Does anyone know how to disable the lower thermostat on a Marathon water heater. Have red and yellow wire going to two screws on the bottom of the thermostat and one black wire to the top of the stat -- plus the green ground wire. The reason the lower thermostat needs to be disabled is that there is not a second tank. The Marathon water heater is connected to a desuperheater that is connected to a geothermal Waterfurnace heat pump.

I would think disconnecting one of the leads to the lower heating element would get it out of the picture.

That said, are you sure you want to do this? My concern would be that during shoulder seasons - spring and fall periods of very limited heat pump operation - there would be very little or no DSH recovery. During those conditions, hot water capacity would be drastically reduced to the volume contained aboive the upper element - 1/4 or less of the total tank volume - barely enough for a single short shower.

Engineer,

Since the lowest setting for the bottom element is 110°, my client wanted to know how to disable it.  For the time being he has decided to turn off the power to the water heater and let the desuperheater try to heat all the water he and his wife needs.  We need to do something because it appears that the desuperheater is lowering the water temperature in the water heater while the lower and upper element was set at 120°.  Is it possible for the desuperheater to lower the water temperature when it is using 120° water that has been heated by the water heater?

Your suspicion is easy to confirm - buy two Topfin aquarium thermometers ($10 at PetSmart), or similar and tape probes to water in and out lines. If DSH leaving water is cooler than entering water after system has been running for 5 plus minutes, you have a problem.

If the unit is two stage, it is quite possible, even likely, for the DSH to pull heat out of the tank if the pump control circuit isn't smart enough to dump the pump when temperatures don't favor heat transfer in the desired direction.

This is but one of several reasons many of us demand preheat tanks.

My main heater (not a Marathon) has been off since early June - DSH has carried the whole load. That might work in AL as well.

As I mentioned before, pulling one or both leads off the lower element and wirenutting them should take that element out of the picture while letting the upper one continue.

Alton, do you have an Envision unit?

Curt, it's my understanding that an Envision unit won't transfer heat in the wrong direction due to an extra sensor. Is this correct?

Alton, we chose to go with a water heater timer to limit the use of the heaters in our Marathon. Since the Marathon holds heat extremely well, we have found that there is always enough hot water for us. We set the timer to engage before major hot water use times to ensure there is enough hot water. If the timer is set correctly, there will be enough hot water to carry you through other periods of use as well. That said, we opted for a larger tank (85 gallons) to help this work. Others with timers have expressed similar results, although a buffer tank may still be the optimal solution (but not for those who don't want a second tank for other reasons.)

IMO disconnecting the lower element is not a wise choice.The tank has been designed to prevent water temperature stratification. A better choice is to add an additional tank in series.This setup is no different than a well designed solar storage.

Geome - Envision has an algorithm that looks at water temperature, liquid line temperature and operating stage and then decides, based on differential temp, whether to run the pump. This is (somewhat) detailed in publicly available docs.

acwizard - in one word, No!! Electric storage water heaters are in fact designed to foster, promote, and depend upon thermal stratification. Plumbed properly, they deliver hot water of consistent temperature at flow rates many times their hourly recovery rate until the contents of the tank are nearly exhausted. This feature is predicated upon maintaining thermal stratification.

Plumbed properly, a DSH supplying just a small fraction of the heat rate of a conventional water heater element via water heated just a few degrees per pass can use the stratification design built into a conventional storage electric water heater: DSH water is withdrawin up the cold water dip tube, heated a few degrees in the DSH coax, and then returned via the tank drain fitting. Only after water has circulated several times will it rise via natural convection (lowered density) to the top of the tank for use by the downstream tank or directly by house residents.

I agree that the (far) better choice is an upstream preheat tank, but not for the reason you cite.