So completely replaced my underground line in the fall of 2014, replaced the nominal 1” pex with Logstor, that helped some. Still had a very large delta T when running the snow melt and the house was calling for heat.
Got around the last few weeks and completely replumbed my basement with 1” copper and used long radius elbows where necessary. The old system was 1” pex with everything plumbed in series which had to be cutting flow down.
Have three secondary loops right now, with a fourth in place and capped off for in the future for radiant heat, baseboards, heating a whirlpool tub, etc.
Now with with everything up and running, I have no problems maintaining a consistent 20 degree delta T, using a Taco Delta pump and before it couldn’t keep up. Sometimes seen a 30-35 degree differential or more.
Now for the strange part, the old fan coil was in a existing duct that was set up for a wood furnace we used to use to heat the house. Back draft damper in it and the gas furnace so one or the other or both could run and not interfere with one another. Problem was that duct work used a old belt drive blower, every time I tried to speed it up any it got very noisy, tried changing it once but didn’t help so left it run slow. Running that slow though if it was cold enough in the mornings it may have took 4-5 hours or longer to bring the house from 67 to 72.
The one I was using was a 20x20, the one that I placed in the gas furnace ductwork (the “normal” place) is a 15x24. The old one with the lack of air flow would still pull 20 degrees or more of heat out of the water, the new one pulls a little less than 15 and runs for half as long or less. Had a LOT warmer air out of the ducts with the old one, but you could barely feel any air movement, this old house is just old enough I figure it still loses enough heat that the old set up on a very cold day was just barely adding a little more heat than the house lost.
I still need to add a on delay timer to the furnace fan circuit as it feels like a cold draft when it first starts as the secondary loop pump hasn’t had time to get hot water thru the whole loop yet and warm up the fan coil. The wife has said something about it already so I suppose that is next.
EDIT: Forgot to add I used a generous amount of immersion type mechanical thermometers in the system as well, one at the line coming in, then one after the set of T’s for each secondary loop to track just what pulls how much heat out of the water.