Posted by Jason Peterson on September 28, 2001 at 14:02:47:
In Reply to: Re: Temperature limit control posted by Harold Kestenholz - Hydronic Network on September 27, 2001 at 15:50:24:
: Actually, the reason a hot water boiler cycles quickly on the high limit is that there is insufficient water flow through the boiler to take the heat away from the boiler to the radiation. This occurs more often in mild weather with a multizone system as not all the zones are open while the boiler puts its rated input into the boiler at every cycle.
: Two-stage thermostats eliminate this problem in multi-zone systems. Proper circulator pumping rates with attention to the proper boiler sizing will eliminate it in single-zone systems. Use a thermometer to check if the water leaving the boiler is coming back from the radiation within 20 F degrees difference. If the temperature difference is more than 20F, the circulator is not moving enough water through the radiation and the boiler overheats.
Thankyou for your response! The temperature has warmed up here, so I will have to measure the temperature differential at the boiler inlet and outlet later.
I have a single zone system. If I'm not getting enough water moving through the loop, how much is enough? Is there a recommended flow rate (in gpm or liters/minute?). I'm trying to determine how to properly size the recirculation pump.
Since this is an old boiler, is it possible that scale buildup within the sections has limited water flow? If so, is there a standard way to flush an older system (with some acidic solution, for example) to remove scale buildup?