I have a system designed by NRT in my own house. An underfloor system where the water target temp is 110 degrees at my design temp. We used a weil McClain Ultra (modulating boiler with outdoor reset).

I also own a 3 family rental house that has slantfin baseboards and had a 20 year old boiler that I just replaced with the same Weil McClain ulta.

When running Slantfin - thin fin baseboards is there any reason I shouldnt use the outdoor reset and let it run less then 180 degrees on warmer days?

Is it a case that a BTU is a BTU and since less BTU/h are lost on a warmer day I can heat the water less or is their some lack of effiecieny when running this type of baseboard at a lower temp then 180.

If I can run it at a lower temp I would draw the conclusion that I should turn the boost feature of the boiler off? The bost feature raises the target temp I think 10 degrees every 20 minutes the boiler runs that cycle. My heat cycles seen to average about 45 minutes to go from one degree under the setpoint to one degree over so the boost function always comes into play if it is turned on.

The is an old house, with no design calcs but I am monitoring proliphix thermostats. I use inthrma.com to log data every minute from the unit and graph it so if I were to turn off the boost feature I could track if the unit wasnt keeping up. With this is mind I dont have a major concern that my miniumn target temp would be too low when listening to the outdoor reset.

you want to run this boiler as cold as you can, so definitely use the outdoor reset feature. if you are willing to fiddle especially, the boost feature is a counterproductive waste of time.

that's simply a 'don't call back' feature, basically.

however, below 120 or so baseboard output falls off a cliff, so for practical purposes I usually spec a minimum temperature of 100 for baseboard, and a max of whatever is needed on "design day".

the only downside to outdoor reset is slower response time if you want to turn up thermostats or the like.