Peteman
New Member
If it's a few minutes it's not a problem if it's many tens of minutes it's a problem.
For high thermal mass systems it's sometimes necessary to install a thermostatic mixing valve on the bypass branch to establish a minimum entering water temperature (EWT) on a cast iron boiler. An EWT of 130F is usually safe for a gas fired cast iron boiler's heat exchanger plates, but sometimes result in excessive flue condensation in masonry chimneys if the flue is oversized, or located on an exterior wall (where the masonry is cold) rather than inside of conditioned space. Setting the EWT to 140F is usually enough to prevent the flue condensation too (with exceptions to prove the rule.)
Hi Dana - Thanks for all your replies to my questions. Learning this new old house heating has been interesting but I think I've got the main house heating figured out. With some adjustment I think I've got a good situation now. I have adjusted the 3 branches (far run wide open, other two halfway) plus throttling back the bypass about halfway. All radiators working well now and there appears to adequate mixing of return water and supply water to keep the boiler input around 130F. One other question ...
As I mentioned earlier, the previous owner established a separate zone (with separate honeywell valve) for basement heat. The basement heat is a series of copper finned baseboard ... waaaaaay smaller water volume than the main house. The problem is when I try to heat just the basement with it's thermostat (main house not needing/calling for heat), the boiler temp limit maxes out very quickly and the boiler goes into a very short cycles (on the order of 1-2 minutes of on/off). Seems hard on the boiler and, at the least, very inefficient. Is there any remedy to this short of ripping out the baseboard system and buying big radiators to heat the basement? I'm kind of stumped how a person could reasonably heat a basement with baseboard heat given a 100,000 btu boiler.