Upsized heat better for night setback?

I know I am asking a lot of questions, but its because I keep getting great answers. Thanks in advance, everyone.

I will downsize my cooling from 3 tons to 2.5 or 2 tons, since I made my house more energy efficient. I don't change my temp setting much in the summer.

But in the winter I regularly set the temps back from 70 to 60 from about 9pm to 6:30 am. The temperature rarely drifts all the way to 60 during that time--if the furnace comes on before 6:30 am, I know it is really cold outside (for Charlotte).

I assume I should keep my gas furnace a bit oversized so it can get back to normal fairly quickly each morning, and that my current practice of doing the night setback saves money. I would not think there is a big downside to a larger gas furnace like there is to a oversized heatpump.

Will I be getting everything out of whack if I have a three ton gas furnace with its associated blower paired with a 2 ton heat pump in a dual fuel system?

Is a three ton gas furnace going to say 36,000 btuh output, or are they measured differently? I am confused because one quote says 3.5 ton furnace at 57,000 btu. I would have thought 3.5 tons was 42,000.

PS-- Sorry--Meant to make separate thread.

 

Is a three ton gas furnace going to say 36,000 btuh output, or are they measured differently? I am confused because one quote says 3.5 ton furnace at 57,000 btu. I would have thought 3.5 tons was 42,000.

PS-- Sorry--Meant to make separate thread.

Normally a furnace is rated in BTUs and cooling in tons (but you may also see cooling specs in BTUs). The 3.5 ton number you see on the furnace is the blower rating, meaning it can support a 3.5 ton (42000) cooling unit (blower has a max of at least 1400 CFM).

Blower speeds (CFM) are set separately for heating and cooling.

 

ECM = Electronically Commutated Motor. It's a DC motor. Here's a pdf with more info
http://www.nailor.com/pdf/ecm_1.pdf

VS blower motors are ECM. not all ECM are VS like X-13

 

Th eproblem with over sizing a gas furnace is that you waste energy and you might short cycle the unit. What you should do is install a variable speed blower furncae(ECM) with a modulating gas valve. This will give you the best effeciency. Carrier has a hybrid system for this. I believe it is their Infinity sysytem.

Gas furnaces are rated in btu's for heat. The blower is rated in cfm. The heat pump a refrigeration unit is also rated in tons and btu's. One ton is equal to 12,000 btu's. You have to remember these are seperate items. One is a gas furnace, the other a refrigeration unit.

 

Th eproblem with over sizing a gas furnace is that you waste energy...

Hmm. I'm a little simple so I don't quite understand this. Does the gas flame become less efficient as the furnace size increases? Does the flame become hotter and waste more heat up the chimney as the furnace size increases?

Let me get this straight. So if my house is losing heat on a 40-degree day at a rate of 8,000 Btu's/hr and my furnace is a 60,000 Btu output furnace, isn't it oversized? Does that mean it's less efficient or that it just runs for a shorter time? What about when it's 10-degrees outdoors (assuming a 0-degree design temp)? Isn't the furnace still oversized? But less efficient?

I don't think so. Oversizing of a furnace (warm air furnace, NOT to be confused with a boiler) has absolutely nothing to do with efficiency. It does have everything to do with comfort. Smaller flames lead to less Btu's/hour delivered into the home, which means the furnace run cycle is very much longer and therefore eliminates the on/off/on/off/on/off short cycles. With a multi-stage furnace it also means that there is a calculated outdoor temperature at which the 1st stage can no longer provide enough heat to keep the home at set-point. That's when the furnace is running without turning off but rather begins to cycle the 2nd stage. Now that's comfort but the number of Btu's needed and delivered doesn't change and the efficiency of the burner doesn't change and the cost of operation doesn't change. Unless I'm missing something. Do tell.

 

You don't need to specify an ECM motor if you buy a variable speed furnace, as all VS furnaces have ECM motors.

There is one ECM motor, the X-13, that is ECM but has fixed (multiple) speeds so cannot ramp the speed. The Goodman GME8 is one example of a furnace with this motor, but I know there are others. The X-13 motor is very efficient, but not VS.

 

The X-13 is not an "ECM" motor. It is similar but has a different method for maintaining output. It does not control CFM output as well as an ECM and is a lower quality motor.

EDIT: Yes, oversizing the furnace DOES affect efficiency. The more oversized the furnace is, the more it will cycle on and off. While the steady state combustion efficiency is the same, most of the sever losses occur during startup while trying to heat up the heat exchanger. Smaller furnace will run longer which in turn leads to operating with a fully heated heat exchanger for a higher % of the time.

 

Thanks, everyone for replies. I now know what ECM stands for, and how it works, and that I should get one.

I also understand that oversizing a furnace is not as bad as oversizing ac, but still far from optimum. However, I feel a little oversized is not going to be a bad thing in this case. Since I am going dual fuel, gas will only come into play when it is either very cold and during morning recovery and so won't short cycle much. The heat pump will then take over to maintain temps most of the time.

With the heat pump and doing dual fuel, is it still worth it to get a two stage or modulating burner? My initial inclination was to get a two-stage burner.

 

Yes.

Actually, allmost all (or all) furnaces with VS will be 2-stage so you won't have much to decide on that.

In an case, 2-stage will usually be more comfortable and quiet.

ETA: Also, when you are looking at specs, be careful of advertising terminology. If it says "multispeed" that's probably a PSC motor, with selectable blower motor speeds that are set by your installer. If it says "variable speed" that's going to be ECM. The X-13 motors are a little harder to discern, but if it says it's ECM but not Variable Speed, that's probably a X-13. IMO, the X-13 is just as good as the VS, if you don't care about the ramping capability of the VS motor.

 

The X-13 cannot maintain CFMs throughout the static pressure range as well as an ECM motor. That is the biggest negative of the x-13 vs the true ECM motor.

 

Ah, I forgot about that, the X-13 is a constant-torque motor, right?

From the charts I've seen though, the X-13 does way better at maintaining CFM than the PSC does.

 

For what's it worth, an ECM motor is also a very expensive motor when compared to the proven, simple, been around for ever, induction motor. About 7X more expensive.

 

No not quite 7x the cost.

If my customer has a standard PSC blower motor go out I can replace it with the original or with an ECM (evergreen motor) for less than double the price of original.

If their oem ECM motor goes bad its actually about the same cost and can be less than replacing a standard motor due to there being less labor.

And ECM motors have been around for about as long too. Just weren't used on resi hvac equipment until the 80s.

 

heatpumps

Question for everyone. I just installed a 2ton gibson heatpump with aux heat. The temp is average of 35 to 45 degrees outside. My customer is keeping it at 70 degrees in the home. Is it sound normal to have a run time of 30 to 40minutes

 

I think you misplaced your question here, but ...

Whether this is "normal" depends on the heat loss of the house. But more importantly, if it's maintaining the setpoint at the design low temperature or below then it is sized right. Long runs times are fine and can result in better comfort.

 

The biggest problem with oversizing your furnace is temp. spikes get exaggerated. Very hot then cool back and forth. size it right and walk away from it.