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New boiler, a blend of old and new

nuke621 Member Posts: 1
First of all, thank you all for this forum. Over the last 2 years I have referenced hundreds of discussions on this forum in my passion for learning more about hydronic systems. I am an electrical engineer by trade, but am fascinated by all the controls on these systems. Dad’s a retired plumber and pipe fitter, we talk at length on this subject (he hates PEX, lol, I’m bringing him around). I bought the Modern Hydronic Heating textbook and dug in.

My home is 2000 sq ft, 4 bed/1 bath upstairs, full basement, built in 1904 in Saint Louis, MO. All windows replaced upstairs, quality storm windows on all original wood windows downstairs. Attic insulation removed, air sealed, and replaced with R-50 blown in. House is brick on 1st floor, plaster on brick, no insulation. 2nd floor is framed with various forms of wall construction, 50% is insulated. Re-sided on second floor and had Tyvec put under all siding. I did some real basic J-Calcs and came up with around 85K BTU heat load on 0 degree design day.

Weil-McLain 140K BTU cast iron boiler on original gravity fed system in 2 pipe reverse return. Main distribution is 2” iron pipe with really cool fittings, no pipe insulation unless random 100 year old newspaper counts. SX-30V 14 gal expansion tank. System holds between 100-125 gallons. I emptied it into a 5 gal bucket when I drained it. High limit at 180F, never have seen boiler over 140-150F. System is connected to EcoBee thermostat. Even in the mid-teens, system runs around 9 hrs in 24 hr period. When up to temp, see an average of 5-10 difference in return temp. Taco 110 circulator. Since adding Spirovent and Watts air vents, system is largely air free, needing only to bleed at beginning of season, if at all. See cast iron BTUs below. Stair radiator is made of 1 1/4” cast iron pipe 9’ long with custom ends, see photo. No idea on radiating BTU of that.

30 gallon atmospheric vent water heater is 20 years old and needs to be replaced soon. Would like to replace with indirect and mod con. I think I can get away with operating most of the time below 140F, so would get the benefits of condensation mode. Goals of project in order are: comfort, efficiency (lower long term bills), a hobby (just kidding), upfront cost. Bonus is abandoning atmospheric vented appliances and capping masonry chimney. System was a mess when moved in, too small expansion tank, too hot upstairs. Fixing all that and adding EcoBee, TRVs upstairs and adding 2 hr off delay on circulator has heat distribution\comfort working great. During boiler replacement would go ahead and change all valves downstairs to TRV (some rooms could be set cooler).

Have heat pump downstairs and Mitsubishi mini-split units upstairs, neither has cold weather defrost, so turn off when freezing/below 40F. EcoBee is coordinating boiler and downstairs heat pump, have set for boiler to take over at 40F. Upstairs does get chilly in shoulder temps, 40-60F, if I forget to turn mini-splits on. Don’t like leaving them on when freezing. Would likely abandon EcoBee and let boiler do outdoor reset with a simple thermostat downstairs. During upgrade, I think adding TRVs downstairs and adding more controls can fix this problem. Would leave Mitsubishi on all the time, but below the radiator TRVs. Sensor upstairs can call for boiler if needed, can be placed in hallway which usually cools off first.

The hot tub and assorted pump and feed water feeder will be install at a later date, but would like to pipe in heat exchanger and valve off.


Thoughts on design? I’ve attached two drawings.

Is using closely spaced tees ok for the boiler loop on the water heater? I added a hydraulic separator in the cast iron radiator loop because I’d like to keep the circulator going to move the heat around even if satisfied and there is a water heater call.

I was thinking a Grunfoss Alpha2 15-55 for the circulator on the cast iron radiators and also the hot tub, it has an integrated check valve. Since the Knight uses a Grunfoss UPS15-58FC, I’d use that on the water heater loop as well and it has an integrated check valve. This way I’d have a backup to rob if anything failed.

I’m torn between 85K or 110K Knight boiler. I’ll do a much more detailed heat load calc, but want to lock down system design.

Old parts:
38” x 9” 3 column cast iron radiator sections
50 sections of cast iron radiators
45 sections of cast iron radiators

95 sections (plus kitchen)
5 sq ft per section (38”, 9” 3 column)
44541 BTU @ 140F
82541 BTU @ 180F
Didn’t add Stair pipe radiator BTU?

New parts:
Lochinvar 110K Knight Fire tube boiler, floor mount, KHB110N or 85K
Lochinvar 27 gal indirect Squire SIT030
Caleffi DirtMAG
Danfoss TRVs
Axion Pressure Pal MF200