Options for heating my shower
Hello,
I have a 25 amp, 240 volt panel for my 3800 watt electric water heater for my tiny house. Unfortunately, I live in THE most expensive area in the country by far, for electricity. Rates are 50 cents + per KW. Only one provider and they are running a monopoly.
Anyways, I am regretting getting my electric water heater because it will cost so much to run.
Gas/propane isn’t really an option as this point because I do not want a propane unit in my house and I don’t want to have to cut open my walls for venting.
Does anyone know of any tankless electric water heaters or point of use water heaters that would turn on only when needed to be heated?
All the units I find are take way more than 25 amps.
Thanks!
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Replies
Grey Wolf,
You can certainly buy a point-of-use water heater rated at less than 25 amps. Here is a link to a 12-amp model manufactured by Bosch:
https://www.homedepot.com/p/Bosch-2-5-Gal-2-Year-Electric-Point-of-Use-Water-Heater-ES-2-5/206428467
There are two points:
1. Electric resistance water heaters all have the same efficiency. Switching from a tank-type electric resistance water heater to a point-of-use water heater won't significantly lower your electric bill. The only thing that will lower your electric bill is using less hot water.
2. A point-of-use water heater with a low amp rating may force you to use less hot water, so that may be what you want. It won't heat up a strong shower to comfortable temperatures -- only a low-flow shower that comes out of the showerhead in a trickle. The warmer your incoming water, the better a low-amperage unit will work.
Tankless units need to be able to heat the water instantaneously as it's coming out of the unit. That requires a LOT of heat. I'm having a tankless water heater installed in my new house, as it's both the auxiliary heat for the heat pump as well as the domestic hot water. It's 240k BTU/hr.
That's roughly 70 kW or nearly 300 amps. Whole house tankless electric water heaters need lots of juice... :)
Grey: yours may be the rare situation where solar hot water makes sense.
If you have at least 4-5' of vertical drain downstream of the water heater you can get ~50% heat recovery or better with a drainwater heat recovery heat exchanger. It's a DIY-able project for those with plumbing skills. That cuts the electricity use for showering in half- still expensive, but better.
Standby losses on electric tanks are really pretty small, and can be made much smaller by insulating ALL of the hot water distribution plumbing, as well as the nearest 15' of incoming cold water plumbing to at least R3, which is code-minimum for new construction under IRC 2015. R4 or R5 would be even better, and still financially rational at 50 cents/kwh.
Whole house tankless electric water heaters may eventually cost you BIG money when residential demand charges become the standard. (The utility Eversource just had demand charges approved in a rate case in Massachusetts for homes with rooftop solar.) Demand charges work slightly differently in different places, but it's a billing line item based on the highest energy use during a specified short interval that occurs any during the billing period, often 15 minutes.
This is a common feature of commercial electric rates, but fairly rare so far in residential rates, but is a concept being heavily pushed by utilities whose revenues are threatened by flat or falling net electric energy use. If you are assessed a demand charge of say, $5 per kilowatt (not kilowatt-hours) for the average power used during the heaviest- use 15 minutes in the month, a 50-70kw electric tankless gets to be pretty expensive, even for 6-8 minute showers, compared to the 3.8kw your tank uses.
If you have a space for it a heat pump water heater may make sense. Is your climate warm enough to use one of these outside of your conditioned indoor space? The discontinued GE "geospring" was available for under $1000... we have one and it is going strong.
Also it may be possible to program a Geospring (it has an internet port) to avoid the demand charges that Dana mentions...
Rich, I was thinking the same. The Rheem HPWH is still available and the latest version is advertized at a COP of 3.5. One of those combined with DWHR as Dana suggests could cut a hot water bill by 1/6th or so. It's an investment up front, but $.50/kWh is brutal. Rough calcs:
8 min shower @ 2.5 GPM = 20 gallons
20 gallons @ 8.3 lb/gal = 166 lbs of water
166 lbs @ 50F temp rise = 8300 BTU
8300 / 3.412 = 2433 Wh, or 2.4 kWh per shower
two showers / day for 30 days = 146 kWh / month
146 x $0.50 = $73/mo for shower water
If the equipment above was to save roughly $60/mo, you'd be looking at roughly a two year payback, maybe a little more depending on installation labor. This is assuming the climate is favorable for a HPWH to operate near its peak efficiency, or that the home's heating appliances have enough added capacity to handle the HPWH load.
Guys, Greywolf lives in a tiny home--you know, like a wood-framed RV trailer. I'm sure he does not have space for a heat pump water heater, and if he did, the cold air it dumps into the space would have a major impact. The Sanden split system would work, but it's expensive.
Grey, I agree with Stephen Sheehy--yours might be the rare case where a solar hot water system could make sense. I recently designed a small off-grid cabin for a client and included a schematic design for an outdoor cistern and roof-mounted solar heating panels. An outdoor cistern (often a big polyethylene drum) would not stand up to freezing weather but you could set it up when the weather cooperates, since you're in a relatively mild climate zone. You would still want an electric water heater for the times when the solar-heated option won't work.
Sounds like a good application for PV solar - with net metering if possible, otherwise directly connected to the water heater.
At $.50/kWh, I'd look at drainwater heat recovery even if I had to pump the drain water up to get the required heat exchange area.
I'd also be running at a high water pressure - so that 1 GPM showers are acceptable.
Even in a Tiny House there may be enough room for an EcoDrain A1000 or B1000 horizontal drainwater heat recovery unit. The heat exchange efficiency isn't as high as with a 4' or taller gravity film type, but at 50 cents/kwh there is still reasonable payback, both in terms of cash and "apparent capacity" in showering mode.
https://ecodrain.com/en/products/B1000/
https://ecodrain.com/en/products/A1000/
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/drainwater-heat-recovery-comes-age
Butler Sun Solutions has some fairly simple to retrofit self contained solar thermal solutions that MAY be compatible with your existing water heater, if it's at least 3' tall:
http://www.butlersunsolutions.com/solar-water-heater
Niagara Earth Shower heads flow 1.25gpm and still give a great shower with my 60/40 psi well pump pressure switch. They're $8 on amazon. Based on Lance's calculation that would save you $36.50/month if you're presently using a 2.5gpm shower head.
After that I'd put an extra R20 around the water heater, including under it, to substantially cut the standby loses. Also, make sure you have heat traps installed. Most water heaters should come with them, but mine didn't. Together those should cut somewhere in the area of 30 Watts of standby power use, or almost $11/month*. Heat traps are about $10 on amazon and the Frost King SP57/11C is $20. For R20 you'd need two water heater blankets, but at $20 that's almost a no brainer to me. Add in some pipe nipples, pipe insulation, and add on taxes, and it might come to about $100 total plus a few minutes of your time.
*30W x 1kW/1000W x 24hr/day x 30days/month x $0.50/kWh = $10.8/month
From there I'd see about DWHR if it's at all possible. But so far that should save you about $47/month. To me, that seems like a great place to start and doesn't involve much investment.
Guys, thanks for all of the valuable input.
Martin - I was actually thinking about this today - that on on demand heater may not save much on the electricity bill (because it takes so much power to heat the water. The tank water heater may run more, but for shorter durations.
Dana- Good points here. I'll have to look into how SDGE charges.
Rich - Absolutely no space for a heat pump water heater inside. Outside its very warm during the day. There a few days a year where it gets to 32 degrees at night but not more than a handful. How much more efficient would this be and what is geospring?
Lance - not a bad idea, but also important to note that I am the only one in my house so I do not want anything bigger than 30 gallons and also my entire house is on 50 amps. Any options meeting those parameters?
Michael - That would be great. How much did that cost and can pretty much any electric water heater be powered by solar or do I have to buy a special type? This type of setup would be desirable.
I do want to point out that if at all possible, I would like to avoid cutting into my walls. Something simpler, the better..if that's doable. I have a lot to learn about solar power so my expectations may be unreasonable.
I'll respond to the rest of the posts tomorrow....falling asleep over here!
In San Diego (where the plumbing freeze-up risk is effectively zero), a batch water heater plumbed in series ahead the electric tank is (comparatively) cheap, low maintenance and highly effective. It's the go-to solution in the warmer Latin American countries, and in much of Asia (even not so warm parts of Asia.) If it's going up on the roof you'll have to verify the structural loading aspects, and perhaps limit the size of the water volume of the unit to suit, but even a 25-30 gallon batch heater can cut a large fraction of the electricity used for heating water.
Tommy87 (are you the same person as Grey Wolf?) My design was strictly schematic--as in, "put a tank here, a showerhead and enclosure here, and roof panels here". It's a cool little timber-framed cabin but once we had a basic design the builder took over. We didn't even get to the point of deciding between evacuated tube, flat plate or batch-type solar collectors. I would recommend a flat plate system but I'm not an expert. (I grew up with solar hot water but it has never made sense on my projects.)
Solar water heaters often use a super-insulated tank, which can include an electric element for backup such as this: http://www.rheem.com/product/solar-water-heaters-universal-solaraide. For the lowest-tech solution, you could use an insulated tank without an electric heating element.
Solar water heater tanks are also usually huge, taking a large percentage of space in a Tiny House, which is another reason to go with a batch solar water heater (with the solar tank outside) which is totally freeze-safe in his San Diego location. Some batch water heaters come with electric element back-up, but if it's plumbed ahead of the existing water heater, the existing water heater's heating element is in a more appropriate place.
The location inferred from "I'll have to look into how SDGE charges." (see: https://www.sdge.com/ )