replacing 1 large boiler with 2 smaller ones

dronic123

We are thinking of replacing our cast iron (C.I) atmospheric boiler with 2 smaller boilers. This is for a multi unit building where the tenants have been complaining over the years that it is too hot in the late spring and early fall. It seems difficult for us to control the heat with this single (

large) standard C.I. and we thought that we should take advantage of the change to try to modulate the heat output.

So we figure we could manually switch on the boilers—a 100K boiler in the early fall and late spring, a 200k boiler in the early and late winter and both boilers in the depth of winter. Perhaps there is a way to do this rather automatically with some outdoor resets?

We have have looked at a single modulating boiler but we really like the idea of the simpler boiler(s), which we can service ourselves at our own convenience. In addition, if one of the boilers goes down, we will still have heat in the boiler system. So the idea is to use 2 Mid efficiency boilers to create the modulation.

Can you help and comment on the piping design and any other considerations we should look into?

Particularly—do we have the mixing valves and the boiler circulators in the right spot?

We are moving the system circulator to the supply side. Does it work better there?

hot_rod

the piping is correct. Tekmar has a number of boiler staging controls, compare here.

https://www.watts.com/our-story/brands/tekmar/multi-staging

bburd

It sounds like you have a control and/or zoning problem, rather than a boiler capacity problem. If you replace one large boiler with two both plumbed to the same system that serves both apartments in common, this will not solve your issue.

If you install a separate boiler for each apartment as its own zone,with its own thermostat, that should fix it.

Beyond the boiler, how are the radiators piped?

PC7060

…and what type of emitter are you using. Seems like a good opportunity for modulating boilers especially if you have high mass radiators.

Kaos

As @bburd said, this is a controls problem not a boiler problem. If you existing boiler is in good shape, there is no reason to replace it.

You need outdoor reset control with boiler protection. This will adjust the heating loop temperature based on building load so people won't overheat in the shoulder season. Bonus is it prevents banging and clanging from pipe expansion as the heating loop temperature is kept even. Even bigger bonus, lets you run your boiler at lower temperature a good part of the heating season which saves fuel costs.

https://www.contractormag.com/hydronics/article/21155243/look-like-a-pro-with-reset-control

Steamhead

https://forum.heatinghelp.com/discussion/comment/1818365#Comment_1818365

This.

@dronic123 , where are you located? We might know someone who can help…………….

dronic123

@bburd: boiler is leaking slightly; we need to get ready

We could replace it with a 300k CI boiler, outdoor reset and a boiler protection loop as you suggest but feel it would still run too hot in the spring and fall. Even with a boiler protection loop, the boiler will still run short runs. So everything is the same with a 100k boiler—it will just run longer at a lower heat output—-just like a modulating boiler. Wouldn’t that be a good thing?

dronic123

@PC7060: thanks for your comment. We really would prefer to avoid the modulating high eff boiler and stay with the mid eff boiler(s) which we have had good luck with over the years and we can service it ourselves. But the question is —-can we create some manual modulation by using 2 boilers instead of 1? What do you think? Will it work to help with the modulation?

dronic123

thanks for your comments

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1818351#Comment_1818351

"Multi unit" may mean more than 2 units. If this is a 10 unit apartment building, then are you suggesting 10 boilers? That could get costly. I believe that outdoor reset is the best answer. Perhaps TRVs on the hotter units could also help. Need more information about the building would help.

If you have a one pipe monoflo® system, then the TRVs are off the table.

I just helped someone with a Lochinvar 2 boiler system that has a built-in communication system for multi boilers. They are modulating condensing boilers and therefore can operate at very low temperatures without fear of low temperatures causing problems associated with a CI boiler. The outdoor reset is built into the boiler controls.  This system does not need additional Tekmar controls to get where you need to be.

And I believe that 2 smaller boilers are better than one large boiler.

If you need a new boiler anyway, go for the 2 boiler option 

dronic123

@EdTheHeaterMan: yes many small units; probably originally a 100 yr old, gravity fed, coal/oil/ now gas boiler; one huge pipe in and one huge pipe out, (probably monoflow tee’s? At that age?—100 years old?)

Can you just use an outdoor reset ( 2 of them? ) to block or or energize the electrical feed to the boilers? How do you adjust the temperature on the outdoor reset—-on the device itself?

EdTheHeaterMan

one huge pipe? are we talking 2" or are we talking larger?

A 2" pipe with a circulator can move up to 500,000 BTUs of heat per hour when there is a 20° temperature drop from the supply to the return. 2" Copper pipe 450,000 BTUh

Gravity pipe sizes are much larger, but a gravity system will not work in a tall building. 3 story above the basement at best. And over 100 years ago, it is unlikely that you have monoflo® tees.

So how many small units? more than 10? How many radiators total, over 30? Are they cast iron radiators? The TRV idea may be of interest to you.

dronic123

@EdTheHeaterMan: this building is flat roof, 2 storeys plus basement with units in the basement; boiler is in a pit 8 ft below the basement; approx 10 units; both pipes covered in asbestos?; probably 3 or 4 “ main pipes ;with big flanges; currently a 500k American standard; a lot of new air infiltration prevention, new windows, doors, insulation. We are guessing at the 300k but we could adjust as we get more heat loss info; probably 40 water style tube rads. If we need more heat we could go to 150/280k but the issue is : the 2 boilers vs 1 boiler. For that matter maybe we could do even better with 3 boilers! The boiler room is huge (probably there was an original coal gravity boiler) so there is lots of room for a second boiler. In addition we will have to lower the boilers on ropes and 2 boilers are easier than one twice as heavy. So we think this 2 boiler set up would solve the problem of the modulation with the simplicity of the mid eff boiler. You have to climb down into the pit on a 12 ft ladder so tech support is limited.

PC7060

https://forum.heatinghelp.com/discussion/comment/1818377#Comment_1818377

As Ed mentioned, ODR is a good option with built in control present in many CI boilers.

dronic123

@PC7060 : I can see how a loop would work with a mixing valve and an outdoor reset (I think) but are we not still faced with a shorter heat cycle if we use a large vs a smaller k boiler? Wouldn’t, it be better in the spring and fall to use a lower k boiler so it would turn on for a longer time and put the heat in the rads more slowly? I thought that was one of the best features of the modern hi eff boilers?

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1818410#Comment_1818410

Your suggestion of 3 simple boilers is better than 2 different size boilers. The need to automatically change from the small boiler when outdoor temperature is between 45° and 60° then change over to the other boiler when the temperature is between 30° and 44° then all three when below 30° sounds good in theory, but to manually do that change is not something you want to depend on, and the automatic control would leave a lot of wasted heat in the transition between 44° and 45° when abandoning the small for the large then back to the small every day in November and December then again in February and March. Just look at any daily forecast for those months and see how many times the high is above 45° and the low is below 44° on any given day. That is probably going to happen about 33 days out of each of those months (Except February maybe only 32 days in that month 🤣).

I toyed with that same idea over 30 years ago and decided that the change over would be less efficient than boilers of the same size that would keep operating when the next stage is needed. Just swap which boiler is primary every other cycle…. That is called Lead/Lag and Tekmar has a control for that. So three 100,000 boilers is better than two boilers of different sizes. IMHO

dronic123

@EdTheHeaterMan : yes I like the idea of 3 same k boilers. We have lots of room for them. How does the tekmar control work? When the “second” or “third” boiler would turn off could we make its boiler circulator still run to get the heat out of it? I guess the problem is that the input water to that boiler is always going to be hotter???

EdTheHeaterMan

one design might be to use a 4 way mixing valve to feed the system (all the radiators) with outdoor reset temperature water. That would involve a Tekmar 360 control to operate the 4 way valve. Now that is going to keep your radiator system form overheating the building. that can be connected to a single boiler system.

The 2 or 3 stage boiler system then is controlled by a Tekmar 260 for a 2 boiler system or a Tekmar 275 for a 3 or 4 boiler system. The 360 will send the call for heat signal to one boiler or to the Tekmar multi boiler control (260 or 275) to do the staging of the boilers. I do not see a benefit of a purge cycle on any particular boiler when it is operating in conjunction with another boiler, because that non functioning boiler will just be acting like an additional radiator on the system that is connected to a chimney to vent heat added to that boiler by the operating boiler.

There are other ways to do this, but this is going to fit your needs to use simple boilers that you can service and operate with in house staff. I will work on a piping diagram and wiring diagram to connect all these parts together. Do you have a boiler preference? I will include that wiring diagram in my mock up, otherwise I will just put a boiler with T T to connect the control wires to.

EdTheHeaterMan

Since there is less than 450,000 BTUh net going to the building system, the mixing valve will be a 2” three way valve This valve needs a motor to automatically open and close the valve as needed to maintain the mix temperature to the radiator system The motor for this valve is a Tekmar 743.  In order for the valve to know what position will deliver the proper system temperature and also protect the boiler from low temperature return water I specify the  Tekmar 360 to operate the mixing valve 

As I said before, that 360 can operate a single boiler, but in your case we will be sending the call for heat to the boiler to a Staging control. Tekmar 275 can handle 2, 3 or 4 boilers and will also operate the boiler(s) at a reduced temperature with a minimum return water temperature.  So you have 2 ODR controls.  One for the boilers that will not go below  a 130° return water temperature. And the other to reduce that temperature to the radiators in order to keep them from overheating when the minimum boiler temperature is too high.

This may take some time to design the whole thing so look for my design in a few days.   Here are the links to the Tekmar parts.   It took me a year to understand the illustrations and how to actually connect the wires, back over 30 years ago.  But once I understood them, I have been able to make easy to follow wiring diagrams for others to follow.  

• Here are the links to the Tekmar parts so you can begin to source them.  https://www.supplyhouse.com/Tekmar-724-2-Brass-4-Way-Mixing-Valve-4873000-p
• https://www.supplyhouse.com/Tekmar-743-Actuating-Motor-for-34-Way-Mixing-Valves
• https://www.supplyhouse.com/Tekmar-360-Floating-Action-Mixing-Control-4173000-p
• https://s3.amazonaws.com/s3.supplyhouse.com/product_files/275%20install.pdf.
• the replacement for this control is the Tekmar 294 if the 275 is no longer available
• https://www.supplyhouse.com/Tekmar-294-TEKMAR-Boiler-Control-One-tN4-Four-Stage-Boiler-DHW-Setpoint

Do not purchase anything until others critique my diagram and piping design. @hot_rod and others may have a less expensive ideas.  But some are all going to push the ModCon boilers that have this stuff built in.  That is what I would do in your situation. But that's me.

dronic123

. @EdTheHeaterMan :thank you very much for taking an interest in this project

EdTheHeaterMan

This would be your piping diagram according to Tekmar. All 2" copper, except where the individual boilers are connected in 1" copper to the 2" shared piping. The 2" 4 way valve will add the boiler protection needed for the return piping BUT having the Boiler mixing valve to ensure the return gets up to 130° ASAP, won't hurt. I like. the Califfi 280165A valve set at 130°.

Click on the picture to zoom in for details

The basic wiring logic is on the piping diagram however the nitty gritty diagram with all the terminal numbers is a little more involved. I just wanted to post this to see if there are nay other ideas out there. I did not include the fill valve location however I believe it is best located on the 2" common pipe just before the P1 circulator and the temperature sensor probe. The CST on the diagram are Closely Spaced Tees not to exceed 12" about 6" should be just right

Kaos

I would stage this upgrade. You can install the outdoor reset bits @EdTheHeaterMan graciously sketch up there to the existing boiler. You will need these even if you replace the boiler down the road, might as well get it in now to fix the comfort issues.

Run the system as is and get through a heating season with the building envelope upgrades. You can then do a fuel use based heat loss calculation:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

This will tell you how big of a boiler you need. A 10 plex is not that large, there is no way on green earth that each unit has a 30000BTU heat load. My guess you'll find that you can get away with a single larger residential boiler for the whole place.

EdTheHeaterMan

@Kaos , I believe you missed a few posts here. @dronic123 has a single boiler in a 100 year old multi family building, that is showing signs of a leak. To me, that means it is time for a replacement. The management wants to stick with cast iron atmospheric. They are simple to work on. That way, in-house staff can do most of the repair work. The original query was to use 2 boilers of different sizes to equal 300K NET. Load calc already completed. The old boiler is 500K NET. The multi-stage new to equal 300K NET

There is no existing control system that will stage a small boiler then large boiler then both boilers to get three different firing rates. So I suggested three small boilers to make three stages of boiler input. Tekmar already has that control system, so no reinventing the wheel.

The problem that @dronic123 wants to solve with this 3 boiler design is to keep the system from overheating the apartments. I believe, as do others, that the overheating will not be solved by the staging of boilers alone.  There are over 40 radiators in the building so TRVs can get costly. Outdoor reset is a better option IMHO. So I thought a Tekmar 360 to control a 4 way valve, and a Tekmar 275 would be the perfect match up. But I came to find that some of the suppliers have discontinued offering that control in favor of the newer Tekmar 294 staging control. 

Now that you are up to speed, I was looking for other options to do three cast iron boilers and offer ODR to the radiators.   I have offered injection pumping systems in the past but never actually installed one.  I did have one oil burner client that has the 4 way valve for their ODR system and remember that they were very satisfied with it.  

Just looking for suggestions to connect an old gravity radiator system that was converted to a forced hot water system using one boiler 20 or 30 years ago.   That system is overheating the place and I have this as a starting point for others to comment.   Any thoughts?    

hot_rod

a 4 way mix valve has two mix points, so it should give you ODR temperature control as well as accurate return temperature protection.

With the proper control and sensor location.

Doesn’t tekmar have a 3 boiler staging control?

Kaos

https://forum.heatinghelp.com/discussion/comment/1818500#Comment_1818500

Assuming the OP is somewhere in the north west, say 6000 HDD days with a 5F design temp.

If we start with 300kBTU/h that would put the place at 5kBTU/h per heat degree.

So in a year would need about:

6000*24*5000=7200 Therms of heat.

Assuming this is supplied by an 80% boiler, that is about 9000Therms/heating season. So if the OP uses about that much than 300k is correct. My guess based on similar place in colder climate (1907 12 plex with no insulation), the number is about 1/2 that.

No need to guess boiler size, calculate. A single right boiler with outdoor reset will be a heck of a lot cheaper install.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1818533#Comment_1818533

thanks for your insight

The Steam Whisperer

How I approach these is:

1. Get a realistic design heat load. I usually work back from the historical fuel usage to get this number.
2. Typical winter design heat loads only require 60% capacity for about 90% of the heating system.
3. I use this as the design target for the first stage boiler.
4. You will usually also find that this capacity is about what you need for freeze protection is you are down to a single boiler when at peak design conditions ( Hold the building about 45F at 0F, in my area, Chicago)
5. Install a first stage boiler with this capacity, often leaving the original boiler for use only a couple days a year during the extreme temperatures

or

Install 2 boilers each at 60% of design load for full back up.

7. Set up the boilers with return temperature protection piped in paralell with each other, but P/S to the system main. If doing full outdoor reset of the system, use return mixing valves (i.e. Esbe) or if doing partial outdoor reset, manually set bypass on the boiler. This really depends on the peak operating temperature of the system. If you really need 180F water at design ( extremely rare) then mixing valves probably will be needed. However, if the design temp of the system is much lower, a manual bypass is often all that is needed.

For example, my church has a 1950's steel pipe in concrete slab radiant system. Design supply temperature for the system is very low ( well under 100F, especially with the added insulation and new windows), so I use a fixed bypass on the boiler, with the boiler piped P/S to the system loop. The bypass sends nearly 100% of the supply water back into the boiler and only a small amount out to the main system loop. Since almost all the supply goes back to the return, the boiler warms up very quickly and then as it gets up to temperature, the delta tee between the boiler loop gets high enough to start transferring significant heat to the system loop. The boiler generally will be running about 180F, while the system loop is about 80F.

I wouldn't worry too much about the losses that may occur when on the edge of the capacity of the first boiler and need to bring on the second stage( like on cold nights) because the btu stored in most package gas fired cast iron boilers are quite low. For a 150,000 btu model its only about 1.5 to 2.5 gallons of water and 350 pound of cast iron, which only hold about 1/10 the btu per pound of water. Using a 2 gallon boiler at 350 pounds of cast iron, that's about 52 btu's/degreeF to change the boiler temperature.

With a well sized 2 stage atmospheric gas cast iron heating plant with outdoor reset, fuel savings over a previous typical single oversized atmospheric cast iron boiler installation typically run 30 % or more. This savings level and the simplicity and long life of cast iron atmospherics is why I don't think condensing boilers are worthwhile in applications where staged boilers can be used.

dronic123

@The Steam Whisperer : thanks for your comments and comprehensive suggestions

I am a bit of a newbie —-can you fill in some details about full vs partial outdoor resets

in my diagram at the top post I tried to do P/S and parallel boilers using CSTees and hydronic ( non powered I think) mixing valves for the boiler protection loop. Would you have any comments or suggestions? is that the way you would do it? Would you in your example put the 60% boiler first or second in line from the return?

When you take the mixing valve out of the box and try to install it does it install directly the way I have shown it ——A B C ports——or do you have to manipulate the piping around the ABC ports?

The Steam Whisperer

Partial outdoor reset is resetting the supply water temperature down to the lower limit the cast iron boilers can operate at continuously. This is typically about 130F for small package atmospherics ( 110-115F return) , but there are commercial boilers that can go down to about 90-100F. Full outdoor reset would be resetting water temperature right down to the room temperature in warm weather. Most older systems, particularly radiators, don't need supply temperatures above 140F at design, so partial reset is often useless unless the system is zoned. This particularly holds true for multi-unit buildings because the internal heat gains tend to be quite high along with the heat loss being relatively low per sq ft of space. Here in Chicago, the old 1880 to 1940 3-story brick multiunit buildings usually only need about 15 to 20 btu/ sq ft at 0F outdoor. You have to remember the system is sized to heat the building assuming there are no internal gains and the sun never shines… so way too big for typical usage of residential buildings.

To pipe the boilers in parallel, the supplies are joined together before reaching the supply tee and returns are joined together before reaching the return tee (P/S tees). A check valve is installed on each boiler supply where they join together. This piping arrangement keeps the return temperature to each boiler the same, which can be important if the system has to run at high supply temperatures.

For the mixing valves, we have used return mounted ESBE thermostatic valves for these little boilers and just follow the manufacturer's instructions.

hot_rod

this is a good option for piping multiple boilers. The separator give you 4 important functions, air, dirt, magnetic, and hydraulic separation. An ECM delta P circulator for the zoned heat emitters on the other side of the sep

IMG_0806.jpeg

dronic123

@EdTheHeaterMan , @The Steam Whisperer, @hot_rod : thanks for all your recent comments. Give me a moment to reflect and digest these 3 excellent ideas.

dronic123

@hot_rod : because the three boiler circulators are not in series you don’t have to isolate them from each other? And the LLH is able to separate all 3 from the system pump?

I had originally set the 2 boilers in series in my top post because I didn’t know any better—thanks for the diagram. Is the any situation where the boilers would work better in series?

EdTheHeaterMan

In your original diagram, the boiler pump is hydraulically separated from the system by the closely spaced tees. Likewise each boiler pump is separated from each other by the closely spaced tees.  And the system pump is hydraulically separated from the boiler pumps by the closely spaced tees. 

The closely spaced tees are at the arrows that say "Webstone CCT Purge". You actually got that right. Good design. But there is a better design. Here is a video that indicates that better design. (around time stamp 46:40)

https://www.youtube.com/watch?v=EZL94XKTQoM&t=1425s

dronic123

@EdTheHeaterMan ; @hot_rod Ed, the diagram in the "lecture" looks similar to the diagram above put up by hot-rod—-plumbing the boilers in parallel—I guess. so the question is: whether there are any circumstances where plumbing the boilers in series would have an advantage? Perhaps if you were using a copper finned boiler first (that didn't need a boiler protection loop) and it would heat up the water to protect the cast iron boiler?

I drew it up again in parallel

do you think we could get along with one boiler protection loop to the “right” of the LLH? I don’t known how it could fit with a 4 way mixing valve to keep a lower water temp in the shoulder season.

dronic123

@EdTheHeaterMan : I was having trouble with your drawing
I guess if the mix sensor reads too hot —it pulls in the return water

And if the boiler sensor reads too cold it draws in the new hot

supply water and sends it to the boilers if you can help me here as if I were a cocker spaniel!

To work does it have to be to the left of the CSTees?

The Steam Whisperer

With the low loss header diagram with mix valves on the boiler returns, all you need is a simple outdoor reset and staging control… No 4 way system mix valve system required.

On your mix valve diagram, I see you still have mix valves on the boilers… I am guessing an oversight. With this setup, you'd need a boiler return water sensor ( since boiler return temperature is the limiting factor, depending on flow rate into boiler) and then system supply for outdoor reset. I suspect the control would make boiler return priority over system supply.

However, without mixing valves on the boiler returns, the boilers can be hot thermal shocked when the boiler pump first starts up….. a boiler on stand by at 65F can be hit with 180F return water at full flow.

Copper fin boilers do not like low return temperatures, they condense very easily on the tubes closest to the return connection. They are probably better with thermal shock than cast iron, however.

hot_rod

piped in parallel they all see the same return temperature as SW mentioned, I prefer that with non condensing boilers, compared to the close tee method where temperatures are different at each set of tees

EdTheHeaterMan

After looking closely at the diagram I made for you earlier, I thought about a major flaw that is built into the design.  When you place boilers on a primary loop in series, the first boiler on the primary loop must do a lot of work because it has the coldest return water.  The second boiler does less work because the return water for that boiler is being heated by the first boiler, and so on down the line.  See the design with hypothetical temperatures added to the diagram.

You can see how the first boiler is always going to be the workhorse whenever two or three boilers are operating.  The second and third boilers will then short cycle because the return water temperature is so high, there will be almost no work for them to do.  

The better design is to put all the boilers on a manifold that is connected to the primary loop by closely spaced tees.  Like this. 

See how all three boilers get the same temperature water off of the manifold? So if only one boiler is operating or all three boilers are operating, the return water will be the same for each.    

Here is a video that explains this issue

https://www.youtube.com/watch?v=CKyJcYlfC7Y

You can do the ODR for all the radiant loops with the 4 way mix valve. If you are interested in getting a slightly higher temperature to the air handlers… you can do a second set of CST off of the primary loop just before the radiant floor loop(s) as shown here.

Set the ODR curve on the Tekmar 294 to handle the higher temperature needs like 140° to 150° for the air handlers and the DHW priority 180° . Set the mixing valve ODR to the radiant floor needs. This control wiring design will do what you need and reduce you fuel usage by over 30% IMO.

dronic123

@EdTheHeaterMan : can’t thank you enough for your interest— very very helpful.

Looking at your new diagram—if we didn’t have an air handler, could we eliminate the middle loop and the circulator/pump (and one set of CSTees)?

I set up my first drawing with Individual boiler protection loops and mixing valves (I thought non-powered hydronic temp sensitive mixing valves) —-now with the 4 way mixing valve, could we drop the individual boiler protection mixing valves?

Or could we leave the individual boiler protection loops (3) and just use a powered 3 way valve to control the system water temp?

EdTheHeaterMan

When using the powered mixing valve (3 way or 4 way) the device that powers the motor on that valve will be doing the ODR for that temperature regardless of the boiler water temperature setting. So you need that control anyway. The control that will stage three boilers can be set without using the ODR feature by setting the curve to maintain 150° or 160° regardless of the needed temperature, but you need the outdoor sensor so the control knows when 2 boilers are needed or all three boilers are needed.

With this in mind, I had an afterthought. Since the boiler staging control has ODR capability anyway…. Why not use it to make a medium temperature primary loop then use it for the air handlers. You don't need to do it, but the capability is there if you want it. The piping design won't change. The only difference is to use some 1" Pex to move the Air Handlers from the low temperature loop to the medium temperature loop.  This will be so super efficient this way and get you the comfort you need on those ubercold days.

I like the non powered boiler protection valves because you may never need water temperature over 120° on the radiant floor system most of the year.  that will leave the return water at less than 110°.  That would mean that the boiler may never get a high enough temperature to create 130° minimum return water. By placing the Tekmar sensor on the 2" primary loop, that water temperature can stay at 120° or 130° while the boilers ramp up to 140° to get enough temperature for the bypass to close a little and send some hot water out the the Primary loop. 

The hypothetical temperatures I posted on the recent diagrams were based on a very cold day. This illustration shows what a mild day's temperatures might look like. 

The Tekmar sensor on the primary loop may stop the boiler from adding any more heat once the primary loop sensor reads 110°. Or it may keep it on if that one boiler can maintain the needed temperature without shutting off.  That is what the sensors do: Cycle on and off to maintain a temperature. When one boiler is not enough then that one boiler will stay on non stop and the second boiler will cycle on and off. Until it gets warm enough to stop or cold enough to bring the third boiler on.

EdTheHeaterMan

You will need to include Flo-Check valves and I added the indirect. if you use 1-1/4" piping that you can operate 2 boilers for faster recovery. If you use 1" then you only need to operate one boiler and the Tekmar will figure that out for you with the proper settings.