If your furnace is located in a basement/utility room, especially if shared with laundry, I hope the two 'vents' you refer to, are not there. Likewise, the grill before the filter would not be a good idea. If there is a vented water heater in the same area, you could back-draft exhaust fumes into your air system.

In such a situation, outside cold air would be' pulled in' since the area would be in negative pressure, and you would be paying to heat it.

Cracking open the panel allows (possibly dirty) air to bypass the filter, and could cause clogging of your air conditioning coil.

High efficiency furnaces need to blow a lot of air, so a lack of return could increase noise and certainly reduce efficiency.

Could you post the model #s (or size/efficiency) of the old and new equipment?

V

Thanks for the reply. The 2 new vents were upstairs in the living room and one bedroom. The grill before the filter would be about 4-5 feet from the water heater. I believe the water heater is vented since the top opening is just below a pipe that goes through the roof and to the outside? What you say makes sense about installing the grill there.
My house is small - about 1100 sq ft above ground and 768 in the basement. Half the basement is the laundry room and where the furnace and hot water heater is. I had a 1978 Luxaire, Model GSU082MF. This ran fine, but I wanted to get AC and, gain the efficiency of a new furnace. I replaced it with an American Standard, 2 Stage Gas with variable driver blower and coil. If I'm reading my paperwork correctly, the furnace is model #AUH2B060A9V3VAC, the AC is Model # 4A7A3024D1000AA, and the Coil is Model #4TxCB025BC3HCAA.

Are the upstairs vents ducted to the furnace, or are they open from one area to another area (center hall, for example)?

Being located near the laundry is less desirable, as fumes (bleach, fabric softener, etc.) would be drawn into the airstream (not good for the heat exchanger).

Hope you were provided with a two stage thermostat. When used with a single stage stat, the furnace stages up after x minutes, whether it needs to or not. A two stage stat decides when to stage up depending on load, and when to stage down. Results in longer cycles (more efficient) and greater comfort.

I could not find specs on your 2 furnaces. Might need to have some modification of the return ductwork.

V

Both vents hook into a larger duct work that runs almost the length of the house. From what I can tell, this is the main return duct work. I cleaned this last night (previously cleaners left cuts into it that I could remove) and saw where both returns hooked into this.
The laundry machines are probably 12 feet away from the furnace, so hopefully that is far enough.
I believe it is a 2 stage thermostat - it says it is for 'single-stage and multi-stage heating and cooling systems'. The model for this is ACONT602AF22MA and I could find it online.
I checked the furnace model number on the unit and I have it right. So, I looked at the installer's guide and among other models, it is UH2B060A9V3VA - the prefix of A and suffix of C are not there.
If I have to modify my return duct work, I assume it is to add more airflow?

Both vents hook into a larger duct work that runs almost the length of the house. From what I can tell, this is the main return duct work. I cleaned this last night (previously cleaners left cuts into it that I could remove) and saw where both returns hooked into this.
The laundry machines are probably 12 feet away from the furnace, so hopefully that is far enough.
I believe it is a 2 stage thermostat - it says it is for 'single-stage and multi-stage heating and cooling systems'. The model for this is ACONT602AF22MA and I could find it online.
I checked the furnace model number on the unit and I have it right. So, I looked at the installer's guide and among other models, it is UH2B060A9V3VA - the prefix of A and suffix of C are not there.
If I have to modify my return duct work, I assume it is to add more airflow?