Hydrostat low limit needed?

btree

Hi, our oil company recently installed a Fuel Smart Hydrostat for our Buderus G115, to replace a broken Logamatic. The new hydrostat has outdoor reset, as did our old Logamatic.

The hydrostat has both low limit and high limit settings. When combined with ODR, I think that means the water temp is constrained to be between the low and high limits, but the target temp will depend on, or could depend on, the outdoor temp. (My impression from the manual is the ratio setting controls the slope of its linear heating curve. I can guess but don't actually know how the low or high limit settings affect that.)

The technician recommended setting the low limit to 120 degrees F, because if the water temp is below that, it risks creating flue condensate. I believe he was concerned with condensate in the chimney, but I'm not sure I fully understood the issue.

I wonder how significant the risk is and whether it may be advisable to let the temp go lower. The hydrostat's low limit can be set as low as 110, or can be turned off altogether. We don't use the boiler for hot water.

For the Logamatic, for the last year or two I had the reference temp at 125, with a heating curve of (water temp @ outdoor temp): 91@50, 108@32, 126@14. With that heating curve, the home's heating was fine, and oil usage, normalized by heating degree days, was lower compared to earlier years when the reference temp was much higher (165). I think this implies efficiency may be improved by allowing water temps lower than 120?

(Looking at other posts here, these water temps seem lower than other people report using — maybe our house has larger radiators or relatively more insulation than typical, not sure.)

If it's a bad idea for the system to put it lower than 120, then that's that and never mind. But I was wondering if anyone here had thoughts about it.

Jamie Hall

That boiler, if I remember correctly, is not designed to be a condensing boiler. Therefore… you shouldn't run the boiler and primary circulation below 140. Now that doesn't mean that you can't run the building circulation at less than that, if there is a mixing valve controlled by the outdoor reset. What will happen is that the boiler will run enough (assuming the controls are installed and set correctly) to keep the circulating water where it needs to be.

btree

Thanks. Yeah, the Buderus website describes the G115WS as a non-condensing boiler. (Our boiler is just G115.)

A related thread:

https://forum.heatinghelp.com/discussion/193655/bypass-valve-for-buderus-g115ws-3