View Full Version : How to evaluate if overcycling ?
12-22-2008, 06:08 PM
1. How do you define a cycle ?
2. If the outside temp is 4 degrees Farenheit how often should the furnace cycle. The paramaters are: little wind, t-stat is set for 65, old house (its 1919 so its a little drafty), attic is insulated r-30, partially finished basement (only heating is from the duct work), and approxiamtely 900 square feet on the main living floor where I spend 95% of my time. I generally do not use the basement. The furnace is a Trane XV80. All the technicians who have been out since the install (5 years ago) say the furnace is oversized and the ductwork can not handle the air from the current furnace.
3. If I decide to bite the bullet and downsize how can I evaluate my current cycling to determine the culprit is the furnace and that long-term it makes sense from a comfort and savings perspective to downsize to a 60000.
4. The current cycling in 4 degree weather is the furnace calls for heat every 7 minutes and runs for 15 minutes. The last 4-5 minutes are in second stage. If the temperature is 25 the furnace is called every 13 minutes and runs for appx. 7 minutes.
12-22-2008, 09:33 PM
Every house is going to be different when it comes to cycle rates. There are a number of factors involved. Even thermostats can be different when it comes to cycle rates. Being that your house was built in 1919 and it's "a little drafty", you're fortunate it cycles and stays off for 7 minutes when it's 4 degrees F outside. Have all the technicians who have been out to evaluate your system based their diagnosis after, at least, having performed a Manual J? If not, they're guessing.
12-22-2008, 11:04 PM
IMO your furnace sounds to be over sized for the home if it is running that often and cyclying on and off that often, If your frunace is 2 stage I would think it can be changed to run in first stage longer than it currently is and could be you might just need some modification to your duct work as well. But if you were to change the furnace you may want to consider replacing with a modulating furnace where you will get the furnace to run in low fire for even longer periods of time than what you currently have. The Infinity mod is a nice system where it will run in low fire approx. 85% of the time even when temps drop down to 4* for a OD temp. also a mod furnace can work under poor duct work sizing if you happen to not have properly sized duct work since the furnace will run in low fire and low cfm's, not saying you should have the proper size duct work but it will help with poor duct work should that be the case.
12-22-2008, 11:38 PM
That's the problem. There is no way you can know whether or not the furnace is oversized based on opinion. The only way to know for sure, rather than guessing, is to do a Manual J on the house.
In your opinion, what do you think is causing the furnace to run and cycle at the rate the OP says it is? Rather than modifying the operation of the furnace, modifying the duct work or replacing his existing furnace with a modulating furnace, the OP needs to address his heat loss. That's where he'll save dollars.
12-23-2008, 12:33 AM
The last tech out also increased the fan speed which seems to have exacerbated the situation. Does it makes sense to have the fan at the lowest speed. He only increased the fan speed since I indicated the far ends of my house did not seem to be receiving as much heat as the other parts of the house...
12-23-2008, 10:22 AM
HO: your last post indicates that the ductwork needs to be designed, not just put in!
Manual J = determing heat loss
Man D= sizing ductwork
Man S= sizing equip
one will need to jump back & forth to get best sizing of all
BUT, first 'tighten' the envelope! --
12-23-2008, 04:08 PM
I assume if I call my Heat Tech he could perform all 3 manuals you reference in your email ?
12-23-2008, 06:01 PM
Sounds like your furnace is just about the right size.
At 4° outdoor temp, its running for 10 to 11 minutes in low stage. then 4 to 5 minutes in second to satisfy the heat call.
Thats not a bad cycle rate for maintaining 65° in the house.
Next time you have a tech come out. Ask him if its set up to use the thermostat to control second stage, or if its using the furnace boards timer.
If its by the boards timer, you can have them set it up to use a 2 stage stat.
And only bring on the second stage when first stage can't handle the load.
Keep in mind, your 80,000 BTU input 80% furnace is outputing roughly 41,600 BTUs in first stage. And uses second stage to satisfy the stat at 4° outdoor temp.
A 60,000 BTU input 80% furnace will only output roughly 48,000 BTUs in second stage.
And may not be able to satisfy teh stat when its 4° outside.
Could be a little to low. And hurt your ability to resell your home, if the furnace can't heat your house to 70°.
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