Is it necessary to have a "tempering tank" in line with my regular 82 Gallon Hot water heater when using the Desuperheater on a WaterFurnace Envision Heat Pump.

One Vendor says "Absolutely Necessary",  Another says Not needed, just disconnect the bottom element and add a tempering valve on the existing heater output line in case the water gets too hot (150 deg).

My existing HW heater is a plastic lined "Life Time Warranty" 82 gallon electric.  There are only two of us and rarely have overnight visitors.

Confused -----

Read this thread "Justifying two electric hot water heaters". Let us know if you have any questions.
http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/68358/afv/topic/afpgj/6/Default.aspx#49775
I can write more later, but this will get you started.

You do not need a buffer tank with a DSH. A single tank setup is described in the Envision manual. We have 2 Envision units with 2 DSH's hooked directly to one 85 gallon Marathon water heater. I would leave the lower heating element connected and set to 100f, and the upper set to 120. This will make water available for the DSH to heat. I have no idea why a tempering valve is being suggested, unless this is a failsafe. Our Envision units cut off the DSH at 130f. If you use a lot of hot water, a buffer tank may be warranted, but if you're like us and don't use a lot of hot water, 1 tank can work great.

If the bottom element is disconnected, and there is little or no call for heating or cooling as in springa an fall, the house will only have a few gallons of hot water; the volume situated in the space above the upper thermostat. That is unlikely to prove satisfactory.

A tempering valve should only be necessary if the DSH lacks an upper temperature limit. WF Envision units have such a cutout, but perhaps some models do not.

Hamop78,

We have read through your comments regarding adding a tempering valve to the desuperheater on your WaterFurnace. Our units have a safety device that shuts off the circulator for the desuperheater in the event that the water temperature reaches 130 degrees. The yellow status light will be illuminated. At this time the unit’s hot water pump will be de-energized to prevent excessive temperatures. Hot water operation will resume when the tank cools off.  Therefore, a tempering valve is not required.

WaterFurnace International, Inc.

Per Climate Master (similar to WF in principles):

On tanks with a single thermostat, a preheat tank should be used.
  ... a dual tank ..., is the most efficient system,

WIth a single tank, your water heater might have the tank back up to temperature before the geo system even comes on.  Note that a DSH may not exceed 100F/85F in winter/summer  and the water heater has to do the rest. 

Not clear to me is if the systems stop DSH flow when the tank is hotter than the DSH.  

Posted By jonr on 04 Mar 2010 04:55 PM
WIth a single tank, your water heater might have the tank back up to temperature before the geo system even comes on.

Hence the reason for the lower thermostat settings.  Lower settings allow the DSH to heat a greater volume of water prior to the DSH turning off on limit compared to 120 or 130 settings on both the upper and lower thermostats.  I know you know this jonr, just clarifying for the OP. 

Lots of Good Information on this site and thread --- Thanks to all ---- Based on this I decided to add the tempering tank and found a Rheem Marathon 40 gallon to use for that purpose. With the 82 gallon I have now there should be an almost endless supply of HW especially in the summer.

The vendor that recommended a1 tank system had some other flaws to his proposal so my plan is now set -- WF Envision 3 ton on a closed 4 ton loop system (600 V ft) w/DSH and tempering tank. This should help the inefficiency difference from the pump and dump system that I have hooked to the Hydroheat system.

Thanks again to all

Great site --- Lots of info here