Breadbox + tankless

[domingi]

Hi folks, I have a question.

I live in the Florida keys where sunshine is almost always present. I am planning to replace my old 30 gallon electric tanked water heater and have been investigating a 30 gallon breadbox style passive as a partial replacement.
My big concern would be the few times we do not have a day of sunshine, yet still have family or friends visiting that like to take hot showers regularly.

I am trying to understand how the tankless portion works.

1. Would they run at a partial heat setting if the inlet water temp is high enough that they do not need to run full blast to heat the water to the desired exit temperature?
2. Can they take an inlet water temperature in excess of what their designed exit water temperature is?

My thought was to set-up the passive solar as a feeder(pre-heater) to a smaller tankless model. The tankless would then become something that at peak daylight hours would not even need to turn on to heat water that is already at or above the designed output temperature. My thinking is that the tankless just becomes a back-up to make up the difference in temperature if we had a high demand or an extended period of gray skies and cooler water coming in from the passive.

Since the output temperature of the solar can be quite high, my thinking was that a tempering valve would need to be installed at some point in the circuit. If the tankless can handle the higher input temperatures then it would be placed after the tankless unit. If not, prehaps it would need to be placed between the two?

Or, do the tankless not have the ability to automatically run at a partial heat input setting.
Or am I just whacked in my thinking?
Thanks for your replies.

[Scott D. Plumber]

If you are trying to go solar, there are many good solar systems out there that have electric elements as a back up, and even ones with gas burners for back up. Jomar has a good one. I'm a Tankless guy, prefering Rinnai and Noritz. However, I can see in some places and situations where a solar system would be very attractive. in that situation, the thing to avoid (my opinion) is having to buy TWO complete hot water systems! In other words...pick one. Pick a good one, have it installed properly and enjoy!

[domingi]

Thank you for the prompt reply, I do appreciate it.

My situation is kind of special, I suppose I should have explained it up front. My spouse is all for solar water heating as long as I can guarantee that we will never be out of hot water, which I can't. Since I have been married 32+ years and do not wish to get rid of her or 1/2 of everything I own, I have to come up with a solution that works for both of us.

Here in the Keys, we have very high power cost and very ample sunshine. I thought that even if I could not completely eliminate the carbon footprint of heating our hot water, maybe I could put a substantial dent into it and make a step in the right direction.

But my concern regarding the particulars of the tankless heaters is an unknown for me at present and is a stumbling block I would like to overcome.

Again, thanks for the info folks.

[Dana]

Electric tankless HW heaters are all fully modulating. Gas/propane fired tankless units have a lower limit, which can be an issue with high incoming water temps. If your plan is to pre-heat with a batch heater, electric tankless is the preferred solution.

Be sure to plumb in a tempering-valve/thermostatic mixing valve at the output of the batch heater to avoid scalding temps during the summer. Set the output temp of the batch heater's anti-scald valve to ~120-125F, and the tankless' output to something like 115F, which will guarantee that it doesn't come on until it has to. You could put the tempering valve after the tankless, but the extra pluming of the potentially high-temp water would increase your standby & distribution losses from the solar.

An electric tankless draws a huge amount of power when on, but has a very low duty cycle. You'll likely have to upgrade the wiring and dedicated breakers to handle the load. If that's too expensive a project, plumbing the batch heater in series with a standard electric tank works as well.

Whatever you do, be sure to insulate the plumbing between the batch heater & finish-heat, and as much of the distribution plumbing to the house to improve your solar fraction, reducing standby & distribution losses. 3/4" thick (or thicker) closed cell pipe insulation is preferable, but not often available from retail box-stores. Grainger carries some of it, as do suppliers to the HVAC guys. It's available from multiple sources online, and the minimum purchase quantities usually aren't huge. Something like 15-20% of the hot water heating energy is abandoned & dissipated in the distribution plumbing. Fully insulating that plumbing cuts the loss down to single-digits.

[domingi]

Thank you Dana,
This is the answer I was looking for. We do have an all electric house and currently have an electric water heater on a 30 amp circuit. Many of the electric tankless that I have seen understandably require a huge amount of current.
Our ground water here is never cold, in fact, it rarely drops below 70 F and in the summer is much closer to 90 F, so we do not have far to go to heat it to a comfortable range. I honestly do think that a passive solar will provide everything that we need,, but for those rare occasions, the addition of a smaller electric tankless should fill in the gaps.

[Dana]

Thank you Dana,
This is the answer I was looking for. We do have an all electric house and currently have an electric water heater on a 30 amp circuit. Many of the electric tankless that I have seen understandably require a huge amount of current.
Our ground water here is never cold, in fact, it rarely drops below 70 F and in the summer is much closer to 90 F, so we do not have far to go to heat it to a comfortable range. I honestly do think that a passive solar will provide everything that we need,, but for those rare occasions, the addition of a smaller electric tankless should fill in the gaps.

On those days when the solar is giving you nothing, you'll still need something like 12-15kw of tankless to bring 70F water up to shower temps at shower flow rates- that's over 50amps at 220V.

If you shower at 105F (a typical temp +/- 2F depending on whether you prefer it cooler or hotter), with a 2.0gpm shower head, that's 35KBTU/hr=10.25kw. If the showerhead flow is true to the advertized 2.5gpm that'll be close to 13kw. That's about your lower limit, realisitically, and it'll be drawing ~ 60amps @ 220V. Most people opt for a ~20-22kw tankless (~100amps) to have some margin, but if you're willling to work around it on cloudy days, a 12-13kw unit willl do.

If you decide to buy commercial rather than hack your own batch heater, the Harparis SunCache is a pretty good option in your neighborhood. The fact that the potable water is in a heat exchanger rather than acting as the bulk storage mass makes tepid-water pathogen issues go away. Legionella can take hold in a tank that's stagnating between 85-120F for long periods, and wouldn't be killed by the tankless. With the internal heat-exchanger approach the potable water in the batch heater gets purged pretty much at every significant hot water draw. Summertime daily/weekly peak temps would likely be high enough to kill it off, but in the winter, maybe not for weeks or months. Letting it get up to 150F and stay there for awhile every now and then would be a good practice when living with most batch heaters.