Five years ago we remodeled an older single-story wood-frame SFD of about 1200sf. We insulated as much as possible and replaced all windows and exterior doors. Heating was by numerous electric wall units which we scrapped. We opted to install a new forced-air heating system consisting of an Aqua Therm (First Company, Dallas TX) model 36HBOB horizonal unit which derives it's heat from a 50G high-recovery LPG water heater that also supplies our hot water. All registers and the cold-air return are in the ceiling as is the Aqua Therm unit. Plumbing distance from the WH to the "box" is not more than 10 feet; all water plumbing to and from the box is 3/4" copper insulated with foam.
We're in Northern California where winter morning temperatures of 30F are typical. System worked OK until this year, but now seems to take forever to bring the interior temperature up in the morning. We like to shut the unit off at night which causes a drop to maybe 58F when I get up at 5AM, and it now takes upwards of two hours to get the interior up to a tolerable 66-68F whereas it used to do that within an hour. We always had to be careful to minimize use of hot water on cold mornings (an acceptable trade-off we believed) but now we don't use a drop until the home is close to warm.
I've tried to compensate by cranking-up the thermostat on the water heater, but it's now close to maximum seemingly without much improving the situation and we really don't want the house water that hot for a lot of reasons, mostly safety. We also don't want to leave the heating system on at night as the air-movement noise bothers wifey a lot. The filter at the return is regularly changed and there are no visible filters in/on the air handler box. The air flow from the registers does seem normal and we've made no adjustments whatsoever to the registers.
I'm sorry I don't have any solid temperature measurements (either water or interior duct) for you to think about. I just know that it wasn't this way for the first four years. Anyone got any ideas that I can try before calling an expert?