3 Boiler piping

Wrot

Good Day,
I'm looking at removing one boiler and installing (3) oil boilers in a commercial building. There is already a 3 way valve on the existing system which is controlled by a building control system. The existing boiler water temperature or burner is not on the control system, so I was going to add a 3 stage boiler control. I sketched up a few drawing, but now I'm not sure which one would be the best option. The existing piping is 2" steel pipe.

STEVEusaPA

First a proper heat loss to make sure you aren't putting in 3 oversized boilers. Unless you're goal is to size to 2 boilers and rotate 3, leaving a spare.
I would do it more like this
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_19_na.pdf page 66, diagram 7-13b

Wrot

Heat loss was done, 500.000 Btu, so 3 boilers @ 167,000 Btu will go in and be staged as needed. The 3 way valve is my concern, P1 & P2 are powered all the time. #1 drawing would be like a Primary/secondary system, with each boiler pump & burner starting as the load increases. #2 Primary/secondary with a reverse return pipe configuration on the return piping. #3 is reverse return with water flow through the 3 boilers and the staging control would start the burners as the load increases, but the downside of #3 is the increased standby losses.

newagedawn

dan holohan has a good book on piping more than one boiler, but im getting a blank to the name, its to early,lol

EBEBRATT-Ed

I am big on primary secondary. #2 would be my choice. #1 puts the load on the first boiler. #2 balances the load better between the boilers. #3 I don't like

Today's newer boilers hold so little water I like using a pump on the boiler to maintain constant flow through the boiler when it is firing. Stopping the boiler pump means no standby loss on that boiler

JMHO

Wrot

Well, won't #1 also be considered primary/secondary loops? Each boiler is a primary loop being picked up by a secondary loop? As for the first boiler carrying most of the load, the staging control will take care of that by alternating the boiler firing sequence. Just because the boiler is first in line doesn't mean it's going to fire up first. #2 is a reverse return piping arrangement. But does it have to be? The circulator pumps are being controlled by the boiler control. So is it really necessary to spend more time and money piping #2.

lchmb

couldnt you do #2 and remove the mixing valve? Put a control on to maintain outdoor reset on your supply and inject in with the boilers as needed? My concern with number 1 is if the mixing valve is in full bypass how will the water flow evenly in the system? Would seem like flow would stop at that point? Maybe I'm wrong and just cant see it..been a long week

Wrot

Yes there will be times when the valve may be in full bypass. but remember that the building control system is controlling the 3 way valve, so if the valve is in full by-pass, the zones should be up to temperature.

EBEBRATT-Ed

# 1 is primary secondary but the return water hits #1 boiler first. #1 boiler will be preheating the water for boiler #2 etc. It will work but not as well.

Dwg #2 all the boilers get the same return water temp. works better

lchmb

Still think I'd go with option 2...

Wrot

Ok, I added in the pipe sizes. I suppose that in #1 some flow from the first boiler would migrate into the return of the next boiler, but only when that burner if firing and the boiler pump is going. If 2 boilers are firing there would be no flow going into #3 boiler because P5 boiler pump isn't running. With 2-1/2" piping moving approx. 80 gpm, how much of 16 gpm from the first boiler is going to flow into the next boiler when #2 boiler pump starts? Also with the boilers on a rotation P5 pump will be the first stage.

Wrot

lchmb said:

Still think I'd go with option 2...

#2 will work and give me the 500,000 btu's i'm looking for, will #1 give me the same 500K btu's?

lchmb

maybe..but I think it will be harder to balance.. #2 to me just seems like it will be easier to control for temp

Harvey Ramer

#2 Is the way I would pipe it.

captainco

I am pretty sure #1 won't work at all. Saw a job like that once where a single boiler was replaced with three boilers. Piped like #1 and there gas bill doubled!!

bob_46

Piped a bunch like#1 with good result. Added a flow control valve to each boiler. Lead boiler on outdoor reset with water temp sensor on supply main after last boiler. #2 boiler on outdoor thermostat set at 40º, boiler would run continues below 40. #3 on outdoor thermostat continues below 10º. Outdoor design temp -10ºF.
Three position wafer switch. We would change position once each year to switch boilers to even wear. These systems were designed and supplied by Triad boiler back in the 60's .

Wrot

I'm still not convinced that #1 piping wouldn't work either. One of my concerns it that will the system be delivered the full 500K of the boiler output.The 1-1/4" boiler piping is being pumped up into the
2-1/2" loop piping. The 3 way valve will be controlling the amount of supply water flow going to the main building loop and mixing with the return water back thru the boiler loop where a percentage of water will be reheated.The outdoor reset boiler & boiler staging control will also be staggering the boilers operation.

RayWohlfarth

I would be very careful when using a 3 way valve as it could create a Thermal Shock to the boiler. Most boilers are designed for a 20-30 deree Delta T. I would suggest a blend pump which takes water from the supply and injects it into the return. This preheats the return water and limits the delta T. You could also use a bypass pipe and manually adjust the bypass flow. I would run your piping past the boiler manufacturer so you have youself covered. You could look at the suggested boiler piping from the boiler manufacturer.
Good luck
Ray

Zman

What type of heating system is this? What are the emmitors?
What is the 3 way valve's function? Is it protecting the boiler from thermal shock/or boiler condensation or is it serving as a delta p bipass for the system pumps?

I would first figure this out, then come up with a design. If it is serving a delta p bipass function, freq drives would save lots of electricity.

Whatever you do, do not ignore the BAS. I should be easy enough to have someone familiar with that system tie your new boiler in.

Wrot

There is (1) existing boiler now which just operates at 190*F water temperature, The control company's system, which includes outdoor reset, (but not on the boiler), operates the 3 way valve. It will adjust the 3 way valve to maintain a certain water temperature at a given outdoor temperature. The 3 way valve will protect the boiler from thermal shock as well as the individual boiler pumps. When the temperature in the boiler drops below 140*F the pump will shut off, let the boiler temperature rise and then the pump will start again. This is a basic feature of all the outdoor rest controls that I've seen. So the function of the 3 way valve other than how it operates is of no concern to me, i have no control over it. It controls the loop temperature on the old boiler and it will control the loop temperature with the new boilers. I do know that i need 500,000 BTU to heat the building on the coldest day of the year. So are both #1 and #2 drawings going to give 500K btu to the system? If I didn't have a 3 way valve, then I would say #2 is definitely the way to go. But when the 3 way was to be 100% open, full flow will be going through the 2-1/2" pipe above the boilers, at this point 3 burners would be going and the boiler pumps will be running unless the boiler temp drops below 140*F.
So say that it's -4*F out and they run out of oil because the oil company forgets to fill the oil tank. On start-up, the way the 3 way valve works is, it opens to say 0% to loop and 100 % bypass. As the loop heats up, the 3 way move to 10% loop & 90% bypass, and so on until it builds up to say 80% loop and 20% bypass. So knowing this, does it really matter if the piping arrangement is #1 or #2? Will either piping arrangement limit the btu output of the boilers & not give a total output of 500K btu? Will either one cause an increase in fuel consumption over the other?

Zman

Well,
My issue with the setup is that the 3 way valve is not really changing the temp to the emmitors, it is just changing the flow rate to them. It is not the the right way to do outdoor reset. The end result is just an imbalanced system do to the low flow rates.
The right way to deal with this is to work with the BAS people and have their system actually turn down the boiler temps and repipe so that the flow rates are constant. If lower than condensing temps are desired, you could use a mixing valve between primary secondary.

If you are just interested in a boiler swap, option #2 will work just like the existing setup.

Option #1 will create different supply and return temps to each boiler and option #3 with pumps in series is a bad plan.

Wrot

You are correct, in any of the drawings, the only thing the 3 way valve can do is divert flow away from the main loop. The way that the 3 way valve is set up here, the only thing it can do is protect the boiler from thermal shock. With the existing boiler running at 190*F, no matter what position the 3 way valve is in, the water making it out to the loop will always be 190*F. The new outdoor reset control will protect the 3 boilers from thermal shock. So really the 3 way valve could just be locked in the full flow position with 0 by-pass. The emitters in the building are mostly forced air wall heaters.

hot_rod

A hydraulic separator would be the very best way to pipe at. It gives you all the required functions, air, dirt, magnetic, and hydraulics sep in one device. Multiple Boiler Piping is greatly simplified and the staging control could provide return protection as you mentioned.

Pages 65, 66 of the journal mentioned above has an example.