Boiler Staging

carol_3 · October 2006

Back to your original question--I've been on vacation--you got more than you asked for, eh? Great stuff. Taking it back to the simple question, though, for comfort I think you'd want to use first stage on all the boilers before going to second stage. Depending upon the controls of course, to bring on second stage most use either timing-only (not best for comfort because you may have to wait too long) or time plus degrees away from set point (called proportional plus integral or P + I). Either way you have to wait a bit for second stage to kick in, which isn't good for comfort. And no matter how much talk about efficiency and energy savings, the purpose of heating is comfort. Immediate gratification preferred! :)

Fixitman · October 2006

Boiler Staging Schemes

I have inherited the care of a boiler plant equipped with 6 identical natural gas hot water boilers. Each boiler has its own circulator. It has badly implemented - and unreliable - DDC controls. I'm about to tear them out and start over again.

The boilers in question are each two-stage. However, the present controls run the boilers as if they are each single-stage, for 6 total stages. I'd like to operate the system as a 12-stage system with the new controls.

I can imagine a handful of schemes to stage the boilers, but there are two basic ones:

1. Run stage A of boiler #1, followed by stage B of boiler #1.
Run stage A of boiler #2, followed by stage B of boiler #2.
Etc.

2. Run stage A of boiler #1, followed by stage A of boiler #2,
followed by stage A of boiler #3, etc, until all the boilers
are running stage A. Then run stage B of boiler #1,
followed by stage B of boiler #2, etc, until all 12 stages
are firing.

It seems to me that Scheme 2 offers the best fuel efficiency. It exposes nearly double the heat exchange area per unit of fire as scheme one does, until most of the stages are firing - a rare occurrence. Scheme two makes the boiler's circulators work harder for the amount of heat delivered, so the circulators would be expected to run nearly twice as many hours. As a result, more electrical use, and higher circulator maintenance cost. Still, my instinct tells me that the fuel savings should be larger than the extra life-cycle expense for the pumping. It also seems like the boilers would run longer, more gentle cycles, increasing boiler life.

Yet I've heard several people insist that the 'right' way to stage boilers of this type is Scheme 1. When I ask for their rationale, they've all basically told me, "Because that's how it's always been done." I confess that I've never thought of that as good rationale for anything. In fact, I was dismayed to get this sort of answer from the boiler manufacturer's tech support person. So I'm asking here - where it seems like folks are more thoughtful than that.

Thanks in advance for any information anyone can provide ...

Empire_2 · October 2006

Staging...

Real quick and I'll get back to ya. Why not bank your boilers,....say 3 and 3 using the 2 stage that each boiler has already. You need to figure out you total load and be able to maintain the load during peak hours which will utilizing only your first bank of boilers. Lead/lag control is also a good idea. If I were the customer I would opt for a new control set up. Major money savings possible here.

Mike T.

Brad White_9 · October 2006

I tend to agree with you

that more boilers at lower stages will increase efficiency. The downside or cautionary note is, are these condensing type boilers or conventional which require protection?

If condensing, yes, low-stage each then high stage each. I agree with Mike T. about lead-lag; even wear and tear. There are many other considerations worth asking though. Here are a few:

1) Are the boilers isolable, automatically? Will they act as reverse heat exchangers when not in operation?

2) If automatically isolated, do they have their own safety trim within the isolaton valved portions?

3) Is return water protection necessary to forstall unwanted shock or corrosion (if conventional boilers)

4) If condensing boilers is there anything in the system which will thwart your best efficiency? (Thinking 4-way valves, bypasses dumping hot water into the return uneccessarily, that sort of thing.)

Overall I am sure you have an understanding of "the system" and how it interacts with the boilers; just rather say than not.

And one last thought: "Because it has always been done that way" is the greatest impediment to progress...

My $0.02,

Brad

Tim_33 · October 2006

I know that...

more boilers at low fire is very efficient for condensing boilers. Is this true for non-condensing boilers as well? Or is it design specific in that some are more efficient at low fire than high fire, some or not?

Fixitman · October 2006

Thanks for the reply ...

Maybe there's something I'm missing, but I don't see why arranging the boilers in two banks would be an improvement over leaving the 6 boilers in a single large bank. Where would any additional efficiency come from?

The design peak load could potentially take both stages of all 6 boilers. They provide heating for a city block of water source heat pumps in a northern tier state. The boiler plant works its proverbial kiester off during morning warm-up, then drops its output considerably for the rest of the day.

Fixitman · October 2006

Brad White commented :

When I posted the original question, I thought of it as an "all other things being equal" sort of question. But your response is quite relevant, so my comments interspersed with yours, below :

> ... more boilers at lower stages will increase efficiency. The downside or cautionary note is, are these condensing type boilers or conventional which require protection?

The boilers are not a condensing design - AO Smith DuraMax. (I wasn't going to name the company, but might as well.) This boiler plant will *theoretically* always operate at above-condensing temperatures, except during its own warm-up. Among other things, it handles the heating of the building entrances for a city-block sized complex. It's programmed for a minimum supply water temperature of 135*f. In practice, It dips below that substantially for short periods - one of several reasons for re-doing everything.

> If condensing, yes, low-stage each then high stage each. I agree with Mike T. about lead-lag; even wear and tear. ...

Lead will be rotated among all 6 boilers.

> ... There are many other considerations worth asking though. Here are a few:

> 1) Are the boilers isolable, automatically? Will they act as reverse heat exchangers when not in operation?

Water-sides are not isolated by valves. Just by circulators that run only while and briefly after firing. I'm planning on studying the stack losses this winter. Combustion air is forced, but by a single-speed fan - so too much combustion air most of the time. The stacks are short, so chimney effect is minimal. Still, I'm thinking we will probably want to add flue dampers, and a barometric relief, or damper control of the CA supply, or fan speed control.

> 2) If automatically isolated, do they have their own safety trim within the isolaton valved portions?

> 3) Is return water protection necessary to forstall unwanted shock or corrosion (if conventional boilers)?

This is a major deficiency in the existing design. Besides the building entrances, these boilers handle the water loop heating for several hundred water source heat pumps. Return water in the heat pump loop is less than 60*f during morning warm-up. The design attempted to use a single 3-way valve for both regulating heat pump loop temp and boiler return-water tempering. The valve is too large, and in some circumstances the valve short circuits most of the hot water flow right back to the boilers, causing low flow at entrances, re-heat coils, etc. In spite of that, the existing control system still manages to send slugs of cold return water back to the boiler plant sometimes. I have a fix. Too long a discussion for here, unless someone really wants to hear about it.

> 4) If condensing boilers is there anything in the system which will thwart your best efficiency? (Thinking 4-way valves, bypasses dumping hot water into the return uneccessarily, that sort of thing.)

Yes, as discussed above.

> Overall I am sure you have an understanding of "the system" and how it interacts with the boilers; just rather say than not.

> And one last thought: "Because it has always been done that way" is the greatest impediment to progress...

Yes - and the greatest impediment to understanding.

Fixitman · October 2006

Tim, thanks for the reply.

In my opinion, other things being equal, anything done to improve the fire / heat-exchange ratio will increase efficiency, whether the boilers are condensing or not.

A condensing boiler gets a big bonus from its ability to greatly increase the temperature differential between the fire and the water sides, and gets to collect the heat from any water that condenses. Obviously a large improvement, if you have return water cold enough to take advantage.

Fixitman · October 2006

Thanks for your reply.
I largely agree with what you say ...
There's one difference in the way this boiler plant runs versus many others. It provides morning warm-up for several hundred water-source heat pumps. Once that's done, it practically idles, providing hot water for a few entrances, and for a handful of hot water re-heat coils.

The hot water provides the heat, but in none of these cases does varying the boiler water temperature itself do the temperature control. Some valve or fan controls the temperature. So the boiler water doesn't have to be dead accurate - it just has to keep up with the worst zone.

Furthermore, the morning warm-up - far and away the peak load - all happens before the building is occupied. So we don't have to be dead accurate with the water temperature then either - perhaps even less so.

Interestingly enough, some preliminary calculations indicate that we might save some money by running the water warmer during morning warm-up than at any other time of the day. If the heat pump loop is warmer, the heat pumps' heating capacity is better, so they warm up the building a bit faster. Since we do variable speed pumping in our heat pump loop, we potentially save more money in pumping energy than we waste by running the loops hotter. If anyone cares, I'll have some real data by the end of this winter.

Boiler Staging

Comments

Categories