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# Fixed Flow Rate and Mod-cons

Member Posts: 503
Fixed flow rate

If one can fix the flow rate through a mod con boiler, how wide of a DT is acceptable? Often we read 40*DT, 35*DT, etc. Can one fix the flow rate to 55*DT assuming the control doesn't lock out? From experience 75*DT is the lockout threshold on the knights.

Lastly, any info from the hx manufacturers on minimum flow requirements? Boiler manufacturers tend to say you "need" to use the pump dictated in the install manual.

Discuss....
:NYplumber:

• Member Posts: 7,543
More Delta T?

This one sure has been kicked around pretty hard.

I am really not sure I see the point in super wide delta Ts.

At some point you will get into laminar flows within the boiler and lose efficiency.

I think you get the manufactures recommended minimum flow and design for that.

Carl
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 2,239
Minimum Flow Rate

I am of the opinion that you could drop below the minimum flow rate if you increased the water pressure accordingly. That would keep the entrained gasses from separating with the water right on the inner surface of the heat exchanger. Under normal pressure and laminar flow conditions, that is exactly what can happen, which leads to stress fractures.

I would stay in charted territory and adhere to the minimum flow rate.

Harvey
• Member Posts: 503
throwing numbers around

Lets toss some numbers around.

55,000btu boiler. On 40*dt the flow rate is 2.75gpm.

At 20% the boiler input is 11,000btu. 40*dt is 0.55gpm.

Choose a boiler load rating in the middle, say 50% which is 27,500btu. Run that at 40*dt and the flow rate is 1.375.

So if a boiler is oversized, say 85,000btu and we want to run the boiler at 40*dt at design conditions and set the flow rate through the piping (caleffi flowsetter) for the 55000btu@40*dt, the dt will be wider if it goes into full modulation.

Just stirring conversation and trying to see what the threshold of the boiler minimum flow rate is. With many companies are sharing hx's it shouldnt be too difficult to dig up minimum flow rates before the thing grenades.

Have a system coming down the pike now and want to stretch the limits and get the lowest return water possible from the radiators. Condensing at design with a system running at 160* design temp would be nice.

Time for me to stop babbling and wait for more replies. Thank you Harvey and Carl for the replies this far.
:NYplumber:
• Member Posts: 7,265
Throwing Numbers:

So for the sake of discussion, if you had a 40* DT and 160 degree water going out, but 120 degree water returning, what is the inlet temperature of the last heat emitter on the circuit?

Unless you use a properly designed, installed and balanced reverse return piping arrangement, how do you size the last emitters?

Will Supply & Return manifold blocks connected as direct returns rather than reverse returns give the same performance?

Direct Return= First out, first back.

Reverse Return+ First out, last back.
• Member Posts: 503
edited May 2014
home run

With pretty much equal return temps. Zoned as four or five radiator zones with three radiant zones. Talk abount micro loads...

The radiator loops will be designed off of a 20 dt but surely will be 10dt or less real world. This is not a trv system.
:NYplumber:
• Member Posts: 7,356
The lawyers made us do it

explains the pump recommendations we see in boiler IOM docs.  I was not aware the Lochinvars would tolerate that high of a ∆T.  TT locks out at 40ºF IIRC.  I always size for the widest ∆T at high fire that will keep the boiler and the system happy.
• Member Posts: 996
Stay away from high Delta T!!!

Dues to the small passages in those mod-con heat exchangers, you need to keep fluid velocity through them in the same manner as flat plate heat exchangers. We went from 35F to 25F as a Delta T after a number of heat exchangers blocked (most by our competitors who used smaller piping and much smaller pumps!). You do not use the same pipe size as the heat exchanger connections. One uses the manufacturers recommendations or proper engineering practices for the flow rates required. GHoing to a delta T that is 35 F or greater is looking for trouble.
• Member Posts: 503
edited May 2014
old vs new

Seems like your referring to the "old" hxs not the "new".

My believe is that with the firetube design, flow isnt as critical in keeping the insides scrubbed clean.

Food for thought; the knight boilers have capabilities of controlling dt through a pump on the boiler loop. Im not familiar enough with that feature however from what I gather through speaking to Lochinvar is that the boiler will be allowed to run at its lowest modulation and hunt for that dt. So on the WHN 110 running at 20% @40dt that pump would optimally look to flow 1.1gpm. The question is, will the boiler not allow the control to look for a dt that wide when below a specific input?

Then theirs viessmann across the pond with the dt features in those boilers. What do they do to combat a BOOM when the wide dt & low modulation take place?

Much to ponder.
:NYplumber:
• Member Posts: 991
I have played with this

I just target the boilers minimum flow rate and try to get close to that...

In my own home, I have tested back and forth with different delta ranges, 20 degrees seems to work fine, I have opened it up and closed it down and the cost of the energy per degree day doesn't change enough to see which way it goes...