The Sense of an Electric On-Demand Water Heater
joenorm
| Posted in General Questions on
Hello,
I get asked all the time about on-demand water heating or often home owners will just by a unit and want it installed. They don’t realize that even the small units demand a huge amount of electricity. Not to mention the bigger units that often warrant a whole separate service.
Is there ever a time and place for these? If so, when? It is rare to have such tight space limitations to justify it. The gas version is a different story, I see good applications for those.
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Hi Joe,
It makes sense as an inline backup to a heat pump split water heater (those aren't popular yet, but they're coming). It also makes more sense when you have a PV system, then you're getting "free" hot water.
Otherwise, from a philosophy of water system design perspective, it's inefficient to have one central water heater, and fill all the DHW piping with paid-for hot water, to get to the fixture, then have the rest of the piping leading up to the fixture dissipate the heat that you paid/consumed energy for. Having smaller, point of use units at the fixtures is much more efficient from this perspective. However, usually these smaller units are electric. I've seen small Gas versions in my travels, haven't seen one in North America though.
Electric water heaters are 100% efficient, all the energy goes into the water. Gas water heaters are 80-96% efficient based on which unit you get.
The only reason gas makes more sense is because it's cheaper to operate because gas is cheaper per BTU. So you could say gas is financially more efficient.
Just my thoughts,
Jamie
Jamie,
Standby losses of hot water isn't always entirely wasted energy. Even in this mild PNW climate our heating season is 8 to 9 months. We heat primarily with wood, but our backup is electric resistance, which has the same efficiency and cost as heating water. The tank itself warms our laundry room, and the pipes which run through the floor help heat the whole house.
I have specified small ones for rarely used guest quarters in all-electric homes, and at other fixtures far from the primary water heater, instead of using a recirculating loop. Generally they draw too much power to make sense for primary water heating.
I have an Eemax 20 amp, 2.4 kW thermostatic on-demand water heater installed below my kitchen sink, it's the furthest fixture from a gas on-demand where the hot/cold supply lines are in conditioned space - hence room temperature. With the boost, even full on, "hot" water no longer "feels" cold. And, once the hot water arrives, the on-demand shuts off. I can also turn the faucet on to under 0.5 gal/min and get sufficiently hot water (over 100 F) such that the on-demand is not triggered. That saves both water and gas.
As Jamie B says, back up to heat pump. If I had to do over, I would probably have gotten a tank heat pump water heater and an electric thermostatic on-demand water heater sufficient for one faucet (e.g., 2.5 gal/min) for an endless supply of hot water when needed. Every once in a while, I have a cleaning task that goes faster with hot water (I have two outdoor hot water spigots). Seldom do I (or would I) need two faucets on at the same time demanding an endless supply of hot water. Hence, a supplemental thermostatic on-demand electric downstream a tank heat pump water heater may be a good option.
Eemax makes an 18 kW, 240 V/75 A (around a 50F rise at about 2.5 gal/min), which may be suitable as a supplemental unit depending on inlet temperature and flow rate.
If you drink a lot of tea or pour-over coffee, the sink type unit designed for this purpose makes sense.
I like the small ones for point of use (usually hand washing sinks) purposes, since they're easy to install and require no venting, but I usually use small tank-type units for this purpose. The large on-demand electric heaters are useful in areas where venting is difficult, since the electric units require no venting. They may also make sense in buildings that have only very ocassional hot water use, but small tank-type heters may be better for those depending on exactly what the hot water is going to be used for.
Large electric supplies to feed these power guzzlers can be a problem for anyone with less than a 200A electric service, and even then you might have problems -- utilities frequently feed large electrical services with undersized transformers to limit their (the utility's) transformer losses. Basically the utilities know that a homeowner will probably never even get close to using a "400A" service, so the utility might only supply that service with a 25KVA (~104 amp) transformer (I've seen this many times). The problem is your 25kw on-demand electric water heater would max out that transformer, and while if you burn out the transformer it's the utility's problem to fix, you could have undervoltage issues if you cycle large loads like this.
My short rule list is like this:
Small electric tank-type water heaters for point of use for things like remote sinks
Larger electric tank-type water heaters for ocassional use (guest house, etc.) buildings, with an easy to access shutoff switch.
Large electric on-demand water heaters only if venting is a major issue, or where electricity is available by natural gas or propane are not options.
In all other cases, if you want an on-demand water heater, go with a gas fired one.
Bill
Thanks Bill, maybe you can comment on this issue?
I have Grohe thermostatic mixing valves in two bathrooms, which come with small screens, one fixture has a plastic screen and the other fixture has a metal screen. The on-demand gas water heater (Navien) kicks out large pieces that clog these screens. So bad that I had to pull one mixing valve out every month to clean it (otherwise not enough pressure to get the shower to hold). I then decided to put in an in-line filter to catch the pieces, which resolved the Grohe issues. I have to empty that in-line filter about once every four months. Maybe I'm not cleaning my water heater often enough (I use vinegar and an electric pump)?
Perhaps this is not unique to on-demand gas and would happen for on-demand electric too (given heat exchanger design/temps).
I also have a Nuvo citric acid system, though I stopped using it. It did help with spotting, making water spots easier to clean away, however, I believe the carbon it puts into the water becomes a food source and/or it removes/chelates hardness that would otherwise hinder growth. Tests indicate that citric acid helps keep heat exchangers clean - but I don't like the excess growth of stuff that may be coming with citric acid.
You probably have water chemistry that is hard on the heaters. Note that all those chunks coming out are one of two things: either "water deposits", which is probably mostly calcium, or bits of the heat exchanger in the water heater, which means your water heater is degrading. The filter is a sort of bandaid to clean up the mess, but it doesn't solve the problem.
Hard water would likely mean more mineral deposits, so that calcium issue (among other things, but calcium is usually the big one with hard water). You can use a water softener, which uses a resin to swap the calcium ions for sodium ions. Softened water won't be more prone to growing stuff. That may be your best option. I would recommend a good particulate filter (I use a "dual gradient" 1 micron filter) ahead of the water softener, and possibly an iron and carbon filter too depending on your water chemistry. You have to use a good particulate filter to keep the water softener from clogging. The water softener will greatly help with water spotting and any issues with mineralization in the water system if that's what's going on.
If you water heater is degrading, you have other issues that may be more difficult or more expensive to solve, and you'd need to have your water tested to determine exactly what the problem may be. I suspect you just have hard water though.
Bill
Thanks Bill, I've put the particles in vinegar and they dissolve after some time, they're a mix of predominately whitish particles with some small black particles. Per city water report, total hardness is 181 ppm (CaCO3), with 49 ppm Ca, 14 ppm Mg, and 223 total dissolved solids. I'll try cleaning the heater more often, get some use out of the pump I bought. I'll also investigate some softeners and RO systems.
181 ppm is pretty hard. My well at home is around 350-400ppm or so for some perspective.
A water softener is almost certain to help. RO is good for drinking water (and awesome ice cubes, and also cleaning things), but RO systems tend to produce small amounts (tends of gallons PER DAY), so they don't really help for hot water unless you want to put in a very large, very expensive, system.
Bill
Thanks again Bill, RO can sure be pricey but the results are crystal clear ;-) And, wow, your water is hard.
Besides space savings, I believe on-demand heaters of any fuel have limited usefulness. They’re not more efficient than other options and have limited capacity.
One of my neighbors is a plumber and he strongly recommends an on-demand gas water for those that have gas and an on-demand electric for those without gas but with sufficient electrical service. His thinking is that standard tank models are too inefficient compared to on-demand and he actively discourages hybrid heaters due to how frequently they break down. That is, replacing hybrid water heaters is a notably income stream for his company, so he considers that entire market segment to be suspect.
So yeah, when he redid the plumbing in his own house, he first upgraded his electrical to a 400A service and then installed a large whole-house on-demand for in general use and then several point-of-use for very specific cases.
That's exactly the perception but it's totally incorrect. Gas tank water heaters can be as efficient at combustion as gas tankless. However, most gas tank water heaters sold are bare minimum efficiency because of marketing choices made by their manufacturers - they price the low efficiency tank units low and the high efficiency tankless units high. The 3rd option, high efficiency tanks, are manufactured by the same companies but they aren't marketed the same. Perhaps it'd be too confusing for the average consumer who buys at failure, but the options are available.
Electric resistance keeps the same efficiency and standby losses are low.
Paul,
I live in a rural area of BC that experiences the most power outages in the province. One of the advantages of having a tank heater is we always have a reservoir of hot water that lasts for a considerable period after the power goes out.
The only difference in "efficiency" between tank-type and on-demand type water heaters is that with on-demand water heaters, you don't have standby losses. This means with on-demand water heaters, you don't use any energy to maintain the temperature of the tank of hot water. In reality, standby losses are pretty low for modern water heaters that are fairly well insulated, and in the winter, the standby "losses" are essentially zero, since all that "lost" energy just goes into heating your home -- just as if you made it with your furnace or boiler. This means on-demand water heaters only really offer anything in the cooling season.
If you have a high efficiency tank-type water heater, it may actually cost you less to operate than an on-demand water heater with a lower combustion efficiency. Best case scenario, they are probably pretty close to the same cost to operate. That brings us to the maintenance issue, where on-demand water heaters are typically big losers -- they require more maintenance, and they tend to be less reliable over time.
So when reading marketing materials, remember that the only real benefit of an on-demand water heater over a tank-type water heater of the same combustion efficiency is lower standby losses. In the case of electric water heaters of either type, the ONLY difference is on-demand water heaters don't have standby losses (electric resistance heat is basically 100% efficient). Now remember that standby "losses" whenever you're running your heating system in your home aren't really "losses", since the energy just goes towards heating your home. That means the ONLY advantage in terms of efficiency is related to standby losses over the cooling season in the summer. Not much.
Unlimited hot water for looong showers is another issue, but not one related to efficiency. My personal recommendation is to just go with a big tank type water heater. I have a 70-75 gallon (I forget exactly how big) water heater, and it only runs out when we fill our large hot tub.
To Malcolm's point about power losses: you can get gas-fired water heaters that require no electricity to run, that will run even when there is no power. Those are nice (mine is like that). Personally, I'd get a generator though :-)
Bill
Bill,
Most of the people around us have gone with propane powered 17kW Generacs. That powers everything you want and more. So far we are sticking with a Honda 2000. We have either just got used to the outages, or have been beaten down over time.
If you ever opt for a beefier permanently installed generator (which is really nice when the power quits at 3am in the middle of a nasty storm), I'd recommend you look at the Kohler units over the Generac units. Kohler makes a better quality product in my experience, and cost is about the same.
BTW, once you have a fully automatic, permanently installed generator, there is no going back. You WILL like having it. I'm in an area with frequent power outages too (a total of around a week or a bit more of outages last year, spread between several outages with the longest about 4.5 days or so), and I would not want to be without mine. It's worth keeping a spare oil change on hand (oil and a filter) too if you're in an area with frequent outages. Best to be prepared.
Bill
"His thinking is that standard tank models are too inefficient compared to on-demand"
A simple way to refute that is this: Look at the UEF (uniform energy factor) for a good electric tank heater. It will be about 0.92 to 0.95, meaning there's 5-8% loss. That 5-8% loss is the standby loss. The most improvement you could possibly have by switching to tankless is 8%. But then look at UEF for tankless. Here's a good quality one:
https://www.hotwater.com/water-heaters/residential/electric-tankless/electric-tankless-208v-/-28kw-4-chamber-water-heater-r4la-280x/
And the UEF is 0.92. So it's not any better than a good tank! The money spent on the 400 A service capacity was wasted.