Possible less restriction in new coil [no dirt] causes more airflow and lower temp rise.

 

I had ductwork improved when furnace was installed last year to reduce static. After furnace install it was less than 0.3 in low fire at 1265CFM and less than 0.6 in high fire at 1822CFM (with old dry coil), so I don't think airflow changed, but logically you would think that.

 

I'm gonna guess that they didn't take the static pressure readings at the right places. A CAPT4961 dry coil has a pressure drop of .159 at 1200 CFM, and .232 at 1800 CFM. That would mean you would have some very large duct work.

 

The static numbers I quoted were measured with the new furnace but the old coil. The measurements were taken with a probe on the blower side of the media filter and through a hole in the furnace below the heat exchanger. The hole was where one of the safety switches goes. The switch was reinstalled afterwards. The ductwork is large but the restriction had been primarily on the return. Added additional return and two 5" media filters. Was quite pleased with the new furnace during the last year. Limped through the summer with the leaky 20 yr old AC, but decided to replace the 5 ton with a 4 ton 2-stage, because I was so pleased with the 2-stage furnace. Just trying to figure out why the temp rise dropped with the new 4961 coil. Don.t have a manometer to measure static with new coil but guess it could be higher than old coil, especially wet. Have called furnace installer to come check everything while still within my 1 year install labor warranty.

 

Sounds more like it is lower then the old coil.

 

A CAPT 4961 is a very large coil. Probably larger than your original coil. Larger = less restriction. You didn't loose BTU's anywhere, now just have less restriction & more flow which is a good thing

 

I guess I thought the ECM blower would maintain constant airflow with small changes in static by changing RPM! Not true??

 

Yes, it will.

 

So it would seem that regardless if the static went up or down (within reason), then the ECM blower should maintain the same airflow from one coil to the next.

Assuming the gas flow and air flow remained constant, but the duct sensor reads a lower temp, then the heat got lost on the way to the duct sensor.

Would the new larger coil (and/or refrigerant) be absorbing the heat (more heat), than the old coil?

 

I just love it when people give information out sparingly so who ever is trying to help is always wrong. is it variable speed or an x-13 motor? In my opinion if an x-13 motor and lower restriction evap coil then air flow can increase and give you a lower temp rise.

 

As stated in my original post, the furnace is an Amana AMVC951155. For those unfamiliar with this furnace it is a 2-stage with variable speed ECM blower.

 

More likely the same number of BTU's are heating more air than formerly.

PHM
-------

 

I agree that seems to be the obvious answer, but with an ECM variable speed blower, where is the 'more air' coming from?

I am having the furnace installer come tomorrow to check gas pressures. I have had the furnace for 11 months and had two lockouts occur, with otherwise stable rise temps. The lower temp rise was exactly coincident with evap coil change. I'm baffled. On the other hand, without the duct temp sensors, I would be aware at all.

 

The area where the high limit is may be a turbulent area, and the static pressure read there may have been inaccurate. So that you are moving more air now. Or your gas valve may be out of adjustment.

 

you are moving more air due to the larger and cleaner evap coil. a varible speed motor will ramp up to overcome dirty coils and filters ,if you look at the blower performance chart as the static pressure increases air flow decreases

 

I had the installing company come out and check the furnace and gas pressure.

Backstory: When I had the furnace installed last October the high fire temp rise was 58 degrees and the low fire temp rise was 71 degrees (acceptable range 45-65). The gas pressures were not checked by installer (first mistake). I asked for the tech manager to come and check out install and address the high temp on low fire. I suggested that with a variable speed blower ECM blower, the high and low temp rise should both be 56 degrees. per Amana Technical Manual (available online), so it seemed like the gas pressure was high. Without actually checking gas pressure, he lowered the low fire gas setting to get 56-57 degrees and checked static on low fire (0.27) and high fire (0.57). Things were fine until replacing AC this year (see details in OP).
During this week's installer visit everything looked good with pressure switches when investigating two previous lockouts (suspected maybe water in signal hose that activates gas valve, as igniter would glow but no gas flow on last lockout). Checked gas pressures (first time actually checked). High fire was 3.52, pretty much right on. Low fire was 1.61. It was reset to 1.9 spec. Low fire rise temp is now 54 degrees and high fire is 53. From tech manual, target temp is 56. Close enough. Appears that new coil lowered static (did not re-measure unfortunately) and increased air flow which WAS NOT compensated for by the variable speed ECM blower motor, as you might expect. I think I understand that issue, but will address in a separate thread.