Raytherm Boiler Trips in Extreme Cold

RyanT

Been having trouble for a few years now with my boiler tripping out when the outside temp. drops below -25C or so. The high limit switch trips out and must be reset, it will run for several hours and then trip out again. I've replaced pumps and have had several contractors look at it but it's hard to troubleshoot when it only faults on the coldest days. We run forced air for the main floor, which is heated by the boiler water, and in-floor heat in the basement.
It is a Raytherm boiler, model #H3-0260RR natural gas.
Any tips would be appreciated.

EBEBRATT-Ed

If it's tripping on high limit (water temp) that is a pretty easy clue to the problem.

Tripping on high water temp means your putting more heat into the boiler than can be absorbed by the water.

1. high limit not set properly...around 100C
2. boiler sooted (fire side) or scaled up (water side)
3. gas is overfired (too much gas)
4. pumps not moving enough water through the system due to wrong pump, not piped right or system is restricted, air bound, or not enough water pressure in the system etc

RyanT

Thanks EBEBRATT for the info. After further investigation, I believe I understand the problem, I just don't know how to solve it. Our boiler supplies heat for both our forced air unit for the main floor as well as the in-floor for the basement. It appears that when the forced air stops calling for heat and the blower shuts off, there is an influx of heat in the lines because the in-floor is still calling for heat. This sudden bump is enough to trip the high limit when it's extra cold outside.
The in-floor system was installed in 1990 and isn't zoned properly (3 loops over 2400 sq.ft. as well as one garage loop). This leads me to believe that your No. 4 suggestion is the likely answer. Short of replacing the old in-floor system, which is less than desirable, it seems like the best answer would be to replace the forced air unit with one that uses it's own burner for heat and leave the boiler strictly for the in-floor heat. Does any part of this rambling sound accurate?

Jamie Hall

It's not that there is an influx of heat in the lines -- it's that half of the capacity for using the heat produced by the boiler has gone away. Numb3re 4 up there probably is the right answer, but permanently reducing the heat absorbing capacity by cutting out the forced air unit isn't -- you already know that won't work.

Assuming that there is no simple way to repipe the floor system so that it presents less resistance to flow your best bet is to simply put in a higher head pump.

RyanT

Yes, that's what I mean by an influx of heat in the lines. The capacity to use the heat is halved, thus resulting in a spike of temperature for a short period of time until the pump can circulate the fluid and level the system out again. I'm not convinced a higher head pump is the answer as we have tried increasing it already once and it didn't make a difference. I appreciate your thoughts though. I'll take them into consideration.