David Voros owns a vacation home that isn’t used frequently, but every so often the house sees a lot of guests. What, he asks, is the best way of providing enough hot water for a crowd?
Voros sketches out these parameters in a recent Q&A post: very cold incoming water temperatures, a limited amount of space for a tank-style heater, and limited availability of gas. The house has photovoltaic panels, and Voros would like to avoid running another exhaust pipe through the building envelope.
For all of those reasons, he’s drawn to an electric tankless water heater. Whether that’s a good idea is the basis for this Q&A Spotlight.
Electric models need a lot of juice
The cost of operating a tankless water heater won’t be any more expensive than operating a tank-style heater, GBA Editor Martin Holladay says, but a tankless model will need an oversized electric service, and that can be expensive to install.
“There are also disadvantages for the local electric utility, since it has to provide high-current electricity in short bursts,” he adds, “but you may not care about that.”
Dana Dorsett adds some particulars: a whole-house tankless draws more than 25,000 watts of electricity and needs a 150-amp, 240-volt breaker. The total electrical service for many houses is less than that, he adds. A typical tank-style water heater draws less than 5,000 watts, and it will have relatively low standby losses. And when Voros is away, a tank-style heater can simply be turned off.
Dorsett also points out some of the potential rate implications for Voros, who is a solar customer.
“In many locations, utilities are struggling to recover their infrastructure costs, particularly on low-use or net-zero electricity houses with solar,” he says. “The concept of applying ‘demand charges’ to the billing has been common for commercial and industrial ratepayers to cover infrastructure costs … Demand charges take the heaviest use in half-hour or 15-minute intervals during a billing period and apply a $/kw (not per kWh) multiplier to that period, and it’s often the largest single line item for those customers. This type of billing is being proposed for special classes of residential ratepayers all over the U.S., the most common special class being low-total-use solar ratepayers.”
Should demand charges be implemented in Voros’s area, an electric tankless water heater can become a liability.
“The notion that demand charges are ‘unlikely’ isn’t well founded, particularly for solar ratepayers,” Dorsett says. “As more distributed solar goes on the grid, this has become a utility favorite proposal in rate cases. only made news because it’s one of the largest utilities to be granted demand charges for solar customers so far, but they are far from the first. As utility business models evolve, it’s conceivable that residential demand charges will be approved for all customers in some U.S. utility somewhere before the warranty period of a water heater is up. (No, wait, that has already happened in some local utilities in Alaska.)”
A low-flow shower head could help
Jon R suggests that using a low-flow shower head, one that draws 1 gallon per minute, could help reduce the size and power needs of a tankless water heater — to something that drew less than 10,000 watts.
And at 1 gallon per minute, Dorsett adds, a 10-gallon under-the-cabinet tank heater that stores water at 140°F would be more than enough for an 8-minute shower even if it wasn’t big enough to fill a tub.
“You’re touching on something that I’ve thought about a few times,” writes Calum Wilde. “Hot water systems at this point have two distinct uses; everyday low volume uses like dishwashers (3-5 gal) and showers (5-20 gal), and high volume uses such as filling a bathtub (35-80 gal). There’s such a huge divide between those uses I’m surprised we aren’t seeing more ideas for a way to split those uses to give better efficiency for both.”
Consider more than one heater
If an electric tankless heater is a less-than-best option, Voros says, what about the possibility of putting a short water tank in the sealed crawlspace beneath the house? Or, he adds, even two tanks?
Trevor Lambert does have two water heaters, and while it’s nice to have the extra hot-water capacity, it comes at the cost of of the second tank plus the standby losses it incurs.
“If you can get a single tank big enough to handle your needs, that is preferable,” Lambert says. “The problem with any electric tank is that the recovery rate is painfully slow when compared to natural gas. If you get two tanks, I would say having them in series makes more sense than having them independent, serving different areas. There will be a lot of times where the entire demand is from one area, and having them in series allows both to serve that demand.”
Holladay recommends that if hot water will be needed in two rooms that are more than 20 feet apart, it might make sense to consider installing one heater near each room. Otherwise, he says, a single tank is the best choice.
Losses through ‘abandoned heat’
Standby losses in an electric water heater are fairly small, Dorsett says. Increasing the volume of the tank doesn’t change that very much.
“Abandoned heat and distribution losses in the distribution plumbing usually adds up to quite a bit more than the standby loss of a 50-gallon electric tank, and doesn’t go away when going tankless,” he says.
To reduce those energy losses, it’s worth installing at least R-3 worth of insulation on all 3/4-inch distribution plumbing, as well as on any 1/2-inch line of some length. In addition, he says, insulate the temperature and pressure relief valve, and the first 5-10 feet of any outflow piping from the valve.
“Actually, the right way to install a tankless electric water heater is as close as possible to the shower or sink where the hot water will be used,” replies Holladay. “Since most homes with tank-style water heaters have a significant length of pipe between the water heater and the fixtures that require hot water, I would argue that switching to a tankless electric water heater almost always reduces ‘abandoned heat and distribution losses’ significantly.”
Although Holladay isn’t advocating that Voros install electric tankless heaters, he does suggest that it’s important to locate each heater close to the fixture or room it is serving.
It’s still a lot of electricity
Low-capacity faucets that limit the demand of hot water may not be enough to justify a tankless electric heater, Dorsett argues. For example, a kitchen faucet drawing 1.5 gallons per minute, and assuming a 70 F° water rise, would still need a tankless heater with a 75-amp, 240-volt breaker.
“A pair of those would add more than 150 [amps] to the service drop capacity requirements,” he says. “That’s already 100% of the capacity of the panel in my house, and could be more than the existing panel capacity at David’s vacation house. The transformer serving my house (and seven others) does not have sufficient capacity to bump all eight houses on it to 200 amps, let alone the 300-350 amps or so per house it would need to run eight multi-tankless houses.”
Compare that demand to a Level 2 electric car charger, he says: most of them draw less than 50 amps. A variety of programs are emerging that use car batteries and smart car chargers to smooth out loads on the grid, and they will become more commonplace in the years ahead as the number of electric vehicles grows.
“But you can’t do any of that with a tankless water heater,” he says. “It draws what it needs, when it needs it, and would not provide satisfactory service if it could be interrupted intermittently for grid management. It’s just a very heavy, intermittent load that forces the grid capacity infrastructure requirements higher. With no possibility of using it for improving the power quality on the distribution grid, it’s just a problem that other infrastructure needs to be put in place to solve, the opposite of ‘green,’ despite the very low standby losses.”
Our expert’s opinion
Here’s what GBA Technical Director Peter Yost added:
I don’t think there is a stand-out solution in this situation (the situation being intermittent use, with the intermittent use sometimes including lots of users). The very cold incoming temperatures, limited tank space, and limited availability of gas make this a domestic hot water perfect storm. The points already made regarding the huge draw of electric tankless beg for some sort of flywheel system — that is, a system that includes a substantial amount of hot water storage — as part of the DHW system for this vacation home.
I like the “staged flywheel” system: two tanks installed in series. It seems the only way to cover, at least to some degree, the large number of users scenario. Rigging these up to a remote wireless activation system makes sense so you can bring the system up to temperature just before you or all your friends and family show up.
I think the very cold incoming water temperature means that an on-demand hot water circulation system is a good fit for this vacation home as well.