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.