Adding Baseboard heating zone to existing gravity boiler

greenplain07

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?

EdTheHeaterMan

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.

Big Ed_4

You will need to install a separate boiler to make it work …. Two different animals …

HVACNUT

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?

EBEBRATT-Ed

You have to have enough pressure in the system to get the water above the highest point of the new attic piping.

Big Ed_4

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 ….