Off-grid water heating
We are looking for the best way to heat water off grid. I would like to avoid propane. We have a 14.5kw solar system. I’ve been going back and forth on whether to get an anll electric hot water heater and change out the elements to lower wattage ones or get a heat pump hot water heater.
My concern with the heat pump hot water heater is we have a 20×36 (two story) house on a slab and an open floor plan, aside from a small closet and bathroom downstairs, so noise from the heat pump would be pretty noticeable. We also would need to come up with some sort of drain for the water heater. I also have concerns about the heat pump cooling the house down too much. Right now we only have a wood stove for heat and some mornings in the shoulder seasons are somewhat brisk but not cool enough to warrant starting a fire. A heat pump may bring that temp down too much. I would also be concerned about it just generally cooling in the winter time and having to burn more wood because of that.
Obviously the all electric water heater would use more electricity but we’d be producing more than we need in the summer time with the solar system, however maybe not so much in the winter. While it would be nice to have hot water year round, we do have a water reservoir for our wood stove if it came to it.
If anyone has any advice or insight into what the better option would be, that would be helpful. I always appreciate everyone’s input on this forum.
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There are some more expensive units that may be worth looking into, if they solve specific problems.
There are heat pump water heaters that have a monoblock outdoor unit, and circulate water between that outdoor unit and the indoor storage tank. SANCO2 makes those systems, and AO Smith also appears to be joining that market later this year. There's no interior mechanical stuff, so they're quiet and they don't rob any heat from the interior. If you don't have battery storage for your solar system, freezing water lines between the outdoor unit and the tank may be a problem.
There are also solar assisted heat pumps, with a black collector panel (for the refrigerant) on the exterior and a small interior heat pump between the panel and tank. The heat pump apparently doesn't need a drain as the condensate happens at the exterior panel.
Smart Solar is one company that makes them, and SAHP from the UK is another.
I don't think anyone in the North American market is selling a standard split system heater heater that is very common in the rest of the world, with a mini split type outdoor unit and refrigerant lines to the indoor unit.
I think you can find WH with factory installed low wattage element. Note the lower wattage element will not change the total number of kWh used by the WH it will lower the peak demand while extending the run time so the power used is the same.
The number of BTU the HPWH removes is a pretty small number. I think people over estimate how much cooling the unit will do.
As far as the sound goes only you can decide if you can live with it.
Note the HPWH will use about 75% less power.
You might want to consider a dual fuel RV water heater. This would let you use your excess summer solar and gas when the wood stove if not going.
https://www.greenbuildingadvisor.com/question/best-hot-water-heating-system-for-off-grid-year-round-cabin-zone-4c
https://www.geappliances.com/appliance/GE-Profile-10G-RV-Dual-Fuel-Water-Heater-PR10DSATBA
Walta
I'm going to bet that in the middle of the day your solar system has more output than you use. I would think if you can calculate[and manage] your hot water usage you could time the water heater to run mid day and it would supply your needs for the next 24 hours. Also noise at noon is less bothersome than midnight
If you have one low wattage and one high wattage element, you can set the low wattage element to maintain a minimum acceptable temperature, then run the high wattage element as a sort of "dump load" when your solar system has extra capacity during the day. This way most of the "work" of heating the water is done with what is basically surplus electric production from your solar system, and that's done relatively quickly (high wattage), but the low wattage element makes sure you are OK during stormy periods of if you have a lot of demand at night. Use the water heater as a sort of thermal battery in this way.
Bill
There are resistance water heaters meant to work with solar cells. They bypass the inverters and run directly off of the DC produced by the cells. I don't have any leads other than knowing they exist.
That's an interesting niche product. I wonder if they were invented/introduced during the early days of lower efficiency inverters where the gain from inverter bypass would be sizable (like 15-25% or whatever from a 75-85% efficient inverter) instead of the newer inverters that run at 90-something % efficiency.
Actually, I guess the inverter capacity could be downsized for lower cost, if the water heater is the largest house load or needs to be a parallel load. I'm probably just showing my lack of solar knowledge.
A lot of it is just cost/benefit. Typical inverters these days are well over 90% efficient, probably 94+% for most. It's usually not worth much to spend a bunch more on a niche product to only gain 5-6% or so.
Note that you can run "regular" water heaters on straight DC at the SAME VOLTAGE as the AC waveform. RMS AC voltage is defined as the voltage of AC that will produce the same energy into a resistive load (like a heater) as the DC voltage would. That means a 120V AC sinewave has peaks up to around 170v or so, but it puts the same energy into a heater as a 120V DC circuit would.
The issues that come up running a "regular" AC water heater on DC are that most AC motors will NOT run on DC, and many/most relays and protective devices (fuses and circuit breakers) CANNOT interrupt the same power levels on DC as they can on AC. The heating element itself though can run fine on DC, as long as you can provide the required voltage. You can run at reduced voltage, but then you reduce capacity: a 5kw 240v element running on 48v DC is only a 200w element, for example.
Bill
One would think with proper current protection the risk of damage to a simple resistive water heater is minimal. I have a feeling that putting a 1500+ watt heating element on the output of a solar panel will drop the effective voltage to zero if there is not enough sun to power it.
The risk of damage is probably small, but overcurrent protection is still required. The easiest/cheapest way to do it would just be to use a fuse with a DC rating for the operating voltage of the system. It's not difficult to find such fuses, and many will work in standard fuse holders. Note that automotive fuses are NOT NOT NOT suitable for use with these higher voltage systems! Proper fuses will be catridge types! You can also get circuit breakers from sources like Airpax (which is a division of someone else now).
Bill
If you just put a resistor across a solar panel, and you size the resistor so that at the panel's rated voltage it draws the rated current, all the electricity generated by the solar panel becomes heat produced by the resistor.
When the output of the panel drops, the voltage drops too, but you don't care because you're just heating water.
The appeal of these systems is that they don't require any controls at all and capture 100% of the available power. I haven't seen one in years, but I don't seek out off-grid solutions either, so I don't know if they are still popular or if advances in solar technology has pushed them aside.
The key is a special heating element. To heat 50 gallons of water by 50F in an 8 hour day you need about 750W continuous. To get that with 48V DC you'd need a 3 Ohm resistor. In a 240V system that would be a 16kW heater drawing 80A, much more than you'd see in a water heater.
Or if you put four 200W panels in series to get 192V you'd need a 49 Ohm resistor, which is only about 1000W on 220V, much smaller than a typical water heater. In either case a regular heating element isn't going to work.
The issue is you need a thermostat to turn off this DC power. You can't use the built in as it will plasma first time it opens under load.
Something can be put together with DC rated contactors and fuses but you are still looking a science fair set up at best.
I would stick to regular AC powered water heater or go for a 48V low voltage unit controlled by the diversion controller. For the AC solution there are also devices that can control the amount of power the tank uses based on available PV and building load.
If you want hot water in the winter, PV only won't work as there whole week where a whiteout storm will mean the array barely makes enough power to keep the lights on.
This means some fuel burner of some kind. You can still use excess PV the rest of the year but will require a two tank setup.
My thinking would be with a resistance tank and a propane fired tankless heater in series. The tankless would be downstream the tank and if set to run at lower temperature than the thermostat on the tank, it would only ever fire if the tank runs out. This would let you get most of your hot water off PV but have a backup in case there is not enough sun.
If you can swing it I would just install a heat pump water heater, maybe build it a small closet with some sound protection and duct it. If you get something like the Ao smith it has an app the lets you set it to resistance for like 100 days. If you are worried about the cooling in the winter or find shoulder season is too much just set it to resistance for that period and then get the cooling gains for the summer.
Also if you don’t have enough electricity in winter, use heat pump mode and just heat your house a little more.
Solar systems are to the point where it makes more sense to size your system and components appropriately rather than getting super creative with appliance selection.
Hi Cottagebuilder: perhaps it would be worthwhile to minimize hot water BTU usage first in an off grid setting. Plumbing a drain water heat recovery device into the system, such as a Power Pipe, can recapture 50%, 60%, or even 70% of the heat BTUs which otherwise go down the drain when using showers and sinks:
https://renewability.com/wp-content/uploads/2021/05/Power-Pipe-Spec-Sheet-Rev2.0.pdf
As well, switching to lower flow shower heads such as the 1.25 gpm Niagara Earth Chrome model could cut shower hot water usage by 50% compared to a standard 2.5 gpm model.
The new heat pump hot water heaters are quite efficient (some with Uniform Energy Factors in excess of 4) and more quiet than earlier versions. For example, this brand can be ducted and 15 amp models are available:
https://files.myrheem.com/webpartners/ProductDocuments/87F03DD1-9AF9-4E61-B825-DEF5BEF19055.pdf
Ducting the HW heater during the shoulder seasons could help re morning chilly conditions.
JJ
Thanks for the input everyone! I think we’re going to install a heat pump water heater and see how that works out for us. It’s too bad the SANCO2 units are so expensive because those seem perfect. My guess is in 5 or 10 years the technology will be much better and cheaper but I would rather not wait that long for hot water.
Ultimately, you'd want to use a device like Harvest Thermal which use a heat pump water heater as an air-to-water-heat pump for both heating your home and for domestic hot water.
https://www.harvest-thermal.com/
Domestic hot water combined with heating is a poor fit for air-to-water heat pumps.
Most people who work with hydronics got their start working with boilers, it's tempting to view a heat pumps as just like a boiler, only noisier. But they aren't, they have very different operating characteristics. This is an idea that works well with boilers and doesn't work well with heat pumps.
Compared to having a standalone heat pump water heater, a combined system saves nothing on operating cost, saves no space in the boiler room, costs a lot more to install and yields inferior comfort. There's literally nothing to recommend it.
"My guess is in 5 or 10 years the technology will be much better and cheaper but I would rather not wait that long for hot water."
The SANCO₂ heat pump water heater was first available for purchase in North America in the summer of 2016. 9 years ago and it has failed to corner the market seems unlikely to now.
Walta
SANCO just seems like a really strange company. We get lots of people here who go down the road with them spending a lot of time figuring out the ins and outs of their system only to be told in the end that it's not going to work in their situation. It clearly has a limited use case, but rather than sell to that use case and try to screen buyers right off the bat they seem to prefer to pitch their product as a universal solution.
Not sure if people are aware ...
https://eco2waterheater.com/about-us/
excerpt:
ECO₂ Systems LLC was founded by John Miles and Maho Ito in January 2020. Formerly employees of Sanden Corporation of Japan, John and Maho were tasked with introducing Sanden’s CO₂ Heat Pump Water Heater products to North America in 2015. Since then, they have worked to establish and grow the market for these innovative products
In January 2020, John and Maho negotiated the purchase of Sanden’s water heater business in North America, re-branding as ECO₂ Systems and opening the door to the introduction of complementary products from other manufacturers.
ECO₂ Systems is committed to providing comfort to our customers through the development and distribution of high quality, high-efficiency heating, cooling and hot water products using low GWP refrigerants, minimizing the environmental impact.
Here is a list of distributors:
https://eco2waterheater.com/where-to-buy-professionals/
I'm wondering if the AO Smith/Panasonic entry into that market will have the same problems and/or hurdles. Just entering at a significantly lower price point would maybe turn some people's heads toward a system of that type (no idea what the price is going to be, obviously...).
So far nothing much info to find except the release video from their trade show booth, and the press release from Panasonic.
https://www.youtube.com/watch?v=oI-J9T2pzMs
https://news.panasonic.com/global/press/en250210-2