JHZR2
Staff member
The design temperature for the outdoor reset in my boiler, for this area, is 14F. We were right around 14F today, so I was playing with some of the settings to try and optimize the efficiency of the boiler operations. We keep the house at 64-65 when we are home and awake, and 62 when gone or asleep. We augment with some electric heat in the baby's room when it is cold, but we generally like to sleep cold, so it works.
We have a peerless purefire modulating/condensing boiler, with indirect water heater. It is really efficient and we are really happy with it. We have had it installed for a few years now. It is set up with two zones - one that feeds all the cast iron radiators in the home, and the other which feeds the water heater.
What I found was that with the circulator pump set to lowest setting causes the boiler to hit the setpoint too quick, and then it will cut out until the water temperature is back in specification. The circulator pump is a make and model recommended by the manufacturer for use with our model boiler, and the lowest speed setting is acceptable per the owners manual but I suppose our pressure drop in the piping is sufficiently high to reduce flowrate. There is a minimum flowrate required, so hitting the setpoint too rapidly without even modulating would indicate too slow a flow, right??? Looking at the pump head vs flowrate chart, it appears to confirm this.
At the mid-speed setting, the delta T (supply - return) is around 34F from initial firing until some time after the water setpoint is reached, and the boiler starts modulating. 40F is "optimal" for efficiency apparently, but I couldnt hit it.
At the high speed setting, the delta T decreases, at which point it starts to modulate "early".
So this tells me that the circulator (which again is a brand and model recommended by the boiler manufacturer for this boiler) is a good recommendation but slightly oversized for this application, in that it is either too slow or just a tad bit too fast on the mid setting, to get to the optimal delta T. I suppose I need a VSD for the circulator
Can I induce some voltage drop in its line and slow it down slightly???
The thing is this: At setting 2, where I have a pretty good delta T, the boiler is very slow to hit the setpoint. The outdoor reset active and set for cast iron radiators is currently specifying 157F of a maximum 160F water temperature, with ambient around 16F currently. For example, after running maybe 10-15 minutes, 100% gas input, with the home around 65F (thermostat set to 69), the water supply temperature reached 151F when the setpoint was 157F (was lower before that and the whole test started with the system equilibrated and supply and return at 107F. After 25-30 minutes, the setpoint is reached and the boiler starts modulating down the gas flow... But the delta T also starts shrinking from 34 to 30-31 and then less as time goes by (because less heat is radiating into the space due to lower delta T in the living area). Is that too slow or too fast to hit setpoint?
Of course this is a mod/con boiler, and boilers are more efficient apparently at lower water temperatures and higher delta T. But its also beneficial to be modulatng so that Im burning less gas overall.
Soooooooo... Is there a good rule of thumb as to how long it should take to hit the water temperature setpoint and begin modulating? Also, is there anything else I can do to get my delta T higher and for longer, including steady-state operations where the boiler is modulating down at the water setpoint? Of course I dont want the delta T to be too great, or else it is probably worse for the heat exchanger in the boiler.
Thanks!
We have a peerless purefire modulating/condensing boiler, with indirect water heater. It is really efficient and we are really happy with it. We have had it installed for a few years now. It is set up with two zones - one that feeds all the cast iron radiators in the home, and the other which feeds the water heater.
What I found was that with the circulator pump set to lowest setting causes the boiler to hit the setpoint too quick, and then it will cut out until the water temperature is back in specification. The circulator pump is a make and model recommended by the manufacturer for use with our model boiler, and the lowest speed setting is acceptable per the owners manual but I suppose our pressure drop in the piping is sufficiently high to reduce flowrate. There is a minimum flowrate required, so hitting the setpoint too rapidly without even modulating would indicate too slow a flow, right??? Looking at the pump head vs flowrate chart, it appears to confirm this.
At the mid-speed setting, the delta T (supply - return) is around 34F from initial firing until some time after the water setpoint is reached, and the boiler starts modulating. 40F is "optimal" for efficiency apparently, but I couldnt hit it.
At the high speed setting, the delta T decreases, at which point it starts to modulate "early".
So this tells me that the circulator (which again is a brand and model recommended by the boiler manufacturer for this boiler) is a good recommendation but slightly oversized for this application, in that it is either too slow or just a tad bit too fast on the mid setting, to get to the optimal delta T. I suppose I need a VSD for the circulator
The thing is this: At setting 2, where I have a pretty good delta T, the boiler is very slow to hit the setpoint. The outdoor reset active and set for cast iron radiators is currently specifying 157F of a maximum 160F water temperature, with ambient around 16F currently. For example, after running maybe 10-15 minutes, 100% gas input, with the home around 65F (thermostat set to 69), the water supply temperature reached 151F when the setpoint was 157F (was lower before that and the whole test started with the system equilibrated and supply and return at 107F. After 25-30 minutes, the setpoint is reached and the boiler starts modulating down the gas flow... But the delta T also starts shrinking from 34 to 30-31 and then less as time goes by (because less heat is radiating into the space due to lower delta T in the living area). Is that too slow or too fast to hit setpoint?
Of course this is a mod/con boiler, and boilers are more efficient apparently at lower water temperatures and higher delta T. But its also beneficial to be modulatng so that Im burning less gas overall.
Soooooooo... Is there a good rule of thumb as to how long it should take to hit the water temperature setpoint and begin modulating? Also, is there anything else I can do to get my delta T higher and for longer, including steady-state operations where the boiler is modulating down at the water setpoint? Of course I dont want the delta T to be too great, or else it is probably worse for the heat exchanger in the boiler.
Thanks!
