Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Aquastat on a buffer tank

imatellerslie Member Posts: 111
a geothermal heat pump and cast iron radiators. The radiators will be run using pex with a home run system to manifolds next to the buffer tank. I'm planning one circulator on the secondary side, with TRV's on the radiators.

I am planning to use an aquastat on the buffer tank with an outdoor reset control for the geothermal heat pump. It occurred to me the other day that if all TRV's are open, the volumetric flow rate on the secondary side may be more than on the primary side. If this happens, this aquastat will "see" the return temperature from the radiators.

I don't think this will be a problem, since the TRV's will throttle flow and significantly reduce the secondary volumetric flow rate as they are satisfied, causing the secondary flow rate to drop to less than the primary. Then the buffer tank aquastat will see temperature of the heated water from the geothermal unit, and everything should be fine, I think. However, I don't have the experience you guys have and was wondering what your thoughts were. Am I going to be ok here, or might I have problems?

I have 22 radiators (127 MBtu output at design) and plan to add radiant flooring in a 12x15 bathroom (1.6 Mbtu heatloss) and an 15x25 kitchen (14 MBtu heatloss).

The geothermal units (2 of them) are 11 gpm each on the hydronic side.


  • Paul Rohrs
    Paul Rohrs Member Posts: 357
    Some experience.....

    I would say your basic logic is correct. We have done several jobs with the same primarysecondary as a staple. I would highly recommend a Tekmar 150 setpoint control in lieu of your aquastat. I have seen some Geo-thermal setups not extract heat quickly enough and the internal controls of the geothermal will lock out on high pressure (overheat). I have poured over some wiring schematics of a few units and you need to be cautious when sending the Pump Relay end switch (?) to the control on the geothermal. I recommend the T150 because you can control the tank temperature precisely as well as the differential.

    Again with CI radiators, you will want to use a Y-strainer to protect the rest of your pumps and equipment. I can send you a JPEG of the basic layout we have used successfully. How are going to incorporate the reset temperature? Use of a modulating valve? Injection mixing? or reset the buffer tank temperature? Which control for reset?


  • Brandon
    Brandon Member Posts: 3

    I'm not an expert on radiators, but doesn't take a minimum 140 degree water to "radiat" A geo isn't going to be able to provide that. I've had a couple of instances were people have done it and been disappointed with the results. I could wrong, but you might want to check in to it.

  • imatellerslie
    imatellerslie Member Posts: 111
    The radiators are oversized enough...

    that the EDR is high enough to provide the necessary heat down to 10F with only 120F average radiator temperature. The geothermal unit I'm looking at can provide 130F, so it looks like it should work.

    I'm planning to use a 5 ton econar DualTec (combination forced air/hydronic) and a 5 ton econar hydronic unit. This allows me to use the DualTec for A/C, and I could even use it for forced air heat if I wanted. I expect that I'll find it much more comfortable to heat with the radiators though.
  • imatellerslie
    imatellerslie Member Posts: 111


    I was planning to use the reset to control the buffer tank temperature. The geothermal unit efficiency goes up pretty quickly as the temperature of the hydronic side fluid through it goes down, so it makes sense to send the cold water coming back from the radiators right to the heat pump. I hadn't worked out the details on the control package yet. It's going to be a pretty complicated system with two heat pumps. One will be dedicated to the hydronic system, and the other will be an Econar DualTec, which is a combo forced air/hydronic heat pump. Thanks for the tip to use the y-strainer. Do you have any brand you have used that you could recommend? If you could send that picture you mentioned, I'd appreciate it.
This discussion has been closed.