1

My home furnace is currently controlled by a programmable Honeywell thermostat, but it regularly thrashes the furnace by turning the furnace on for just ten or twenty seconds.

It seems that the thermostat will register a one-degree drop in temperature for a few seconds, turning the furnace on, but then switch back up to the previous temperature a few seconds later, turning the furnace off again. Presumably, air currents near the thermostat produce rapid swings in temperature of around one degree.

After some investigation online, it seem that what I need is a thermostat that supports "variable swing" or "deadband" -- that is, heat to a given temperature (e.g. 25 degrees), then let the temperature cool to a different temperature (e.g. 22 degrees). Apparently Honeywell thermostats do not support this.

Which thermostats allow a temperature range to be programmed? I don't want to spend hundreds of dollars on a smart thermostat; a simple and cheap programmable thermostat would be ideal.


Update: the thermostat is a Honeywell Pro TH4000 series, most likely a TH4110D.

  • 1
    AKA: Adjustable differential. Robertshaw RS3110 – Mazura Feb 18 '15 at 0:34
  • 1
    Where's the hysteresis when we need it? – bib Feb 18 '15 at 0:54
  • I've never seen a thermostat that turns on/off with a change of 1 degree. In most cases if; for example, you have the thermostat set to 72. The temperature will have to drop to 71 or 70 before the heat is called, and the heat call will not stop until the temperature is 73 or 74. I've never come across a thermostat that calls for heat at 71, and is satisfied at 72. The thermostat may be malfunctioning. – Tester101 Feb 18 '15 at 2:43
  • 1
    Venstars have both dead band and cycle per hour parameters. – alx9r Feb 18 '15 at 3:20
  • Bib mentioned hysteresis. Wikipedia has information on hysteresis in control systems. – Steve HHH Feb 19 '15 at 5:24
3

Looks like many Honeywell thermostats have a "cycles per hour" setting instead of a deadband setting. Not exactly the same thing but roughly analogous. If your thermostat has that setting you could try changing it.

But if the cycle time is 10 seconds, I'm not sure the deadband is really the problem... even 1º should take longer than that. Do you have a hot air vent blowing directly on the thermostat or something? Or possibly some other problem with the furnace not lighting?

  • There doesn't seem to be a model number on the back of the thermostat, but the manual says it's a Honeywell Pro TH4000 series (TH4110D). I can't find any mention of cycles per hour in the manual, but it's still under warranty. I'll call Honeywell and ask about it. Thank-you! – Steve HHH Feb 19 '15 at 5:17
1

Defective thermostat certainly seems like the most likely option here, so probably any reasonable programmable thermosat will work more correctly than this, no need for fancy and pricy "smarts."

One possible solution would be to insert some time-delay relays into the loop to enforce minimum on/off times. A simpler one would be to replace with a more functionally correct 'stat. Even the old mercury bimetal stats had sufficient hysteresis to keep this sort of thrashing from happening.

It's also possible that your current 'stat does have adjustments that are mis-adjusted which would help with this. Whether or not you could find documentation on them I don't know, and you have not mentioned a model number. If you pop it off the wall and have a look at the back you might see something, or you might get lucky if you look the model number up online. Often older stats will have somewhat obfusticated terminology intended to keep you calling your heating guy rather than adjusting it yourself. If your heating guy wasn't too bright this could backfire (after all, he knew how to adjust a mercury 'stat, so why learn anything new? Followed by the "newfangled junk" not working, just as he predicted while throwing the directions in the trash.)

0

I have the same problem. Set to 60°F it will fast cycle 15 minutes on at 60°F, and 10 minutes off at 59°F.

So I called Honeywell. Their answer: build a custom thermostat that has an "anticipator circuit", with hysteresis like the old Mercury thermostat. "All Honeywell thermostats have a +-1°F or +-.5°c and are not adjustable" said tech support.

I did a test. With my gas forced air furnace and my Honeywell RTH221B thermostat, I set the switch on the back to water water/electric, instead of gas /oil.

Result: with set point at 68°F, it will turn on at 67°F, and off at 71°F.

I believe this is the fix you were looking for, unless you have a heat pump.

Gumby stay flexible.

  • I tried to edit your answer into something resembling English; please edit further if I didn't get it right. – Daniel Griscom Mar 29 '16 at 20:51
0

I know this is old, but it is surprisingly hard to find this information on the web.

I had the same problem with a Honeywell thermostat in my old apartment. It was located by the door and the temperature at that location would change by a degree frequently, which was a major problem because my forced air heat was very loud. I spoke to Honeywell and they do not make products with variable swing, so I went to a different company.

One solution for you is LUXPRO PSPA711a http://www.luxproproducts.com/pdfs/PSPA711a_NL_ENG_WebManual.pdf

It has a variable swing that can go between 0.5 and 4.5 degrees (the 0.5 is marked as .25 because it starts heats when the temp is 0.25 degrees below the set temperature and shuts off when temp is 0.25 above). This worked for me and I believe Lux makes other thermostats with variable swing so check out their models. I ended up not liking this thermostat because each day has 4 different periods temperature settings, and everyday is programmable. If you want to change the weekday temperature, you have to sort through 28 different periods and adjust 5 of them. Works for people that need to adjust each day, but this was overkill for my needs.

Lux doesn't always highlight that variable swing in the product description, so read the manuals.

0

I have a very long house with long duct runs. This reveals itself as a problem with my Honeywell 7600 thermostat which has a very narrow hysteresis. I was almost ready to throw the thing away but I discovered that the ducts run under the floor nearby the 'stat, and so heat rises up the wall is mounted on but is captured under the thermostat box bottom forcing it up through the stat. To solve that problem somewhat, I spaced the thermostat OFF the wall, allowing the warm air to pass BEHIND the thermostat instead of through it. This means the heat runs a few minutes longer and that has made a big difference with the rooms at the end of the the long runs!

0

Is your vent pointing directly at your thermostat? that would affect the temperature reading when the system is on.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.