Condensing lp gas boiler short-cycling

WD31

LAARS NeoTherm 600 used in a school in central Va.

Below is the information for this issue. I am in search of some solutions, any input is welcome.

I have suggested that the school contact the engineer for assistance, but I am also seeking advice from this community as the engineer may not be as up-to-date on prevailing methods concerning condensing boilers.

BAS has boiler enable and provides an outdoor temperature.

The setpoints from the engineer are 120 degrees minimum and 160 degrees maximum.

The operating temps are attained by a 4 – 20mv signal on a sliding scale. The minimum and maximum is derived from the engineer so at 35 degrees or below the boiler uses the above temperature setpoints. As the outdoor temperatures rise the minimum/maximum temps. drop according to the 4 – 20mv scale.

There are 2 air handlers

AC-ACT is solely dedicated to the gymnasium and it has a hot water coil

AH – 118 has a hot water coil that is used mainly for morning warm-up and 13 VAV’s to take care of classrooms.

Based on our knowledge that the boiler minimum firing rate is 120MBH a small number of VAVs calling for heat are an insufficient load for the boiler at its minimum.

An example from observations after boiler set-up.

OD temp. 48

Mv reading 6.9 – 7.3

Cut-in temp 111

Modulation 129

Cut-out temp 136

Building return loop water temp 110

Boiler supply loop water temp 128 – 133

The piping arrangement is primary secondary.

As you can see the delta between building return loop water and boiler supply loop at start-up is an 18 degree rise. Not much for a 600mbh boiler.

Cycle time 1 min 44 seconds from shut down to start-up. The unit achieved set point 133 degrees in 30 seconds or less.

School system is trending the boiler on/off cycles and as of yesterday it was over 100/day.

It is my observation that the system is not utilizing all of the heat energy remaining in the building loop piping (110 degrees) to provide heat to the VAV’s.

My theory is the current return loop water temperature carries enough heat to satisfy the needs of the a classroom and should not call on the boiler to fire until it has exhausted that heat, say down to 80 (an arbitrary number) degrees. If the return loop water temp is low enough the boiler may modulate lower and run longer to attain its setpoint thus mitigating or eliminating the short cycling.

Let me know your thoughts.

WD31

hot_rod

you are correct in noticing that the connected heat emitters dictate the boilers operating condition

Ideally the boiler turndown would be close to the smallest heat emitters load

Two smaller boilers, a buffer tank are options

Many of the modulating boilers have control features like step firing , boiler derate, anti short cycle timers to address your concerns, read through the control program options

WD31

Bob,

Spoke LAARS engineering this model has a 5:1 turndown, minimum 120mbh, and cannot be derated. To add to the schools misery this is a new install (2years old). Have had to replace the SOLA controller 6x, just found out the school has funky power 57v is not good for SOLA controllers!

There is no room for a buffer tank that large and I told them to refer it back to the EOR.

I also suggested 2 300mbh units and with a 5:1 turndown gives them @ 60k btu's and some redundancy. Not an option, yet.

Thanks and have a great holiday.

WD31

JUGHNE

Many LP gas burners have a low gas pressure switch.

Are some of them auto reset? IDK

But have you monitored the gas inlet pressure as these short cycle?

Just a WAG….