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is outdoor rest control a viable option

a viable option when there is only a standard cast iron water boiler installed? i'm thinking about the flue gas condensation. Seems one wouldn't be able to reset to much and still provide the deltaT. Ive never installed outdoor reset so i'm curious. I normally work just in existing homes about 80 years old. more repair than design.


  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    With 80 year old homes...

    I imagine that you are dealing with mostly single zone cast iron radiation systems. I install lots of boiler replacements in these types of systems. Doing a load calc and checking it against available radiation usually results in design day supply temps around 150F. Bucket time confirms this. If it is going to continue to be single zone with no TRV's and with a boiler sized to the current heat load, the temperature rises so slowly in these systems that the boiler runs very cool throughout its firing cycle. In one particular gravity conversion with a copper tube boiler sized to load and a design supply of 180F (house had no insulation at all), on a typical winter day the boiler fired 20 minutes at a time with the system temp only rising about 20F throughout the firing cycle. To me there appears to be little need for reset on these systems because they are self resetting. In warm weather they run cooler and in cold weather warmer just because of the huge thermal mass of the system. I usually set them up with the boiler piped P/S with a Danfoss/Esbe thermix valve on the boiler return for boiler protection and use a standard thermostat with a long anticipator setting (2 times amp draw). If the owner desires a setback stat, I install a Honeywell Chronotherm. If you want, you could use a variable speed pump setup for boiler protection without outdoor reset. On the tekmars I use I just set the supply temp target for slightly above the peak design supply temp, set the minimum return at about 120 to 125 (117F min return temp to cast iron Dunkirks) and switch the control tosetpoint control instead of outdoor reset.

  • Tom Meyer
    Tom Meyer Member Posts: 300
    Outdoor reset controls

    Outdoor reset controls provide additional features to the overall control strategy for the system. Many times this results in cost savings to the homeowner. Every one of my designs have ORCs.

    Condensation is a result of the return water temperature. If you make provisions to maintain approximately 130 deg F water return temperature, flue condensation shouldn't be an issue. Take a hard look at each system's requirements.

    Senior Designer/Trainer
    Precision Hydronics Corporation
  • thanks BP&Tom (n.m.)

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