Sanden Sanco2 vs. Standard Heat Pump Water Heater
Trevor_Lambert
| Posted in General Questions on
This seems like it should be obvious, but cost aside, is a Sanco2 going to be significantly better than a standard HP water heater like a Rheem? This would be on the warm edge of zone 6. House is heated by mini splits, so the Rheem would theoretically still run in heat pump mode in the winter, but at reduced efficiency. There is no basement, so all the heat taken out of the house needs to be replaced.
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You can compare the COP between the two units if you have the COP of your mini split.
If I got my math right, the combined COP of mini-split to heat-pump water heater is 1 / ( 1/SplitCOP + 1/WaterHeaterCOP - 1/(SplitCOP*WaterHeaterCOP) ). So if both are around 3, you get an overall COP of 1.8.
Checking your math, assuming COP 3, for 27 units of heat into the water, you use 9 units of electricity for the water heater and pull 18 units of heat from the interior of the house. Replacing those takes 6 units of electricity, so the COP is 27/(6+9) = 1.8.
So yes, you got that right.
Oops, just saw this was a 1-year-old thread. Oh well, the math is still correct a year later!
Heat pumps move heat energy, they don’t create it. So if you’re putting 27 units of heat into the water, you’re taking 27 units of heat from the house. The energy cost to move that heat is 9 units of electricity. So the combined COP is 1.5. And that’s only when it’s moderate temperature out. When it’s cold and your mini split heat pump is running at a COP of 1.5, then your indoor heat pump water heater is no better than a cheap electric water heater.
"So if you’re putting 27 units of heat into the water, you’re taking 27 units of heat from the house. "
That's incorrect. By conservation of energy, the electric energy you put in also has to go somewhere. It goes into the water.
I suppose you’re right to a degree, I misunderstood the exact definition of a heat pumps “COP”. Realizing how these mfg are rating these, I had to dive in further and see if any scientific study of realistic permorfamce was studied. Heat pump water heaters work input have ~30% losses that are actually just heating up the surrounding room’s air. Obviously this heat is then recaptured by the heat pump and eventually makes it way into the water. But in reality, 100% of electricity input to the system does not get transferred to heat in the tank, some it is lost to the air and room around the unit. In a warm climate these units are amazing. In a cold climate, 1/2 the year you’re robbing Peter to pay Paul.
Trevor,
I too have been trying to determine whether the Sanden is worth the expense as well. Your question sent me off researching and calculating. The long tedious version is available, but suffice to say that the answer may depend a lot on your hot water needs and climate zone location.
The overall system COP/efficiency for the Sanden was tested by a Washington State University team and for Portland at least, it was COP3. For whatever Heating Zone 3 was it fell to COP2.6.
I live in CZ6b at high altitude and have air density adjustments to account for. Still, the general gist of the report is "Resistance is Futile" get an HPWH of some form. One will save at least a third of electrical costs for garage locations, about half of costs for interior locations and a very slippery to calculate third of electricity costs (maybe less) for a HPWH stealing the heat of home heating mini-splits.
The Rheem, if it follows the energy management pattern of an older unit tested by NREL in 2011, tends to focus on happy customers getting hot water quickly. To do so, programming will kick in the resistive elements to make the recovery rate acceptable. Heat pumps with 4200 BTU output simply don't raise water temps fast enough for rapid repeat volume access. Tedious charts available. Programming choices aside, the system COP of the old Rheem came in at 2.5. Whether the test conditions match your local conditions is debatable.
An interior HPWH that is "steals" home heat may exact a small penalty during milder weather, but on a freezing February night the penalty goes up. The best new hyper heat minis seem to achieve COPs of 2.5 down to 5F, below that is fuzzy data. The cost analysis that I have done so far indicates that the net cost savings below 5F can be 25% or less. Lots of ill defined variables to deal with.
The Sanden units are capable of COP 5 or even 6 in warm weather, but appear to fall into the 2-3 range when below 10F. Their chart is quite confusing, but it will still make hot water down to -20F. The real trick for them seems to lie in being able to raise the water temperature almost 100 degrees all the way down to 5F. In combination with stratification tactics and a large tank, the first hour rate is very large and the recovery in keeping with posted values for Rheem offerings. Sanden does this without any resistance elements, period. So efficiency award goes to them. Whether the cost of a separate outdoor unit is worthwhile, I have yet to determine.
A few other items to note, the Sanden does not move air in the house. The Rheem will. Is this a comfort or noise issue for you? Another item, Sanden has a GWP of 1 if it leaks the Rheem has one north of 1400. The Sanden will not contribute to any cooling needs during the summer the way an interior Rheem will. Is this worth the COP difference? I am still working on that answer.
For my recent application the choice was the Sanden in a sunny breezy, or the Rheem/Rudd in the still air basement (the coolest place on the property). The Sanden would likely be the winner just based on the temperature of the source air. The Sanden does not move basement air, which avoids tedious questions of if it's a good idea to circulate or pressurize the basement air.
Note the COP of the 83 gallon tank is a lot better than the 43 gallon. Stratification is higher in the tall tank, and thus supplies cooler water to the heat exchanger.
The Sanden unit is worth it if you need a bit of heat. For example you can heat a low load house with it, which would save buying another heating appliance.
I could also see it working in a smaller two story place where the main floor is heated by the Sanden and the upstairs a single mini split. The cost of the extra mini split could go a long way for paying for the unit plus the bit of hydronics.
>"The Sanden unit is worth it if you need a bit of heat. For example you can heat a low load house with it, which would save buying another heating appliance."
That's easier said than done- the engineering isn't trivial. CO2 doesn't undergo a phase change, and needs much higher delta-Ts than R410A etc to operate efficiently. (Show me a Sanden combi solution that runs an average COP better than 2, and maybe it'll be "worth it".)
I might take you up on that challenge, just need to find a project that is a good fit.
Delta T and low return water temperature is easy to create, just need to design it in.
The irony is the sealed nature of the Sanden unit makes it ideal for.... RFC134a coolant. There's nothing to leak, no connectors, no tubes... the CO2 seems like an Achille's heel in a way. Overall I still love the units...
> is a Sanco2 going to be significantly better than a standard HP water heater like a Rheem?
When the power fails and the former is destroyed by freezing, the Rheem will be significantly better.
Conversely, when half the Rheem fails and all of it needs replacing, the SanCO2 will look a lot better. In a non-freezing climate, the chance of a SANCO₂ freezing is non-existent.
https://www.eco2waterheater.com/
Just a quick note as the original question and most of the comments are a year old. A new generation of Sanden's HPWH is now shipping in the United States, and is somewhat more efficient than what was on the market last year. Here's a link to some useful information:
https://www.eco2waterheater.com/technical-docs
As of this writing, the generation 3 units are not for new installs. The G4 units are backordered, with a bunch of units stuck at the port of Long Beach on the MARESK ALFIRK.
I installed a Rheem hybrid WH a year ago. Off-grid solar here, so efficiency is critical. I have the input air for the heat pump coming from the attic; free heat and some of it is probably leakage "lost" from the below-heated space anyway. The efficiency of the Rheem is simply amazing. It hardly registers as a draw when running. We got the 80 gal, best $2500 I ever spent.