Best Efficiency: Multiple Condensing Boilers

Fixitman

I'm part of a team doing an energy upgrade on a 70's commercial building. It has 5, three-year-old, 2 million BTU, modulating, condensing boilers. Turn down is quite low. They are on a single circulating pump, piped reverse return, with good quality balancing valves.

Except in unusual circumstances, leaving water temps are under 110*f. A large fraction of hours water temps are under 100*f. Entering water can be in the high 70*s to mid 80*s The controls basically run all 5 boilers in parallel when warm water is needed. It's not unusual to see all 5 boilers running at 97* leaving, for instance, with all 5 producing water at about that temperature. Flows can be quite low, so the boilers operate turned way down much of the time. There is no provision in the current configuration for staging the boilers.

The two engineers I'm working with both insist that this arrangement is not efficient. They believe the boilers would achieve better efficiency if they were each separately pumped, and staged so that each boiler reaches full fire before staging in the next. I believe the existing installation is superior from a fuel efficiency standpoint.

It seems to me that the current installation must often be condensing most of the produced flue gas latent load. In addition, stack gas temperatures are low - indicating low losses. They wouldn't be nearly this low if the boilers were producing the same specification of hot water by running fewer boilers at full fire and mixing. Also, it seems like much less heat exchanger surface will be below the flue gas dew point, so less latent gain.

The two engineers are now going about re-designing the system to work in the way they favor. I'm an employee of the owner - a long-time commercial service guy, not a PE like the other two guys. We've traded rationales, but no one has had a change of mind. I feel like this boiler plant may be about to get a makeover that will not only make it less fuel efficient - but with added cost of designing and executing the makeover.

Is there something I'm overlooking here? Or do I need to start making my case more forcefully?

Thanks in advance for any wisdom anyone can provide.

Doug_7

Talk to boiler manufacturer

Someone needs to talk to boiler manufacturer about what boiler controls, pumping and piping give the highest overall efficiency with multiple boilers. Don't need to re-invent the wheel.

AERCO point to the "Inverse Efficiency Curve" of condensing boilers - that is, unlike atmospheric boilers, condensing boilers are slightly more efficient at low load than they are at high load. So it is more efficient to have multiple condensing boilers all running and sharing the load than to have some running at 100% and the other boilers shutdown.

Atmospheric boilers are much less efficient at low load, so what the engineers propose would be perfectly correct for atmospheric boilers, but is not necessarily correct for condensing boilers which are actually more efficient at low load.

AERCO also has a unique pumping requirement for condensing boilers unlike conventional atmospheric boilers.

Break out the boiler manufacturers part-load efficiency curves. Look at the part load efficiency vs full load efficiency. This should settle the debate.

Consult manufacturers literature and talk to the boiler manufacturer.

Doug

Henry

Multiple staging is more efficient. You mention that the temperatures are low. So therefore one boiler will probably be sufficient to maintain the temperature. The problem with your setup is that there is some jacket and piping heat loss, small but it all ads up. On our commercial instalations, we have found that a proper P/S with a header and individual pumps to be most efficient. We have done several retrofit as required by your PEs and the savings have paid for the work.

As for AERCO, we see them as a problem and not a solution. They are a maintenance nightmare. You may contact me for more information.

Glen Aspen_2

your thermal/combustion gains -

might be quite small given your discriptions. A condensing boiler at low fire is the most efficient firing system possible; so if it is meeting your heating needs and there are not other considerations to initiate a change - do not fix what is not broken. That said - there may be some electrical gains available by changing piping and installing smaller circulators rather than one large circulator (helps with the redundancy factor as well.) Multiple boiler staging has its detractors and its supporters - it really depends on your temperature requirements and your normally expected return temps. I'd press on with the discussions with the engineers and ask for gaurantees of performance - even a ROI schedule; I reiterate - do not change that which is not broken.

Fixitman

Re: Talk to boiler manufacturer

Thanks for your reply.

I suppose we could haggle about whether establishing a general principle regarding condensing boiler installation is re-inventing the wheel. I was hoping to avoid depending upon a specific manufacturer's recommendation since, compared to a sound general principle, each manufacturer sorta invents its own wheel. (The other difficulty for me: as a rep of the building owner rather than as a contractor, I find that some manufacturers don't take people in my position seriously. It's amazing how little cooperation some manufacturers and/or reps give when they're talking to someone who will not personally be sending them a check.) That said ...

I did talk to the boiler manufacturer eventually, and looked up literature for a few others as well. Given what I've now waded through, this seems obvious: Multiple condensing boilers that can turn down enough to stay in condensing conditions will be most efficient operated as a group rather than staged at full fire. Gas savings are significant.

Fixitman

Re: Thermal/Combustion Gains

Thanks for your reply ...
I'm now convinced that the boiler piping "upgrade" plan will cause a significant loss of efficiency. The current installation runs below the 135* condensing threshold 80+% of the time. Reconfiguring it as planned, I would expect it to run above the condensing threshold 70+% of the time.

The two existing circulators (lead/lag) are constant speed, and at least twice as large as they need to be. I will advocate a plan to replace them with two new, smaller VFD-driven circulators (lead/lag).

There is much in this heating plant that needs work - but IMHO, boiler repiping does not belong on that list.

Randy Baerg

maximizing mod/con boiler plant efficiency

There are several things to consider when trying to gain the highest efficiency from your Mod/con boiler plant. In no particular order:
1- Piping- The best method is to have the boilers piped in parallel, primary-secondary. IE The system is piped in a loop with return back to supply keeping your existing pump for building circulation. Cut in two large tees primary-secondary (within 4 pipe diameters apart) to the main loop, these two branches will be the supply and return to the boilers. Pipe your two boilers in parallel on these branches, each with its own pump. You will now have eliminated standby losses when only one boiler is firing. You can also gain some extra efficiency by pre and post purging the boiler pumps. (This also helps prevent short-cycling which can be an issue with this piping method.) Your existing piping in reverse-return can not provide these benefits.
2- Staging / modulation – Most Mod/con boilers are more efficient at lower temperatures and it is generally better to have both boilers running at a low firing rate than have one at a higher rate. Thus it is better to do parallel staging vs. sequential staging. Also you should have a control that is capable of matching the boiler firing to the load. Only fire one boiler if that is all that is required to maintain the temperature.
3- Temperature – By the low operating temperature you mention it sounds like you have some method of targeting a low operating temperature. If not then consider using an Outdoor Reset control to automatically lower the Supply target temperature as the heat loss drops in warm weather (add Indoor Feedback for even lower targets). Obviously the lower your operating temperature the more chance you will be condensing and getting the benefit of the latent heat.
4- Short-cycling – an important thing often seen in boiler plants is short-cycling which can have detrimental effects on efficiency and on component lifespan (probably more so). There are several ways to help prevent this from occurring. One is to have the differential automatically adjust to the load.
5- Warm weather shut down – automatically turn things off in warm weather.
6- If possible it is also beneficial to synchronize the building heat loads. Instead of random loads, it is possible, with newer technology, to coordinate the operation of zones so they place a steadier load on the boiler creating longer and more efficient cycles. This may or may not be possible depending on mechanical components and wiring.

Once you have decided on your piping there are controls available like the tekmar 275 that can parallel stage and modulate the boiler burners and pre-post purge the boiler pumps. Also provide Warm weather shut down do lower fuel and electric pumping costs.

Randy Baerg

maximizing mod/con boiler plant efficiency

There are several things to consider when trying to gain the highest efficiency from your Mod/con boiler plant. In no particular order:
1- Piping- The best method is to have the boilers piped in parallel, primary-secondary. IE The system is piped in a loop with return back to supply keeping your existing pump for building circulation. Cut in two large tees primary-secondary (within 4 pipe diameters apart) to the main loop, these two branches will be the supply and return to the boilers. Pipe your two boilers in parallel on these branches, each with its own pump. You will now have eliminated standby losses when only one boiler is firing. You can also gain some extra efficiency by pre and post purging the boiler pumps. (This also helps prevent short-cycling which can be an issue with this piping method.) Your existing piping in reverse-return can not provide these benefits.
2- Staging / modulation – Most Mod/con boilers are more efficient at lower temperatures and it is generally better to have both boilers running at a low firing rate than have one at a higher rate. Thus it is better to do parallel staging vs. sequential staging. Also you should have a control that is capable of matching the boiler firing to the load. Only fire one boiler if that is all that is required to maintain the temperature.
3- Temperature – By the low operating temperature you mention it sounds like you have some method of targeting a low operating temperature. If not then consider using an Outdoor Reset control to automatically lower the Supply target temperature as the heat loss drops in warm weather (add Indoor Feedback for even lower targets). Obviously the lower your operating temperature the more chance you will be condensing and getting the benefit of the latent heat.
4- Short-cycling – an important thing often seen in boiler plants is short-cycling which can have detrimental effects on efficiency and on component lifespan (probably more so). There are several ways to help prevent this from occurring. One is to have the differential automatically adjust to the load.
5- Warm weather shut down – automatically turn things off in warm weather.
6- If possible it is also beneficial to synchronize the building heat loads. Instead of random loads, it is possible, with newer technology, to coordinate the operation of zones so they place a steadier load on the boiler creating longer and more efficient cycles. This may or may not be possible depending on mechanical components and wiring.

Once you have decided on your piping there are controls available like the tekmar 275 that can parallel stage and modulate the boiler burners and pre-post purge the boiler pumps. Also provide Warm weather shut down do lower fuel and electric pumping costs.

Fixitman

Primary/Secondary

Randy,
Thanks for your reply. I agree with this, mostly. I'll reply point by point, though out of order ...

3, 4, 5, 6. Our DDC control system should allow us to fire the boilers at the minimum rate required to satisfy the worst zone. IMO that's a more efficient method than outdoor temp reset, which typically allows the hot water setpoint to go higher than the worst zone requires. We will also fire the boilers only if at least one zone requires heat. This is also an improvement over the typical outdoor temp lock-out, which often has the boilers firing when there is no demand at all.

2. This description matches pretty well the way our controlsy alread fire the boilers.

1. In our case the branching on the reverse return piping is very short, and well insulated. All the boiler piping is very compact. It's my guess that we can't justify a conversion to primary/secondary with the savings realized by the conversion. I'd expect that payback to be quite long. I might well go for primary/secondary if I were building this plant from scratch. However, I have some arguments with primary/secondary as it is often executed - especially for water source heat pump installations. Typically, Boiler 1 steals some water out of the header, and puts it back in a few pipe diameters downstream, as you describe. So, when Boiler 2 fires, it steals some water from the same header, downstream from Boiler 1. As a result, the "return" water going into Boiler 2 is warmer than the water entering Boiler 1. Boiler 2 operates less efficiently as a result. Worse yet for Boilers 3, 4, 5 ... Likewise, any flaws in the heat exchanger design that dislike cold inlet water will likely show up much earlier in Boiler 1 than the other boilers. This is true even if the firing order is rotated. A way around this problem would be to split the header going past the boilers and rejoin it again after it goes past. One of the split streams provides "return" water to the boilers, and the other stream accepts the supply. All boilers then get the same inlet water temp, no matter what the firing order or number of boilers operating. Note that this arrangement is actually fairly similar to ... reverse return. Kind of a hybrid of the two.

Randy Baerg

piping/control clarification

Fixitman,

Your points are well taken.
Here are my thoughts to your reply, for what they are worth...

Points:
3,4,5,6 - I agree, if your DDC has a method of tracking the worst zone requirement, that is better than straight outdoor reset which may or may not be dialed in perfectly. I often add an Indoor Sensor to the control to provide 'indoor feedback'. This allows the reset to be further modified by the building temperature and how far away it is from setpoint to provide the perfect supply temperature. This sounds similar to what you're doing.

I agree it is best to try and use an interrupted demand, so the control only fires if at least one zone is calling. This is definitely the preferred method to reduce costs of heating the water and also pumping it. This interrupted demand is usually provided by end switches wired in parallel to the control demand. For even better operation the operation of the zones can be 'Synchronized' which eliminates the random calls from various zones. By synchronizing the calls you can dramatically reduced boiler short-cycling which increases efficiency and reduces wear and tear on the boilers.
The other thing I might add here is that in some larger buildings with long runs and large mass the wait time for heat with interrupted demand can be too long. This of course is mitigated by the fact we are running hopefully at the lowest possible temperature. In this case the coldest zones should be calling pretty much all the time.

Unfortunately in many buildings the end switches and wiring are not available for an interrupted demand, especially in retro-fits, so you are forced to go with a permanent demand.
You can still provide a warm weather shut down which can help quite a bit.

1. Perhaps my piping explanation lacked some clarity. The method I described was not to pipe each boiler primary-secondary individually into the main loop. In this case you are absolutely correct to point out that the boilers then get a progressively hotter return temperature which reduces condensing benefits. The method I described (or tried to!) is parallel primary-secondary. That is, all boilers are piped in parallel to each other with their own pumps. The supply and return manifold serving all these boilers is then piped into the main loop, ala primary-secondary. There is only one set of pri-sec tees tapped into the main loop. This provides the hydraulic isolation between the main loop and the boilers and allows each boiler to get the same return temperature water.

Hope my ramblings make better sense this time.

Unknown

I just want to make sure there is not a misunderstanding.

#2, parallel staging is a staging control, and it should be done so that flow does not go through all boilers all the time as you are currently set up. Basically parallel staging starts with one boiler, like regular staging, but when the boiler demand is high enough to bring on two boilers at a lower, consistent firing rate, it will do so, rather than waiting for one boiler to max out (sequential staging).

Your current setup would, if fired like that, have one boiler heating the other 4 boilers in addition to your load... not efficient. so the repipe is necessary to do proper paralell staging.

Fixitman

It's my take that there are two ideas here, each of them worthwhile. Parallel staging, as you described it, is one idea. The idea of isolating the boilers that are not running is another idea. Each is a worthy goal on its own, and they are complementary.

In my opinion, it is quite worthwhile to do parallel staging, even without isolating the boilers that are not running. We are discussing isolating the non-running boilers using a butterfly valve at each boiler. The particular boilers in our installation have air/fuel pathways that won't allow much convection through the boiler. The vessels and piping are well-insulated.

I think the losses through piping branches and the boiler are likely to be pretty low - especially at our fairly low operating temperatures. Ideally, I'd like to see each boiler with its own circulator, or an isolation valve of some sort.

It remains to be seen whether the budget will allow for either those upgrades. I'm not yet convinced that either one has good payback. There are many problems with the existing system that will pay back more quickly.

Fixitman

It's not easy to describe some of these piping arrangements verbally - especially because there is no completely agreed-upon naming system for different arrangements. Thank heavens for drawings.