Ask Me Help Desk - Is this normal? : thermostat & furnace question

Hi all...

Firstly, I read a lot of posts before asking this question... I saw some similar questions but didn't find the answers I needed...

It would be very helpful for me if the follwing could be answered (I'm a first time home owner, in the process of learning stuff, pardon my lack of knowledge):

(PAYNE FAU -- 80% -- NG -- 6 years old -- start cycle looks fine)

1. When I turn the heat on, (lets say HOLD at 68F) and the temp is at 59F, the furnace starts, then after it reaches about 61/62F it turns off, the thermostat says system off, it then kicks in after 2 minutes... Does this mean it is getting over heated?? I can increase the hold temp by 2F (in this case to 70F) or turn off & on and it would turn on immediately, which should not happen if its overheated right?
Is this normal??

2a. After it reaches the final temp 68F and it turns off, sometimes it turns back on in 1 min, is this normal??

b. It also turns off and on every 10 minutes, is this normal? (note the temperature display in the tstat does not change)... Is the tstat TOO SENSITIVE, responding to changes under 1F??

3. It takes 30 minutes to raise the temp from 57F to 60F (experimental) with 1 stop in the middle, the house is 1700sq ft, don't know if this is the normal rate at which these furnaces work... would bringing a HVAC person help make my furnace more effficient?

From the other posts I gather that it might be dirty heat exchanger, faulty gas value or bad flame sensor... is that true?

Answers to these would be very helpful... thanks for your time...

-Anand

Go to the furnace. Jumper the R and W contacts. If the furnace comes on and stays on as long as the jumper is in place, the trouble is in the thermostat or wiring. There is an adjustment in the thermostat that I have never touched, and usually suggest leaving alone. Dig some more here looking for instructions on setting the anticipator.

If the furnace shuts off with the R and w jumpered, then look at the high limit switch.

Thanks Labman for the reply!

I will short the jumper and see if it runs continuously... Is is the same if I short them at the thermostat or the circuit borad? (should be, I'm assuming)

I did an experiment today, running the furnace to heat the house from 57 to 67, the furance ran for 1 hour straight to raise the temp from 57 to 64 and then stopped for 2 minutes then continued for 20 more minutes to bring it to 67 (finally), so I think this 2 minutes break would be normal because of over heating rt?

>>> I'm very concerned about the time it takes to heat the whole house... 80 minutes to raise the temp from 57 to 67??

After heating to 67, it started cycling OFF-ON for 6mins-4mins for the first hour and slowly shifted to OFF-ON 10mins-3mins the second hour.

>>> I think it might be the problem with the heat anticipator, but I can't find the adjustment to increase the resistance on it (its a honeywell thermostat)...

I will do more experiments and update the thread... Thanks again...
(I've scheduled a checkup with my heat company on 12th, hope they can figure out the issue)

Connecting the R and W at the thermostat does not eliminate problems with the wires, unlikely in this case.

Thanks, I will do more experiments and update the thread...

You need to look into the installers guide for the thermostat, many have settings for differential and heat anticipator. Set the differential to + or - 1 degree.

Thanks!

I wish I had the user manual... I can't even find the model number :(

Anyway, when I set the thermostat to High efficieny furnace, it cycles 15-7 (off-on)
And when I set it to gas furnace less than 90% efficient, it cycles 9-4 (off-on)

I need to figure out how to set the differential (will look for the manual online if I figure out the model)... I'm almost sure I can't adjust the heat anticipator, because there is not one...

Thanks again guys for the help!

Found it (the model number & troubleshooting guide)!!

Here is the answer:
--------------------
The thermostat is calibrated to hold temperature within one degree of your set temperature to maintain an even room temperature. Because of this, the thermostat may seem to cycle more often than older thermostats that had a wider room temperature swing. Many people can feel a temperature change as low as two degrees. Since your Honeywell thermostat holds the temperature within one degree, the room air temperature remains steady and comfortable. The energy savings comes from setting the temperature back during the day when there is no one at home or at night when everyone is sleeping.
Every heating system type will deliver heat to the house at a slightly different rate. The thermostat provides you with the flexibility to set the cycle rate adjustment to match your specific heating system, whether it is gas or oil warm air, high efficiency warm air, electric warm air, or baseboard hot water.

A cycle rate is the ideal number of times a heating system will run, in an hour, to maintain temperature within one degree. For instance, a gas or oil furnace has a recommended cycle rate of 6. With a cycle rate of 6, the heating system, at a 50% load, will cycle 6 times per hour. This breaks down to about 5 minutes on and 5 minutes off. The actual on and off time of the heating system will vary as the load on the heating system varies.

The installing contractor should have set the correct cycle rate on your thermostat. If you have questions on the cycle rate setting on your thermostat, honeywell suggests contacting a heating/cooling contractor.
-----------------------

Quote:

Originally Posted by ramasund

Thanks!

I wish I had the user manual... I can't even find the model number :(

Anyway, when I set the thermostat to High efficieny furnace, it cycles 15-7 (off-on)
and when I set it to gas furnace less than 90% efficient, it cycles 9-4 (off-on)

I need to figure out how to set the differential (will look for the manual online if I figure out the model)... I'm almost sure I can't adjust the heat anticipator, coz there is not one...

Thanks again guys for the help!!

Has the thing always worked that way, or at least since you bought the house? If you were sold a house with a messed up thermostat, you may be able to make the seller send somebody to fix it.

Now that you have the model number, you might dig a little deeper yourself. Even if you don't have instructions, cycling through the settings could turn up the heat anticipator. Although I said such settings usually are better left alone, this may be a case for resetting it. Normally if a furnace once was running right, a problem is more likely to be a dirty flame detector or a sticky relay than the heat anticipator needing adjusted.

It was probably cycling the same number of times an hour, except I didn't notice... It has been more than a year since I bought the house, so no luck there :)

I don't think Honeywell has a good record on providing technical details. If you can't find anything to help from this link, you may have to call a contractor. https://www.askmehelpdesk.com/electr...als-37098.html

Or perhaps you could use that link to choose a friendlier brand of thermostat. The reality is that to stay in business, a contractor may need to charge $50-$100 to come and reset the old one. They have outlandish fixed expenses.

I was reading this post and I was trying to find if you did the test that Labman suggested of jumping R and W?

I think that I would not expect a residential system to be designed in that way.
My house is set to +/- 1d (from what I can tell), and does not change temperature that often, even though the main thermostat is close to the main door.

Unless your thermostat is located at a strange location where temperatures could change drastically, it should not turn on and off that often.

Perhaps Nothernheat or Labman could confirm this, but I do not think this should be common nor be an exception case. I think the focus has to be to on what is making it go on and off mainly.

I hope this help the process.

Quote:

Originally Posted by Felrio

I was reading this post and I was trying to find if you did the test that Labman suggested of jumping R and W?

I think that I would not expect a residential system to be designed in that way.
My house is set to +/- 1d (from what I can tell), and does not change temperature that often, eventhough the main thermostat is close to the main door.

Unless your thermostat is located at a strange location where temperatures could change drastically, it should not turn on and off that often.

Perhaps Nothernheat or Labman could confirm this, but I do not think this should be common nor be an exception case. I think the focus has to be to on what is making it go on and off mainly.

I hope this help the process.

What Labman wants to do is, confirm the furnace will run a long/complete cycle. Red wire is 24 volts, connecting it to white sends a signal to the furnace heat relay (whatever that may be). If furnace does not cut out prematurely the problem is with the stat.

I think those things come set from the factory so that in most cases they don't need adjusted. I don't quite understand why they need reset, but in some cases they do. Once set right, they are unlikely to change. Better to check out other things first. However, I think this is clearly a case where it does need reset. I wish somebody that knew how to set it would post. Jumpering the R/W is a quick furnace/thermostat check.

The few thermostats I have installed worked fine out of the box which leads me to assume in most cases it is like carburator, a French word for leave alone.

Here are a few other ideas. Is the furnace oversized? Is the hole behind the stat insulated or sealed, you would be surprised how much cold air can drop down a stud wall, particularly with poor construction practices.

I'm sorry, I haven't done the R & W short test... The reason I didn't was because the furnace runs for 45 minutes straight without overheating (then stopped for 2 minutes, might be because of overheating) and then re-started again and ran for 30 minutes before reaching the desired temperature... so I figured it was not the furnace overheating... I'm sure the problem is with the tstat...

I also figured the cold draft is not the reason because when I turn it off it takes 45mins to 60 minutes before the temp changes by 1F (in the thermostat display). I'm going to seal the small hole behind to see if that is going to help (but doubt it).

The factory defaut is set to 6cph (cycles per hour) so it should cycle 6 times an hour right? I have changed the settings to 90% eff furance and it cycles 4 times/hour.

I did find thermostats with adjustable swing/differential without fixed cph, this will make sure the tstat turns the system ON only when the temp drops by one whole degree (1F).
(buying one of those would be a good investment)

Appreciate all the help guys...

Quote:

Originally Posted by ramasund

Thanks!

I wish I had the user manual... I can't even find the model number :(

Anyway, when I set the thermostat to High efficieny furnace, it cycles 15-7 (off-on)
and when I set it to gas furnace less than 90% efficient, it cycles 9-4 (off-on)

I need to figure out how to set the differential (will look for the manual online if I figure out the model)... I'm almost sure I can't adjust the heat anticipator, coz there is not one...

Thanks again guys for the help!!

The quote from the manual is identical to my Honeywell CT3200 thermostat's. Only settings are through 1 switch and some screws on the back, and there are inscriptions on the backplate indicating the settings. Electric or fossil fuel, and the equivalent of cycle settings.

In the original note you mention that the thermostat indicates "system off" when the furnace shuts down briefly as it heats the house from 57F to 68F. That is normal for when the thermostat is not calling for heat. If the thermostat was calling for heat and the furnace was shutting off due to overheating, then the thermostat would, I believe, still be indicating that it was calling for heat by highlighting "system on" (even though the furnace was off). If I am interpreting your description correctly, then I do not believe that the furnace is overheating (I believe that in the R-W wire test the furnace will continue to heat as long as those wires are connected).

As for the issue of taking a long time to heat the house, at 80% efficiency this is a single stage furnace. Then the heatup time is being determined by the heat output of the furnace except for the brief (1-2 minute) shutdown as the temperature gets near the setpoint. The only effect the thermostat is having is that the brief shutdown is adding 1-2 minutes to about an hour heatup. A pretty small effect. I can't comment on whether this is normal or not for this thermostat. My CT3200 heats to the setpoint without a pause, but my furnace is oversized and it gets to 68F in about 10 minutes from 57F. My guess is that your thermostat is behaving as it was designed and shutting down the heat briefly as part of an alogrithm to prevent temperature overshoot.

The last issue is the cycling rate/time when the furnace reaches the setpoint. You describe shorter cycle times when the thermostat is set for an 80% furnace (6 cycles per hour) and longer cycles when the thermostat is set for a 90% furnace (4 cycles per hour). While the numbers don't equate exactly to 6 and 4 cph respectively, they are pretty close to those values. So in this regard the thermostat is working correctly. Considering the relatively long time that it is taking to heat up the house, the furnace is not oversized for the house and, if it is an 80% furnace, I'd leave the thermostat set at 6 cycles per hour. My CT3200 holds temperature rock solid at 6 cph.

In summary, then, the issue with slow heating is a consequence of the heat output of the furnace relative to the heat requirements of the house and not the thermostat. And an hour and 20 minutes to raise the temperature 10 deg F (8 min/deg F) is not unusual, I think. The 2 minutes of shutdown are not, I believe, caused by the furnace overheating (still do the wire test to confirm, though). I think that the thermostat is operating normally and briefly suspending its call for heat as the temperature approaches the setpoint. Finally, the thermostat cycle setting appears to be having the proper effect. Longer cycles in the hi eff. Setting and shorter cycles for

Thanks a lot for the lenghty response, really appreciate your time!

Quote:

Originally Posted by bkspero

In the original note you mention that the thermostat indicates "system off" when the furnace shuts down briefly as it heats the house from 57F to 68F. That is normal for when the thermostat is not calling for heat. If the thermostat was calling for heat and the furnace was shutting off due to overheating, then the thermostat would, I believe, still be indicating that it was calling for heat by highlighting "system on" (even though the furnace was off). If I am interpreting your description correctly, then I do not believe that the furnace is overheating (I believe that in the R-W wire test the furnace will continue to heat as long as those wires are connected).

This is an excellent point, the thermostat does say system off => it the T-stat and not the furnace overheating causing it to turn off... The very fact that I see it 2*F before it reaches the set temperatures proves that its trying to prevent overshoot.

Quote:

Originally Posted by bkspero

As for the issue of taking a long time to heat the house, at 80% efficiency this is a single stage furnace. Then the heatup time is being determined by the heat output of the furnace except for the brief (1-2 minute) shutdown as the temperature gets near the setpoint. The only effect the thermostat is having is that the brief shutdown is adding 1-2 minutes to about an hour heatup. A pretty small effect. I can't comment on whether this is normal or not for this thermostat. My CT3200 heats to the setpoint without a pause, but my furnace is oversized and it gets to 68F in about 10 minutes from 57F. My guess is that your thermostat is behaving as it was designed and shutting down the heat briefly as part of an alogrithm to prevent temperature overshoot.

68F to 57F in 10 minutes? I would love that :)
Maybe it just needs service, gas company is coming for an annual tuneup, will ask them.
The size is fine I think, doubt they would have put a small one for the house size (the house is 1700sqft, which is not too big in the first place)
I'm guessing there might be some cold draft or duct leak or blocked/dirty duct...

Quote:

Originally Posted by bkspero

The last issue is the cycling rate/time when the furnace reaches the setpoint. You describe shorter cycle times when the thermostat is set for an 80% furnace (6 cycles per hour) and longer cycles when the thermostat is set for a 90% furnace (4 cycles per hour). While the numbers don't equate exactly to 6 and 4 cph respectively, they are pretty close to those values. So in this regard the thermostat is working correctly. Considering the relatively long time that it is taking to heat up the house, the furnace is not oversized for the house and, if it is an 80% furnace, I'd leave the thermostat set at 6 cycles per hour. My CT3200 holds temperature rock solid at 6 cph.

Does your turn on 6 times / hour?? Based on the size of yours if it turns on/off 6 times and if your insulation is good, won't it overshoot??

I read somewhere that it is the max limit and not necessarily that many on-offs/hour... is that correct?

I think its not too effective to turn it on and off that many times... The t-stat that works based on swing/difference would be much more effective...
The temp drops at 1.5*F/hour when its off, so my furnace needs to run only for 10 mins/hr to maintain the temp, but it runs more because of 6cph...

I'm guessing the short cycling maybe because of a hole behind the t-stat where the wire came from, although I didn't feel cold draft from that hole I feel it might have affected it a little bit. The hole has been sealed now...

Quote:

Originally Posted by bkspero

In summary, then, the issue with slow heating is a consequence of the heat output of the furnace relative to the heat requirements of the house and not the thermostat. And an hour and 20 minutes to raise the temperature 10 deg F (8 min/deg F) is not unusual, I think. The 2 minutes of shutdown are not, I believe, caused by the furnace overheating (still do the the wire test to confirm, though). I think that the thermostat is operating normally and briefly suspending its call for heat as the temperature approaches the setpoint. Finally, the thermostat cycle setting appears to be having the proper effect. Longer cycles in the hi eff. setting and shorter cycles for

I'm going to set it to 3cph and see, the fan runs 2 minutes after the furnace is off that sends out non-warm air (not really cold), the effect of which increase if the cph increases...
I'll experimetn with 3cph or maybe get a t-stat which supports swing/deadband/difference.

Thanks everyone for the time! I will update the thread once NW Natural comes in for a FREE annual tuneup.

I did one more experiment... Took the thermostat to a room with electric personal heater, raised the room temp to 67 and when the t-stat was on hold 67, it did NOT cycle at the rate of 6 cph (6 on-offs in 1 hour).

So I think it might be the hole behind the tstat that caused it to short-cycle...

So 6cph does not mean guaranteed 6cph, its more like 6cph max (I THINK).

Having an oversized furnace is nice when heating from a large setback. But it is not so good when trying to control temperature. In an extreme case it is impossible to control temperature, as the room overheats before the thermostat can respond and cut off heat. My case is less extreme. It can hold temperature, but the burner may run only 1 minute per cycle. I've been told that such operation is less efficient than normal operation such as yours.

I do not think your furnace needs service for slow heating. Eight (8) minutes per degree is not unusual.

My furnace cycles more frequently when the thermostat is set for a furnace of <90% efficiency, and less frequently when it is set for a furnace of >90% efficiency (it is actually 80% eff.). When set for <90% eff (more, shorter cycles). It holds temperature rock steady (set it at 68 and it stays there). When set for >90% efficiency (fewer, longer cycles) and temperature cycles between 67F and 69F (when set at 67F). I can't be sure that one setting is exactly 6 cycles, and the other is exactly 4.

I have opted for fewer, longer cycles because I am trying to increase efficiency and a 2 deg F cycle is acceptable to me. If I had a smaller, 80% furnace, I would set the t'stat at 6 cycles and hold the temperature steady while getting the design efficiency.

I don't know if the cycle setting is a max limit or a hard setting.

Your cycles aren't short. Even when setting for <90% efficiency (6 cycles) you are getting about 8 minutes on and 4 minutes off. And my guess is that you are timing the time that the fan is on. The real issue is the burner-on time. You mention that the fan runs about 2 minutes after the burner turns off. So your real cycle rate is about 6 min on and 6 min off. That is perfectly normal for an 80% furnace heating against moderately cold outside temperature (say about 30-40 F).

Plug the hole behind the thermostat with a piece of cloth or sponge. My guess is that it will have little effect, but its good practice anyway.

It is normal for the fan to operate for a minute or 2 after the burner shuts down. It is extracting the last bit of heat that it can from the still-warm heat exchanger metal. That was one of the features designed into furnaces to get the efficiency up from about 65-70% to 80%.

Try 3 cph if you want. I will not change the time it takes to heat from 57F. It will cause the furnace to run longer when it is holding temperature constant. It may or may not cause the room temperature to cycle more than is comfortable to you.

I have another zone with a thermostat with deadband settings. It cycles a minimum of 1 degree at the tightest control setting (1 deg F deadband). As I mentioned, the CT3200 can hold temperature constant within a degree. With the right size furnace, I would prefer the CT3200.

Good luck