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What’s the best way to save energy used for hot water?

OpusC | Posted in General Questions on

So I’ve collected a whole bunch of information about my existing water-heating situation, but I’m having trouble working out how to compare the cost/benefit for ways to save energy. I’ll detail my existing situation first, and then list the options I’ve come up with. Any clarification will be greatly appreciated!

I live in Northwest Connecticut in a house with a damp, unconditioned, uninsulated, 950-sq.ft. stone basement containing no mechanical systems other than our 12-year-old 50-gallon GE electric water heater (w/ 4500-watt and 3380-watt coils). When it’s around freezing temperatures outside, the basement is about 50 degrees and the incoming temperature of our municipal water supply is in the low 40s. Our shower runs for a good half hour per day (at about 110-degrees), and is directly next to a 4-inch vertical PVC drain that runs directly down to about where the water heater is in the basement. I have to run a dehumidifier all summer to keep the basement at a bearable level of humidity. My house is 100% electric, with Mitsubishi mini-splits for heat, and I just got a 10.2kW grid-tied  PV system installed. 

So so I figure that I have a few options, and may even choose to implement more than one over time, but want to try to do the one with the biggest ROI first.

1) I could I nstall a vertical 4”x48” DWHR device between my shower and my existing water heater.  While repairing a blocked drainpipe this week, we were unable to use any hot water, and I saw my electricity use drop by what would amount to $2 per day, and that doesn’t even account for the rate hike we’re  getting in CT next month. With the high price of copper, a DWHR would cost me about $1000, but I would install it myself. I found some charts and calculators online, and roughly figured out that my shower is possibly(?) using 64,000 BTUs per hour, and a DWHR in the size I’m looking for could recover 33,000 BTUs per hour, but I don’t know if the recovery time can be compared apples to apples to the time it takes to bring the water heater back up to temperature.

2) I could install a heat-pump water heater. I have friends with similar old stone basements who have seen huge savings with a heat-pump water heater, but they also have an oil furnace, so their basement is quite warm. I wasn’t  sure if  my 40-50 degree basement would be warm enough, and if it would just be stealing heat from my house above. I do like the idea of the bonus dehumidification in summer though.

3) I could insulate my water heater and/or build an insulated enclosure for it. Right now my water heater just sits on the concrete floor of my cold basement. I imagine just placing it on some rigid foam could help a little, but I would be willing to build a small closet insulated on all sides to contain my water heater. Alternatively, how can I tell if it’s safe to just add an insulating blanket just around the water heater itself, and would that make a difference?

Which option seems like the best investment? Are there any options I haven’t thought of? 

Thanks!
Rob

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    1> If you have more vertical space and can accommodate a taller DWHR the "payoff" will be quicker. The current performance leader out there is EcoDrain's V1000 series, knocking Renewability's PowerPipe series off their long held perch:

    https://ecodrain.com/en/products/v1000/ (A 4 x 48 delivers 57% return, a 4 x 60 delivers 64% return for not a whole lot mor money.)

    vs. :

    https://canada.renewability.com/collections/r4-series/products/r4-48 ( 49% for a 4 x 48)

    https://canada.renewability.com/collections/r4-series/products/r4-54 (53% for a 4 x 54)

    https://canada.renewability.com/collections/r4-series/products/r4-60 (56% for a 4 x 60, comparable to the 4 x 48 V1000)

    2> The heat pump water heater will work great if you insulate the foundation walls with 2" of HFO blown closed cell foam (~R14, almost a code-min R15). That costs about $2.50- $3 per square foot- it's not cheap, but it'll cut the summertime dehumidification needs in half, and will raise the average temperature of the basement.

    Insulating the foundation is "worth it" going forward, even if you don't go with a heat pump water heater. It will lower the space heating and dehumidification costs, and will deliver a warmer floor upstairs.

    2a> Even without insulating (but at least air sealing) the basement walls the heat pump water heater will be a bit less efficient than it would be in a warmer basement, but will still reduce overall water heating costs by more than a 4 x 60 V1000 would.

    3> The heat losses out of a legal minimum efficiency plain old electric water heater are too low to make retrofit insulation "worth it".

    But insulating all of the attached plumbing (including the closest 5' of cold feed) to a code-min R3 or better will be worth it, whether you have DWHR or a heat pump water heater or not.

    Bottom line: If you only have the funds to do just one, #2 is the best bang/buck, even better if you insulate the foundation, and #3 buys you next to nothing.

    But with a heat pump water heater the extra showering time performance of #1 would ensure that you can do just fine with a 50 gallon HPWH even when you have multiple houseguests increasing the amount of time the shower is used. At CT style electricity prices DWHR would still pay for itself in under a decade of 30 minutes/day use even with HPWH.

    4> Of course nothing is cheaper than installing ultra-low flow showerheads. The NRCan test protocol on the DWHR is at 2.5 gpm. The better low flow showerheads run about 1.3-1.8gpm. At the lower flow the efficiency of the DWHR goes up from the tested rating, and you're using less water overall.

    1. calum_wilde | | #8

      Dana, I read one manufacturers specs for heat loss and it said their top end model was IIRC 53w. Is that the heat lose through the plumbing and the walls of the tank or just the walls of the tank? If it is the walls only, an R10 blanket and a sheet of rigid foam could cut that in half. That would be about 232kWh/year. At $100 for the blanket and rigid foam and $0.15/kWh that would be about a three year ROI. Am I wrong?

      1. Expert Member
        Dana Dorsett | | #9

        The standby loss specifications are tank only, and at a specified temperature difference between storage & room, typically 140F water, 65F room, a delta-T of 75F. In a 50F basement you'd have the same losses with a 125F storage temp (which is usually adequate & generally safe.)

        An uninsulated basement in NW CT will have a mean annual air temperature considerably higher than 50F. An insulated but unheated basement in NW CT would likely have an annual mean temp north of 65F.

        Insulate the tank if you like, but most of the time the money is better spent elsewhere.

        Even with "heat traps" the distribution losses of uninsulated pipes will usually exceed the standby losses of an UEF 0.92 water heater.

        1. calum_wilde | | #10

          Thanks

  2. Jon_R | | #2

    +1 on low flow shower heads. I run at a verified < 1 GPM with high pressure and it doesn't bother me. I could understand others wanting 1.5 GPM.

    I can't speak for those rinsing long, thick hair. Maybe shower heads should have a boost button.

    1. calum_wilde | | #7

      We're using Niagara Earth 1.25gpm shower heads and my wife has never complained about rinsing her long thick curly hair. We switched from 2.5gom deltas.

  3. OpusC | | #3

    Dana, thanks so much! Your detailed breakdown of the situation is awesome (as I would expect from you).
    And, guys, I’ll try to get a low flow shower head in there as my first step...but I’ll have to sneak it past my family! 🙂

  4. Robert Opaluch | | #4

    A couple other ways to reduce water usage during showers significantly, conveniently and cheaply (which you may have already d0ne or considered):
    1. A shutoff valve at the showerhead, which allows occupants to turn the water down to a lower flow rate, or to a trickle while soaping up (without adjusting the hot-cold water mix). For example $9 easy to install shutoff valve:
    https://www.amazon.com/dp/B07F1C6CSF/ref=sspa_dk_detail_2?psc=1
    2. A hot water detection shutoff valve that will stop the water flow once hot water finally arrives at the showerhead. Often people turn on the hot water, and let it run until it reaches the showerhead, but then it runs hot water for a while until someone notices, or finally gets into the shower. This automatic hot water temp detection value reduces that wasted hot water by stopping the water flow when hot water first arrives. For example, $14 valve:
    https://www.amazon.com/gp/product/B0017YXIKC/ref=ox_sc_act_title_3?smid=ATVPDKIKX0DER&psc=1

    Payoff would be quick. Easy to install. Works with other hot water saving alternatives described earlier.

  5. OpusC | | #6

    Thanks for the tips, guys. I would never have known that there were shower heads with features like this. I think I’ll definitely look into the low-flow ones with the push-button trickle valves or multiple flow settings.

  6. tommay | | #11

    Most shower valves have a volume control, use that. Also check the anti-scold setting, if set to low you are mixing a smaller volume of hw with colder incoming winter water forcing you to use a hotter setting even though the water won't get any hotter.
    The biggest drawback for any water heater in the winter is cold incoming water. If you can get your hands on another tank to sit at room temperature or find a simple way to preheat incoming water, that may save you some.

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