New furnace required - advice welcomed

[PandaBear]

Also - Here is the sequence of operations for a "multi-stage gas valve" gas furnace-

Call for heat
Fire burner 100%
Fire furnace/evap fan
Run fan/burner at 100% for a minimum TIME (5 minutes?)
If setpoint not satisfied, continue at 100% unless temperate within ____ degrees of setpoint



My problem with this is most furnace burners are grossly oversized for homes in the southeast. Firing at 100% for 5 minutes will reach the setpoint 95% of the time, resulting in the gas valve and blower speed never seeing a reduction. This means your multi-stage gas valve is never utilized.

This is my experience.....

I will never again buy a gas furnace and I'm a big fan of gas.

Yes, I suspect this is the reason why my 2/F 1450 sqft house (with cathedral ceiling so about 1700 sqft worth of volume to cool) in a 60-70F winter climate ended up with a 100K BTU 1 stage furnace. They were spec for a possible 3.5-4 ton AC back then and the matching air handler would be for a 100K BTU furnace.

However, if you do not need that big of an air handler to match the AC, I think installing a smaller furnace would be the right way to go. Southeast may only need something like a 32K BTU furnace for a house my size but the AC would need to be 4 ton still, so if I were to install the same system again without going too high end like infinity / variable size unit from Lennox, I would probably just hard wire a dual stage furnace to a "low" for heating and then let the AC run like a normal dual stage AC.

Based on what you said about the Lennox earlier, I would stay away from Lennox and instead use other more stupid dual stage furnaces. Infinite "steps" heating but with a 5 min max burning before it slow down is either mal-function or poor design. Most stupid 2 stage furnaces or ACs should not be like this.

 

[brianl703]

However, if you do not need that big of an air handler to match the AC

My Trane TEM6 air handler (just air handler with strip heat coils, not furnace) has fan speed dip switch settings for condensing unit sizes from 1.5 ton to 5 ton, all with high-medium-low CFM rates.

The installer left it set at the factory default which is 5 ton HIGH. I have a 4 ton and ductwork sized for a 4 ton so it was moving way too much air, to the point where the return grille was making excessive noise from the airflow.

I reset the dip switches to 4 ton MEDIUM and that fixed that problem. They also got the 50 amp and 60 amp circuits swapped around which caused it to trip the 50 amp breaker once or twice...but that was easy enough to fix as well.

I expect that the furnaces from Trane with ECM motors have similar adjustability. So with this equipment there shouldn't be a need to use a larger furnace just because the AC is larger.

And, double-check what the installer did. They do make mistakes.

 

[hemitom]

I tell ya what - since you seem to know so much about this, why don't you help me figure out if it is indeed incorrectly installed.

I'll get more info and pictures. For now, it's a 92% AFUE furnace, I believe it has a modulating gas valve (maybe it's 2-stage), 2-speed evap fan motor. Lennox, made in 2015, installed in October 2015. Hooked to a 2-stage, 15 SEER Condensing unit. The thermostat the is 2015 Lennox WiFi thermostat. Lennox iComfort Model 10F81

This is how it operates - Call for heat, the burner fires at 100% and fan at full speed. This will continue for 5 minutes minimum. I have tested this multiple times. Indoor temp at 62, setpoint at 69 (IOW, we've lowered the temp so the furnace doesn't run while we are away). System runs, for five minutes and the indoor temp might see a 3 degree rise to 65. I'll lower the setpoint to 66 and I can hear the system drop down a notch in fan speed. I'll bump the setpoint to 67 shortly to keep the burner on and it will continue in "low" speed.

The problem is when the setpoint is 69 and the indoor temp drops a half degree from setpoint or so, it fires at 100% and runs for 60-90 seconds. Yes, two problems in play - grossly oversized burner size for our area and it's in 100% mode.

I've asked the installing contractor about this and they've been out to check it and have assured me it's correct. I've also had a large Lennox-based contractor service the system and ask them, they say it's operating like it should.

Brianl703 is correct !
the furnace should fire on low and lower blower speed then if the T-stat is not satisfied in a predetermined amount of time it will kick on to high heat and higher blower speed.
look on your control board you will see switch's for first and second stage heating with taps for levels of blower speed .This is exactly how mine works

 

[hemitom]

That entirely depends on how the thermostat is set up. I would think that most properly configured thermostats would start on the low stage, and only switch to the high stage when it is taking too long for the temperature to rise.

Now if some dipstick got the W1 and W2 terminals swapped around...then it would do exactly as you described.

You're correct fully defeats the purpose of having a multi speed furnace, Why would it start and run on high makes no sense !

 

[brianl703]

You're correct fully defeats the purpose of having a multi speed furnace, Why would it start and run on high makes no sense !

I think it's a configuration error. And given that I can't find the installer manual for his Lennox thermostat, well, that doesn't make me want to go out and buy any Lennox equipment....

 

[PandaBear]

My Trane TEM6 air handler (just air handler with strip heat coils, not furnace) has fan speed dip switch settings for condensing unit sizes from 1.5 ton to 5 ton, all with high-medium-low CFM rates.

The installer left it set at the factory default which is 5 ton HIGH. I have a 4 ton and ductwork sized for a 4 ton so it was moving way too much air, to the point where the return grille was making excessive noise from the airflow.

I reset the dip switches to 4 ton MEDIUM and that fixed that problem. They also got the 50 amp and 60 amp circuits swapped around which caused it to trip the 50 amp breaker once or twice...but that was easy enough to fix as well.

I expect that the furnaces from Trane with ECM motors have similar adjustability. So with this equipment there shouldn't be a need to use a larger furnace just because the AC is larger.

And, double-check what the installer did. They do make mistakes.

"Larger" is probably what they did back in the days (my furnace was from 1994) before 2 stages, separate setting for heating vs cooling, etc. I think I read my ancient furnace installation guide / manual and they do spec the amount of CFM for each setting, and for the amount of tonnage the CFM needed, and it was right at the border between 1200 CFM and 1600CFM where I landed. Had I install a 4 ton AC the CFM of my furnace would be correct for a 80% furnace. Had I install a 3 ton AC with a condensing furnace I would probably be able to go to a 40/60K BTU furnace with a 1200CFM fan instead.

Just wondering, is putting too big of an ECM for the same duct going to kill it faster? or actually depends on how you set it? or a big ECM set to a lower flow actually last longer than a correct size ECM set to max it can handle?

 

[tbm5690]

Brianl703 is correct !
the furnace should fire on low and lower blower speed then if the T-stat is not satisfied in a predetermined amount of time it will kick on to high heat and higher blower speed.
look on your control board you will see switch's for first and second stage heating with taps for levels of blower speed .This is exactly how mine works

My t-stat calls for stage 1 vs stage 2 based on the temp rise specified it seems...if it's set for 68 and I increase to 69 it'll kick stage 1, but if I go 2 degrees or more above the current set point it will call for 2nd stage and then ramp back down to stage 1 when the measured temp is one degree below set point until it reaches the desired temp. Must depend on the thermostat programming?

 

[brianl703]

Just wondering, is putting too big of an ECM for the same duct going to kill it faster? or actually depends on how you set it? or a big ECM set to a lower flow actually last longer than a correct size ECM set to max it can handle?

The ECM is variable speed so it runs according to how it's set. Theoretically, if it's not running at max output, it should last longer. The ECM motor in my Trane is a constant-CFM (aka constant airflow) unit so it will speed up to maintain the selected airflow in cubic feet per minute. That means that if the ductwork is inadequate (too small, too long) it will speed up more to compensate and maintain the set CFM. And it will also speed up to maintain airflow as the filter gets more clogged and dirty. Obviously there is a limit to how much it can speed up, and it probably won't last as long running at 100% of it's output (if nothing else, you would expect the bearings to wear out quicker).

EDIT: I have a zoned system, and if only the upstairs zone is calling/active, the air handler uses about 100 watts more than if both upstairs and downstairs zones are calling/active. This shows that the ECM motor is speeding up, and therefore using more power, to move the same amount of air into only the upstairs zone. (The air handler is in the basement).

 

[hemitom]

My t-stat calls for stage 1 vs stage 2 based on the temp rise specified it seems...if it's set for 68 and I increase to 69 it'll kick stage 1, but if I go 2 degrees or more above the current set point it will call for 2nd stage and then ramp back down to stage 1 when the measured temp is one degree below set point until it reaches the desired temp. Must depend on the thermostat programming?

My t-stat calls for stage 1 vs stage 2 based on the temp rise specified it seems...if it's set for 68 and I increase to 69 it'll kick stage 1, but if I go 2 degrees or more above the current set point it will call for 2nd stage and then ramp back down to stage 1 when the measured temp is one degree below set point until it reaches the desired temp. Must depend on the thermostat programming?

that would be correct!

 

[akmac]

Just had a 30 year old Lennox replaced with a new Lennox unit back in December. After the initial sticker shock, I'm totally satisfied. Old unit was also 80% efficient and new one's 96% 2-stage. My installer explained the need for a liner was because the water heater doesn't produce enough heat to dry out the chimney. That old furnace was putting a lot of heat up the chimney, so it would dry things out when it ran. I only paid $500 for the liner installed. I will say that although the house is noticeably more comfortable and the furnace is whisper quiet, I haven't noticed any reduction in my gas bill. My gas utility also gave an $800 rebate for the upgrade to more efficient furnace. Installer offered a 10 year parts & labor warranty. I had heard the common complaint about Lennox parts availability, but if the old one lasted 30 years, I'm not worried about the new one. As for the ECM blower motor, the sales guy told me that the old AC blower was costing me a couple of dollars a day to run, while the new one would be pennies per day. I never tested the actual current draw of the old one, so I'm not sure if that was just a sales line, or what. It definitely is quieter.

 

[brianl703]

the old AC blower was costing me a couple of dollars a day to run

Most of them consume around 500 to 800 watts from what I've seen. Probably depends on whether it's a 1/3 or 1/2HP motor.

However, at a 500 watt power consumption and 12 cents per kwh, it would have to be running about 32 hours a day in order to cost $2 a day.

Obviously, there aren't 32 hours in a day.

In a more realistic scenario where it operates, say, 12 hours a day, the cost is 72 cents. And if 6 hours a day, 36 cents.

 

[PandaBear]

Most of them consume around 500 to 800 watts from what I've seen. Probably depends on whether it's a 1/3 or 1/2HP motor.

However, at a 500 watt power consumption and 12 cents per kwh, it would have to be running about 32 hours a day in order to cost $2 a day.

Obviously, there aren't 32 hours in a day.

In a more realistic scenario where it operates, say, 12 hours a day, the cost is 72 cents. And if 6 hours a day, 36 cents.

We have 26 to 42c/kwh in our area depends on which time of the day it is or what "rate plan" you sign up for, so it "depends" I guess.

Then again our labor is expensive too, so installing the latest and greatest may cost so much you will never get your investment back.

 

[brianl703]

We have 26 to 42c/kwh in our area depends on which time of the day it is or what "rate plan" you sign up for, so it "depends" I guess.

Then again our labor is expensive too, so installing the latest and greatest may cost so much you will never get your investment back.

I think 12 cents/kwh is the national average. Or was. I still pay that much.

 

[Cujet]

Keep in mind that the "efficiency" claims lead one to believe there will be huge savings. The reality is that even 30 years ago, the science of heating was well refined and a properly maintained older system will be relatively competitive. The combustion process won't be significantly better. The heat transfer "might be" a bit better. The combustion related airflow might be considerably better. But we must remember that the furnace is often not the only consumer of fuel.

Another point made above, variable speed fans, motors and circuitry will fail, and is expensive. A $100 motor becomes $700.

 

[PandaBear]

Keep in mind that the "efficiency" claims lead one to believe there will be huge savings. The reality is that even 30 years ago, the science of heating was well refined and a properly maintained older system will be relatively competitive. The combustion process won't be significantly better. The heat transfer "might be" a bit better. The combustion related airflow might be considerably better. But we must remember that the furnace is often not the only consumer of fuel.

Another point made above, variable speed fans, motors and circuitry will fail, and is expensive. A $100 motor becomes $700.

Other than going from non condensing (80%) to condensing (90%+), the main savings is really "right sizing" the system. If you were using the right size furnace in the past even a new system going from 80% to 96% will only save you 16% * 1.25 = 20%, and ECM may save you another 20-30% on the electric part (say $1 a day over the old one, but more like 20-50c a day).

 

[brianl703]

Other than going from non condensing (80%) to condensing (90%+), the main savings is really "right sizing" the system. If you were using the right size furnace in the past even a new system going from 80% to 96% will only save you 16% * 1.25 = 20%, and ECM may save you another 20-30% on the electric part (say $1 a day over the old one, but more like 20-50c a day).

If one has a tract house, one would do well to get an energy audit done. Thermal camera, blower door, and the rest of it.

Sometimes (maybe even usually) they're in such a hurry to slap those houses up that they forget things like caulk and insulation. I've seen it myself.

And then when all those issues get fixed, you might find that you don't need as big of a furnace or air conditioner...and your energy bills have gone done, maybe substantially.

 

[Number_35]

Most of them consume around 500 to 800 watts from what I've seen. Probably depends on whether it's a 1/3 or 1/2HP motor.

However, at a 500 watt power consumption and 12 cents per kwh, it would have to be running about 32 hours a day in order to cost $2 a day.

Obviously, there aren't 32 hours in a day.

In a more realistic scenario where it operates, say, 12 hours a day, the cost is 72 cents. And if 6 hours a day, 36 cents.

I run the blower motor 24/7 because the HRV uses the HVAC system's ductwork to supply fresh air and exhaust stale air. If we assume a 1/2 HP motor, that's 373 W (assuming the electric motor is 100% efficient, with no friction losses - in reality, the power consumption would be higher). But let's assume a perfect motor anyway - per month that's 0.373 kW x 24 hrs/day x 30.5 days/month = 273 kWh. Our electricity is cheap here - about C$0.08 per kWh, so I'd pay about C$22/month.

If I can save half that with a more efficient blower motor, that's not insignificant.

 

[Number_35]

Just put a new power vented hot water tank in and cap off the chimney pretty much the same price as putting a liner in the chimney.

I asked about that - a power-vented hot-water tank is legal here, but they start well above C$2K - well above the C$1200 I've been quoted for a chimney liner! My water heater is not much over 3 years old, so I'd hate to change it out anyway.

I had thought the power-vented tanks might not be legal in that they would shut down in the event of a power outage, but there's a safety device that shuts off the gas supply to the water heater if the powered vent loses power.

 

[brianl703]

I run the blower motor 24/7 because the HRV uses the HVAC system's ductwork to supply fresh air and exhaust stale air. If we assume a 1/2 HP motor, that's 373 W (assuming the electric motor is 100% efficient, with no friction losses - in reality, the power consumption would be higher). But let's assume a perfect motor anyway - per month that's 0.373 kW x 24 hrs/day x 30.5 days/month = 273 kWh. Our electricity is cheap here - about C$0.08 per kWh, so I'd pay about C$22/month.

If I can save half that with a more efficient blower motor, that's not insignificant.

If you run the blower motor all the time, the majority of your energy savings with ECM motors is because they can run at a lower speed for ventilation. So they might only pull 100W at the lower ventilation speed vs the 500W they might pull when running at normal speed. PSC motors are incapable of this, they use about the same amount of power no matter what speed they're running at, and they can't slow down as much as an ECM motor can.

My fresh air ventilation system is on a timer. I currently have it set to run 5 minutes per hour. If outdoor conditions are good (low humidity, mild temperatures), I'll set it to run continuously. If the HVAC system isn't running enough heating or cooling calls for it to run the set amount of ventilation per hour, then it'll turn on the HVAC system fan at low speed (and open the fresh air damper and turn on the fresh air fan to pull air from outside).

 

[Number_35]

I'm not too far south of you and sounds like our houses are nearly identical. I have a 60K BTU Trane S9V2 installed 2 years ago...love it and seems to be perfectly sized.

Good to hear! I'm really impressed with how well we're managing without the furnace. Running a couple of little electric blowers during waking hours, and we've had the oven on when we're home. Lots of solar gain the last couple of days fortunately! And it's a lot milder too, compared to a week ago.