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Upstairs radiators are very oversized
I've been working on my two pipe steam system for the past several years and I've gotten to the point where the system is nicely balanced most of the time. When outside temperatures are above about 20 degrees, the system heats nicely and evenly without building any more than about 2 oz/sq in of pressure. None of the radiators get filled enough for any of their steam traps to close at these temperatures. All of the radiators get hot within a couple of minutes of each other and I don't see any temperature differences between rooms greater than about a degree. It's very comfortable.
This all goes out the window when a cold snap arrives. Calls for heat get longer and the upstairs bedrooms can climb up to about 78+ degrees while every room downstairs maintains the 71 degrees that I set on the thermostat. I did heat loss calculations for every room and compared them with EDR of currently installed radiators, and made adjustments for radiator boxes where they exist. What I found is that all of the downstairs radiators are properly sized, while each bedroom has a grossly oversized radiator. According to my estimates, I should have about 12-13 EDR for the guest room and the nursery, and about 25 EDR total for the master bedroom. As it is now, the nursery and guest room each have a big 30.25 EDR rad, and the master bedroom has two of these monsters for a total of 60.5.
What I think is happening is that when colder weather creates long heat calls, the downstairs radiators reach capacity and the traps close, redirecting all of the steam upstairs to the oversized rads which still have open traps.
I tried installing Danfoss TRVs on these four rads a couple of years ago and did not care for them. I changed the valves back to Hammonds this year. The TRVs worked during cold snaps, but during more typical 20 degree + temps, they left the rooms cold. These valves do not open nearly as far as a standard radiator valve and I think they restricted the steam flow enough that it just went to fill other radiators. The TRVs were installed properly, sticking out sideways away from the radiator.
Right now I'm managing this by throwing an MDF slab on top of the radiators and draping a light blanket over them. It's comfortable up there now, but as soon as the cold snap is over I'm going to have to take the slabs and blankets off. I'd like a way to solve this so I don't have to mess with these radiators throughout the heating season.
Some ideas I had:
1. Put the TRVs back on, and throttle every other radiator with a standard valve to try to match the flow restriction of the TRVs. Seems like it might be hard to do effectively.
2. Build radiator boxes upstairs. This would help, but probably won't give me the 50% reduction I'm looking for.
3. Buy some correctly sized radiators and replace the oversized ones. Would probably work best, but would be expensive and labor-intensive.
4. Switch from 1 cycle per hour to 2 cycles per hour to shorten heat calls. Not sure if this would work, but seems worth a try.
Any advice on the best way to handle this problem?