beenthere: Thank you this is indeed helpful. I will go back in about a week and they guaranteed up front in writing that they will change the equipment out if I am not satisfied so I need to gather ALL my facts. Therefore I would like to use this next week to become more educated.
The old furnace was natural gas at 162K BTU input. Perhaps 40% efficient would have been 64K useful heat output as a guess. The thermo calc sheet provided by the installer said 60K but that was based on the original 1800 SQ ft public records not the 2300 it is now with the closed in a ducted porch.
So if I understand you correctly I should expect (with right sized furnace) the low fire to run much longer than 33% of the time? The furnace I have now (105K) spec is 73K low 97K high. next one down is the 90 at 63K low 84K high, then the next down is 75K which is 52K low and 70K high and lastly the 60K which is 42K low and 56K high.
So it seems that the 75K or the 60K would be the ones in the running, not the 90K
I do understand that a different A/C and blower considerations also are important but I want to set that aside for the moment and focus on the heat. They will also change out the A/C.
So based on what I have provided does it seem reasonable that either the 60K or 75K units would be the areas of possibility?
Not unusual for an oil furnace to be 2 to 3 times as big as the home/structure needs.
At 33% duty cycle in first stage, a 60,000 BTU 2 stage would have been more the size this place needs.
Not knowing all of the particulars, I was thinking 60-70K as well. And yes, we see it often on older homes here in PA where a 150K furnace can be properly replaced with a 60k.
Unfortunately, I have to get involved when contractors replace that 150K with the largest condensing furnace they can get, usually a 110k condensing furnace. The new furnaces need a lot more air because the temperature differentials is so much less. Then we have the fact that so much more heat is being put into the house rather then going out the vent pipe.
Unfortunately again, furnaces do not fire up being as efficient as they have the potential to be. That is why all testing is required to be done after 15 minutes of constant run time. It takes that long for new furnaces to actually reach their potential efficiency.
By the way, beenthere, NCI is doing a CO/Combustion certification class in Coatesville at the end of February. This three day class really teaches about what we are discussing here. If you are interrested, shoot me a pm.
The thermostat will only bring on 2nd stage when the thermostat calls for it, but the furnace will go into high fire after a certain amount of time regardless of what the thermostat is telling it to do.
There are now furnace controls that will anticipate heat need and turn on the furnace "before" the thermostat even calls for heat. Currently, all York brands have furnaces with these controls in them.
The Nest thermostat changes it's programming depending on activity in the home. Any change in activity causes the Nest stat to start reprogramming. During any reprogramming, the Nest stat will call for 2nd stage of heat whenever it is recovering temperature and the difference in temperature of what the stat is calling for and what the temperature is withiin a certain variance for a certain amount of time. The temperature differential as well as the amount of time is a constantly moving target on the Nest stat, unless a household is absolutely timed exactly the same every day and the outdoor ambient temps never change.
Robo, could you go into more detail maybe start a thread on the pro side to explain the 2 stage circuitry you are talking about. I know on some furnaces if you don't use a 2 stage stat it will time low fire like you say but from what I understand if a 2 stage stat is used the thermostat controls firing rate. Please elaborate as I am Mr. Don't know it all...lol
Robo, could you go into more detail maybe start a thread on the pro side to explain the 2 stage circuitry you are talking about. I know on some furnaces if you don't use a 2 stage stat it will time low fire like you say but from what I understand if a 2 stage stat is used the thermostat controls firing rate. Please elaborate as I am Mr. Don't know it all...lol
Sounds like it would be a good pro-section conversation. There is a lot of confusion about what actually controls furnaces now days. I will start a thread titled "Multi-stage furnace controlling"
Sounds like it would be a good pro-section conversation. There is a lot of confusion about what actually controls furnaces now days. I will start a thread titled "Multi-stage furnace controlling"
OK so still sizing up my options before I go back the the contractor. One fear I have is, lets say the 105 is replaced with the 75K BTU unit. And then stage one runs shortly (12-15 as ROBO says) then it jumps to 70K output. Seems I would be in a similar mode as I am now with 73K mode the majority of the time. Meaning running at 70K BTU.
After a little investigation at the property, I feel like the wind noise acceptable tolerance level in heat mode (not cool) would be less than 900 CFM with the duct work as it currently exists, and lower better yet. This is just based on trying the fan only mode fro the various DIP selections for heat rise and not running the heat. Looking at the specs of these two RGRM-07*MAES RGRM-07*YBGS. I can imagine lower blower CFM also could make it more challenging to get sufficient flow to the second level.
Any opinions about these two models pros and cons?
I stand by every comment I made from a standpoint of working as a manufacturing and HVAC brand rep, including the brand you have in your home.
Hmm.
Here's what the manual says about the OP's furnace:
Call for 1st stage heat
a. The “R” and “W” thermostat contacts
close and the control module runs a
self-check routine. After the control
module verifies that both sets of
pressure switch contacts are open, the
induced draft motor starts on high until
both the low and high pressure switch
contacts close (a maximum of 60
seconds).
b. After a 30 second pre-purge the spark
igniter energizes and the high fire gas
valve opens, lighting the burners.
c. After the burners light, the remote
flame sensor must prove ignition. If the
burners do not light, the system goes
through another ignition sequence. It
does this up to 4 times.
d. The furnace immediately drops
inducer and gas value to low fire after
sensing flame. The main blower starts
on low-heat speed 20 seconds after
the flame is proven. The furnace continues to run on low fire until the
call for heat is satisfied or the 2nd
stage contacts close. If the 1st stage
call for heat is satisfied the gas valve
closes and the induced draft motor
continues running for a 10 second post
purge time. the main blower runs for
its blower off delay (this timing is field
adjustable). See Figure 44 for switch
settings"
You must have been reading a proposal for future design changes. Don't sweat it, nobody's prefect.
Last edited by hvacrmedic; 01-28-2013 at 03:59 AM.
Even though this is a ECM 2 stage furnace- it's still 105,000 btu's & has a 5 ton drive.
This system is grossly oversized & will NEVER work properly.
But my biggest concern is the installer pulling all the return air into 1 side of cabinet on a 5 ton drive unit. I don't have installation instructions in front of me but I do know Rheem doesn't recommend bringing all return into one side of cabinet on their larger units. Our policy is 3 ton & under for single side return, 3 1/2 to 5 ton needs either a platform to come in the bottom or 2 returns, one on each side.
This thing is starving for air & then you add a media filter?
No such thing as no room in a basement or celler. If furnace couldn't be elevated for bottom return, it should be installed as a horizontal.
You can bring it all into the bottom or split it up on both sides of furnace.
Thanks for the info. Having 5 ton capabilities does not mean it is operating at 5 ton as I understand it. Even the next two sizes down have 5 ton "capabilities". Due to height restrictions they could not raise the furnace. I thought the MFG requirements were above 4 ton to have air intake from both sides. Would brining the input in on two sides of the furnace improve the noise of the return? Thanks
OK so still sizing up my options before I go back the the contractor. One fear I have is, lets say the 105 is replaced with the 75K BTU unit. And then stage one runs shortly (12-15 as ROBO says) then it jumps to 70K output. Seems I would be in a similar mode as I am now with 73K mode the majority of the time. Meaning running at 70K BTU.
After a little investigation at the property, I feel like the wind noise acceptable tolerance level in heat mode (not cool) would be less than 900 CFM with the duct work as it currently exists, and lower better yet. This is just based on trying the fan only mode fro the various DIP selections for heat rise and not running the heat. Looking at the specs of these two RGRM-07*MAES RGRM-07*YBGS. I can imagine lower blower CFM also could make it more challenging to get sufficient flow to the second level.
Any opinions about these two models pros and cons?
The smaller capacity furnace will run better because even when it is forced into higher fire periodically, it is only to keep the heat exchanger above dewpoint. Every system can be a little different in engineering as to how it will protect the heat exchanger. One thing is for certain, the more a furnace is calling for high fire, the better sized that furnace is and the more efficiently it is going to operate.
Tomorrow (Friday Jan 1) I meet with the sales and installers at the premises.
Based on what I have learned here I should ask for the following.
Since I am at a mile high how much does that downgrade the equipment BTU capacity ? (someone here answer that?)
Why was a 105K unit installed with the documentation provided showed a lower heat loss calc
How was the heat loss calc done as I did not see any measuring going on
What is the manual J
Why is all of the return air run into one side (unit cannot be raised due to A/C unit and cellar height.
How was a 4 ton A/C unit selected
Were static pressure tests done and do they fall within mfg specifications
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Originally Posted by beenthere
