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ChrisFoster

02:08PM | 11/20/02
Member Since: 11/14/02
3 lifetime posts
Bvhvac
I have a high efficiency Carrier furnace that a local 'professional' dealer installed for the previous owner of the house about 7 years ago. It would probably be appropriate for Minnesota, but here on the Northern Cal coast, it's way oversized. Even with the fan at full (read noisy) speed, it overheats and the control unit cycles it on and off. The output air is hot enough that the ductwork creaks and groans from heat expansion as the furnace cycles.

Filter is clean, all ducts unblocked.

The dealer's solution is to sell me a lower output unit -- an expensive solution.

I was wondering if it's possible to re-jet the unit with the high altitude installation kit - this looks to reduce the BTUs by over ten percent. Or are there any other tricks to reduce the BTU rating?

superheat

06:49AM | 11/25/02
Member Since: 11/24/02
11 lifetime posts
You could change the spuds to the next size down and drop your blower speed, but anything you do to that furnace will void the warrenty and might make it unsafe or at least fail early.


Your problem is probably a dirty evap or undersized duct system.

ChrisFoster

06:07PM | 11/25/02
Member Since: 11/14/02
3 lifetime posts
You're kinda right about the duct system being undersized: the system is undersized for the furnace, but the right size for the house (there's no humidifier, and the ducts and filter are absolutely clean).

It's really the furnace that's too darn big for the house. Every estimator that I've tried says 60K BTU is plenty for where I am and the square footage of the house, but the furnace itself is 90K. Either the geniuses at the so-called "professional" HVAC contractor didn't realize that central California isn't Minnesota, or this was the furnace what they had lying around. Based on the number of times I've had to crawl under the house to fix flex ducts that have come apart where they joined them to with "duct tape" that's dried out, either is a good chance.

The warranty has long since expired, so that's why I'm thinking about putting in the jets for 8000 foot elevation. I'm just not certain whether that will simply reduce the output (my hope), or if it will end up acting like an engine running too lean and blowtorch itself to death. Since my option is to fork out the money to buy a proper sized furnace, I'm thinking that modifying this one and running it 'til it dies is the best choice. Though I really hate abusing machinery that's the victim of a lousy sales and installation company.

superheat

09:45AM | 11/26/02
Member Since: 11/24/02
11 lifetime posts
I agree with the HKestenholz. If you want to replace it, replace it. It will probably get through the winter after you docter it up, but what if it doesn't. CO piosening is one concern. You could hurt the furnace to the point that it can't cantain the fire inside. Many things could go wrong. It is just too dangerous to chance.

superheat

09:47AM | 11/26/02
Member Since: 11/24/02
11 lifetime posts
One more thing about sizing. The number and size of your windows makes a big difference. Orientation to the sun makes a big difference too.

Dumwun

10:06AM | 11/29/02
Member Since: 11/28/02
2 lifetime posts
If the furnace is cycling due to the limit switch opening it would seem you have inadequate airflow for your furnace. Dirty filters, undersized ducts or physical obstructions could be the culprit there. To decrease the input could cause problems at the burner not allowing enough air to mix or burning to occur in the venturi throat. That could be dangerous and not advised. We are also going on the assumption that the cycling is occuring due to the limit switch opening. I would use a multimeter to verify where the circut is opening. If it is the limit switch I would probably add an air dump to raise the circulation volume. Look for the short cycling to also be from the heat anticipator in the thermostat or inturruptions or obstructions in the forced draft vacume tube (if so equipped). PG&E services your area and could probably help troubleshoot your system.
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