i am going to be heating a 85 gallon DHW storage tank with a 2-ton water unit. what instrumentation would be required to maintain a given water tempurature in the tank? would need a high level cut off and a low level cut in. anybody have any experience with this? also, what kind of temps can i expect to get?

hedgehog,

I'm not sure I fully understand what you are putting together, but if you are looking to just turn the heat pump on and off based on the tank temp, why can't you use a normal electric water heater thermostat?

Regards,

Eric, thanls for the reply. i am heating my house with a 3-ton forced air heat pump with a desuperheater, and a 2-ton water unit. the 2-ton will heat my basement floor, garage floor. in my climate the desuperheater should provide most if not all my hot water in the winter months, it will provide a little in the summer months, but very little in the spring and fall. i want to add a third zone to my 2-ton, the same tank the desuperheater heats, a 85 gallon marathon. downstream of my storage tank is a 29kw electric tankless water heater. i want to minimize the amount of electricity i use to make my hot water. the only thing hooked up at this point is the 3-ton forced air unit. the desuperheater still is not hooked up. i am not sure what temps a guy can expect to to heat my tank with a 2-ton heat pump. my water tempurature here is around 50, even if i could keep it from getting any lower than 90 or 100 it would drastically cut down the amount of power my tankless water heater uses. i am not a hvac installer, but understand basic electrical. from what i understand my 2-ton water heat pump would be turned on by a 24/240 relay that is activated by either the garage or basement thermostat, will the existing thermostat on my tank work with 24v? i would assume i would use the bottom one. i was looking at honeywell aquastats, but not very many of them have a tempurature range compatable with what i need. any thoughts or ideas are appreciated.

hedgehog,
 
I see no reason why you couldn't remove power to the lower heating element and use its thermostat to switch the 24 volt circuit to turn on the heat pump. The thermostat on the lower element is adjustable, so you could set it for the temp you want.

Around my neck of the woods the building codes frown on mixing heat pump water with domestic water. They require an exchanger to be used here. Point being, you may want to check your local codes.

I have a Marathon 85 as my hot water source.  The lower element is disconnected.  I use a desuperheater that supplies 100% of our needs during the winter months and through the summer.  During the spring and fall, like you pointed out, the upper element covers our needs. 

I understand that you have the tankless unit, but I doubt you will need it.

Good luck with the system and keep us posted on how things are working out.

i anticipated the code requirments of mixing my radiant water and my domestic water, i have a brazed stainless flat plate heat exchanger that has 1" ports and about 8 ft2 of surface area. i do not have my marathon wired in at all, i purchased it in ND, unforunatly most US electric appliances are not CSA approved for use in canada, so i couldnt wire it in anyways, i just checked the lower thermostat, i think the lowest setting is 110, not sure what temps a water to water heat pump is capable of producing in this type of system, i am sure i will lose some effincey with the heat exchanger. i dont know, maybe i should just try the desuper heater for a year. how big of heat pump are you using? does the desuper heater hit its high temp cut out often? i am curious since we both have the same tank. thanks for your input