally wouldn't go through the hassle of adding a small 4 gallon tank at your sink.


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## thompwa (Aug 26, 2019)

burntfire said:


> I guess I'm a bit confused here. You currently have a whole house POU but you want to add a small one for your kitchen? Are you gas or electric? Being your in Alabama I'm assuming natural gas.
> 
> At the end of the day your large POU doesn't care about turning on/off even for a short time; that's what it's designed to do. If you're worried about wear/tear you could theoretically do what you're talking about. Disconnect the hot water line being fed from your large POU from your sink and add a small POU and connect to your cold water line and add an electrical line.
> 
> I personally wouldn't go through the hassle of adding a small 4 gallon tank at your sink.


Yes I'm on natural gas at my house. I think the real issue is two fold:
1- the time the standing (cold) water in the line takes to clear the say 30-40' of 3/4" line from the main tankless unit to the kitchen sink while we wait on hot water at 1GPM is wasteful and by the time that water clears the line we are already done doing washing whatever quick thing we needed to wash.

2-Turning on hot water at the sink calls for the 200,000btu boiler to kick on, heat a bunch of water, kill a bunch of dinosaurs, and then leave a bunch of water I paid to heat in the line between the fixture and the tankless unit. I don't actually even know if a point of use tankless or tank unit would be more cost effective, but that's what i'm trying to figure out. Our water coming in from the meter averages at around 50 degrees so it would take a pretty decent size electric unit to bring that up to 110-120.


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## MasterMech (Sep 24, 2017)

MrMeaner said:


> ...
> @mastermech both my last house and current house are slab on grade. no basement but all plumbing running was under slab. Might look into the additional water heater and constant flow pump for instant hot water.
> ...


I agree that's prob the best way to ensure hot water on demand and reduce water usage. It does concern me from an energy efficiency standpoint however. Powering the pump, and of course, all the heat that would be dumped into the slab/ground. It doesn't take long for the line to cool off once the flow is stopped.


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## Ware (Jan 28, 2017)

thompwa said:


> ...but seems wasteful to fire up a 200,000BTU tankless unit just to wash a few baby bottles or rinse a few dishes.





thompwa said:


> 2-Turning on hot water at the sink calls for the 200,000btu boiler to kick on, heat a bunch of water, kill a bunch of dinosaurs...


That is not really accurate. A 199,000 Btu/hr Rinnai unit, for instance, has a minimum firing rate of only 15,000 Btu/hr. I think the comparable Rheem units dip down into the 11,000 Btu/hr range. The modulating burner adjusts its intensity to match the demand.

So unless you're pushing somewhere close to the maximum flow rating of the water heater (~11 GPM for a 199,000 Btu/hr unit), you're not burning NG at a rate anywhere near 199,000 Btu/hr. Using a sink at 1-2 GPM, you're going to be much closer to the minimum firing rate - which, by the way, is significantly lower than the ~40,000 Btu/hr burner in a 50gal storage tank water heater.

As for wasted water, here are the volumes for common PEX sizes:


1/2" = 0.92 gal/100ft
5/8" = 1.34 gal/100ft
3/4" = 1.84 gal/100ft

You can do the math for your scenario, but it's probably safe to say the delay is more annoying than the actual value of the slug of cold water wasted. Here where I live, my combined water and wastewater rate is $8.32/1,000gal - so the gallon or so of cold water in 100ft of pipe is worth about 1 cent.


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## Ware (Jan 28, 2017)

We're putting a Rinnai RUR Series in our new house. It has a built-in recirculation pump, and is WiFi-enabled so you can control/schedule to avoid the wasted energy of recirculating 24/7. You can control it from the mobile app, a push button at a convenient location (e.g. near a sink), motion sensor, or even voice activation (e.g. Amazon Alexa or Google Home).

If your house isn't plumbed with a dedicated return line, they have a thermal bypass valve you can install:


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## MasterMech (Sep 24, 2017)

Ware said:


> We're putting a Rinnai RUR Series in our new house. It has a built-in recirculation pump, and is WiFi-enabled so you can control/schedule it so it's not wasting energy recirculating 24/7. You can control it from the mobile app, a push button at a convenient location (e.g. near a sink), motion sensor, or even voice activation (e.g. Amazon Alexa or Google Home).
> 
> If your house isn't plumbed with a dedicated return line, they have a thermal bypass valve you can install:


I think my FIL's previous house (in NY) used a similar unit to power a hydronic heating (baseboard convection) system and the domestic hot water demands. No re-circ loop however. One "issue" was the loss of the thermosiphon effect he got from his old tank unit as almost all of his fixtures were a level above the unit. Bi-Level/Raised Ranch style home.


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## thompwa (Aug 26, 2019)

Ware said:


> thompwa said:
> 
> 
> > ...but seems wasteful to fire up a 200,000BTU tankless unit just to wash a few baby bottles or rinse a few dishes.
> ...


Ahh. Thanks. This actually makes so much more sense now. I am new to this tankless lifestyle so I wasn't accounting for the modulating burner. That makes me feel a lot better and also makes me feel like A whiny American with my first world problems of having to wait 60 seconds for warm water.


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## g-man (Jun 15, 2017)

thompwa said:


> , kill a bunch of dinosaurs,


FYI, they are already dead. you can bypass this step.


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## quintafresnos (Mar 3, 2021)

I took ours out and installed an electric boiler instead. We have solar panels that offset our electricity bill.


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