Couple Tepid Radiators?

[Peteman]

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.

 

[Dana]

To take the full 100,000 BTU/hr without cycling at all takes about 200' of typical baseboard. But you also have the thermal mass of the boiler and it's water content to work with. Even though the burns are short (and not very efficient), as long as the time between burn cycles are long enough that it's not delivering more that 10 burns/hour (5 burns/hour would be better) it's not a big problem for the boiler. Measure the burn times to within 5 seconds precision n0t "...1-2 minutes on/off), and the total amount of time between the start of successive burn times. If it's 2 minute burns but 15 minutes between successive burns it's in "who cares?"territory. If it's 55 second burns starting 4 minutes apart it might be worth doing something about it.

To maximize the utility of that thermal mass, if possible set the differential swing between the high and low limits to as wide a difference as possible. (Read the manual to figure out if/how that can done.) That will maximize the burn times, as well as the dwell time between burns when it's just the fin-tube zone calling for heat. Since this boiler has an internal indirect water heater there may be some peculiarities about the control setup that make it less flexible, but a retrofit heat purging boiler control such as the Intellicon HW+ can usually stretch that even further, if needed.