boiler bypass pipe question

[Deleted User]

dont see boiler bypass piping installed on gravity hot water boilers? yet according to the boilers gauge, the water drops down to around 100 degrees between cycles.. we normally work on steam boilers so i was wondering at one of our customers home why her boiler (gravity hot water) didn't have a bypass pipe..seems the water on the return is almost always cool coming back..cant be much flue gas condensation cause the boilers probably 50 or more years old..shouldn't there be flue gas condensation? or is there something special about gravity hot water boilers i'm not grasping here?

[Deleted User]

Gravity circulation

is slow & easy. Stack temperatures on those old fellas are normally much higher than on newer boilers. Condensation problems were almost unheard of until we started tightening up heat exchangers and scrubbing the stack gasses to sauna temperatures.

hr

Converted gravity and

high mass radiant systems are where you really need to be careful. Especially with todays more efficient, small HX cast boilers.

Here is an article from B&G on the subject.

http://www.bellgossett.com/press/thermalshock.htm

hot rod

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[Deleted User]

so what happens

when you replace the boiler with a newer model? is flue gas condensation now an issue when it wasn't with the old boiler?

[Deleted User]

Oh Yeah.

Pull out that old gravity Am. Std w/ a 50 gal water content & replace it w/ a new modern, whippersnapper, low mass, 3 gal H2O tea kettle. Reduce those 2 1/2" supplies & returns to 1 1/4". Now add a nice new circulator. Can you say "chilled water?"

That is why most boiler manufacturers have piping schematics for their products that resemble the wiring diagram for the Mercury Space Craft. Bypass loops, balancing valves, primary/seconday pumping, zone valves w/ small bypass circuits, 3 & 4 way valves, stategically placed temperature gages, etc. They are there to keep the boiler water & flue gas temperature above the point of condensation.

The Germans said "enough." They passed a law mandating no new systems w/ design temps above 130 degrees. They're designing into their systems the product we are still, by & large, trying to keep out. Condensation.

[Deleted User]

so the option

on a gravity boiler replacement would be either a condensing boiler or piping it in a manner like siggy shows in one of his articles, primary/secondary with a 3-way thermostatic valve to raise boiler return temps? i gather i couldn't take a new boiler and just ''pipe it in" if there is going to be a pump installed..but if its going to stay gravity only, i should be okay plumbing out both supply taps full size and using both full size returns, wouldn't i? or is it best not to?

[Deleted User]

If you are going to

install a low mass boiler w/ a circulator, then an alternative piping schematic is a must. Pick one.

Some folks still replace boilers & stay gravity. If so, your best bet would be to use a dedicated steam boiler w/ the large supply & return tappings. The steamer should have the larger water content & adequate stack temperature you want.

Condensing boilers & old gravity systems, IMHO, are not a good marriage. HR or ME might be able to lead someone through it. However, the few I have seen died prematurely. I'm a little old fashioned. A boiler should last at least 15 years.

[Deleted User]

thanks Ron & Hot Rod

i appreciate the info..

Keith_5

gravity system condensation

Ran into a severe condensation problem converting from oil to gas. The original boiler vessel stayed but the owner wanted to convert to gas. I sized the burner for the load of the building not the water content of the system. Couple that with the higher water content of gas and I had a wet mess on my hands. The other obstical to over come was the reduced demand due to the new owner swapping out the windows and adding insullation. I was able to clean up the majority of the problem by installing a bypass pump to come on when ever the boiler fired. Some tweaking regarding the Gpm flow thru the bypass loop and the gravity loop was required. Learned my lesson to walk slowley into gravity conversions. Keith