Adding Baseboard heating zone to existing gravity boiler

I have an older home with an existing gravity hot water heating system (Pre-1940's). The natural gas boiler is a newer (1990's vintage). I want to add a new baseboard heating zone for an existing attic remod (that has no working heat)…….but I don't want to impact the existing gravity system, which works fine. I estimate that I need 15,000 BTU of heat and the existing 130,000 BTU boiler should be able to supply the needed heat.
I want to avoid using electric baseboard or heatpump due to high electric prices in my area and limited electric circuits in my panel.
What options do I have? Are there valves or pumps that can support the new baseboard zone without impacting the existing gravity system? Do I need a heat exchanger tied to the existing gravity system with the pump on the baseboard side? Or should I just get a heating capable water heater…..or combi-boiler?
Comments
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You can do that with a pump and a thermostat to operate the pump. Your first step is to locate the 3/4" or even just 1/2" supply and return for the new zone at about the same place on the supply of the boiler and the return of the boiler. This is so you don't interrupt the gravity flow of the rest of the system. see illustration to follow
Then you control only the pump with the new thermostat. do not operate the burner with the new attic thermostat. The attic thermostat will call for heat and operate the pump to circulate the water at whatever the temperature is, The attic will only get heat when the main system thermostat powers the burner. So there is a possibility that the attic may be a little cooler than the main house, but that is unlikely. Since the main burners will operate longer that the attic zone needs, that thermostat will keep the attic from overheating by stopping the pump. You may want to get a pump that has a internal flow check so you get minimum gravity flow thru the attic loop.
By adding a Tee fitting before the boiler drain and before the relief valve, as shown in this illustration you create a path for the pumped zone that will not interfere with the gravity zone.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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If the bleeders on the rads are old, or painted over, replace them while the new zone is being done.
Is there a compression tank in the ceiling near the boiler, or is there something in the attic?
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You have to have enough pressure in the system to get the water above the highest point of the new attic piping.
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The problem is the gravity is a large mass low temperature cold start system . The high limit is set to about 160*. Which the system will only reach on the coldest day of the year . It heats very slowly with its mass to prevent over heating and to give a flat heat curve ..
The attic with baseboard heating will be a high temperature low mass system which runs on 180*-200* . Installing over sized radiators sized the same as the main system would be better but the heat cycle will be crazy dogging off the gravity with the higher heat loss of a converted attic room ….
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I was searching the topic and saw this post, I see it is from August 2024, so yes this may be an "old post" but I'm just here to say that at some point, someone connected a slant fin baseboard right to my gravity system, no separate zone. It works pretty well shockingly, even though it is breaking some rules. The heat unfortunately is short lived as baseboards don't hold heat like a cast iron rad would so again, this is not a firm solution. But gravity tops out at three floors so it could be done in your situation but again, warmth would be limited. I actually utilize a portable plug in Intertherm Hot Water Electric baseboard set to 500 watts on most normal cold days. When frigid temps hit, I bump it up to 1000 watts.
I'm curious what route did you end up going? @greenplain07
Lifelong Michigander
-Willie
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