How are multiple boilers controlled?

delcrossv · March 5

None of the systems I work on are big enough to rate two boilers, so this is a general question.

If i have a primary boiler that handles the load on most days, and a secondary for use with the primary when things get colder, how is the secondary told to fire?

(Aside from "dang it's cold,flip the switch" kind of control)

I've seen controllers for 2 boilers but not clear on how the black box does its thing.

mattmia2 · March 5

it measures the swt and fires a boiler if it is below setpoint. if it is more than a certain amount or is below for a certain period of time it fires a second boiler. many systems call with a voltage that modulates the firing rate of the boiler. usually they are the same size boilers and it uses some sort of algorithm to call different boilers every day or week or something like that. some systems know about how much is calling and can make decisions that way. there are everything from simple twinning controllers that just get a heat call and control the boilers to building management systems that have control of all the room thermostats and zone valves and other components of the system.

pedmec · March 5

It depends on what boilers you install. Most mod-cons have the software built in and all you have to do is daisy chain the wiring from one boiler to the next. One boiler will be the master and the remaining will be slaves. You just have to set up the cascade feature.

Another set up is to buy a boiler control such as Tekmar, Heattimer, or Taco etc. that is just a small sample of controls. I'm sure others will have other brands.

delcrossv · March 5

Steam system, that's why I'm drawing a blank.

HW, makes sense to use swt. I don't know what a Tekmar steam control, for example, measures to kick on boiler #2 on a heat call. Outdoor reset? But that would have to be manually entered.

mattmia2 · March 5

small steam systems with multiple boilers are usually controlled on pressure. there are modulating pressure controls that control a pot based on system pressure.

delcrossv · March 5

https://forum.heatinghelp.com/discussion/comment/1846796#Comment_1846796

So, if there's no pressure rise, it fires the second boiler?

mattmia2 · March 5

there has to be hysteresis in there so it is within certain ranges and probably time in that range. for a lot of reasons systems that are large enough to require more than one boiler usually run on a couple psig.

EBEBRATT-Ed · March 5

Steam systems you can do different things. You could use a two stage thermostat (or two separate thermostats set at different temperatures)

On commercial jobs like schools with multiple boilers run on pressure you can buy Lead Lag controls.

So, when the system is started up (no pressure on the system) both boilers fire, As the pressure rises the lag boiler is shut down on pressure and the lead boiler continues to run if it gets up to its pressure it shuts down. If the lead boiler cant keep up with the load the pressure drops and the lag boiler fires again. Preferred utilities make a lead lag control that alternates the lead and the lag boiler.

Heat timer & Tekmar have controls for steam that work on condensate temp or steam establishing temp but I don't know if they have multiple boiler controls. Heat Timer might.

I also made my own lead lag control that someone can put together with 2 pressure controls and a couple of relays. See attached. It doesn't alternate but you can run both boilers on lead or both on lag or have boiler 1 lead and 2 lag or vise versa. Lead lag is good because you run 1 boiler on lead and 1 on lag. If the lead boiler fails to start the pressure drops and the lag boiler fires to maintain some heat.

If you use lead lag the idle boiler will fill with condensate from the steam condensing in the idle boiler so overflow traps are a good idea. That is also why Preferred made their alternating model.

LEAD LAG.pdf

Long Beach Ed · March 5

We worked on a church complex with three steam boilers. There are also three motorized independent zone controllers. The three boilers were set to fire on pressure, with each one cutting out as the pressure increased.

Overflow traps on each with a feed pump.

A beautifully engineered 1924 system.

Unfortunately the "expert" before us replaced the three boilers with a single horribly oversized boiler. They called us because their fuel costs tripled and the place wouldn't heat.

SlamDunk · March 5

I run four commercial HPS boilers off of a lead-lag dispatcher. The best management system would have a pressure transmitter on the header somewhere in the midpoint of the steam distribution system to iron out fluctuations. If the lead boiler dropped Xpsi, the dispatcher would fire up a lag boiler. If the lead boiler was maintaining pressure but running between X% -100% for Y minutes, it would also fire up a lag boiler and the lag boiler would run for Z minutes before shutting down. The variables are programmable to suit your needs.

A good dispatcher would give you the option to run boilers in unison or base load. Base load is when the lead boiler fires at rate and the lag boiler runs at a programmed lower rate.

The ideal operation, in my opinion, would be all boilers running around 75% on the coldest days and one boiler running around 75% on the hottest days.

delcrossv · March 5

Thanks everyone. Learn something new everyday. 😀

jumper · March 6

Fifty years ago a human isolated off line boilers, both steam & HHW. Return; supply; and flue. Safeties prevented firing if stuff was not back online. Especially flue.

How are multiple boilers controlled?

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