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Zone that doesn't heat properly
I replaced a Weil Mclain HE4 propane boiler with a Lochinvar Knight KBN106 about 5.5 years ago. I have 6 zones now but had 4 zones before I replaced the HE4.
Zone 1 = 466 sq. ft. with 16' of baseboard slant-fin radiators and 3/4" copper pipe under the slab.
Zone 2 = 475 sq. ft. with 20' of baseboard radiators
Zone 3 = 367 Sq. ft. with 17' of baseboard radiators
Zone 4 = 407 sq. ft. with 15' of baseboard radiators
Zone 5 = 840 Sq. ft. with pex in concrete garage floor
Zone 6 = 547 sq. ft. + stairwell with 38' of baseboard radiators
The smallest zone (3) will not maintain above 65 degrees and the thermostat thermometer never reads above 58 degrees even though an accurate thermometer in the middle of the rooms reads 65. The zone is almost always calling for heat. This is the case whenever the outside temp is below freezing. No other zone has any trouble heating the space up to where the thermostat is set. Zone 3 is also the closest zone to the boiler. The zone worked fine before replacing the HE4.
Before I called for help I had cleaned radiator fins, flushed the zone, replaced the thermostat, replaced the pump for the zone and switched positions on the controller.... all with no change. After the Tech support guy at Lochinvar suggested monitoring the delta T for the zone (which was negative as it turned out) I installed a backflow preventer in the return line - - still no change. The next tech suggested I change the position of the expansion tank connection - still no change. He also suggested flushing out the anti-freeze and cleaning the system, refilling with only conditioned water. Left the cleaner in for 2 weeks then refilled with water and conditioner. Still no change. As I see it there is nothing left but to tear out the piping and replace with new clean pipes. when flushing the zone there seems to be no restriction in flow through but there must be something restricting flow that limits the amount of heat the radiators emit. I have had 4 different heating Techs come to fix this problem and none of them could figure out how to correct this issue... everything done so far really hasn't even improved the situation noticeably.
Has anyone run across a problem like this - - any suggestions?