replacing 1 large boiler with 2 smaller ones
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?
Comments
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the piping is correct. Tekmar has a number of boiler staging controls, compare here.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
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?
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Bburd
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…and what type of emitter are you using. Seems like a good opportunity for modulating boilers especially if you have high mass radiators.
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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
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This.
@dronic123 , where are you located? We might know someone who can help…………….
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting0 -
@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?
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@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?
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thanks for your comments
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"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
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@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?
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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.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@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.
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As Ed mentioned, ODR is a good option with built in control present in many CI boilers.
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@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?
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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
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@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???
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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.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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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.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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. @EdTheHeaterMan :thank you very much for taking an interest in this project
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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
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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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:
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.
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@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?
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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