Surprised with system running a low water temp for a ci boiler you haven't had flue and burners rusting out. With being oversized = short cycling ,using inside air for combustion efficiency 70%. Condensing boiler running return water temps under 130* is where they get the 94% + efficiency and with modulation can track water temp closer and not short cycle. How many different loops does the system have brand and model of pump.
With high mass radiators and a low load it probably won't really short cycle, but the duty cycle will be extremely low. If the boiler output temp is 120F it's raw combustion efficiency will be over 90% until it's so corroded that the turbulence inducing structures on the fire side of the plates have corroded completely away. I suspect the real temp on the boiler is much much higher than 120F if the last combustion analyzer test showed 80% steady-state.
Basement is semi-finished, though the one external wall in the boiler room is bare cinderblock, so maybe it would be worth it to insulate that side?
Absolutely
yes! The boiler room is probably the warmest room in the house during cold weather, and has a high heat loss at any temperature. Cinder block walls are notoriously air-leaky, and have an R value of about 2.
Spray foam contractors would have to charge quite a lot- the set up and break down time is the same whether they're installing 200 board feet (10' x 10' x 2") or 2000 board feet.
If you want to DIY it on the cheap it's possible to get there with 1.5" foil faced polyisocyanurate board (R9-R10) strapped to the wall with 1x4 furring through-screwed to the cinder block with TapCons (be sure to use washers so the furring doesn't split), hanging 1/2" wallboard on the furring. That adds a bit less than 3" of thickness to the exterior wall assembly. Be sure to observe the manufacturer's clearances to walls/combustibles, and if the vent stack is on that exterior wall, the code required clearances from the vent. (It's sometimes OK to insulate right up to vent pipe using rock wool, but never fiberglass or foam.)
Start by putting the foam board on the wall temporarily with blobs of foam board construction adhesive (available at box stores), keeping the bottom edge of the foam a half inch off the slab (where it could wick moisture if the slab were damp). Tape the seams with a high quality foil HVAC tap (Nashua 324A is available at most box stores). Seal the top edge of the foam to the foundation with polyurethane caulk or foam board construction adhesive. It's also important to air seal the foundation sill to the foundation, and the band joist to the foundation sill & subfloor, etc. Sometimes that's more easily done with combinations of cut'n'cobbled foam board & small DIY spray foam kits like FrothPak. Polyurethane caulk is the better material for sealing wood to wood, or wood to concrete/cinderblock for gaps under 1/2".
And that's where my concern has been - at 120 water temp (both the aquastat and the temp/pressure gauge on the boiler show this) I am worried about the life expectancy of the boiler. I mean, it's been running for 21 years (I assume) this way.
My other issue is finding a company that doesn't scratch their head every time they see this system. They generally say, 'well, if the house is staying warm at 120, it should be fine.'
If I went mod-con, what would you recommend?
Thanks,
John
The final choice really depends on your local manufacturer & installer support there is, but...
Small fire-tube type mod cons are pretty easy to retrofit into cast-iron replacements, since they can usually handle the higher flows and can be pumped direct. The lower the minimum-fire input the better, independently of the max firing rate. There are several fire tube mod-cons with ~80KBTU/hr max firing rates that can throttle back to 8K. The value leader in my neighborhood is HTP's UFT-080W- it's cheap, reliable, and comes pre-plumbed with a secondary port for running an indirect water heater. (The Westinghouse WBRNG080W is the identical equipment under the paint, sometimes distributed through the big orange box store.) Lochinvar's KHB085N or WHB085N are pretty similar in modulation range, and have a lot more bells & whistles, capable of programming different temperatures for different zones, etc, but it usually quite a bit more expensive. (The KHB055N would cover your likely load, but has about the same minimum firing rate as the KHB085N, and would be slower for heating an indirect.) There are others.
Navien's NHB-80
water-tube boiler is usually competitively priced and can work here too, but MUST be plumbed primary/secondary (with an extra pump) due to the high pumping head of the water tube heat exchanger. It's somewhat more difficult to install, and more sensitive to the competence of the system designer/installer, but done right they have a good track record, with pretty good distributor & installer support in my area. There are other water tube boilers with big turn down ratios that would work too.
A Taco 007-F5 uses nearly 10x the amount of the drop-in replacement Taco 007e that uses a high efficiency ECM drive DC motor, which will pay for itself many times over the lifecycle of a pump- in high electricity price areas it pays for itself every YEAR! They run about $140-150 at internet pricing. There are also smarter pumps with programmable ranges out there that would work too, offering a bit more flexibility to how the system is operated, starting at ~$200 that might be considered. Even if using primary/secondary configuration with a water-tube boiler, replacing the -007 with a Taco 007e or Taco VR1816 (smarter pump) would be "worth it". Alternative similar pumps like the Grunfos Alpha2 15-55F or the AquaMotion Einstein AM55FVL would work too.
As long as the 18 port manifolds isn't 18 separately controlled micro-zones, each with it's own thermostat it's fine to pump a fire-tube boiler directly. If there ARE 18 thermostats even the fire-tube boilers would likely need to be plumbed primary/secondary to meet the minimum flow requirements of the boiler under all conditions.