Tankless electric water heaters & size of units
I’m building a small multi unit residential building with individual water heaters. I want to go all electric and prefer tankless for space and efficiency purposes. Unfortunately with the small size of units we don’t have enough room for the heat pump WH ventilation requirements.
Our MEP is concerned that tankless will not meet the demand for our 3-br 3-ba units. We also have 1br units but I assume that is less of a concern.
For tankless, they are proposing the Steibel Eltron Tempra series Trend or Plus. 3br size is the Tempra 29 and 1br size is Tempra 24.
We are in climate zone 4 – East Coast.
What do you think, will these WH meet the demand for the larger units? Would it make sense to use a traditional tank for the 3-br units and a tankless electric for the much smaller 1-br units?
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In zone 4 you will need 36 kWh per unit just for water heating. Just be sure the engineer at the electrical utility understands this load is expected and his equipment is sized accordingly.
Note if the water heater is pulling 36 kWH, back up strip heat 30kWh, car charting 15kWh, induction stove 12kWh plus the normal loads means a 200 Amp panel will not be enough. So budget for the larger panels and feeds.
Walta
You should not use tankless heaters for this application. Find a way to fit a 30 or 40G tank, pure resistance is fine. Efficiency gains from tankless heaters are minimal to non-existent, so you’re adding significant cost for no real benefit.
If you want more space, this might be a good application for a Sunamp heater. They store energy, like a tank, but using a phase change material so less volume is needed for the same energy.
In my area, I get this question a lot. The units are quite cheap. The panel and transformer upgrades are not. Given the supply chain issues with transformers we're seeing 6 -12 months delays in getting projects powered up, more impacting when asking for upgrades to existing infrastructure.
This ends up being a very expensive way to save money
Thanks everyone, this is really helpful feedback.
I knew about the higher energy demand but hadn’t thought about the delays involved and I know that can be a big deal here as well. This building will be pulling a lot more because of multiple units, so that will make it worse.
I’ve never heard of Sunamp, so another thing for me to get started researching!
Does the building layout make a centralized solution impractical? For example: https://www.smallplanetsupply.com/waterdrop
I have an EcoSmart 27kW in my house in South Carolina and it has had zero problems. I can take a shower while the washing machine and/or dishwasher is running and not have cold water. If you have colder ground water that same brand has a 36kW but it seems over kill to me. I find the 27kW overkill I only bought it because it was on sale on Amazon for $30 cheaper than the 18kW one I had on my wishlist. Also, the delay is pretty much nonexistent, I only have to run water for maybe 5-10 seconds for it to be hot with the exception of a spare bathroom which is 80ft away, which would take a long time even for a standard water heater. I love my tankless, just make sure you are aware of the hidden costs of electrical install (40amp breaker per 9kW) and the cost of running (Energy Star sticker pegs the 27kW at $400 per year vs a 80gal HPWH is like $110). That last point though depends on usage. Our power bill actually went down when we installed the tankless since we only shower twice per week typically, and if I shower more than that I often do a cold shower for the health benefits
I have the tempura 24. It’s good and reliable but if you want to run hot water for more than 2 things at once it slows down. You would need at least a 29 for the larger unit. I would think it’s more expensive to store hot water than to make it on demand but I don’t have a comparison. The other good thing is with on demand you never run out of hot water but the down side you are likely to use more water.
We have a tempra 36 and are mostly happy with it (decided to go with it both b/c of space constraints and occasionally needing a very large amount of hot water at once for a large bathtub).
One thing I didn't anticipate which is a downside is that the larger units also have a higher minimum flow rate before they will kick in, which means that if you want hot water at the kitchen tap you have to run the tap on high; and our bathroom taps don't actually draw enough water to trigger the unit to turn on at all. So it's definitely worth trying to right-size your unit rather than going with a larger one than you need just in case.
Are the units separately metered? Do the tenants pay the utilities or are they included? Is it a condo or rental?
If you're paying the bills I recommend the High Sierra 1.25 GPM showerhead. Installing those on my house was the single biggest energy saving thing I've ever done.