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Boiler High Limit set too low?

DJDrew Member Posts: 89
We had a company come out to discuss an air-to-water heat pump for our home with cast iron radiators. They mentioned that running the water temp at 140 degrees or more is not ideal for long periods with the heat pump, but more reasonable for 120-low 130s water temperature.

As an experiment to see if our house could handle the low 130s, for the past month we set the high limit at 132. (Cast Iron, natural gas boiler with a 3way return bypass that is set at 122 degrees). With the lower high-limit temperature, I noticed that the heating cycles increased from 45 minutes to 75 minutes, but there was no difference in comfort aside from the stack effect in the house decreased. The boiler itself runs solid for the first 30 minutes of a call for heat and then does 10 minutes on/12 minutes off.

Based on these results, does this experiment proved that our house would probably do well with the lower water temps an air-to water heat pump would provide?

Also, is there any issues in leaving the existing boiler set at the lower set point? The previous high set point was 143 since we didn't want to worry about the kids burning themselves on a radiator, it's only about 10 degrees difference than the old set point. Thoughts?


  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    It would seem that with the outdoor temperatures which you have had during your test that yes, your house can manage with the lower temperature water in the radiators. It won't, of course, when it gets colder -- something you might want to consider.

    That said, you are doing your poor boiler no favours by running it that cool. It's condensing all the time, and will quite happily rot out about twice as fast as it otherwise would have -- or faster. Likewise your exhaust flue, if it's metal. Don't do it. Much better, so long as you are using that boiler, to run a mixing valve to lower the circulating water temperature, and let the boiler cycle. You might even consider adding a buffer tank, so that the boiler cycles will be longer.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    SuperTechGGrossBig Ed_4
  • DJDrew
    DJDrew Member Posts: 89
    edited March 5
    Thanks @Jamie Hall that is good to know. Even under normal circumstances, I think the boiler left to its own devices rarely gets above 135 when the weather is warmer than 20 degrees - and it only does so because of the existing mixing valve. The mixing valve is only on the return line to keep it above 122 degrees. Before that was installed, it would run for 45 and never get above 115-120 degrees and satisfy the house Tstat. We had -5 degree days in a row and the house was satisfied and the highest the boiler could get the water temp was 139/140 for an hour, boiler only ran 8-9 hours those days.

    I'll put it back to the previous max of 143 (which it never hits) and put some sensors on the supply side to log max temp needed based on outside temp. Would be an interesting chart.