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Basement loop 1of 3 not heating

I have a loop that isn't heating up.

The floor is in the basement , is concrete with three loops, about 900 square feet total. I didn't install the floor, but the loops seem equal in area. The pex is 1/2 connecting to a three zone ICMA manifold. My system is around three years old and the zone worked normally for the first two winters. Also i live in Alaska, temps can be -30 but its been very warm (25-15) for the last three weeks. I noticed the problem at the end of last winter.

The flow meter moved at about .50 when normal would have be around .80 or so. A week ago I did isolate the zone and connected the water hose and flushed around 60 gallons through it, via a bucket to keep it out of my septic. I did seem to push a small amount fine grit and air . The water from my well is around 40 degrees, very chilly. The flow seemed to improve, however the section of the slab really didn't heat up once it was set for regular use.

So today I did the same thing with isolated hot water from the boiler and bucketed another 80 or so gallons through the section. The return water never really heated up... much. It did improve slightly from around 55 to 62 degrees or so. I then closed the system back up drained a little air out, and set the floor at 66 with just the one zone on. I would also add that while flushing the hot water (120 degrees) I did get a steady stream of very small bubbles.

After running the one zone for two hours the basement air temp did get to 66 degrees (a 4 degree rise) and the return out -flow ran up to around 68 degrees. The floor is just slightly warm to the touch but not as warm as it was in the past or the other two zones. Also my return out-flow temps for the good two zones average between 90 and 75 when the floor has called for heat for a while. The flow meter did improve and is now equal to the other two zones at around 80.

Am I on my way and the slab just takes a long, long time to heat up?

Should I flush more until all fine bubbles are gone?


Thanks , Peter in AK