# how does a modulating furnace modulate?



## raylo32 (Nov 25, 2006)

I am looking to upgrade my HVAC and going through the usual research between single stage, 2-stage, and modulating furnaces. I have read some of the threads here on this already.

I have also looked at the thermostat connection schematics for the York 9C series furnaces and see that generally it just takes a single stage thermostat "w" wire connected to the Modulating Furnace control board W terminal.

So all the board ever sees is just a typical call for heat. How then does the furnace decide at what blower speed and flame % to run? Is it just a timed thing or are there other inputs and sensors in the furnace? I really like the idea of extended lower % running for comfort but wonder just how it works and if it is adjustable to any extent? 

I understand how 2-stage units work with 2-stage t-stats so just trying to figure out how exactly a modulating furnace works so I can compare the 2.


----------



## beenthere (Oct 11, 2008)

The York mod uses algorithms based on how long previous runs were, and what firing rate it reached.


----------



## raylo32 (Nov 25, 2006)

Interesting... any idea how it responds when you turn the heat up at the end of the day after having it setback to a very low temp during the day? Seems the recent history wouldn't really be relevant. Does the control board itself have any ambient detectors that would help with its logic since the t-stat only supplies a call for heat?




beenthere said:


> The York mod uses algorithms based on how long previous runs were, and what firing rate it reached.


----------



## yuri (Nov 29, 2008)

The Lennox SLP uses an algorithm method with a 2 stage stat. If the house temp drops 2 deg F it will kick into high fire so you don't lose ground. Same furnace can also be used as a 3 stage with 35/70/100% firing rates with a single stage stat. Doubt if most people could tell the difference anyway as the cycles get longer with 3 stages. None of the mod furnaces allow for adjustments of stage lengths. They are INCREDIBLY quiet with an ECM Variable speed motor and comfortable. If you want a large setback then the 2 stage stat version is a better idea as it can go to high fire quickly. Not sure what York uses for that scenario.


----------



## raylo32 (Nov 25, 2006)

Yes, I am looking for quiet and long runtimes. That will be great for comfort. The York manual does not show any t-stat connections for heat other than the single W wire. Nor does it talk about the modulating logic. I like the way the Lennox can take the 2nd stage t-stat input and ramp up when needed. Maybe the Yorks does this but it isn't obvious how. I still wonder if the modulating furnace control board contains any RTDs that would report ambient conditions and help it make decisions. For instance upon a call from heat from 60 degrees, assuming it knows it is 60 degrees, after sitting idle for hours the firmware might realize that the user wants to heat up fast.




yuri said:


> The Lennox SLP uses an algorithm method with a 2 stage stat. If the house temp drops 2 deg F it will kick into high fire so you don't lose ground. Same furnace can also be used as a 3 stage with 35/70/100% firing rates with a single stage stat. Doubt if most people could tell the difference anyway as the cycles get longer with 3 stages. None of the mod furnaces allow for adjustments of stage lengths. They are INCREDIBLY quiet with an ECM Variable speed motor and comfortable. If you want a large setback then the 2 stage stat version is a better idea as it can go to high fire quickly. Not sure what York uses for that scenario.


----------



## yuri (Nov 29, 2008)

I won't mention any brand names but one starts with R. Some of these/those units have more technology and sensors in them than a spaceship and lots of ways for them to go wrong and fail. Point being that you may want to stay away from any unit that has that many sensors to fail. I sell Lennox and used to do a lot of Carrier/Bryant which I like. Not sure of their system. The Lennox unit is fairly simple and uses a vacuum operated gas valve to modulate. Very simple method. Some units use a $500 proprietary tstat and varying DC voltage very expensive gas valve. Cost a fortune to fix later. Salesman won't tell you that when selling it to you. The more complicated you make it the more it costs to repair and VERY few techs know how to troubleshoot some of those units either.


----------



## raylo32 (Nov 25, 2006)

Yes, simple is almost always better. However.... I have the ultimate simple now, a 16 year old base Bryant split with one speed blower, fixed orifice, and type B vent. Any of these new systems would be a whole lot more efficient and comfortable but more complex of course.



yuri said:


> I won't mention any brand names but one starts with R. Some of these/those units have more technology and sensors in them than a spaceship and lots of ways for them to go wrong and fail. Point being that you may want to stay away from any unit that has that many sensors to fail. I sell Lennox and used to do a lot of Carrier/Bryant which I like. Not sure of their system. The Lennox unit is fairly simple and uses a vacuum operated gas valve to modulate. Very simple method. Some units use a $500 proprietary tstat and varying DC voltage very expensive gas valve. Cost a fortune to fix later. Salesman won't tell you that when selling it to you. The more complicated you make it the more it costs to repair and VERY few techs know how to troubleshoot some of those units either.


----------



## yuri (Nov 29, 2008)

Really depends on how long the contractor that installs it has been in business. If he pays top dollar he can get the better skilled techs also. The lowest priced guys attract the lame techs. If you buy a Lennox from a certified larger Premier dealer some have hidden access to 24 hr tech support to talk a tech thru any problems with any of their units and an incredible database called DaveNet with loads of tech info etc. The dealer has to pay for that info but it is worth it for the customer. You get what you pay for.


----------



## beenthere (Oct 11, 2008)

raylo32 said:


> Interesting... any idea how it responds when you turn the heat up at the end of the day after having it setback to a very low temp during the day? Seems the recent history wouldn't really be relevant. Does the control board itself have any ambient detectors that would help with its logic since the t-stat only supplies a call for heat?


No, it just starts out the same as though you hadn't turned up the temp. Then as it runs longer then it anticipated, it increases firing rate more, until the stat is satisfied. Its next run cycle it will start out at a higher firing rate, and then as it satisfies the stat quicker it recalcs its next run time to start at a lower rate.

You may or may not notice the longer first run time. But your comfort won't suffer.


----------



## JJboy (Oct 12, 2010)

yuri said:


> Really depends on how long the contractor that installs it has been in business. If he pays top dollar he can get the better skilled techs also. The lowest priced guys attract the lame techs. If you buy a Lennox from a certified larger Premier dealer some have hidden access to 24 hr tech support to talk a tech thru any problems with any of their units and an incredible database called DaveNet with loads of tech info etc. The dealer has to pay for that info but it is worth it for the customer. You get what you pay for.



I do not agree with you at this point Yuri. I am a good tech without experience. I can not do **** work. Some experienced technicians are sometimes not good. 

I understand now why is so hard a new technicians to get job in this field. My clients doesn't have any complain about my work so far


----------



## veesubotee (Nov 22, 2008)

The 'R' brand has been around the longest (about 15 years or so) and is known to be reliable.

It all depends on the dealer. While shopping for mine, one dealer stated that he didn't recommend the mods as they run 'all of the time' and use more gas. HOGWASH.

Another stated that they could install one if I wanted one, then specified a generic thermostat.

Speaking of thermostats, I've seen the genuine modulating thermostats selling on the net for under $200.


----------



## beenthere (Oct 11, 2008)

The Rheem mod was the first residential mod furnace. The Rheem mod is only a 90% efficient furnace though. Someday they may improve its efficiency. But right now it is the lowest efficiency mod on the market. As far as I know, no Rheem mod has had a heat exchanger failure yet, so that says a lot of good things about it. Without its mod stat, it is reduced to a 3, or 2 stage furnace depending if your using a 2 stage or single stage thermnostat on it.


Are the mod stats you see on ebay the older versions of it?


----------



## veesubotee (Nov 22, 2008)

beenthere said:


> The Rheem mod was the first residential mod furnace. The Rheem mod is only a 90% efficient furnace though. Someday they may improve its efficiency. But right now it is the lowest efficiency mod on the market. As far as I know, no Rheem mod has had a heat exchanger failure yet, so that says a lot of good things about it. Without its mod stat, it is reduced to a 3, or 2 stage furnace depending if your using a 2 stage or single stage thermnostat on it.
> 
> 
> Are the mod stats you see on ebay the older versions of it?


Actually, the efficiency of the RGFD (mine, which is 3 yrs old) units vary by size. Granted, they are not 95%, but my '90' is rated at 94.1%.

Going by memory, (at least) one site was selling the model 412, which is the latest 'non-communicating' version. Not saying that they aren't selling them, but I haven't noticed the stats on Ebay.

For the benefit of the OP, the older stats communicate in the sense that they 'instruct' the furnace as to how much heat to deliver--they just don't display error codes, warnings, etc. as the communicators do.


----------



## beenthere (Oct 11, 2008)

They use pulse width modulation, very accurate control of furnace modulation to heat loss of the house.


----------



## Jackofall1 (Dec 5, 2010)

beenthere said:


> They use pulse width modulation, very accurate control of furnace modulation to heat loss of the house.


Really that sounds like something from Star Trek or Back to the Future, I would have thought it would be Proportional, Intregral and Derivative (PID) controlling the functionality between the output, variable and setpoint.


Mark


----------



## beenthere (Oct 11, 2008)

Jackofall1 said:


> Really that sounds like something from Star Trek or Back to the Future, I would have thought it would be Proportional, Intregral and Derivative (PID) controlling the functionality between the output, variable and setpoint.
> 
> 
> Mark



PWM has been around for a long time.


----------



## Jackofall1 (Dec 5, 2010)

I always thought PWM'er (an inexpensive alternative to a VFD) was the device that would drive the ECM motor to variable speeds and the PID is what drives both the PWM'er and the voltage output to the gas valve, is this not the case in residential mod furnaces?

Mark


----------



## beenthere (Oct 11, 2008)

VS blowers can be controlled by either PWM or by temperature.

PID can be used to control the PWM of a thermostat.


----------

