Gravity Conversion- Condensation Concerns

Bob Billmyre

I'm just finishing a boiler replacement on my home's gravity system, replacing the old coal to oil retrofited unit with a smaller oil fired Dunkirk. Added a B&G Series 100 circulator and bypass loop. Because of the large volume of water, I've started out with 80% of the water running through the bypass. In spite of this it takes a long time to raise the water temperature to the 180 F aquastat setting. In fact, so far this year it has never gotten anywhere near that hot.
Am I running a risk of condensation problems? How hot does the system need to get to avoid condensation? I noticed in Dan's books that he recommends a minimum temp of 160 F in a discussion of the bypass loop, but in another place he talks about running systems with an output temp as low as 150 F.
I have the option of two larger nozzle sizes on my burner, but I was going for efficiency and trying to avoid short-cycling.

kevin coppinger_4

Bob I am not an...

expert but I have some experiance w/ gravity systems. As long as the boiler temp stays above 140F you should be in good shape. Gravity systems are not designed to run w/ high temp. water in them anyways. Is the bypass you have a system bypass( most of the water returning to the boiler) or a boiler bypass (most returning to the system)? A couple of stategicly placed water t-stats will help you in seeing what goes on. It might take a whlie to get the right balance. Keep an eye on it...kpc

Mike T., Swampeast MO

Cast iron boilers driving gravity conversions don't seem prone to condensation problems--even when reset to temperatures WELL below 140° and with zero bypass.

Should a condensation problem occur it will likely show itself first in the joints of the flue. Keep a close eye. Most likely you will see nothing, but I wouldn't increase the firing rate because (as you see) gravity systems can and do try to "reset" themselves.

Bob Billmyre

The bypass currently sends 80% of the supply back into the return. I have Tstats on the return, before and after the bypass connection and also on the supply. It will take a while to get a feeling for the best settings.