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Recommendations for kitchen/half-bath tankless heater

whitenack | Posted in Green Products and Materials on

Hi all,

Back in 2012, Martin posted an article (https://www.greenbuildingadvisor.com/blogs/dept/musings/are-tankless-water-heaters-waste-money) about how tankless water heaters aren’t really worth the money. I have a couple of questions..

1.) Has there been changes in pricing to change the recommendation?
2.) Is this article focused just on whole-house heaters, or are point of use heaters also not worth going tankless?

Building a house in Zone 4A. The majority of the plumbing is on one side of the house, and I have a Heat Pump Water Heater located directly under the bathrooms. The kitchen is on the other side of the house, so I was thinking a small point of use heater to serve the kitchen sink, dishwasher and half bath would be efficient. Thoughts? Recommendations for a good tankless heater?

Thanks

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Replies

  1. user-2310254 | | #1

    I think the real question is do you want a tankless water heater. A high-efficiency conventional tank like a Rheem Marathon or a heat pump model is going to be more cost-effective. It's also easy to solve distance issues with an recirculating line tied to a switch or proximity sensor.

    Just as an FYI, Consumer Reports has a 2015 assessment of tankless models, and the issues Martin raised in his article are still present.

  2. GBA Editor
    Martin Holladay | | #2

    Clay,
    The decision on how to proceed depends on several factors, including:

    1. Does your house have access to natural gas?

    2. How expensive is electricity where you live -- cheap (10 cents / kWh) or expensive (22 cents / kWh)?

    One possible solution to your dilemma is to supply your kitchen from the heat-pump water heater -- but to also install a small electric point-of-use water heater near the kitchen sink. The point-of-use water heater will eliminate the long wait, and will boost the temperature of the water in the pipes until the water arrives from the heat-pump water heater.

  3. Jon_R | | #3

    If you don't need much water, an undersink 120V electric tank heater is very easy to install. Electric tankless typically requires a new electrical circuit.

    For whole house, I'd run the numbers. For example, the local store is selling a condensing tankless water heater for $916 and a non-condensing power vent (required in my case) tank heater for $800. I'd do the work myself, so saving $42-$121/year is clearly cost effective.

  4. Expert Member
    Dana Dorsett | | #4

    There is really no such thing as a good ELECTRIC tankless for several reasons:

    The instantaneous load of even a smaller one (too small to run a full flow shower) is huge, making it the largest load in almost any home (unless you have a ~15 ton air conditioner.) The amount of grid infrastructure required to support loads that big are significant. Some utilities states are looking at re-structuring residential rates to include "demand charges" that applies a per-kilowatt fee based on the heaviest 15 or 30 minute period of power use. A tankless hot water heater would become a liability under that type of rate structure.

    There are peak flow limitations with an electric tankless that can prove unacceptable, depending on how you use hot water in the kitchen. For instance, a 12kw point of use tankless can only deliver ~40,000 BTU/hr, so if you're filling a sink with 110F water in winter when the incoming water temperature is 40F (a 70F delta) your flow rate is limited to (40,000/70=) ~570lbs/hr, which is 68.5 gallons per hour, or a bit more than one gallon per minute. That's fine for washing your hands or a quick rinse, but pretty lousy for filling pots/tubs.

    Even a 12kw tankless requires a dedicated 240V/50A circuit, which takes some pretty fat wire.

    Add it all up and a 3-5 gallon under sink tank-let starts to look pretty good. If you feed the cold input side of the mini-tank from the existing hot water distribution plumbing from the HPWH it will save some energy during heavier draws, and would in most cases save a modest amount of kwh from the bill.

    Plumbing in a gas point of use hot water heater would potentially give you a lot more flow, but they're bulky and between the larger bore gas plumbing requirements and the venting requirements they can be pretty expensive to install.

    All tankless HW heaters are prone to developing mineral deposits on the heat exchangers. To keep flow & efficiency up they can require periodic de-liming (particularly if you have hard water), making them a higher maintenance than a tank.

  5. whitenack | | #5

    Thanks for the replies.

    There is no access to natural gas. The house will be all electric.

    Electricity is cheap here. After taxes and surcharges it is 13 cents / kWh. Plus, I have plans to install solar electricity at some point.

  6. Expert Member
    Dana Dorsett | | #6

    Of the locations where residential demand charges are being proposed most strenuously, it is initially being applied to solar customers only, based on the notion that solar customers don't pay their "fair share" of the grid infrastructure costs unless demand charges are applied. That has been shot down by regulators in some locations, but it keeps coming up.

    https://www.greentechmedia.com/articles/read/New-APS-Rate-Case-Seeks-Mandatory-Demand-Charges

    http://frontiergroup.org/blogs/blog/fg/how-high-residential-demand-charges-could-slow-growth-rooftop-solar

    http://www.utilitydive.com/news/comed-jumps-on-the-demand-charge-train-with-new-illinois-proposal/418735/

    Of course you could buy a shiny-new battery system with your solar to flatten out your load profile even more to avoid demand charges. (Demand charges on commercial rates are what's driving behind-the meter grid attached storage in the US so far.) Tesla announced Friday that their new 14kwh / 7kw- peak PowerWall will have a lower installed cost than their initial 7kwh version. The 7kw of peak output is still only about half the instantaneous power draw of a point of use tankless, so it wouldn't flatten that load completely unless you only used the tankless when the sun was shining, and had sufficient PV output to cover the other half.

    A point of use mini-tank is still the lowest risk and lowest cost solution, despite the standby loss.

  7. user-2310254 | | #7

    A tankless electric water heater with enough capacity to serve an entire house would likely draw 120 amps. (Most panels are rated for 200 amps so you would have to upgrade your electrical system for the higher demand.) If you are considering PV, I would look at a conventional electric tank such as the Rheem Marathon.

  8. Expert Member
    Dana Dorsett | | #8

    Steve: In the initial post Clay informed us that...

    "...I have a Heat Pump Water Heater located directly under the bathrooms."

    Which is perfect from both a PV and potential future demand-charges point of view. He's just trying to figure out how to run some water at the other end of the house without having to call a full gallon of water into the distribution plumbing when just a cup or two is needed at the kitchen or half-bath.

    Since the water heater is located under the bathrooms, it's worth at least considering a drainwater heat recovery heat exchanger to pre-heat the water to the heat pump water heater and shower with the outgoing water flows. That will give the water heater much greater "apparent capacity" during showers, and would shorten the recovery times. The recovery time is often an issue with heat pump water heaters.

  9. Yamayagi1 | | #9

    I second and support Martin's answer (No.2) This is exactly the approach I will be using in my new Gloucester home. Ducted HPWH (bargain discontinued Aergenerate ATI66) directly under the two stacked bathrooms, and feeding into the domestic water manifold. Kitchen across the house, so HW from manifold will feed the Steibel Eltron DHC-E microprocessor/SCR temperture controlled 12KW point of use heater under the kitchen sink. Probably could get away with the 8 or 10KW model, but it is what I got some time back. Dana- No, it is not a 50A feeder. A 12KW uses 50A, so requires a 60A feeder and breaker on 240 volt. No.6 THHN AWG Copper under most circumstances. An 8 or 10KW can be wired on No. 8 on a 40A 2 pole breaker. Dana is correct about the high load on the utility systems. I would not recommend this approach unless you have a 200 Amp service entrance. And if everyone did it, it could compound the utilites' challenges. Does that make this unwise or unethical? I haven't figured that out yet.
    An alternative approach to the kitchen issue that may be more prudent, though, is to plumb the house on home-run lines from a manifold close to the water heater, and keep the diameter of the home run- especially to farther locations, small- 3/8" if local authorities will permit it. This may give you satisfactory hot water response time if your kitchen is not too far away...
    Recommendations? The Stiebel Eltron DHC-E line has been around for some time and are not too expensive. They are an established reputable company. Their Tempra line is much more expensive.

  10. whitenack | | #10

    Thanks for the responses so far.

    Regarding service entrances: We had already planned on going with a bigger capacity. I don't know what is bigger than 200 Amp (300? 400?), but our builder had mentioned that he was going to put in a larger system than 200 to make sure there was enough space for current and future needs.

    Dana, yes, we have a drain water heat recovery system located just below the 2nd floor bathrooms, which is right above the HPWH. I am counting on that "apparent capacity" bump. I was able to immediately see the return on investment for the heat recovery system because the larger capacity HPWH was going to be the same price as the smaller HPWH plus the heat recovery. So, it is your opinion to just stick with a small tank type heater for the kitchen/half bath?

    James, thanks. The DHC-E units is what I had in mind. They seem to be a better choice than their competitors, and seem to be cheaper than tank units. I had not considered tying it into the other heater. That would solve one issue, where the water takes a while to heat up, but it would still leave a lot of hot water in the lines after the water is turned off.

    So what am I missing about tankless heaters, that tank type is a better option? If I am already going with a bigger electrical service, and I am already going to have to run the electric since it is new construction, what am I missing? Since it is new construction, is installation costs going to be that much more complicated? What is different other than having a larger electric line run to the tankless than to a tank type?

  11. Expert Member
    Dana Dorsett | | #11

    Yes, stick with a tiny tank for the local kitchen + half-bath. You don't need gobs of "endless hot water" volume, and even if you needed more than the volume of a 5 gallon tank from time to time if you feed it from the main tank (rather than the cold water distribution) you have it. And it would be delivered at high flow rates when desired.

    Forget the tankless. The potential for getting smacked with demand charges in the future, and the pathetic flow limitations (even with a 25kw unit) isn't enough deterrence?

    Also, what's the point of opting for a big-amp electric service? Installing a larger electrical service than needed forces the utility capital expenditures to be higher, since if they hook up a 400A service they have to install the infrastructure to be ABLE to deliver that, even if the as-used draw never breaks 40A. That's a cost that usually gets built into the rate structures. Utilities like bigger than needed services because they have guaranteed profits for most capital expenditures, but it's not necessarily the right thing to do (for you, or other ratepayers.)

    In a high-R house with right-sized HVAC and reasonable appliances & lighting it's unlikely that you would exceed 50A of draw very often (if ever.) But if you installed an electric tankless it would happen pretty much every day, especially if you installed a 30 kw whole-house type tankless. If you like to see the service drop to the house twitch and dance in the air every time you turn on a high flow of hot water, install one of those!

    200A of 240VAC is 48,000 watts, which is a RIDICULOUS amount of instantaneous power. That might have made "sense" for a McMansion with all incandescent lighting (except for the HIDs on the poles lighting up the backyard stadium :-) ) with multiple large screen plasma TVs and a ridiculously oversized central air conditioning to move all that heat out during video gaming parties. Most homes (not even high-efficiency homes) do just fine with 100A service, and a true energy-sipper house would do just fine with 50A. What would you even DO with the other 150A? Some big radiant heaters so's you can comfortably hang out in the back yard in a Speedo during January snowstorms, mayhaps? ;-) Maybe you want to be able to charge 3-4 electric vehicles all at the same time without a spendy battery backed DC charger?

  12. Jon_R | | #12

    Would be interesting to read more about occupancy based re-circulation. Heat from a whole house HPWH is lower cost than point of use electric resistance heat - but how it all adds up is unclear.

  13. whitenack | | #13

    Thanks, Dana (again), for the amazing replies. If you are ever in central KY, look me up. I'd love to buy you a beer or something as a thanks for all the help throughout my project.

    I haven't had time to google it today, but I would imagine there is an article out the somewhere that discusses the energy consumption comparison between a tankless and tank-type heaters. When I get time I will look that up. Maybe I am overestimating the energy savings.

    Thanks for the info on electrical entrances. Maybe I misunderstood the builder when he said he was going for a larger entrance. Maybe I assumed that 200 was the current standard and "larger" meant something more than that. But maybe he is saying "larger" meaning 200. But I thought I remember him saying something larger than 200. Anyway, this is great information and I will do my research and talk about it with him.

  14. Expert Member
    Dana Dorsett | | #14

    Kentucky is one of the handful of states where demand charges for commercial ratepayers are high enough that it's financially rational to buy racks of batteries with smart controllers to avoid them:

    https://www.greentechmedia.com/articles/read/5-surprising-states-where-commercial-energy-storage-works-today

    Don't know how keen Kentucky utilities & regulators are about applying demand charges to residential PV owners, but it's not worth the risk, since it seems to be a growing trend. They'er duking it out in discussions in neighboring Illinois right now:

    http://midwestenergynews.com/2016/11/01/in-illinois-energy-bill-drama-demand-charge-is-central-and-evolving/

    Jon R's idea about setting up a recirculation system to call water from the HPWH instead of a local mini-tank may have some merit if it's a type that only calls the hot water when flow is detected, or called by pressing a switch.

    I'm not sure when I'll be in Kentucky again (it's been decades since the last time), so I guess you can drink my share when it's all done! :-)

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