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My house was built in 1969. I have pipe and fin registers in my house. In Dec. of 2012 I replaced the original furnace with a Navien NP-240A (199,000 Btu's heating approx. 2500sq. ft.). The first winter the system kept both circuits (bedrooms on one circuit, living area on another circuit) at the temperature set on the thermostat.

In winter (2) I noticed the bedroom circuit was not keeping up (actual temp 2-3 degrees lower than thermo setting). I checked to make sure there was not any air in the system (no air was found). The bedroom circuit has a crawl space, the living area does not but has a basement that is not heated. I thought that must be the difference. By winter (3) I have insulated the pipes in the crawl space (R7). This did not make difference.
I placed thermometers on the outside of the input pipes and return pipes for each circuit, with insulation covering the thermometer probes. I found the input temps to be approx. the same, but the return temp from the bedroom circuit was significantly higher (10-15 degrees) than the return temp from the living area. This said to me the heat for some reason is not being dissipated on the bedroom circuit. During winters 2 and 3 I have tried water pressure from 12-15psi.

Winter (4) I was having the same issue with the bedroom circuit and then just yesterday the living area circuit did the same (2-3 degrees less than thermometer setting).

The bearing assembly for the circulating pump was replaced in Dec. 2011. Expansion tank pressure is 13psi.

Hopefully someone has a solution to this problem.
 
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