View Full Version : Cold and Rheem Mod
12-04-2006, 09:43 AM
I saw a recent post by Doc Holiday who said that at 6 degrees F. the mod furnace should run all the time. Lately, I've had a few nights at 10-13 degrees, and the furnace is cycling. It's comfortable, however I was expecting that at those temps the Rheem mod furnace would maintain my 69 degree set temp by running continuously like doc holiday said.
I appreciate there are a lot of unknowns, but I'll do my best to answer any questions. To clarify, I have the Rheem touchscreen mod tstat. My calc of heat loss if the temp was 0 (if I remember correctly) was 55K BTU, and I had a 75K Rheem mod installed. The house is 1900 sq. feet.
12-04-2006, 12:42 PM
Your mod furnace is probably oversized. I think its very critical to size a modulating furnace right or you won't get the benefits from it. I think some of the other chaps here can back me up on these sentiments?
You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals
12-04-2006, 01:32 PM
We had a high of 15 F last couple of days and my mod didn't cycle off all day. Never been so comfortable. We have a 4500 sq ft modified 2 story, with 90,000 BTU Rheem Mod and 3 ton A/C
12-04-2006, 01:35 PM
Make sure you don't have any heat ducts close to the thermostat.
12-04-2006, 01:39 PM
Are you sure you're unit is modulating? You may have a setup issue. If it is modulating, the heat loss at 16 degrees may be less than the lowest furnace output allowing the unit to cycle still (signs of an oversized unit).
12-04-2006, 04:47 PM
I don't know how to tell other than the readout on the furnace circuit board flashes a code that says that it is modulating. At it's lowest modulation the furnace would put out about 29K BTU.
Based on 2heat's response, my furnace sounds oversized. (I tried to get a Mod J done by the contractors I called. Only one was done and it came back at 90K BTU for a 1900 sq. foot house in St. Louis.)
12-04-2006, 04:56 PM
How long of runs are you getting? Are you using a setback at night.
12-04-2006, 05:45 PM
I set back from 69 to 67 at night.
Since it was really cold, it took about 20-25 minutes to go from 67 to 69. It shut off for 10 minutes and then came on for 15 minutes. I didn't keep track of it after that, but I can.
12-04-2006, 06:02 PM
I don't know what teh next smaller mod is, but it would probaly be too small.
So I wouldn't worry about being a little over.
12-04-2006, 06:20 PM
At that cold out, even a slightly oversized furnace should not cycle.
You might have to tell us the model of the thermostat. Hopefully, they didnt install the non-mod touchscreen. You cant tell the difference by looking at it. It could be that the wiring to the thermostat or furnace is wrong OR it is configured not to modulate.
look in the little window there is some red letters or numbers that light up on the board when there is a call for heat, is it a lower case h or a capitol H?
12-05-2006, 12:05 AM
Good hearing from you. It's a capital 'H'.
12-05-2006, 09:25 AM
Originally posted by dancarne
Only one was done and it came back at 90K BTU for a 1900 sq. foot house in St. Louis.)
Yikes, that sounds oversized.. I have a 1,700 sq ft home, and I am running on 60k
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12-05-2006, 12:00 PM
How Far away is the thermostat from the closest heat register? Capital -H- means your in Modulation mode.
[Edited by 2heat on 12-05-2006 at 12:24 PM]
12-05-2006, 12:07 PM
The closest register is in the bedroom across from the thermostat. The register is about 15' feet away. The return is about 5' away. That room does run hot. I could cut back the flow some.
As I mentioned, I did my own hvac-calc and it came out to 56K BTU. Conventional wisdom was that was too close. At the moment I wish I hadn't listened to conventional wisdom.
DThe house is comfortable. The reason I posted is it looks like I may be oversized or not modulating correctly.
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