HVAC question about muti-stage vs two stage (w/ECM) furnace

We just got a shiny new furnaceinstalled yesterday to replace our previous 40 yr old model that was oversized for the house. After only one night, I feel like I can tell a huge difference. Temperature was much more consistent, air circulation was better, overall comfort improved by large degrees of magnitude.

But there is something that still escapes me.

The new furnace is two stage, with an ECM motor and matching communicating thermostat. As a result, the blower ramps up and is precisely controlled in terms of air flow based on what is needed. The ECM motor is far more effecient, electronically, than a PSC motor - and probably much more electronically efficient than the previous blower that was in place.
I get all that. Electricity savings. Got it.

I also get the idea of a variable gas valve, that opens and closes in small amounts depending on how much gas is needed. Need 30% capacity? Open valve 30%. Need 60%? Go with 60. Need full operation? Open full bore. Saving $$ on gas. Understood.

However, if the furnace is two stage - how does having a variable speed motor improve efficiency in terms of gas usage? It would seem to me that a dual stage furnace, where full blast operation (stage 2) is 60,000 BTU and normal operation (stage 1) is 40,000 BTU (or whatever) - that it is going to use the same amount of fuel at the lower setting regardless of the speed of the blower motor. So if you only need 20,000 BTU (or some amount of heat lower than the 1st stage is capable of producing), and the blower is running at some relatively low speed - where does that extra heat that was generated go?

With the flue outlet still producing a white plume, I’d imagine that heat goes somewhere other than right up the pipe and to the outside.

I’m not sure I’m stating my question too awful clearly - I guess I’ll see what the responses are and refine based on what I see.

I don’t think the variable speed motor is used that way. My reading indicates that the blower is set at a certain capacity, which the controller “thinks” of as 100%; the blower circuits then start and stop the motor using a “soft-start/soft-stop” profile. The soft start, running at, say, 50% of capacity, reduces humidity during AC operation, and a well-chosen AC will operate almost continuously at 100%. I don’t think it does much for you during heating. The motor speed is set according to the requirements of the system; there is no variation in demand.

The main benefit of an ECM is that it uses less electricity at full capacity than a PSC motor; it also runs cooler, which is great for an AC system. I don’t see how varying blower speed during operation would be of any benefit – it’s not moving heat, it’s moving air, and the volume of air that needs to be moved does not change unless you remodel the house or the ductwork.

A low speed is set for continuous air circulation. A higher speed for the 1st stage. A higher speed for the 2nd stage. With more heat from the burner you can move more air through the heat exchange and get the same temperature of air out at the duct. It’s all about keeping the air at a more constant temperature and not having bursts of heat that will cook the person’s leg in front of the duct.

Who told you that a variable speed blower on a multi stage furnace saves on gas useage?

There were multiple options available when we were picking what we wanted. In a nutshell, they boiled down to:

  1. single stage valve, single speed (basically what we had - but more appropriately sized for the house)
  2. dual stage valve, dual speed motor. My impression of this was that during low operations, the motor runs slowly, and during higher heat demands, the motor runs faster
  3. dual stage valve, infinately variable speed ECM motor. This is what we ended up getting. It is supposed to ramp up and modulate motor speed based on what the communicating thermostat tells it - which goes through a learning process to determine how quickly it heated up to desired temperature last time, and adjust accordingly for maximum efficiency.
  4. multi stage valve, variable ECM motor. Like option 3, but more precision on how much gas is used.

In reading various websites, it appears that the variable speed motor can indeed change, and not just run at predetermined set points. Example.

So while the motor speed can change, and at one moment be 40%, then go to 55%, or 64%, or 75%, or whatever - the gas valve has only two settings. If the valve is running at low capacity, and the blower is running at, say, 60% - the temp coming out of the register is X degrees. If the valve is running at low capacity, and the blower is now running at, say, 40% - now the equation for heat transfer is different - so I’m curious what that results are. Warmer air, but a lower flow rate?

I don’t know if this will help…

Imagine 2 **exact **sauce pans, sitting on your gas stove. In one you put 1 quart of water, in the other 2 quarts of water. You turn on both burners to the exact setting. Both pots of water are being subjected to the same heat output of the 2 burners.

Both pots of water are seeing the same BTU input then. In other words, the same amount of heat is being transferred to the water in each pan. They have the same *heat content.
However, they don’t have the same temperature. Because of the greater volume of water, the temperature in the pan with 2 quarts will be lower. It can be said the larger volume of water “dilutes” the BTUs or “disperses” them over a greater area.

The same thing applies to air. If the air volume decreases------and the gas output stays the same-----the temperature increases, although the heat content stayed the same.

So your perception may be that the heat is different because the temperature is different (and it is) but the heat content is the same.

In any event, the temperature for a gas furnace is typically between 105° and 125° and I would imagine that the ECM is ramping up and down with the gas valve to maintain air temperatures in that range.

So I think it is helping your comfort, but I don’t think the ECM is helping gas usage in any appreciable way.

I agree with Raindog. ECM is about comfort.

If you’re referring to the fan flow rate, yes.

Heat output is the same.

Efficiency of gas usage is the same.

But, because its being transferred to less air, the temperature will be warmer.

(although the fan will be using less electricity…)

After making this OP, and reading up on it some more, I realized I was a little confused in my initial thinking about the ECM and gas usage efficiency.

I often overanalyze things, and this becomes a problem when I read things that are not descriptive enough - such as those types of things written in articles for laymen, or marketing material. At a high level, those types of texts basically appear to say that with a variable speed motor, efficiency is improved - and since they generally speak about gas efficiency (via multi-stage gas valve, proper sizing of the units, etc) prior to mentioning the ECM motor, I drew the conclusion that the ECM motor also helps reduce gas usage. However, they are speaking of electical efficiency.

In this case, there was something in the back of my mind about that initial conclusion that didn’t make sense - so I just needed to either figure out what I was missing, or be validated in thinking that gas usage doesn’t change with the ECM motor.

Good point, and all true.

There is the appliance’s gas efficiency (and vis a vis older appliances), and then the electrical efficiency (here too, vis a vis older appliances) and also the overall combined efficiency gains; gas and electric.

An ECM motor draws about 1/4 to 1/3 the current of a similar PSC motor, so even the electrical savings are significant.