Condensing Boiler Tripping Out on High Fire? 1

[michael333]

Hello,

I have (3) P-K condensing boilers (MACH Model C-300 w/ ENVI Control) for a small job. Each C-300 is rated as follows:
Output 276,000 BTU/hr heating capactiy at full-fire
Flow rate @ 20 delta T: 28 GPM
Flow rate @ 40 delta T: 14 GPM
Each C-300 has its own isolation valve associated with it.
I stated in my sequences for each boiler to control its own isolation valve to OPEN/ClOSE based on operation. B-1 is master, the other 2 boilers are the slaves.
Stated in my sequences 140 deg F maintained to the system, 120 deg F returning.
2-way control valves at all 30 boxes, 4 FCUs, and 2 AHUs with recovery wheels.
Also, have a bypass valve out in the HW system.

Total peak heating load for facility: approx 650,000 BTU/hr
Had 2 inline pumps, operating in parallel, providing approx 68 GPM @ 41 Hz to the heating system at full-flow according to a submitted TAB report.

When I was onsite 2 weeks ago, I observed all 3 boilers ON to maintain setpoint of 140 deg F and modulating their isolation valves accordingly. It was 55 - 60 deg F outside. I would've thought that only 1 boiler would be operating to maintain the 140 setpoint, not 3 of them. Also, noticed 46 Hz for each of the inline pump VFDs operating in parallel.

Contractor is telling me that the lead master boiler B-1 is tripping out on high-fire when its associated isolation valve opens up for flow to the boiler. Tells me its a design issue.

Can anyone speak to this? Has anyone ever ran across this issue before with a similar design? Any advise or input would be appreciated.

Thanks,

 

[Drazen]

can you describe a little more what you call isolation valve? i cannot understand how it is modulated. if flow varied according to fcu valves tripping and pump modulation, design should take care to ensure minimum flow through boilers when they are on, and their isolation valves should have on-off duty.

moreover, what tripping out on high-fire has to do with all boilers working?!

than, finally, what do you think is a problem with all three boilers running? if your supply and flow temperature difference matches design temperature difference that means capacity matches load. only when you notice your return temperature rising you will be able to check firing sequence. you can simulate it artificially by manually closing fcu valves.

 

[DRWeig]

I have put controls on a bunch of condensing boiler systems with more than one boiler. The manufacturer and mechanical engineer call for the maximum number of boilers to run for any load without running them below minimum fire (maximum turndown). As an energy modeler, I do the same.

Have a look at the boiler performance curves. Highest efficiency occurs at lowest load, which corresponds to highest return water temperature at constant supply temperature. The ones I've been working with this week have about 90 percent efficiency at 175°F return temperature and full load. It's up in the 97 percent range at 25 percent load and 100°F return water.

Nuisance tripping I've dealt with almost always has been due to low water flow. The trip is on high temperature lockout. Once, though, it was a damaged safety circuit on the manufacturer's control board.

If the flow switch is too close to the isolation valve, it will experience lots of turbulence as the isolation valve opens. It could be flapping around, in which case the control board would take action.

Your TAB was done a while ago. Check the strainers. They can look like they are wearing socks not long after starting a new system.

Your bypass valve out in the system might not allow enough flow when open with all the two-way valves closed.

Is this system primary-pump only, or is it primary-secondary? Primary-only is tough to control and keep the boilers above minimum flow.

At 46 Hz, your pumps are running at 77 percent of full speed. That means 59 percent of full flow. Check the control loop that deals with the VFDs.




Best to you,

Goober Dave

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[Drazen]

DRWeig, it's interesting to hear your comment even if i do not get it fully. over the years of observing i would say american and european practicy differs the most in domain of hydronic systems.

i cannot fully get why someone would go to as high as 175 F return temperature. that looks very high to me. as regards to high temp tripping, i mostly have low flow switch in systems i deal with. high temp tripping requres maintenance technician to unlock it with special key and reports on the cause.

 

[DRWeig]

You're right on both items, Drazen. High temp cutout requires a manual reset and it's good to have it keyed for a qualified person.

175 F is very high, I only cited it for reference -- I don't see it in HVAC systems. It's just the high end the performance curve. If you start out at 180 F supply temperature at full building load, the reset downward as the load drops, you get the much lower return temps. See attached for a pretty good Lochinvar boiler. The performance chart is on page 4. The maintenance techs used to call that brand "Lock-out-var" years ago. They had a tendency to trip on hi temperature very often. I think they are very reliable lately, though.

Example: 4 boilers at 2 MBTUH each. If the load is only 2.0 MBTUH, it's much more thermally efficient to have all four boilers working at 0.5 MBTUH (25% load) than one boiler full-blast (100% load) or two boilers half-speed (50% load). The pump and draft fan energy are very small in comparison to the heat energy saved. The effect is multiplied by resetting the HW supply downward to get the return water as low as you can.

A flow switch is best I think, but they're hard to calibrate just right. If the load on the boiler is decreasing slowly, the hi-temp cutout might act first?

Best to you,

Goober Dave

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[ChasBean1]

I would need a diagram and field review of the system. One thing to consider: the world has gone to variable primary systems versus primary / secondary systems on the promise to save a dime. This unfortunately comes at the risk of robust operation. If you have a variable primary system and the boilers have individual control, cycling their own isolation valves, there can be times when the boilers satisfy the system temperature and all shut off, which is normal. But for this system configuration the result can be dead-head of the loop. I’ve seen this on the last three jobs (and counting). Post more detail and a system sketch if you want better help…