Dave Davis, thank you for your reply. I believe this is an important topic to understand as a home owner. I've read on this forum that for a properly designed AC system and at the outside design temp, the unit should run a long time. Seems to me that would be similar for a properly sized furnace.
Plan ahead I always say. Who knows when the next ice age will hit. Then that furnace will probably be too small.
engineerguy, and wyounger are right, your over sized ALOT, by what you posted in you time and design temp.
The reason for the temp over shoot is its too big.
As fas as is it harming the furnace, it could be, the limit may be shutting the burner off, and the stat may be satified before the limit resets.
jrbenny, I understand what you are saying and thats why I thought the unit was way oversized. I would prefer the longer on-heat cycles than the quick high burst of heating. Also, if I had a smaller furnace, the CFM's would more closely match my ac unit. Right now the heating cfm's are higher than my ac cfms, which in my understanding its usally the other way around. The furnace supports a 4 tons of air and thats why the cfms on heat or higher (to keep temp rise of heat exhanger within spec).
EDIT: Gee...look what got typed while I was typing...
haha ...you guys beat me to the punch. I went so far as to register (and stop being a lurker) in order to respond to this, but the last few posts say it all very clearly. When BTUH's lost <AKA> Home Heat Loss = BTUH's replaced <AKA> Furnace Output, the furnace will run all the time and the homeowner will be as close to "comfortable" as he/she will ever get. This calculation is also true for heating COST and is not effected by "size" of furnce or run time (when viewed in simplified terms and not taking into account the lost efficiencies due to short cycling).
I might as well take this opportunity to say hello to the forum as well - I have been watching and reading for awhile and must say this is a great place to kill some time and see what is happeing out there across the country. It is refreshing to see there actually are some reputable contractors out there who do care about the integrity of our industry!!
Over and Out ...
These replies are great and I appreciate your input. Anybody got a hugh house that need a good used furnace? LOL...
For the benefit of Dave Davis. A burner that runs for 8 minutes in an hour will not use near as much fuel (gas or oil)as a burner that runs 3 times in an hour for 4 minutes each. 12 minutes minus 8 minutes= 4 minutes savings every hour. And with the extra cycles in an hour there is a certain amount of start-up "waste" involved, not to mention the additional 'wear and tear' on the units various parts...
Shoot52; A manual J is what your so-called installer should have "reached" for before installing this 'brute'
If HE used it properly in the first place, then YOU would not be having this problem. savvy?
LOTS of Equipment
Do you mean specfiying needs and 'shopping' to address the specs is not the correct approach?
Originally posted by dave davis
We were lucky to have a super company work with us on our recent oil burner replacement, but this is also illustrative. Our present house had a 90k btu/hr 80 pct in place but only needs 45k btu/hr or so, and the smallest Thermopride available was 70k btu input 87 pct with a .5gph nozzle.
You can't always get the exact right equipment for the requirement and need to compromise.
It's Not Rocket Science, But It is SCIENCE
with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
Don't know where you're going with that question but I mean that as I understand it in specifiying a furnace, you first identify the heat loss. This is an estimate, not an exact science. You then determine what equipment will meet the need, in our case a high efficiency gas furnace of smaller btu than the oil furnace we purchased would have met those needs more closely, ie max btu/hr output closer to max btu/hr required.
Unfortunately other parameters may rule against the optimum equipment, in our case we opted to stay with oil, wanted a high quality piece of gear (as recommended on this forum), and the smallest available of the equipment that the company we trusted recommended (Thermopride) was somewhat larger in output than our (estimated) requirement.
So we compromised. We probably get 60k btu/hr out of our furnace and only needed 45k. We should never be cold. The furnace will likely never run non-stop on the coldest winter day. We're happy with the install and the furnace because the contractor worked with us to understand all these parameters, and the compromise was up front and understood by both parties. Not all contractors are as communicative/patient, and some would simply oversize the furnace to prevent a call-back. We homeowners need to understand the theories to protect our investment, and find a contractor that can be trusted.
Tha't what I meant.
You can set the t-stat to be less agressive or more agressive to cycle the furnace more frequently.
Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced
Freezeking2000, yes i see that option "0680" in the setup menu. Right now its set at the default 2. 1=less agressive temp control or 3=more agreesive temp control. Would you suggest option 1?
I believe that the majority of responses have been that my system is definitely oversized. Is it stupid of me to think about replacing the furnace? The CFM's can't be reduced because it would be too close to the upper end of the temperature rise specifications of the heat exhanger.
since it's already in and only 7 years old, your options are to replace with the proper size for maximum comfort or live with it for another 10 yrs. who knows what will be available then?
if you replace, go with a 2 stage , or better yet, modulating furnace for maximum comfort..
if this is the home you intend to die in, it may be worth the investment at this time to properly size the system and enjoy the benefits.
if you are moving in 5 years, forget and live with it.
also, running your fan continuously may help in evening out the temps throughout the home. give it a try and see how it works.