Glenn Stanton... Boiler Bypass Piping

Boilerpro

your seminar got me to revisit boiler bypass piping on Hot water gravity conversions. I pulled out data on a typical system. Heat load, 55,000, 620 edr columnar radiation, assume a 100,000 input 80,000 output boiler, uninsulated basement piping, 130gal water volume, 4900 lbs piping and radiators. Running about 8 gpm through system (extra flow to help compensate for piping losses), wowuld yield about a 20F delta tee across the system. 55,000 / 620 edr = 89 btu/edr yielding about 140F average radiation temp on the coldest day of the year.... supply = 150, return = 130. bioler average temp is 140F. NO boiler Bypass needed under these conditions. Now lets look at typical day conditions about 50% output needed....90 x .5 = 45 btu/edr.... average rad temp =115F. At 1/2 load, delta tee through system is 1/2 full load delta tee, so supply is 120F, return 110F. Boiler cycle length is 1 cph or aboiut 30 min. System resonse time is based on mass. 4900 lbs steel/ cast iron) x .11 (multiplier for conversion to equivalent water weight) = 539 lbs = 1066 lbs water=1600 lbs water. It takes 1,600 btu to raise system temp 1F. 80,000btu/hr /60min x 30min on cycle = 40,000 btu per cycle
40,000 btu/ 1600 btu/F = 25 swing in systm temp between cycle. AT end of on cycle, return to boiler is 110F + 12.5F (1/2 of swing)= 122.5 F.....Bypass is now needed. The warmer it gets the more the byapss is needed. This is why I don't like boiler bypass....under light load conditions it no appears to nolonger provide low temp protection.


Boilerpro

Unknown

All the more reason

for injection mixing with a boiler return sensor. Although we were covering the boiler bypass. it is the system bypass that would br used on the gravity system. I agree that the mixing valve does a better job due to changing system demand conditions. Hope this helps.

Glenn