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My Undersized Boiler Experiment

mikeg2015 Member Posts: 1,194
From my other previous posts, I gave some info on my replacement of my oversized 375k 1983 HB Smith G300. Connected load was estimated at 710 EDR. Boiler EDR 1250. After some adjustments and adding TRV’s operating load is 680, with 150 on TRV’s. System operated at <1oz until radiator fully heated then I shut it down at 12oz to prevent short cycling.

Replaced it this summer with a salvaged 200k Hydrotherm. Install drop header and added an indirect tank on a hot water loop. Have a 2 stage gas valve, run on hi stage heating, unless call for indirect. Was going ot add a pressure switch to drop heating to low stage at 2-3oz, but it will never get there.

The Good: Boiler never builds any measureable pressure on a 12oz gauge. Nothing. I’ve managed ot balance the radiators so they mostly heat up evenly except when all TRV’s are calling, the 2nd radiator on the loop will not get any steam (might need to get raised to pitch lateral up a little more).
I get some hissing of vent on hot restarts (main vents still closed, but system full of air). I might stop using the upstairs thermostat as an alternate Call and only use the downstairs which is set ot 1 CPH, which works really well since the system heats up so slowly. Will need to run more, but I appear to be using less water. Used to add a couple gallons a week. There were some minor leaks I could see on the old boiler. The even lower pressure should reduce any minor valve stem leaks I have (on my list to repack them). Boiler output estimated at 556. With 25% pickup (it’s a large vapor system, 3” header), that’s 417 to the radiators. Even with all the TRV’s closed, it still won’t ever fully heat every radiator. Average Heat loss on the coldest windiest days is estimated at 110,000BTU peak assuming I attempt to heat all rooms fully. 90k if I keep unused rooms cooler. SO boiler output with indirect tank demand is closely matched.

I used Gifford Loop, with a Y fitting just above the normal water line. Seems to hold a fairly stable water line despite needing more skimming and a small steam drum.

Indirect tank works very well. Despite less than optimal supply and return location, as long as I keep the boiler filled near the normal level and not the lower level that the auto fill keeps it at (boiler only connected at bottom and across steam drum at the top), I can get about 100,000 BTU out of it with 45F cold water, 140F setpoint, Boiler Out 185F, 8GPM, 25F delta T. This is close to the output of low fire.

The Bad: Currently only able to run 170k input on hi fire. The fire box runs really hot. If I set gas pressure to full 3.5”, the flame sensor oxidizes and I start to damage the silicone cable sheathing going to the flame rod/egnitor assembly. No roll out, draft seems OK. Flame looks good. Might still have a lot of scale, and I think the burner design on the Hydrotherm sucks. I might change the orifices and convert it to a 175k (which was how it was sold as the next model size down, same block and burners, but smaller orifice). Might burn better that way.

The Hydrotherm only has a single outlet. SO the drop header is critical IMO, but still results in some surging on startup, that would likely be there even if it was skimmed perfectly.

Still need to skim more, but no skim port. There’s one extra 3/4” port on the right side, but couldn’t get the plug out and hestant to drill it out just yet, but might get there eventually. I’d added boiler cleaner 3 times now and it seems ot be improving. Kettering sound has reduced each time. Unfortunately I didn’t install a 2” cross on the return (my biggest regret). I might have a lot more scale than I thought. The boiler did sit open for 9 months but cursory inspection looked like it was just surface rust. But thinking now that it was a good layer.

I did brush the surface rust really well off the heat exchanger surface before I installed the boiler. Spent a good hour on it and basement is fairly dry and has dehumidifier in summer.

Take-away - an undersized boiler is possible on a large vapor system with a very well vented main and the smallest vents possible on the radiators. But I theorize it begins to rely on pipe pitch for even distribution as much as venting rate and preferential flow is a definite concern but hasn’t been a real issue. TRV’s can effectively be used for zone temp control of individual rooms. However once they close, the radiator will keep heating to the point its vented, since they won’t break vacuum, but the room will also delay heating or skip cycles, so with a high mass home, it works well.

Overall I’m satisfied with it and the current piping layout should transfer well to a new boiler some day in the future.


  • Steamhead
    Steamhead Member Posts: 16,272
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Fine... My craftsmanship and housekeeping in my own home isn’t great. It’s still a work in progress. Still insulating pipe. So please easy on the criticisms of sloppy insulation and wiring work and general workmanship. I literally learned to sweat pipe 6 months ago and this was my first project with piping over 3/4” and never worked on black pipe over 1” before. Did this all by myself without an veteran assistance other than pictures on here. So this is what you get when you turn a apprentice loose without supervision. Haha.

    Yes, the rollout switch location is modified a bit. It trips it in the original location due to radiant heat. No issues on that bracket.

    I repaired all the refractory with wet blanket ceramic.

    The pressure gauge literally never moves unless I overfill the boiler, so I know it reads accurately.

    No hammer on the indirect unless hte strainer starts ot plug. I ended up installing a sweep 90 inside the boiler, modified a copper bushing to sleeve it to 1-1/4” then used a threaded brushing to make it all work, and sweated it in place. This way it drawing away from the bottom of the boiler a little so less bypass from the return.

    Will be interesting ot see how long the SS Alpha lasts in this application. I mounted it vertically and placed it after the strainer to give it a fighting chance. I get 8-11GPM on low speed depending on water temp. I probably should have install a 1” check after the coil and a 1/2” check on the bypass. I get a little reverse ghost flow through the bypass line towards the indirect when steaming.

    I tried to use Tee’s wherever possible and installed boiler drains. Have a 1/2” tie in for eventually connecting direct to my kitchen sink and dishwasher which are above the boiler room.

  • acwagner
    acwagner Member Posts: 505
    edited November 2019
    Cool post. I love the data.

    I'm trying to work out how undersized you are. Did I get all this right?

    High Fire input = 170K BTU/Hr
    High Fire output = approximately 140k BTU/Hr
    Max Connected Radiator Load = 163K BTU/HR
    Indirect Water Heater = ????
    Max Building Heat Loss = 90k - 110k BTU/HR

    So, you basically have a boiler that is sized for building heat loss, not for max connected radiator load, if I understand it all correctly.

    What's the building envelope like--original or have there been improvements with storms windows and insulation? It sounds like your radiator load is more than your heat loss.
    Burnham IN5PVNI Boiler, Single Pipe with 290 EDR
    18 Ounce per Square Inch Gauge
    Time Delay Relay in Series with Thermostat
    Operating Pressure 0.3-0.5 Ounce per Square Inch

  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Yes, boiler NET output less pickup matches heat loss. But most of the pickup is in conditioned space (2nd floor risers exposed and basement header creates sort of like radiant floors as most of hte heat stays at the ceiling.., so really it still exceeds it. There are poor quality storm windows but they are moderately sealed. I have something like 50+ windows including transoms, <25% window area. House is 3600sqft plus full attic with high ceilings with fiberglass insulation under attic floor and foam board on attic ceiling (eventual project to finish it). Construction is 3 course structural brick with a 1/2” air gap between the face brick and the 2 structural courses. Probably an R3 or so. Sill plate is covered by a cut stone face. Doors have improved weather stripping so pretty well sealed.

    Radiation is oversized because it was a coal fired vacuum vapor system (still have coal bits above the rafters in the coal bunker room in the basement. Have one original Hoffman 2 vent and the main vent may have been original too. Was so green and corroded I couldn’t read it and had to cut it out. I think oversizing hte radiation by 30-50% was common so you didn’t have to shovel coal as often.

    Indirect load is about 100k on smaller draws and recovery with 185-190F incoming water temp. Probably peaks at 110k on longer draws with cooler water in Feb. depending on incoming water temp and draw rate. I have the boiler drop to low fire on a DHW call so it’s basically priority. I’d rather have the boiler not steam then have only 10-30k going out to the header. Doesnt’ seem to cause any issues since there’s no pressure built in the header anyway although air does start to draw back into the radiators and header. I was more concerned about short cycling and overshooting since the aquastat is not optimal location and tends to read low while the pump is running. Other odd wiring configuration is that the aquastat will maintain the boiler at 110-130F at all times using high fire. I have a recirculation loop (every house should have this) and only a 40 gallon tank, so I think it calls for DHW every 4-6 hours even in standby.

    I might drop the tank temp to 130F in summer and aquastat down a little to reduce standby losses. Lo fire is called with aquastat reads 145-160F (but it’s really 15F hotter than that.

    Thinking about ditching 2 stage and switch to a step open valve like it originally had. It lights off a little strong right now.

  • mikeg2015
    mikeg2015 Member Posts: 1,194
    edited November 2019
    Some historic home “Porn”. Shots of the house including right after it was built around 1907. Also some radiators and a picture of the boiler when I found it just after scrappers had already cut the header and knocked it over. And the TP holder from the company that installed it. They are still in business but only commercial and industrial now.

    The wall radiator I drilled and tapped a new vent on the 2nd row so it only heats 1/2 way up. It was roasting the whole stairwell and upstairs foyer.

    Yes, some idiot painted the radiators brown and installed that ugly tile floor. It was the early 90’s. 4 of them are still the original bronzed color. Gorgeous. One other was repainted bronze and looks descent. The rest are poop brown. Future project to paint most of them so they stand out.

  • ethicalpaul
    ethicalpaul Member Posts: 4,822
    Look, a pop-up ad on the TP holder! The more things change, the more they stay the same
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
    mikeg2015icy78Erin Holohan Haskelllchmb
  • mikeg2015
    mikeg2015 Member Posts: 1,194

    Look, a pop-up ad on the TP holder! The more things change, the more they stay the same

    The company is coming up on their 150th anniversary and offered $150 for it. It’s neat to have, but they are more likely to preserve it and worth more to them.

    I might ask them if they have any old steam boiler stuff in trade instead.
  • ethicalpaul
    ethicalpaul Member Posts: 4,822
    Phone number: 220 LOL
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    edited November 2019

    Phone number: 220 LOL

    It was an early exchange. At one point a local tycoon started his own phone service to compete with Bell and some businesses had 2 phone numbers for a while. They eventually drove him out of business and he went bankrupt, having sold his other primary business years later. At one point he even had the first power plant in town below the bluff near his home. They charged by the number of light bulbs you had. In my attic there’s a board with a sheet of asbestos coated cloth where the original main service came in when my house as built. It had both gas lighting AND electrical service. There was a pile of old fuses there as well. I theorize it was the only panel at first with just some lights upstairs only almost as a novelty. Then was later expanded and main service relocated to the basement and fed to a sub panel upstairs.

    My favorite find, electrically, was a 240V feed to. Assume an old, small electric range. It wasn’t a dedicated feed, rather they took two 120V circuits on opposite legs that were running around the basement for general lighting use, and combined a hot leg from each to make a 240V feed at a set of fuse holders at the ceiling. I think it was called a Carter system. Not easy to trace or troubleshoot. Fused neutrals are also a little tricky and there no color coding of wires.

    SOrry, not heating, but cool old house stuff.
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    I opened up the skim port! I knew there was plug on the back of hte boiler opposite where the control are. I have to drill and cut laterally to remove it. Bonus, Turn out it’s 1” (thought it was 3/4”).

    Threw on a 1” nipple, then a Tee with a 1/2 boiler drain facing down and plugged the other end. It’s been skimming for the last hour. I left DHW hot water loop active, so it will stay warm. Probably let it skim maybe 2 more hours and just leave the hose attached so I can skim more in a couple weeks.