Thermostat questions

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Old 11-04-07, 01:54 AM
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Thermostat questions

I have a 92.9% efficiency natural gas furnace that I had to replace the thermostat on recently.
The first thing I did was try the cheapest digital thermostat (Hunter Just Right model 42999) and it took care of the problem, but used batteries and wasn't every easy to move the heat/off/cool switch.

I went out and got a Honeywell Digital Round model CT8775C and it's so much nicer in every way except it cycles the heat too short.. the Hunter would (on the shortest span setting) run it for at least twice as long and less frequently, raising the display temp from the set temp to 1 above it (reading on thermostat) and cutting off.

The Honeywell doesn't have "span" settings, instead in the manual it says: Heat Cycle Rate

for: High Efficiency Warm Air (90%+ efficiency), Hot Water, Heat Pump: Cycles Per Hour 3. DIP Switch 2 On

for: Gas or Oil Warm Air (factory setting): Cycles Per Hour 6. DIP Switch 2 Off.

I have it set on 6 cycles per hour since it didn't state natural gas under the high efficiency warm air setting...

Wouldn't it make more sense to set it to 3 cycles per hour and hopefully make it have longer running times so there's less wasted fuel warming the system up before the blower turns on?

Sorry about the lengthy post, and it's my first one! So glad I found this forum, it looks like I'll be spending lots of time here
 
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Old 11-04-07, 06:04 AM
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Originally Posted by LCR4e View Post
I have a 92.9% efficiency natural gas furnace .



for: High Efficiency Warm Air (90%+ efficiency), Hot Water, Heat Pump: Cycles Per Hour 3. DIP Switch 2 On
Your furnace is a high efficiency furnace, so yes you want to set it to 3CPH


You did a good thing of taking the Hunter back, hear too many mixed results on them.
 
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Old 11-04-07, 10:48 PM
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Thanks for the input!
Well at 3 cycles per hour instead of running for as short as a little less than 3 minutes and cycling as often as every 10 minutes, it's running for about 7 mins and there's about a minimum of 40 minutes between cycles.

Both nights for comparison were right around 40 and the thermostat was set at 70.

Think that might save a little on the gas bill?
 
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Old 11-05-07, 06:12 AM
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Should help a little bit on your gas bill.

Just make sure your air filter is kept clean, and seal any leaky windows and door ways.
 
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Old 11-06-07, 07:26 PM
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Yup change them every month. I just use the new type fiberglass that's supposed to filter a little better than the old ones. The pleated filters make too much noise in the downstairs intake.

You know the funny part is that the original thermostat always cycled short too (pretty close to the 6 cycles per hour setting on this new one). It was put in and programmed by the installer of the system 6 years ago. When he came to check the ac the other year, I remember him saying how it was set at a one degree tolerance but that he could change that if we wanted. Of course that was with the ac.. not sure if it would have effected the heat.

It looked just like the Honeywell Digital Manual CT500, not sure if it has that same model number on it though. The reason I had to change it out was partly because the cycles became even shorter and also if I pushed it up a few degrees it would cycle off before it reached the set temp.
 
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Old 11-09-07, 07:34 AM
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I was just reading on this page: http://www.toad.net/~jsmeenen/thermostat.html "The Honeywell thermostats that you buy in the Home Store are no where near the same quality as the ones that you would buy elsewhere. I know this because I have replaced so many of them and I have had customers call me for service only to find out it is the Honeywell thermostat they bought in the home store (Lowes, Home Depot ect. ) that is the problem."

not sure how old that info is but hopefully they've improved their store quality thermostats?

Also I've been thinking that the 3 cycle per hour limit might not be such a good thing when it gets down into low temps.

Would there be any other type of digital non programmable thermostat that might be better quality and have a temperature swing limit instead of a cycle limit?
 
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Old 11-09-07, 08:12 AM
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Originally Posted by LCR4e View Post
I was just reading on this page: http://www.toad.net/~jsmeenen/thermostat.html "The Honeywell thermostats that you buy in the Home Store are no where near the same quality as the ones that you would buy elsewhere.

not sure how old that info is but hopefully they've improved their store quality thermostats?
I think the newer Honeywell sold at Home Centers are better than what you would of gotten 4 years ago. But, in a way he is right, the models sold at Home Centers has less options than the Pro model. But those options that you could get the pro may not be needed for your set up. Options like program blower to come on a scheuduled time, numbers of CPH on A/C mode... ect)
Also I've been thinking that the 3 cycle per hour limit might not be such a good thing when it gets down into low temps.
Not sure what you mean by that? 3CPH does not mean it will cycle times an hour all the time.. It will run longer cycles if the t-stat see that the heating need is higher on a cold blast. or less on a mild day. The 3CPH spread the heating cycle on an avg 3 times on cool day. I have the Honeywell, when we had our cold blast last feb of -0 in the day, -20 at night, my furnace ran no stop, and kept the house at a perfect 68˚ all the time.

Would there be any other type of digital non programmable thermostat that might be better quality and have a temperature swing limit instead of a cycle limit?
Why would you want the swing?? Our body can feel the swing.. I used to have a Lux in my last home, and Rite-Temp in this house when we first moved in, they did the swing, and I hated it. I was feeling cold by the time it kicks in, and then when it get near set point, I was getting warm.

As for Non-programable one, not many out there.. I think you got a good one for your needs. I went and took a look at the manual of the CT87 and T87, you are not gaining anything more with the Pro model of the 87's Stats.
 
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Old 11-09-07, 11:32 PM
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I was just thinking that a swing type thermostat set at a 1 degree swing might cycle a little more often on really cold days and keep the house at a more even temp. (It can get 2 to 5 degrees colder depending on which room you're in and how cold it is outside between cycles. I've added r23 cellulose insulation to the two big attics, weather strips to the windows, sealed leaky switch plates, ceiling fixtures, exhaust fans, duct work, etc..)

You know I thought that as soon as the thermostat called for heat it started using gas, but the exhaust starts first and about 35 seconds later the burners fire up.

One thing I don't understand is why they'd recommend more cycles for a less efficient furnace? I realize the exhaust temp on the less efficient one is hotter, so would the shorter cycles keep it from heating up more and letting even more hot air out the exhaust?
 
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Old 11-10-07, 05:47 AM
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On the older furnace they ran hotter, so of they ran it longer the temp may have a higher chance over shooting the set point since there is a lot of residue heat on the heat exchanger.

The 3CPH is really going to be seen when your heating system at 50% load, then less when you are getting near your design point.

YOu do not want your heating system cycling too often on a very cold day, if it is. then is' over sized.
 
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Old 11-15-07, 05:08 AM
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Thanks for all the help! I had it set on 71 a few nights ago when it was right at freezing outside and it'd cycle back on 23 mins after it shut it off. That's the shortest time between cycles I've seen so far.
I guess the only thing I like about the 6cph setting is it keeps it at a little more even heat throughout the house. Other than that it can get annoying because it will cycle so often and never even get to 100 degrees at the register.
 
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Old 11-15-07, 08:11 AM
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There's pro and con with short or longer cycle.. But overall, the 3 is better.

Once it gets down into the teens, the stat will do a really good job.
 
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