Tankless water heater with no electric requirement?
dfvellone
| Posted in General Questions on
I installed a Bosch Aquastar some years back in my home that operates on lp and uses a pilot light. Doesn’t seem that Bosch makes a model anymore that doesn’t use an electric ignition. Are there any tankless heaters out there that still offer a model with a standing pilot or battery-powered ignition? Thanks, Daniel
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Daniel,
I live in an off-grid home, and over the years I've had two propane-fueled tankless water heaters that didn't require any electricity. When my last one died, I couldn't find a replacement on the market. I finally threw in the towel and bought a propane-fueled Rinnai water heater that needs to be plugged in to a 120-volt AC outlet.
Martin,
Could you give me an idea of the functioning of this unit? It's hard to get info on that from the manufacturer. Does it require constant ac or will it wake my inverter from search mode when I turn on the hot water? My inverter is in search mode most of time - lights, fridge are dc. Will it require my inverter to be actively converting dc to ac constantly? Thanks, Daniel
Martin,
that's my dilemma - We have a modest off grid system that provides excess power from April to October, but from October through winter our budget's pretty tight. I had to finally relent to an ac submersible this past summer - it's our first regular use ac appliance- and though it's going well we're feeling it. We've been using the Aquastar for 14 years now and are admittedly spoiled by it.
Just a thought for both of you, have you thought of putting a flow switch in the hot water line and using it to control the electrical supply to the water heater? I’m not sure if the water heater would “like” this, but it would be a way to ensure the water heater is only powered during times of actual hot water demand.
Bill
Propane or natural gas tankless water heaters that need no grid power are common in the developing world, and many of those models are available in the US if you know where to look. Marey (a US company based in Puerto Rico) has several decent quality models (almost all imported from China) that require no grid power to operate, but there are others. The big box stores carry them in some parts of the US (not just P.R.), but they can also be ordered from web stores.
https://marey.com/english/products/gas-water-heaters/duct-exhaust.html
Dana,
Thanks for the information on Marey water heaters. Too late for my house... but good to know about.
Check out RV water heaters. Or get yourself a small solar set up, panel,controller, battery, inverter. You may be able to get a set up cheap that will last for many years to come and you won't have to depend on outside sources for power and may be used for other applications if needed. If you already have a "modest" off grid set up, I don't see the problem.
Many thanks for the input and particularly to Dana for the info on Marey water heaters. I searched exhaustively and somehow never happened upon them. It's sometimes pretty frustrating trying to find appliances that work well in our system. The regular loads that seem to be small can all add up to a significant demand on an off grid system in the west central Adirondacks where we can often go beyond 30 days with no sun in the late fall/early winter. Hate to run the generator.
At mid-winter incoming water temps Marey's biggest-burner off-grid GA16LPDP is only good for ~2 gpm at showering output temps (a solid low-flow shower), a bit less at bathtub filling temps, so other hot water uses would have to scheduled to not conflict with bathing draws. But that sort of energy-use scheduling is par for the course for off-grid living. If the drain plumbing is configured in such a way that a decent sized drainwater heat exchanger can be installed it's possible to get reasonable margin for at least some other hot water use during showers. Filling a bathtub will always be a bit tedious with a tankless burner that small.
For months when there is more sunshine available, a standard electric water heater with heating elements somewhat optimized for the V-mmp / I-mmp of the panels & configuration running it DC can be cheaper and more effective hot water than traditional solar thermal system (cheaper hardware up front, and more reliable.) Some off-gridders going this route select and configure the panels such that a standard 240VAC/4500W heater element runs near the optimal V-mmp / I-mmp range much of the time. The controls can be dead simple (though I would assume there are DC-optmizers that do much better than the totally dumb approach), along with something to interrupt power when the tank hits 175-180F (most are rated for 180F max), and a thermostatic mixing valve on the output to prevent scalding works. I've not personally dug into what it takes to design and optimize those systems, but am aware that it's now "a thing".
Panel costs have fallen enough in the past decade that these sorts of systems can be put together at half the cost of equivalent-output thermal solar. With no plumbing or water exposed to the cold air, the efficiency doesn't take a hit with falling temperatures- it works just as well at -15F as it does at +40F. Solar thermal really IS dead, for anything but the lowest temperature applications, and even those applications are under increasing threat giving the rate of panel cost deflation.
Daniel,
If you are interested in Dana's idea, you might want to read this article: "PV Systems That Divert Surplus Power to a Water Heater."
Most of the article discusses grid-tied systems, not off-grid systems -- but there's a bit of relevant info in there for off-grid homes.
Dana and Martin,
Putting our excess charge into a water heater has long been my desire - in the spring through early fall months we're often at 100 percent before noon with nowhere to send the electricity that continues to come in- but cabin we built is much to small to accommodate the water heaters. We'll be moving into the house we've been building adjacent our cabin will have plenty room for the heaters. Much as we've come to enjoy the on-demand heater- and it does have its drawbacks- the ability to utilize our excess electricity for half a year will likely outperform the on demand's savings. Does that make sense?
Sure, it makes sense: you’re basically wanting to use a tank-type electric water heater as extra storage capacity when your “real” batteries are full. The water heater in this case is acting somewhat like additional battery capacity that can soak up extra PV output. That seems like a good idea to me, why waste the extra energy?
If you have a charge controller with some kind of “batteries full” output signal, you could drive a relay to energize the water heater so that it would start using electricity after your batteries are fully charged. You’d need to be sure it will shut off when PV output drops back down too.
I keep thinking I should develope a fancy solar energy management controller. I’ve designed similar things for related applications in the past. There seems to be a lot of clever ideas on how to utilize excess PV output but not many ways to implement those ideas unless you have a grid tie system where you can take advantage of net metering. The off-grid people seem to have few options that are really optimized to their particular needs.
Bill
They already have them, it's called a diversion load controller.
Water heater with a low voltage battery system is pretty straight forward. I have done something similar but with big load dump resistor.
You can get 24V DC element to fit into a standard water heater. Most water heaters take 2 elements so you can absorb about 2kW of excess PV.
There is lots of info on the web for this:
http://solarhomestead.com/hot-water-tank-heater-dump-load/
You probably still want to keep your tankless heater, just put in series with the tank in case the water in the tank is not hot enough on a cloudy day.
Akos,
Thanks very much for the link. Great info.
A lot of tankless hot water heaters aren't recommended for use above an elevation of 3000'. Are there any that can be used at higher elevations that don't require electricity?
I'm pretty sure there are a lot of Mareys (or similar) installed above 3000' in Mexico, whether that is recommended or not.
For a solar PV water heater to work efficiently the resistance of the heating element has to be near the maximum power point for the panel type. This isn't the same as running some constant voltage into an element. The maximum power point of voltage and current out of the panels will vary with insolation levels, but the panel specs will have what the Vmmp and Immp are under standard insolation conditions, and selecting an element in the right resistant range to match those numbers would be "pretty efficient" without active tracking. (Most battery chargers for PV systems use some amount of max power point tracking.)
There may be maximum power point trackers out there capable of driving a range of water heater elements varying the actual voltage at the heater element in response to the immediate max power point of the panels as conditions vary, but I don't know of an existing product specific to water heaters. Maybe some of the off-grid blogs will have insight into optimizing the power for this application. This guy's thermal maximum power tracker solution addresses those issues, but isn't designed to match or optimize for a fixed resistance such as a water heater element:
http://electrodacus.com/DMPPT450/dmppt-presentation-v01.pdf
Bosch make models called hydropower that use the movement of water through the pipe to generate enough electricity to ignite the gas. I had one on my off grid house in Tasmania, worked a treat.
I don't believe Bosch is marketing flow-generated ignition tankless units in the US anymore. (About a decade ago they were still available.) They also had fairly low max firing rates- 117KBTU/hr, unsuitable for the cooler half of North America, and a fairly high ~28K minimum firing rate, leading to poor temperature control and flame-out at low flow.
Amazon, 189 bucks. Chinese made units run on LP or Natural. They use 2 d batteries. I have installed 2 with no callbacks yet.