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Community and Q&A

Point of Use Possible in Ontario?

Jaz_of_all_Trades | Posted in Mechanicals on

I’m building a house near Orangeville, Southern Ontario (Canadian Climate Zone 2, American equivalent Zone 6?) and am considering my hot water heating options. It will be an all-electric house with good solar PV, and I’m trying to convince myself that point-of-use heaters are the best choice in terms of energy use/cost/payback. I was inspired by Rick Durapau’s post from 2013, but he lives in Austin and I’m not sure if I have warm enough water coming into the house to make it work.

I’ve found a Stiebel Eltron 12 kW heater which claims to be able to deliver 1.5 gpm, which is right at the limit for a shower with low-flow head. Does anyone have experience taking a shower in Canada with these units, or any part of the U.S. with low-temperature water inflow? I don’t want to run cold-only pipes to all the bathrooms only to find that I’m stuck with tepid showers until I retrofit hot-water pipe through my finished wall for the tank heater I have to buy on top of 3 POUETs!

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Replies

  1. user-2310254 | | #1

    Jason. I think you will find this article and its comments useful. https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/point-use-electric-tankless-water-heaters

    Be sure to read the comments.

  2. GBA Editor
    Martin Holladay | | #2

    Jason,
    If you want to buy an instantaneous electric resistance water heater, you will be able to find a model that works.

    The first step is to measure your incoming cold water temperature in March or April (when the temperature is usually lowest). It might be as low as 40F or so.

    Then choose your desired hot water temperature. Let's say that you want 120F water. (Your own desired temperature might be different.)

    That means you want a water heater capable of a temperature rise of 80 degrees.

    Then you need to determine your needed flow rate. In your case, that appears to be 1.5 gpm.

    Next, consult the handy dandy line graph published by Stiebel Eltron. (See below -- there are actually two graphs, one for 240 volts and one for 208 volts). The graphs show that you need to purchase a Tempra 20 model.

    .

  3. Dana1 | | #3

    The infrastructure cost to support high powered intermittent use loads such as electric point of use water heaters increases the size & cost of the power distribution grid, raising the retail price of electricity for everyone, since most residential rate structures are flat, based on total energy, not peak power. While it saves a small amount of energy, it's not necessarily the "right" thing to do.

    If/when residential retail rate structures begin to include "demand charges" (common in commercial rate structures), where there is a fixed charge based on the heaviest use half-hour (other intervals are often used) during the billing period( to PAY for that increased infrastructure cost necessary to support such loads) the cash investment value of a 12-24kw tankless electric will be sharply negative, and the value of heat pump water heaters would be enhanced..

    The standby losses of electric tank water heaters is quite low- the bulk of an energy savings with a point of use tankless would be from the reduced amount of hot water abandoned in the distribution plumbing. But in heating dominated climates that abandoned energy while expensive, isn't totally lost, since it accrues to the space heating load.

    BTW: At about 4600 annual HDD18C, Orangeville is smack in the middle of zone 6 on an average heating degree day basis. (4000-5000 HDD18C= zone 6) .

  4. charlie_sullivan | | #4

    I agree with Dana. For primary water heating applications in most buildings, point-of-use electric is not a good choice. It could make sense for a handwashing sink, but I doubt it's a good idea for most other uses. If you want to do better than a well insulated electric tank, drainwater heat recover and heat pumps are the real opportunities you have available.

  5. itserich | | #5

    I plan to remove all gas and use an electric tankless water heater.

    Bricor seems a leader in low flow shower heads, including many large hotel chains.

    Lowest option at around 0.65 gpm.

    1.125 is the lowest I have seen for a handheld shower.

    http://bricor.com/products/handheld-b110/

  6. itserich | | #6

    Also, be aware, tankless water heaters have an activation rate. The minimum flow of water required to activate. I have seen rates ranging from 0.25-0.5 gpm.

  7. STEPHEN SHEEHY | | #7

    A Tempra 20 (see comment above) draws almost 20,000 watts, requiring two 40 amp breakers at 240 volts. Not running that off rooftop solar!

  8. GBA Editor
    Martin Holladay | | #8

    Stephen,
    Dana has explained why an instantaneous electric resistance water heater might not be a good choice. But it's worth pointing out that such a water heater is not incompatible with a grid-connected PV system.

    The grid-connected PV system (if installed in a location where the local utility offers net metering) produces x kWh of electricity per year. The homeowner uses y kWh of electricity per year. The homeowner is billed for y-x.

    It doesn't matter if the electric water heater uses 20,000 watts for one hour or 2,000 watts for 10 hours. The bill is the same (at least until the future billing system envisioned by Dana Dorsett comes to pass).

    Of course, a homeowner with a Tempra 20 requires a large electrical service connection, and that's expensive to install. But that type of service does not preclude the use of PV.

  9. user-2310254 | | #9

    So would using 3/8-inch lines for showers and sinks along with a minimum 5 foot by 3 inch heat recovery heat exchanger on the drain line make any sense? (Large soaking tubs would probably be a bad idea.)

    I am curious since I considered electric point of use for my current house but was concerned about having to run multiple high-ampere services. I thought I could make it work, but in the end it did not seem to be the best option. I decided to go with a Rheem Marathon tank to serve the entire house. I would have purchased a HPWH except I had no confidence that the plumber would installed it correctly.

  10. STEPHEN SHEEHY | | #10

    Martin: I understand net metering, which I enjoy in Maine. I was just remarking on the astonishing power draw.

  11. Expert Member
    Dana Dorsett | | #11

    Those future billing rate structures with residential demand charges ARE coming to pass, and are more likely to be applied to those with rooftop PV than others:

    https://www.solarquotes.com.au/blog/residential-demand-tariffs-have-arrived-and-theyre-vicious/

    http://www.energymatters.com.au/energy-efficiency/capacity-demand-charges/

    ^^^Australia is just the canary in the coal mine, due to the high penetration of residential rooftop solar^^^

    It's not just an Oz thing:

    https://www.euci.com/event_post/1016-residential-demand-charges/

    http://midwestenergynews.com/2015/12/02/move-over-fixed-fees-utilities-see-demand-charges-as-revenue-cure/

    http://www.utilitydive.com/news/the-flaws-in-the-utilities-push-for-residential-demand-charges/427481/

    Whether you're paying directly for the heavier infrastructure needed to support those load or or not, high power intermittent loads are still a cost-adder to the system as a whole, whether you have PV or not. If you had a big grid-attached battery and grid-smarts to limit the peak draws from the grid it can work fine, but you're paying for your private battery infrastructure, which isn't free either. Smart battery systems that do just that are economic in some situations and are already being marketed and installed in areas where there are hefty demand charges applied to commercial ratepayers.

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

    [edited to add]

    If you can't be deterred from installing a point of use tankless, in your location be sure to have at least 20kw serving any full-bath, or tub-filling times will be insanely tedious, and showers prone to shrieks of something other than joy when somebody steps in to wash their hands while another is in the shower.

    For showering-only (no tub) situations you can get there with a 12kw unit if you also install a decent sized drainwater heat recovery heat exchanger, nothing smaller than 4" x 48" or 3" x 60", (but longer & fatter is DEFINITELY better. ) At those sizes you'll get about 50% of the heat recovered from the drain, and returned to the shower, giving a 12kw tankless an "apparent capacity " of over 20kw. This also cuts the total energy used for showering in half. Without drainwater heat recovery a 12kw unit is only good for about 1.1-1.2gpm flow showers at wintertime incoming water temps, but with 50% or better heat recovery it's good for over 2gpm, which gives you a bit of margin.

    But it does nothing for tub-fills, since the heat is only recovered and returned when the drain is flowing simultaneously with the potable flow. It would take a half-hour to fill even a 25 gallon bathtub in winter with a 12kw unit.

  12. Jaz_of_all_Trades | | #12

    Thanks for this great discussion, guys. It appears that although possible, POUETs are not the way to go up here in Zone 6 (thanks Dana :) ). I wasn't concerned about the infrastructure costs of intermittent high residential draw since my PV is 9kW with significant battery so I thought I'd be covering most of the load, but I don't have full understanding of how the grid is involved in this capacity, so perhaps it is a concern.

    Unfortunately a Tempra 20 wouldn't work as it's a whole-home tankless heater which is not what I'm pursuing. I briefly considered installing those as POUETs but that's a non-starter in terms of cost and draw. Thanks again Dana for the insight on installing drainwater heat recovery with the 12 kw. Those recovery pipes aren't cheap either, about the cost of a traditional tank. It would take longer than the lives of the POUETs to recoup the extra costs. Too bad. I'm trying to find the most ecological choice for all aspects of this build (my own home) as well as support new technologies within the green building movement, but so many times I end up stuck with the conventional choice. It's tough to build a non-fossil fuel home in a heating climate! It looks like a tank with drainwater heat recovery is the best choice.

  13. amach | | #13

    Look into a heat pump water heater (HPWH) tank. I'm in Ontario and pay by time-of-use (TOU): off-peak is 8.7, mid-peak is 13.2, and on-peak is 18 cents/kWh. Right now I have a gas hot water tank, but if I went electric, I'd want to avoid electrical use when it's expensive. If there was a way to run the heat pump more when power is cheap (7pm to 7am), you'd save money and reduce the daytime load on the grid.

  14. Jaz_of_all_Trades | | #14

    Thanks Antony - I did seriously consider the HPWH as it's definitely less of an energy hog than any kind of electric resistance heat. However, I had to decide against it because - as an all-electric house we're using an electric boiler for in-floor heating (I know this forum is very pro-mini splits and the like for energy-efficient heating, but let's save that debate for another time). In the winter, the heat for the water heater would be stolen from the electric-resistance boiler heat, forcing it to work harder, so in essence I would still be heating my water with electric resistance, just with an expensive heat pump as a middle-man. The Canadian Centre for Housing Technology did a convincing study which shows that HPWHs are best when heating with natural gas or another fossil fuel (the less expensive fuel option), especially when there is a lot of waste heat being thrown off by the nearby furnace. Great article on that here: https://www.cmhc-schl.gc.ca/odpub/pdf/68220.pdf
    As Dana pointed out above, standby heat losses are quite low and do contribute to the space heating load in the winter. We designed the house to have short runs to the bathrooms, so standby loss is minimized anyway. Our longest run is to the kitchen - about 30', so I'm considering installing a POUET at that sink to deal with the wait for hot water. Here's a rabbit hole though: can one recoup the energy used to heat the water that's cooled from sitting in the pipe, as well as the embodied energy within the POUET itself, from 15 years (life of a POUET) of water savings (from a well) from one 30' length of pipe?

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