Running both main boiler and stand-by boiler at the same time

[markk1pe]

I was performing an energy audit at a fire station on Kennedy Space Center in Florida when I came across this situation. There are two Fulton 270-MBH gas-fired boilers in the mechanical room for reheat/humidity control of the HVAC system. During the site visits it was found that both boilers were set at 160 °F and both were running at the same time. The design drawing Sequence of Operations states that of the boilers one shall be a stand-by boiler. Note 1 states that the stand-by boiler should be set at 70 °F.

When I contacted the hot water shop supervisor he told me that they have always ran the system that way because the shop only works first shift and no over time is authorized so if the main boiler went down that the fire station (24/7 operation) would be without reheat for up to 16 hours or up to 2 days if it was the weekend. He opined that running 2 boilers wouldn't use any more energy because the water would just heat up twice as fast, same total energy in.

I'm looking for a second opinion. The controls are allegedly +/- 10 degrees on these boilers. One that was set at 160 degrees F was reading 168 degrees.

[rjk_cmh]

Well, if it was engineered as a standby and therefore a single boiler has enough capacity to cover the full load, running both could cause both boilers to short-cycle. However if they need unattended failover capability, there will need to be some kind of control system that can detect a failure and stage on the standby automatically. It sounds like the original design assumed that there would be someone onsite all the time that could switch on the standby.

[markk1pe]

Thank you for your well thought out reply. What does it mean "short cycle" of a boiler and what are the ill effects?

[rjk_cmh]

Thank you for your well thought out reply. What does it mean "short cycle" of a boiler and what are the ill effects?

Short cycling means that the boilers run in short bursts. This can cause excessive wear and tear for a variety of different reasons. Boilers like to have long run times, which isn't possible if they are oversized.

[HVAC_Marc]

Short cycling means that the boilers run in short bursts. This can cause excessive wear and tear for a variety of different reasons. Boilers like to have long run times, which isn't possible if they are oversized.

If they are Fulton Pulse boilers they should condense and short cycling wont hurt them as much.

[rjk_cmh]

If they are Fulton Pulse boilers they should condense and short cycling wont hurt them as much.

Good point, although the model of boiler wasn't specified by OP. There's a few variables possible in this situation and we don't have much info to go off of.

[HVAC_Marc]

Agreed.

[JCN]

I agree. We would need more specific boiler info to answer your question OP. Some boilers can modulate to meet load. Others cannot handle cold return water temps, etc....

[markk1pe]

From the original post: "two Fulton 270-MBH gas-fired boilers"

Here is a link to the literature:
https://www.fulton.com/fultonDotCom/...?ptc=hw&uid=12

[HVAC_Marc]

From the original post: "two Fulton 270-MBH gas-fired boilers"

Here is a link to the literature:
https://www.fulton.com/fultonDotCom/...?ptc=hw&uid=12

Fulton does more than Pulse/condensing boilers, so the size alone doesnt tell us anything.

The short run cycles shouldnt hurt them much. But, they wont reach steady state either for maximum efficiency.

The water temperature should rise at the same rate whether or not you have 1 boiler or 10 running on it. BTUs are BTUs. (dependent upon piping configuration).

[servicefitter]

if you would increased throttling range on standby it should stay off till water temp dropped.

[monu96]

Thanks for sharing a useful post.

[stanbyyourword]

and there can be system issues external causing both to run

[Spitz]

Could a boiler staging control with reset be used? Tekmar maybe.

Sent from my SM-G960U using Tapatalk

[markbrown1]

Why dont you call the expert technician and ask for the help?

[Cool breeze 38]

A staging control would be best. no need for two to run if the load doesn't need it. Boilers do like to run longer then shorter. The flue likes it also. Get past the condensing stage.
A big part of efficiency loss is the start up time.

with staging if one fails the other will kick in.

Now staging and its efficiency does change with piping. is it pump through or primary secondary. if PS are there check valves?

[FultonBoiler]

Your customer would certainly see efficiency benefits with staging the boilers. This could be done with a boiler lead/lag controller or could be set up with varying set points where the lag boiler would start only if the lead boiler fails to keep up with the demand or goes into a fault. Also, depending on the model and burner control, it could be something as simple as relay logic that would start the lag boiler when needed.

[bradluke0]

Whats the pumping situation? 2 pumps ? 1 pump ? If 2 pumps are both running all the time?

[Jim Davis]

Piping and pumping have a lot to do with how the boilers should run. If water is circulating through an idle boiler it will become a cooling chamber and increase fuel usage. The differential should be 20 degrees not 10 degrees! This will increase run time.

Fuel usage is dependent on how the boilers are set up more than just cycling. If they are modulating or High-Low, operating both of them in Low fire can be costly, unless they are condensing boilers. Knowing the combustion numbers of both boilers is critical in proper operation. In reality, the longer a boiler operates, the lower the efficiency. Basic heat transfer rules. The hotter the heat exchanger the less heat it absorbs from the burner. Ever notice the flue temperature keeps getting higher the longer things run? Short cycling only causes excess wear if the burners are set up incorrectly. Most equipment is oversized most of the year. Messing with the firing rates to create longer cycles increases energy use. Control settings (differential) is the only correct way to minimize cycling.

Laziness should never be a reason to operate equipment incorrectly. Stand-by equipment needs to be valves off if not used. Otherwise it should operate normally.