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….

WD31

JUGHNE,

the boiler complies with CSD1 and the LGP switch is a manual reset.

from my perspective the boiler does not have a low enough turndown ratio to accommodate the lowest VAV (5900 btu).

thanks for the help

WD31

LRCCBJ

You state that the supply loop (the primary loop) water temperature is 128-133. Do I understand that the return on the primary loop is 128°F and the supply on the primary loop is 133°F? This would be a DT of 5°F and would absolutely be the source of your problem.

If the RWT on the secondary loop is 110°F, the RWT for the primary loop is IDEALLY 110°F. BUT, if the primary loop is flowing significantly more than the secondary loop, the flow FROM the boiler turns right around and goes TO the boiler together with the 110°F from the secondary loop.

The primary loop flow rate must be equal or less than the secondary loop. In this situation, the RWT to the primary loop is exactly the same temperature as the RWT on the secondary loop. They cannot be different.

bburd

Regarding the power issues with the SOLA controller, it might make sense to feed that through a small UPS, the type used for desktop computers. That would keep the power clean and stable.

EBEBRATT-Ed

I know you said no room for a buffer tank but that would be the fix until you can install smaller boilers. Can you find some room in an adjacent room and pipe over to it?

Unfortunately, your engineer should have spotted this before the install

Kaos

I'm assuming those air handlers are pretty far and connected with big pipes. You might be able to use that as makeshift buffer tank if you wire the pumps to those to run during the day. If you can also set the air handler fans to run very low speed it would add some baseload. The two together might not eliminate the short cycling but reduce it enough so the boiler won't die an early death.

WD31

LRCCBJ,

The primary (boiler) pump is rated at 38 gpm @ 24ft. The building (secondary) pump (2 pumps are lead lag) is rated at 36 gpm @ 40ft controlled by a VFD. There is a DPS in the loop to modulate the VFD to match the load. All of the control valves are modulating. The primary pump is constant.

I think the calls for heat may only be for partial opening of a small number of control valves and thus complicating the issue. Actual observation of the SWT and RWT is only @ 5 degrees. Only on morning warm up does the delta T increase to 20.

Thanks for the help as I have another job under construction with potential similar issues. I am trying to get ahead of it.

Regards,

WD31

WD31

Kaos,

The 2 large coils in the AH's are for morning warm up only. We tried that however, it only causes the VAV's to overshoot their setpoint.

Thanks,

WD31

LRCCBJ

@WD31

The primary pump is constant.

Actual observation of the SWT and RWT is only @ 5 degrees. 

You must put a VFD on the primary pump……………..OR…………..install a butterfly valve to increase the resistance to primary flow until the RWT on the primary loop matches the RWT from the secondary loop (110°). It would, of course, be better if the system properly controlled the VFD but, at the moment, ANYTHING to slow the primary pump would help you dramatically.

The short cycling will be reduced significantly.

nb: If you slow the primary pump excessively, (below the flow of the secondary) you will find that the SWT on the secondary will be less than the SWT on the primary. Obviously, you don't want that if you can avoid it.

Kaos

If running the air handlers overshoots the temp, I would try just running the pumps to circulate water though them as a makeshift buffer. This would not deliver heat to the building but should reduces cycling.