Install of high eff boiler together with mid eff boiler

dronic123

I am re-installing a hi eff mod con boiler. (Max temp at supply of 170 but it is recommended to keep boiler under 160 —probably under 150 or 140 or 130 would even be better. The original idea was to install a mid efficiency boiler as a stand-by/back-up. (Max temp 200 but probably better to run it under 180.

The tappings on the mid are 1-1/4”, on the hi eff 3/4” and the risers on the 1 zone system are 1-1/4”.

——But I would like to see if we could do the install so both boilers could run at the same time (when necessary in the very cold weather).

From first principles, I am guessing this can’t work (WELL), but would appreciate your helping me through it.

If we isolate the boilers from the system pump, I think the hi eff boiler has to be first in line; otherwise the hotter water coming out of the mid eff boiler could make the hi eff boiler redundant right from the start—what do you think??

But then—eventually, the mid eff would create 180 supply with a delta of 20 degrees and I suppose that would really take out the hi eff boiler anyways???? I am struggling to find a way to do this simply. Your comments would be appreciated.

EBEBRATT-Ed

I would install them as you have drawn it. In the spring and fall only the high efficiency will be needed. When the weather is colder the hig efficiency will see the lower return water temp and hopefully still condense some water and will "boost" the return water temp to the mid efficiency boiler.

This is assuming you're not going to change from one boiler to the other by opening and closing valve under normal circumstances

dronic123

I was going to try to find an on-off outdoor reset for the mid efficiency that would close the circuit at say 5 and lower and open it at 6 and higher—- (and of course experiment with the set point). Anyone have a recommendation?

dronic123

Viessmann “strongly” recommends using a low loss header. If we wanted to make them happy, what would be the best design? I can’t see any way to do it without using CSTees for the mid efficiency boiler

The mid eff boiler has big tappings and perhaps small head loss?—-could we pipe it directly if we lost the mixing valve and the little pump?

hot_rod

a hydraulic sep is good option as it give you hydraulic separation , air , dirt, and magnetic separation

No need for the cast boiler to go across the sep, it doesn’t need hydraulic separation. Piped like this it goes direct to load without temperature blending

This would allow either or both boilers to fire, they are hydraulically separated

I would use a control to run the mod con until it is no longer condensing. This is known as as hybrid piping, a high efficiency and mid efficiency piped together

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dronic123

@hot_rod : a little hard to to follow precisely the flow in your last diagram. Do you have 3 pumps?

I have redrawn the install, keeping in mind where the equipment will most easily be located—trying to follow your sketch. Does it match your intention? At least it splits the return flow at a tee. Is that the idea?

dronic123

@hot_rod : sketch

hot_rod

tell us more about the distribution. What type of heat emitters? Is the distribution one zone, or multiple. Any idea of the btu load? What size is the mid efficiency boiler?

My thought is the ,mid boiler sees the hydro sep as a mini buffer. That boiler is controlled by a sensor on or in the sep. Then when the distribution is calling, whatever gpm it requires pulls from the sep/ buffer.

If or when the gpm to the load and the gpm from the boiler are the same. Flow goes from the boiler directly to the load at the boilers temperature, no mixing going on.

Either of these pipings accomplish the same result. On has both boilers paralleled on the A side of the sep. The other has the mid efficiency after the sep.

Whenever the load and boiler output, either, match flow is straight across the sep. It's just a coupling at that point.

On zoned systems the flow may never be the same on both sides of the sep. So the system you are connecting to, may help decide the best piping option.

Ideally you want the high efficiency boiler condensing as much as possible. If efficiency is one of the targets?

If the system always needs 150 of higher SWT? At least the high efficiency brings modulation to the party.

Most high efficiency boilers like some hydraulic separation, to assure their HX is always running happily, flow wise.

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GGross

Whats the reason for 2 boilers here? which model Viessmann do you have, some are adjustable right up to 180, none I have seen are adjustable only to 170, they were all 167 or 176, then current ones are 180 (194 for DHW)

dronic123

@hot_rod : Thanks for the input. It is so very much appreciated; I'm way behind the curve on this stuff.

Distribution:

It appears that the system started out in the thirty's with a more or less huge, coal fired, gravity, water boiler with 25 assorted cast iron rads on 2 floors plus the basement (total sq ft including the basement approx 4,000). A single "zone" system with a 1-1/4" pipe for the return and supply is still being used. At various times over the years, it appears that, the rads on the ceiling of the basement were moved to the walls, a circulator/pump was added, gas was "brought from the street", and a 200K Raypak gas mid eff copper finned boiler was installed.

The Raypak started to leak and was exchanged for a 125K Viessmann Vitodens 100W B1KA combi, mod con along with a 125K SlantFin Galaxy GG mid efficiency to be used as back-up.

A Heat loss at design was done after the install and came in at 128K. Is this what you mean by load? Would you divide that by .7 because at design temp there would usually be no condensing of the Viessmann? While it would be very expensive, and probably not economically practical, we could tighten up the house with new doors and windows.

dronic123

@GGross : Thanks for your comment. Yes you are right about the Viessmann B1KA—its spec top temp is 176F. The spec gpm appears to be 6.2. I am not sure I completely understand how the arithmetic works. Please chime in if you can provide some comment on possible installation—it is very much appreciated.

I really wanted to have a back-up basic boiler in case the Viessmann hi eff broke down or just stopped working for any number of reasons—so that I wouldn't have to worry as much having to get the boiler fixed on an emergency basis.

;

dronic123

@hot_rod: Is there a reason for picking/using the number 8 gpm?

How would the sensor on the sep work?

"Whenever the load and boiler output, either, match flow is straight across the sep. It's just a coupling at that point." Is there expected to be any flow from the hi eff too at this point? or just the mid boiler?

I am really struggling with this but would like to work through it and I am so thankful you have been patient. These are probably issues that are well settled.

hot_rod

If the mid efficiency is a rarely if ever used boiler, I'd just tie both into one side of the sep.

You could run one or both this way.

An example of 4 multiple boiler piping, and a job in Texas with "hybrid "boiler piping. Two high efficiency boilers piped with 3 standard mid efficiency boilers.

They run the high efficiency boilers until they are no longer able to condense. Then switch to the less expensive standard efficiency boilers. For this application it saved big $$ not buying 5 high efficiency boilers for an application where they are not condensing and providing 90% plus efficiency much of the time.

8 gpm was just a random number I used for the example.

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dronic123

@hot_rod : Thanks for your comment. I have drawn out your first suggestion, splitting the boilers around the LLH, and showing just the mid boiler.

I gather the more solid lines in your sketch are headers. If my piping is 1-1/4", would 1-1/2" do as this "header"?

From your previous comment about the use of the LLH as a buffer,———is this perhaps the same thing as the Gil Carson "common pipe" construction from the 50's? Evidently, he gave (there were) lectures on this across the country but I have never had the opportunity to see or read one of them. Does that info still exist???

I have accepted the "isolation" of the pumps theory but I have never really understood how it works in practice—-thinking of water as a bunch of marbles flowing through the pipes etc..

dronic123

I have redrawn the diagram taking out all the elbows ——diagram no. 1 on the left side is the same piping, i think.

Will the LLH still work if the water flow doesn’t actually go through the LLH? If so, It would then be the same as in diagram 2? And 3

Diagram 4, I guess, could be “common pipe” or CSTees if close enough?.

Could the mixing/ boiler protection loop act as an “pump isolator” if the mixing valve always was a little open ( never closed completely)?

hot_rod

No need to have the large header in #1, just make it a large enough tube. With all pumps running, you want the velocity in the header, or that pipe, to be below 2 fps. The lower the better.

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A LLH, separator or closely spaced tees the goal is the same very low pressure drop between connections.

An example of a 2" sep with 40 gpm flow. The velocity is .44 fps through the center tube.

2 & 3 work fine.

I'm not sure what is going on in number 4?

Charts like this from engineering toolbox show you flow velocity in pipe at different gpm.

The Reynolds number Re is what indicates laminar or turbulent flow

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