Hybrid Hot Water Heater, Thermostatic Mixing Valve, Efficiency

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BobD777

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I'm installing an AO Smith HPTU-80N hybrid hot water heater and am looking at the pros and cons of adding a thermostatic mixing valve. I see it's recommended for other types of installations, but is there anything that makes using it with a hybrid different?

Also, if I understand correctly, the mixing valve can potentially provide a whole lot more hot water with a smaller tank--given a higher set temperature. We currently have an 80 gal HWH. Is it more efficient to have an 80 gal HWH running at 120 degrees or a 66 gallon at 140 degrees with a thermostatic valve, and is it reasonable to consider downsizing, despite having a big house with currently 5 living here?

And FYI, I know some of you don't like the hybrids. I've got excess solar pv that will handle the load, a really large, unconditioned basement, and need for dehumidification.

Thanks for your input--
 

Phog

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is there anything that makes using it with a hybrid different

Nope.

Is it more efficient to have an 80 gal HWH running at 120 degrees or a 66 gallon at 140 degrees with a thermostatic valve, and is it reasonable to consider downsizing, despite having a big house with currently 5 living here?

For the same size tank, setting the tank to a higher temperature will result in higher standby losses & be less efficient. So that works in favor of having a larger tank at a lower temperature.

On the other hand increasing the tank size (in general) increases the surface area, so for the same hot water temperature you will have higher standby losses with a larger tank.

With hybrid heat pump water heaters you also need to calculate in the efficiency of the heat pump, which may be slightly different even between two model sizes in the same manufacturer brand series and will be affected by the different storage temperature as well.

To bottom line it for you, you'll probably have a slightly better efficiency with the larger tank at a lower temperature, but it depends on too many variables to say for sure without doing a calculation.

A thermostatic mixing valve is code in many places now and it's not a bad idea to get one installed even if you don't use to it jack the storage temperature up.
 

BobD777

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Nope.



For the same size tank, setting the tank to a higher temperature will result in higher standby losses & be less efficient. So that works in favor of having a larger tank at a lower temperature.

On the other hand increasing the tank size (in general) increases the surface area, so for the same hot water temperature you will have higher standby losses with a larger tank.

With hybrid heat pump water heaters you also need to calculate in the efficiency of the heat pump, which may be slightly different even between two model sizes in the same manufacturer brand series and will be affected by the different storage temperature as well.

To bottom line it for you, you'll probably have a slightly better efficiency with the larger tank at a lower temperature, but it depends on too many variables to say for sure without doing a calculation.

A thermostatic mixing valve is code in many places now and it's not a bad idea to get one installed even if you don't use to it jack the storage temperature up.

Thanks for the reply-- Those are the factors I've been weighing. Speaking of weighing, part of that question was being asked by my back, since with COVID-19, I don't know how I'm going to get help getting the new tank into place, at 307 lbs. Yikes. Not that a 66 gal tank would make much of a difference since it's 289 lbs.

And thanks for summing it up on the thermostatic mixing valve. So I'll definitely add that.
 

Reach4

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I would think that a heat pump would be more efficient if the temperature was not set so high. So I would opt for the bigger tank rather than the higher setting.
 

Jadnashua

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While the surface area of a tank does go up with a larger tank, the area/volume goes up at a slower rate. What's more important is how well the thing is insulated. Newer ones generally are mandated to have more insulation than older ones.
 

BobD777

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Think about a electric fiberglass water heater?

Until you asked, I didn't know they existed. From what I can see they have a tank that will never leak. That's an appealing trait. So why don't they make a heat pump water heater with a fiberglass tank?
 

Fitter30

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Fiberglass tank would outlive the refrigeraton unit. Heres a calculator for hot to cold water mixing just change to gallons and temperature.
 
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