I am modeling a home this week that was built in 1927. Heated (CZ 4) with a 1927 coal fired boiler that was converted to NG in the mid 1960s. AC is 2 units one for each floor. The basement is heated with waste heat and no AC.
The original radiators are still in use. There is no information on the burner unit. It has been turned off for the summer. I can only conclude that 150K - 200K might be in range. The home is 2 stories plus basement. Square design, boiler on west wall. Attached garage on north wall (one story), there is what could be an addition on the east side, 2 stories. Could be original sleeping porch set up also. The two rooms, one up, one down of the addition/sleeping porch use a baseboard unit. All other room use radiators. There is a circulating pump in the south east corner of the basement that must be turned on to operate the 2 rooms with baseboard heaters. There is a thermostatic control for the pump but no one seems to know how it works or where it is. It is a manual turn on or turn off by the home owner.
Should I stick with a standard boiler? Should I consider tankless units? Would a tankless unit be worth installing to cover just the two rooms that at times need to have the circulating pump turned on?
This is the first hydronic system I've audited, so I am in learn mode.
Cast Iron Radiators LOVE modulating condensing boilers. This is a great opportunity for these folks to save big energy.
You can usually run really low temperatures, which means long, super-efficient condensing cycles. A Navien Combi or Triangle Tube combi would be great options depending on potable hot water consumption. The really elegant solution is a mod-con with stainless steel indirect - but may be more $ than they want. And Navien has really figured out on-demand with their buffer tank and pump/internal or external recirc setup, so indirect becomes less of a luxury benefit.
The red herring is the baseboard. It may need higher temperatures for adequate conditioning when the weather gets really cold. Measure it and load calc the rooms it's in. They'll probably require a separate zone.
First, is this a steam or hot water boiler? It sounds like there is not a main circulator pump, this would indicate steam.
Second, these old girls are usually leaking a bit. This can be a serious problem because the coal sludge has traveled from the boiler throughout the entire system. These big cast iron radiators will hold sludge even after running cleaner through the system for weeks. I know this first hand, on a recent job where I replaced a coal conversion with a mod con boiler, I ended up cleaning the hx 3 times before finally removing every radiator and getting them hot tanked at the automotive radiator shop, then repainted and re-installed. The 10-15% increase in combustion efficiency over a standard cast iron boiler will never pay for the increased cost. Lesson learned.
It sounds like what you should do is; pull the boiler and replace it with a standard cast iron hot water boiler, add an indirect tank with a solar option(2 heat exchangers), replace the 2 baseboards with flat panels(Runtal) and add a t-stat.