Outdoor Reset and Thermostats.

Jean-David Beyer

The other day I turned on the hot water tap in my kitchen sink, and no water came out. All the other hot water taps in the house worked, even the ones in my darkroom that is farther along the piping from the hot water heater than the sink is. That localized where the freeze was.



Unfortunately, to warm up that piece of pipe would require removing cabinets under the granite counter tops, and punching through some drywall. That did not appeal to me. So I thought I would raise the indoor temperature, so I adjusted my thermostats up from 69F to 74F and 12 hours later the indoor temperature had not gone up much. Downstairs is a radiant slab at grade, so I expected a delay, but not that much. I then adjusted the maximum reset curve from 120F to 128F for the slab and from 136F to 160F for the baseboard zone. In another 24 hours, the temperature went up to 71F.



Sounds like my thermostats do not do much with the reset curves as I had them. Then the sun came out, the outdoor temperatures made it up to about 18F and the pipe unfroze. It had been 10F below design temperature for about a day.



I have changed the reset curves so that the coldest setting is at 0F now instead of 8F, even though that means the slab can get supply temperatures of 128F instead of just 120F. I do not have hardwood floors, so I can probably get away with this. And if I need to put 140F into the baseboards instead of 136F, I may lose a little condensing on very cold days, but that is better than insufficient heat.



I guess those reset curves are pretty close to the actual load.



In the past 5 years where I have been keeping track, it never went below 8F and that for only an hour or two here. Now it has been down to 2.7F for a while several times and for longer.