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Outdoor reset

Brad White_9
Brad White_9 Member Posts: 2,440
IMHO, there is no better place to be than to have CI radiators and OD Reset. As the other posters have stated, you have to protect your CI boiler. A mixing valve will adjust to what the radiation needs but will send back to the boiler whatever the return water temperature is.

Personally I would (and have) install a 4-way mixing valve so that the radiation and boiler loops are divided. In the alternate, primary-secondary with injection is equally good but in answer to your question, some piping surgery is warranted. The control should monitor the HWR to the boiler to keep it elevated.

Using OD reset on the boiler alone is very limiting, your reset range would be from 180F to maybe 140F on the supply side.

With a decoupling, your boiler can keep that range but "sip" water into the radiation loop side as needed. Thus your radiation side can have water temperatures from 180 F on the coldest day to practially room temperature on the mildest. Add TRV's on the radiators as an added treat.

p.s.: Use 4-Way valves on CI boilers, 3-way on condensing. With 4-way, you will pass HWS back to the boiler which you want with CI but it will stop the condensing in a Mod-Con. This illustrates why the 4-way is more effective at protecting the CI boiler.

My $0.02



  • Josh S.
    Josh S. Member Posts: 9
    Outdoor reset with or with out a mixing valve?

    My father and I are debating weather or not an outdoor reset like the tekmar 256 are worth while on homes with cast iron radiators. Is it worth the money if it is tied into the TT on the boiler or is it necessary to repipe the boiler with a mixing valve? One theory is that the thermostat satisfies long before the aquastat on cast iron radiation, making the reset ineffective without a mixing valve....I'm looking for input.
  • Steve L.
    Steve L. Member Posts: 35

    The 256 allows you to set a min water temp. Typicaly 140 degrees on most boilers. So on a 40 degree day the system may only allow the boiler enough to maintain 145 degrees (or whatever the heating curve calls for). So with this cooler water the thermostat will take longer to be satisfied.
    I actually just used a 256 on a family friends system with big old cast irom radiators. Due to a leak on some piping near the boiler I changed the piping and set up and injection system (I had to drain whole system and take out the leaking section of pipe). But I could have simply used the 256 and kept the same piping for similar results. But With the injection set up I can allow a cooler loop temperature on warm fall of spring days.
  • Timco
    Timco Member Posts: 3,026

    So does the 256 have a sensor that reads the pipe temp? How does it know? Is the Tekmar 260 an equivilant setback?

    Just a guy running some pipes.
  • Steve L.
    Steve L. Member Posts: 35

    The 256 has comes with two sensors. One goes on system piping to sense the water temp. And the second sensor is an outor sensor that mounts outside. The outdoor sensor tells the control what temp is required. The system sensor lets the control when to turn the boiler on and off.
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