The delta-T (=temperature difference) I'm asking for is the difference in temp of the water entering the boiler compared to it's output tempeature. this has to be measured at the boiler, in the loop driven by the primary pump, not the return water from the zones and what's being delivered to the zones.
The reason this is important with condensing boilers is that if the water entering the boiler is warmer than ~125F, you won't get better than 87% combustion efficiency out of it. So if you have a small delta-T , say 10F and the output is 140F, the entering water temperature is 130F, and you won't get condensing efficiency out of it. The water returning from radiation might be 120F, but if the primary loop is being over-pumped, the mixing with the boiler's output raises the entering water temperature too much. Most boilers won't hit the 95% range until the entering water temp is under 120F.
Even though it's the smallest in their lineup, the LX-90 is way oversized for most homes in the US. If you have some mid winter gas bills with exact meter reading dates and amounts, you can look up the weather data for those usage periods and derive a linear constant of heat load per degree-F, (below 60-65F, the typical heatng/cooling outdoor temperature balance point rangef or houses in the US) and derive the heat load at the 99th percentile temperature bin for the location. Sounds complicated, but it's not really. It's basically:
1: Take the fuel used during the meter-reading period, multiply times the boiler's efficiency to come up with a total BTU number.
2: Look up the heating degree-day (HDD) data for your location for that period, and divide the BTU per HDD.
3: Divide the BTU per HDD by 24, to come up with BTU per degree-hour.
4: Look up the 99% outside design temperature for your location, and subtract it from your HDD base temperature (either 65F or 60F- whatever you used when looking up the HDD), for the number of heating degrees.
5:Multiply the BTU per degree-hour times the number of heating degrees, and you have the implied heat load.
If you do it for both base 60F and base 65F HDD you'll bracket the actual heat load fairly reasonably. Only in the rarest of cases will the real heat load be much higher than the bigger number, or much lower than the lower number. Most of the time it'll be somewhere in between.
The minimum modulated output of the LX-90 is about 23,000 BTU/hr, which is well over half the heat load of most reasonably tight homes, and WAY more than the load of individual zones of multi-zoned houses. From an efficiency point of view that's OK if the radiation is high volume radiators, but too often a short-cycling disaster with fin-tube baseboard. For a primer on how to make that assessment,
read this.
Adjusting the outdoor reset curve takes some time and patience, but if you have enough radiation to operate in the condensing zone without short-cycling the boiler, it's WELL worth it! Most boilers are designed to kick up the output temperature when serving a hot water heater zone, and is not affected by the outdoor reset temperature. This is true of the LX-90 as well, if installed correctly. On page
32 of the manual you'll see that the domestic hot water temperature and space heating temperature can be set independently.
If you run the numbers to come up with a heat load at some cold temperature and measure up the total radation, I can walk you through how to set up & adjust the outdoor reset curve. Most installers won't/can't take the time to fully tweak it in, but YOU can. And when you do it ends up operating at the highest possible comfort & efficiency that the system can deliver.
