Furnace Recovery Questions

[Briant73]

Not even two months ago I had a new Lennox g71mpp 70k btu furnace installed. I had three quotes one wanted to do a 90k Lennox g71, another I believe an 75k or 80k Trane. The manual J-calc done by a pro was the house on a minus -5 or -10 degree day would need 72k btu to maintain tempature.

I have setup my thermostat to drop to 66f when sleeping and gone for the day and go to 70f when home and awake. The program reads like this on the weekdays 6:30am 70f, 8:15am 66f, 4:00pm 70f, 11pm 66f. Weekend is just day 70f/night 66f.

The past few days the outside temp is around 20f, but feels like 4f so there is some wind and the sun is nowhere to be found. It seems the furnace is running full throttle according to the led (h100A###) yet the thermostat takes awhile (15-20 minutes) to even go up one degree. The furnace seems to be holding it's own running at h40A### once the desired temp is reached though at times it seems to run a good amount even at h40A###.

My question does this seem normal or could there be an issue with the furnace size, ducts, or something else?

I will watch the recovery this evening but figured I would start this thread now to get any information on this and if I do need to call a pro I will.

[Briant73]

Testing update -

I set my recovery time from 66 to 70f at 6pm. I noticed the furnace kicked on around 4:45pm-5pm and hit 70f around 5:45-5:50pm. The furnace from what I could tell was running h70,h80,h90 then h100 for most of the time. During this time it was dark out, the outside temp was in the mid teens but felt like 0f degrees.

So it took 50 minutes to an hour to go up 4 degrees but once there it seems to only need h40-h44 to maintain it. Does this seem normal and I know I'm asking a lot since you haven't seen my house/windows/insulation/ducts.

[t527ed]

sounds like it is working right to me, heater is sized to mantain a set temperature not to recover from a setback.....;)


not only that but your model is designed to give extended run times so it is doing what it should.

[Briant73]

sounds like it is working right to me, heater is sized to mantain a set temperature not to recover from a setback.....;)


not only that but your model is designed to give extended run times so it is doing what it should.

Thanks for the response, new to this style furnace so thought I'd ask.

[Ladiesman271]

sounds like it is working right to me, heater is sized to mantain a set temperature not to recover from a setback.....;)


not only that but your model is designed to give extended run times so it is doing what it should.



In other words the furnace is to small to do the job. With the variable output furnace, a larger BTU unit would have been better. He needs 72k BTU OUTPUT just to stay even on a cold day. He bought a 70k INPUT furnace.


http://www.lennox.com/products/overview.asp?model=G71P

[Briant73]

I just moved in so this house is new to me. I had 4 HVAC contractors bid, only 3 submitted written paper quotes, one wouldn't give model numbers, one never called back when I called to discuss. The one who gave all model numbers and called back - I went over the quote multiple times with the salesman and tweaked it and questioned everything that was in it. I know the ultimate decision was mine but this company felt strongly that a 70k unit would handle our needs and anything bigger would be oversized when I asked and I made sure to ask more than once and have the salesman come out and look things over two times.

Now we don't get a lot of -5 or -10 degree days here so I don't mind a few days a year the furnace is having trouble keeping up but when it's 10f-23f (though windy out) I think it should keep up. I have sent the installation company an email detailing my questions on this.

[Kevin O'Neill]

Setback thermostats were invented 30 years ago to save money operating extremely over-sized equipment. Set your temperature and leave it. A properly sized unit will save more money running all the time than an over-sized furnace with 4 setback periods.

People think setback thermostats are the cats a$$, but better sizing is more economical in the long run. In my youth, I often saw furnaces that were 3 - 4 times the size needed. A setback thermostat would save a lot on standby losses; and during recovery, the furnace would run longer, getting better efficiency. Several manufacturers reps told me years ago that the labs only tested efficiency after the furnace was running 1 hour to reach steady state conditions.

Sounds like your dealer did good.

[t527ed]

In other words the furnace is to small to do the job. With the variable output furnace, a larger BTU unit would have been better. He needs 72k BTU OUTPUT just to stay even on a cold day. He bought a 70k INPUT furnace.


http://www.lennox.com/products/overview.asp?model=G71P



the furnace is mantaining the set temperature while firing at 40-44% of max firing rate, how is it undersized?????:rolleyes:

[Briant73]

the furnace is mantaining the set temperature while firing at 40-44% of max firing rate, how is it undersized?????:rolleyes:

Last night once it got up to it's recovery point I saw it at h80 once, and then h53, h46 a few times from that time on.

[t527ed]

if it ever gets to where its running h100 24 hours a day and losing temperature in the house then its undersized.....;)


as long as it is holding house temp at your setting quit watching it do its job and get your holiday stuff done....;)

[Ladiesman271]

Setback thermostats were invented 30 years ago to save money operating extremely over-sized equipment. Set your temperature and leave it. A properly sized unit will save more money running all the time than an over-sized furnace with 4 setback periods.

People think setback thermostats are the cats a$$, but better sizing is more economical in the long run. In my youth, I often saw furnaces that were 3 - 4 times the size needed. A setback thermostat would save a lot on standby losses; and during recovery, the furnace would run longer, getting better efficiency. Several manufacturers reps told me years ago that the labs only tested efficiency after the furnace was running 1 hour to reach steady state conditions.

Sounds like your dealer did good.



Sounds like rationalization to me. He does not have a single stage furnace. Explain how a modulated burner can be oversized in this case.

Manual J says 72K BTU is design heat loss (theoretical).

His furnace is a modulated burner 70K BTU INPUT furnace, so the furnace OUTPUT does not even meet the design heat loss calculation.

Building heat loss does not change with furnace size. A furnace with a modulated burner will adjust the BTU output as needed to maintain temperature. The next larger size furnace would have been a better fit.



I wonder what size the original furnace was (as well as efficiency)!

[gcastillo]

It sounds like from what you are saying the equipment you have is working well. Maybe you are nit picking? Maybe the company that wouldn't call you back saw this coming and that's why they wouldn't call you back. I guess it's ok you haven't *****ed at them yet. Going on just what you are saying and not being there sounds like your system is doing well.

[Ladiesman271]

the furnace is mantaining the set temperature while firing at 40-44% of max firing rate, how is it undersized?????:rolleyes:



Wait until it gets cold. Then see how long it takes to recover from that night setback.

Granted, maybe design temperature gets hit two weeks a year. I guess you guys just ignore the coldest days of the year!

[Briant73]

Sounds like rationalization to me. He does not have a single stage furnace. Explain how a modulated burner can be oversized in this case.

Manual J says 72K BTU is design heat loss (theoretical).

His furnace is a modulated burner 70K BTU INPUT furnace, so the furnace OUTPUT does not even meet the design heat loss calculation.

Building heat loss does not change with furnace size. A furnace with a modulated burner will adjust the BTU output as needed to maintain temperature. The next larger size furnace would have been a better fit.



I wonder what size the original furnace was (as well as efficiency)!

The furnace it replaced which was installed when the house was built was a Lennox from the early 70s, everyone seemed to agree that by today standards it was probably in the 50-60% efficiency range and what I can remember it was rated at 130000 or 137000btu. As to the 72k manual J calc. I only was told the results of the one manual J calc, the others any calculation they made they kept to themselves.

[Briant73]

if it ever gets to where its running h100 24 hours a day and losing temperature in the house then its undersized.....;)


as long as it is holding house temp at your setting quit watching it do its job and get your holiday stuff done....;)

I think one thing I can do is make the setback less aggressive and yes so far the house has been holding steady for the most part once a temp is reach with the furnace calling for heat at a reduced rate. Part of the reason for the setback is because I sleep better in a cooler house.

As to holiday stuff or watching the furnace well if it's between christmas shopping in packed stores and everyone out driving or sitting home with a frosty beverage and checking an led every once in a blue moon... :D

I have to say these new furnaces and thermostats are pretty awesome but they do have a learning curve compared to what I am used to.

[Ladiesman271]

Testing update -

I set my recovery time from 66 to 70f at 6pm. I noticed the furnace kicked on around 4:45pm-5pm and hit 70f around 5:45-5:50pm. The furnace from what I could tell was running h70,h80,h90 then h100 for most of the time. During this time it was dark out, the outside temp was in the mid teens but felt like 0f degrees.

So it took 50 minutes to an hour to go up 4 degrees but once there it seems to only need h40-h44 to maintain it. Does this seem normal and I know I'm asking a lot since you haven't seen my house/windows/insulation/ducts.



Leave the setback as is. I setback more than that. Your furnace hits 70F right around your target 6 pm set time, so everything looks good at the present outside temperature.

Just make sure that you hit 70F near 6 pm on a real cold day. If you do hit 70F at around 6 pm, then you are golden.

How do you like the "comfort level" with that modulating burner?

[Briant73]

Leave the setback as is. I setback more than that. Your furnace hits 70F right around your target 6 pm set time, so everything looks good at the present outside temperature.

Just make sure that you hit 70F near 6 pm on a real cold day. If you do hit 70F at around 6 pm, then you are golden.

How do you like the "comfort level" with that modulating burner?

The thermostat intelligence seems to have the furnace heating up so the house is at the desired temp range on time, just takes awhile.

The past few days have been pretty cold for this area, add the windchill and it was pretty bitter out. The average reported temp for the cold months (accuwateher) November 43f, December 33f, January 27f, February 28f with low averages are Nov. 37f, Dec. 33f, Jan. 20f, Feb. 21f.

As to how I like things - So far very impressed with the way it picks what to use and how quiet it operates. Only used the original furnace a few times and at the time part of the cold air return duct was off so plumbers could work on some stuff so not a fair comparsion but the noise level was louder when it ran even with the fan set to low speed. Comparing to my old home that had a high eff. single stage this furnace is a lot nicer.

[DanW13]

IMO your better off to set the stat to your desired set point when OD temps are at or below 10* or sub zero weather for a few reason. One being your indoor furnishings will get alittle colder than when the temps are in mid 20's or higher making yoru furnace have to heat more than just the air in the home. secondly when the temps outside are that cold your furnace is going to run near continuously regardless so there's no sense in setting the stat back with those OD temps infact your furnace will probably run more efficently by just leaving it set at 69* or 70* when it gets that cold. I have the Infinity 96 mod and when OD temps get into the mid to low teens the frunace is running near continuous on low fire 90% of the time so there's no sense in setting it back until OD temps stay in the high 20's to mid to low 30's then I use the set back on my controller.

[beenthere]

First, the coldest 97½% design temp in PA is 2°.
So if they used -5 or -10 to come with 72,000, the 70,000 is more then enough for design conditions.

Yes, in PA, we have temps drop below design every couple years or so.

His furnace is not undersized for PA design temps.

Mod furnaces are not designed for quick recovery.
Recovery will be slow, but as long as its meeting the temp by the time its suppose to, nothing to worry about.

Sealing switches and receps on the outside walls, and repairing or replacing wornout door seals, can reduce the heat loss enough, that it recovers several minutes sooner.

Your mod is working the way its suppose to.