Multi Stage Gas Furnace with Single Stage AC

[sng]

Hi,

I replaced my AC six years ago with single stage 16 SEER Carrier Performance unit. Now I need to go replace the 24 years old 92% AFUE single stage gas furnace part. I am in the DC area and during the summer the air seem a little musky when I first get home, I assume due to the humidity with the AC not running as much when I am at work. Does getting a multi stage gas furnace with variable blower help with this?

In general does it make sense to replace with a multi stage gas furnace when you have a single stage AC.

Thanks!

[Poor Rod]

I don't think it will help with humidity, as the single stage AC compressor isn't going to work well with the lower airflow. When in cooling mode, the system should be using the higher fan speed. The lower fan speeds would be used during heating only.

If you can, getting a new variable speed compressor to go with the new furnace would be best (and will likely be a higher SEER), but I also think that scrapping a six year old system is a bit wasteful. If the humidity is just a minor problem, can you use a stand alone dehumidifier?

[sng]

I don't think it will help with humidity, as the single stage AC compressor isn't going to work well with the lower airflow. When in cooling mode, the system should be using the higher fan speed. The lower fan speeds would be used during heating only.

If you can, getting a new variable speed compressor to go with the new furnace would be best (and will likely be a higher SEER), but I also think that scrapping a six year old system is a bit wasteful. If the humidity is just a minor problem, can you use a stand alone dehumidifier?

Yes, I can use a stand alone dehumidifier.

In general, does it still make sense to get a multistage gas furnace with a single stage AC for the heating portion? Thanks!

[sport]

I don't think it will help with humidity, as the single stage AC compressor isn't going to work well with the lower airflow. When in cooling mode, the system should be using the higher fan speed. The lower fan speeds would be used during heating only.

If you can, getting a new variable speed compressor to go with the new furnace would be best (and will likely be a higher SEER), but I also think that scrapping a six year old system is a bit wasteful. If the humidity is just a minor problem, can you use a stand alone dehumidifier?

Yes, I can use a stand alone dehumidifier.

In general, does it still make sense to get a multistage gas furnace with a single stage AC for the heating portion? Thanks!

We did this in a previous house 25 years ago. The new furnace was a two-stage unit with 4 burners. In most conditions, the furnace used only 2 burners and the fan was on low speed. When more heat was needed, all 4 burners came on and the fan was on medium speed. When we used the AC, the fan was on high speed. It all worked just fine.

[strafe]

Yes. A variable speed blower can help a great deal with humidity. The “standard” airflow for AC is 400cfm per ton. With a humidity sensing thermostat and fan speed under control of the thermostat, the thermostat can slow down the fan speed or overcool slightly when the humidity exceeds set point. Alternatively you could set the airflow to 350cfm/ton all the time for better humidity removal. (Slower airflow means colder discharge air temp and lower dew point. However total cooling capacity decreases somewhat.) A variable speed blower is worthwhile because it maintains the programmed airflow. In my experience with multiple HVAC contractors, every one of them has cut corners and not measured airflow or static pressure. To some degree a variable speed blower overcomes contractor sloppiness.

If humidity is a major concern consider a whole house dehumidifier. Ideally the dehumidifier would have its own return vent and be connected to the supply plenum.

[sng]

Yes. A variable speed blower can help a great deal with humidity. The “standard” airflow for AC is 400cfm per ton. With a humidity sensing thermostat and fan speed under control of the thermostat, the thermostat can slow down the fan speed or overcool slightly when the humidity exceeds set point. Alternatively you could set the airflow to 350cfm/ton all the time for better humidity removal. (Slower airflow means colder discharge air temp and lower dew point. However total cooling capacity decreases somewhat.) A variable speed blower is worthwhile because it maintains the programmed airflow. In my experience with multiple HVAC contractors, every one of them has cut corners and not measured airflow or static pressure. To some degree a variable speed blower overcomes contractor sloppiness.

If humidity is a major concern consider a whole house dehumidifier. Ideally the dehumidifier would have its own return vent and be connected to the supply plenum.

Thanks! Would I have to go to the high end Carrier Infinity thermostat to get the humidity sensing capability? Just wondering how much of an upgrade to the thermostat is needed?

[strafe]

There are multiple options with both communicating (eg Carrier infinity) and non-communicating systems. You should ask your contractor.

I would add that the most important part for humidity control is correctly sized AC. Both furnaces and ACs are often grossly oversized, which leads to decreased comfort from run times being too short. Do not assume the existing system is correctly sized. A “Manual J” load calculation is essential.

Edit: As I read your initial post more carefully, I personally would proceed with a variable speed furnace and get a thermostat that can control the blower speed. Would not replace your existing 6yr old AC unless oversized. Your money may be well spent on a dehumidifier.

[blueoval9942]

Its done all the time in northern climates. The blower speeds for hi/lo heat can be set independent of the blower cooling speed so you are not compromising performance with this type of combination.

[crefwatch]

In northern NJ my Lennox high-efficiency gas furnace (four speed fan) is hooked up so the lowest speed is "Fan [alone, switch on thermostat], Medium is Heat Stage 1, Medium-High is Heat Stage 2, and Fastest is Air Conditioning. No logic, no humidity sensor. Isn't the high fan speed to keep the coils from freezing up when humidity is high?

I know the HVAC company has an incentive to sell stuff (I tend to trust my guy), but they recommended replacing the A/C and the heat at the same time. As labor costs have increased in the last 20 years, that makes more sense than ever. And my current refrigerant is discontinued, so VERY expensive if needed.

[cbs2002]

We just put in a single stage AC with a two-stage burner 97% furnace and variable blower. I looked at variable compressors but the cost differential was about $5K and a heat pump was even higher. The blower speeds are set internally using switches on the unit, not at the thermostat.

The contractor should tell you that a single stage AC is designed to run at a consistent blower speed based on tonnage, and running it lower risks damaging the compressor. How much risk? I don't know. But it's not designed to run at a low fan speed for longer run times.

We are in a more northern climate so the furnace is more important to us than the AC. While I would like to have the humidity control and lower noise of a variable compressor AND blower, the $ just didn't make sense for the 2 months (max) that we use the AC. If I was in DC, I likely would have bought a variable speed system or heat pump. I hope the furnace meets my expectations for more consistent heating and quiet operation this winter!