Electric tankless system too small for a hot bath. Tweak settings, upgrade, add additional unit?

[Sharky-PI]

Hi folks,

I got an EcoSmart Eco11 for various reasons but it didn't occur to me that it (seemingly) wouldn't have the power to run a hot bath (in my defense, EcoSmart don't really warn you about this anywhere). The unit's great otherwise, no limescale nightmares etc like I've seen on this forum, but the lack of bath option is getting me down. Specifically, the missus reminding me about it is getting me down

[FWIW we're in San Francisco, mains water temp is apparently ~62F (average? I guess it's colder in winter?)]

I figure I have three options but I'd be grateful of any tips (including whether I have other options!):

1. Tweak settings (text copied from an email to ecosmart that went unanswered):
   When initially set up, with the both inlet & outlet flow taps fully open, the shower (~1.5GPM) reaches appropriate temperatures, but the bath faucet doesn't. Conceptually the Eco11 doesn't have enough power to deliver 'full heat' (112F for a bath?) at whatever GPM the faucet delivers (lets say 8), and thus the fully open faucet is powered by the mains water pressure pushing more cold water through the heater than it can heat, such that the water is only lukewarm at 8GPM, but hot at 1.5. To try to address this, I gradually closed the outlet flow valve to limit the water mixing, which worked to an extent, but means the flow rate at the bath is so low that the water has cooled before the bath is full. We have the heater thermostat set at 114F, so conceptually I could bump this up to max (140F?) if that means it'll draw more electrical power and thus heat at a faster rate. If so I could then open the outlet flow value until the water coming out the tub faucet stops being full-temp and starts being mixed with cool (and then dialling it back so it's not mixed with cool!).
   Would it make more sense to leave the thermostat at 114F and run the tub faucet for a while so that the heater is working at full heat for that flow rate, then try again to tweak the outlet flow value to see if it can be opened more while still providing 114F?
2. Upgrade:
   Eco11 delivers up to 3.1gpm depending on water temp. Based on my location on the map, I suspect it's more like 1.9gpm. This is fine for showering, though even then the water isn't quite hot enough for the wife in winter. So let's say I need... 8? gpm, for a bath? I guess it's variable but too slow a fill means you're starting to lose heat to radiative cooling as fast as you're adding warm water. Their larger units likely deliver 4gpm for the Eco27 and 5.4gpm for the Eco36. So feasibly I could swap out with one of those?
   
3. Additional Unit:
   Per #2, but since I already have a ~1.9gpm unit installed & paid for, that works fine for everything except the occasional bath, it seems silly to throw it out. Talk on this forum seems to suggest >1 unit can be installed simultaneously? If so, would it make more sense to install another unit, rather than removing the current one? This also means my max combined gpm goes from 5.4 (Eco36) to 7.3 (Eco36 + Eco11)
   
4. In-bath water temp raiser:
   Does such a thing exist? Whereby I'd run a bath with my existing 1.9gpm unit, taking forever to fill up (22 mins if my calcs are right), but have a plug-in free floating heater? I suspect anything that runs on single-mains-outlet power would be pretty weedy? EcoSmart's most powerful 120w option would deliver 0.5gpm so if that's a reasonable expectation then that's 26% more power, which might drop fill time down to 16 mins, but I don't know if these exist in the first place.
   Exothermic chemicals???

Thanks in advance for any ideas folks. Sorting the missus out a bath would get my in the good books for a while!

Cheers!!

 

[Breplum]

You may be able to buy an additional heater like the EcoSmart Eco8 but likely it will cost nearly as much as just getting an Eco27.
I think I recall with Chronotherm electric whtrs, that they don't like preheated water, but I am not very sure about that.
At any rate if you do upsize the amps you will have to rewire. Make sure you have enough available amps in your panel in the first place.

 

[Sponsor]

 

[Sharky-PI]

Cheers pal. I'll ask EcoSmart about the preheated water. That said, if I have the big one after the small one, with the big one turned off by default unless we're having a bath, then when we're having a bath, and thus a full ~8GPM is being pulled through the small one, the big one will only be getting slightly heated water anyway.

If I do go with either upgrade or additional unit, I'd get a sparky and a plumber to sort it out.

Very good point about the amp availability in the panel. Panel photo is here, Eco36 says it needs 4 x 40A. It looks like there are 4 unused bays, and all the small stuff is 2 x 20 = 40 per single bay, so conceptually that works.... but the big stuff (30A bottom right, & apparently 40A top right water heater which is supposed to be 60A (!)) is using double bays.

 

[Jadnashua]

Look at your main shut off breaker. I sort of doubt that you can add a 160A to the panel. It's not all about how many spaces you have, but the incoming service. FWIW, it's not uncommon for the total of the branch circuits to exceed the main breaker, because it is assumed that not all of them will be drawing maximum power, but there are limits.

I'd be surprised if your incoming water was still 62-degrees F in the middle of the winter.

FWIW, a typical 1/2" tub/shower valve generally maxes out at about 5-6 gpm, but not all of that is hot.

If your existing system can't keep up filling the tub (that's understandable), when running, it's already drawing as much power as it can, so raising the desired outlet temperature won't do much of anything for you.

 

[Sharky-PI]

Cheers Jim.

Re "look at your main shutoff breaker", what am I looking for there? Total amperage incoming or something similar?

Very much doubt we're getting 62F now, I 100% agree. I suppose I could measure it...

Ok that's useful intel about the tub valve max, thank you. So (pending looking up my specific valve), if I work to a valve max of 5gpm and my existing system is already producing 1.9gpm of hot water (less in winter) then I'm looking for something that would add the remaining 3.1gpm best case, ~3.6 worst case. So the Eco36 at 5.1 to 5.7gpm is overkill. Whereas the Eco27, 3.8 to 4.2gpm is looking more apt. Eco24 at 3.4 to 3.8gph sounds like it's probably the best choice. That wants 3 x 40A so there's also more chance it'll fit on the fuse box than the Eco36...

Re the settings tweaking, my thinking is/was that my having limited the outlet flow valve might mean the water heater is capable of heating more water than the current gpm amount to the same temp (lets say 104), or the same amount to a hotter temp. But the fact that I have that outlet valve limited and the temp turned up and the shower isn't full heat suggests that not only do I not have extra capacity on the unit, but rather it's already maxed out and can't deliver the ~1.5gpm 104F water in winter when the incoming temp is lower.

Thanks again for the ideas!

 

[Jadnashua]

In a main panel, there will be a big breaker at the top that is based on the maximum power available to your panel. It will shut the power off to the full panel. An older home might only have a 100A service, but newer ones will typically have a larger one, maybe 150A, or 200A, or maybe even more. Note, your base price may vary based on what you COULD use, then there's a charge for what you DO use. So, a larger service may or may not cost you more every month, depending on where you live. But, you usually can't just put in a larger breaker without possibly rewiring the power out to the pole, and if the local transformer is undersized, the utility might have to upgrade that, too (not as likely, but possible). Because electric tankless systems can exhibit some radically varying loads, a large electrical load might actually cause your neighbors lights to dim, too! Utilities don't really like loads that vary radically, especially if they're large. An a/c turning on or running an electric dryer are small fry compared to a big electric tankless.

That one that needs three 40A circuits, means close to 100A instantaneous load variations (80% rule applies, so it's not 120A). That's sort of like 8 typical a/c units all turning on at the same time.

96A at 240vac ~ 78K BTU. Keep in mind a common sized gas fired tankless might be 199K BTU, or over twice as large.

One BTU will raise one pound of water one degree. A gallon of water weighs about 8.4#, so if you figure how much of the volume is hot, you can see how much of a temperature rise you can get. At full output, that tankless system would be drawing over 23Kw. A tank typically is using a 4.5 or 5.5Kw element, so it takes much longer to heat that pot of water, but is not a big instantaneous load on the utility. Note, those ratings are per hour, so a 78K unit would be able to produce 78/60 or about 1300 BTU/minute.

 

[wwhitney]

Electric tankless is not a good fit for filling a tub. As you've discovered, the required heater size is prohibitively large for a typical residential service. So switch to a gas tankless or storage type water heater.

Cheers, Wayne

Edit: say you want to fill a 40 gallon bathtub using 60F incoming water that you want to raise to 100F water. That's (40 gal * 8.3 lbs/gal * 40F) BTUs, or 3.9 kWh. So ignoring for the moment heat loss at the tub, your 13 kW heater should be able to do it in 0.3 hrs, or 18 minutes. If you wan to fill the tub in 5 minutes, you'd need a heater over 3 times as big, which would be more than a typical 200A residential electrical service can handle (and about 3 times as much as such a service would otherwise typically ever see in residential usage).