House heats to 73 no matter what I set it to

HotMN

I have a Lennox Pulse 21 gas furnace. No matter what I set the thermostat to, it shows about 73 degrees (and it feels like 73 in the house, so I believe it). I replaced the thermostat with an inexpensive 4 program Honeywell. The wiring was easy, color coded, and I also used little stickers as I removed them from the original base plate. Before I attached the new thermostat to the new wall plate, it showed 65 as the temp. When I snapped it into the wall plate, it immediately jumped to 73. Once I flipped on the breaker, the heater came on and stayed on for the last 6+ hours, and the temperature reading has gone up from 73 to 76. And it feels like 76! I can turn it off to let the house cool down, but I'd rather not have to manually turn it on and off all to get a nice temperature in the house. And when I'm away on vacation, I would much rather have it at 55 then 75 the whole time. Please help!

Fred

Sounds like a short somewhere, maybe in the wiring? Does the furnace actually go off at 73 and/or 76 or is that where you intervene?

Zman

You might try pulling the t-stat wires off the furnace and see if it still runs. Then, connect those wires and disconnect the t-stat itself. It sound like either bad wires or miswired.

DZoro

Make sure no wires are touching each other inside the thermostat.
D

HotMN

Thanks for the comments. I checked the wiring at the thermostat, and it looks fine to me, no visible shorts. This gets a little complicated, but please bear with me!

I forgot to mention in the original post that there is a 2nd thermostat upstairs, an old fashioned dial with a mercury switch and a bi-metal coil. While I had the heat off yesterday, someone had cranked that one up high, and that was why it went up to 76. But I want to be clear: this was not the root cause of the problem; we've been stuck at 73 all winter, and we have definitely had the upstairs therm set to 50 or 60 for most of that time. Anyway, once I turned that one back down (to 50), the house cooled considerably... but the new thermostat display went down from about 77 to 73. And it has not moved below 73 in the 12 hours since then, even though the house is certainly much cooler than 73 now.

So in other words, the old thermostat would heat the house to 73 no matter what I set it to. The brand new thermostat heats the house to the temperature I program it for, but still displays 73 on its LCD display.

I talked to a technician whose only comment was that it sounds like something to do with zone heating is messed up, or that it is wired wrong. He tried to tell me new thermostats don't have a "B" and an "O" and so I must have wired it wrong, but mine definitely does have a B and an O (it is a Honeywell RTH221).

So the temperature feels good now, but the thermostat still says 73 on the display. Any advice or troubleshooting tips appreciated.

Zman, I will look into pulling the t-stat wires off the furnace. Have not done that step yet. Thanks.

Jamie Hall

I think at this point that what I would do is disconnect the fancy new thermostat completely -- not at the thermostat, but at the boiler or zone valve or pump. You want all the wiring to that thermostat completely out of the picture.

Then I would see if the "old fashioned dial with a mercury switch and a bi-metal coil" can control the system. Not necessarily at the temperature it's set at, since they have to be very carefully levelled when they are installed or reinstalled, but at least control. The reason I suggest this is that that old type of thermostat is pretty close to bulletproof, and if it can control the rest of the system, you will know that the problem is in the new thermostat or in the wiring to it -- and thus get one whole set of possible gremlins put away.

Fred

Just to double check, how old is this thermostat? Does it have a digital display? The last programmable Honeywell I bought had a film (fake) protective cover over the actual display screen and it showed "73" on that film, just to show what the display looks like. You don't have that film still on that thermostat, do you?

nicholas bonham-carter

Good thought Fred-maybe you've nailed the mystery !!--NBC

rjheater

I think the proper technical term for this problem is hmmmm!

rjheater

I once saw a first floor and second floor Tstats wired in parallel, poor man zoning

DZoro

Do you have zoned forced air? If so, the wiring maybe complex. Like the tech that you talked to, one thermostat may need to be the boss using the B terminal. But there is many ways to wire a zone system.....
May have zone dampers open when should be closed and vise versa.
Bring the tech in to have a look.
D

HotMN

These are all good ideas, but unfortunately the mystery is still not solved! It would be funny if the plastic coating were still on the LCD screen on the new thermostat, but it isn't! The thing to remember here is that the old thermostat (not original to the house, but the make and model of it places it as at least 15 years old) worked fine for A/C, but this winter for heat, even if I set it at 68, it would heat right up to 73 and stay there. If I turned it off, as the house cooled the display would accurately show the temperature going down, down, down. So it wasn't "stuck" at 73, but it would always go there when the heat was on. And then a couple days ago I replaced it with a brand new thermostat, and it is doing the same thing! I'm going to call Honeywell because they have a support line, and see if they have any guesses. I still have not done any troubleshooting at the furnace itself (cut me some slack, its been a busy couple days!), but just to simplify things, I did remove the upstairs thermostat, but that did not change anything. I'm going to try to research the furnace itself, and I'll post an update if I find anything (or if Honeywell tells me something good). Thanks for the brainstorming...

Jamie Hall

Let's try to think logically here -- having eliminated the overlay! It is apparent that something in your structure, somewhere, is controlling the heating system so that it reaches, and holds, a temperature of 73. Question is, what.

I repeat my earlier suggestion: disconnect the new thermostat at the furnace from the system, and see what that nice old T87 will do. Will it control the heat? Can you turn that up and down and get different shut off points? OK. If so, that thermostat -- and it's wiring -- are OK. If not, disconnect that thermostat at the furnace, too. Does the system still run up to 73 and sit there? If so, there's another thermostat somewhere hooked in. Don't laugh; it's been known to happen. If not, and the furnace stays off, either the T87 is bad or its wiring is bad. Reconnect its wiring, but not the thermostat. Now what happens? It if runs up to 73 and sits, there's a thermostat connected to that wiring somewhere -- find it. If not, put the T87 back in line. Runs up to 73? Problem with the T87 -- rare, but it could happen.

But let's suppose that when the new thermostat and its wiring are disconnected, the T87 controls the system properly. Reconnect that wiring, but not the thermostat. Now what? T87 still controls? Good -- at least the problem isn't in the wiring or in something attached to it. Try moving the T87 to the new thermostat's wiring. Don't forget to level the T87! Now what?

And report back... please. I'm fascinated!

SuperJ

One thing to check is if a high limit safety is cycling on the furnace. Perhaps it happens to cut out when the space is 73.
Does the furnace have good airflow? Filter good?

Fred

Let's start back at the beginning. You have two thermostats in this house. One on the first floor, one on the second floor.
Maybe it is actually 73 degrees on the first floor near that thermostat. Put a thermometer there and see if you can confirm the temp.
My guess is that the second floor thermostat is the only one calling for heat, to keep that floor at the desired temp, maybe 70? and in order to keep that floor at the set temp, the first floor never calls for heat. It's just a thermometer until and unless you turn the second floor Thermostat to "OFF".

The other possibility is you have that first floor thermostat on a wall that's maybe over the furnace or that may have a duct behind it, in the area and it actually is 73 degrees. Is there a good size hole behind the Thermostat that the wire comes through? You may be pulling warm air from the furnace area. If so, plug the hole or move the Thermostat.

Fred

I also believe the second floor thermostat is the reason the first floor went to 76 and the furnace ran for six hours when you powered the furnace back up. It took that long to bring the second floor back up to temp, to satisfy that thermostat after the furnace was powered off. The furnace ran long enough to add additional heat to the first floor. You can't just add thermostats to create zones in a house. The duct work should also have to be zoned.

HotMN

Thanks everyone for the feedback. Jamie Hall, I wish I had the time and the skills to take your advice, your troubleshooting steps are clear and logical. Unfortunately I keep dragging my feet and today decided I will just call a professional and get it done sooner rather than later. But I wanted to add a couple more details for those who are interested in the mystery.

Even though 73 had seemed like the magic number, in the last couple weeks I have had the new thermostat on the ground floor set at about 60, and it has consistently showed a reading of 70 or 71. Once or twice I have woken up sweating in the middle of the night and gone down and checked it and found it showing 73 again, though. And the upstairs thermostat is still disconnected.

Upstairs there are 2 bathrooms, and they are always much warmer than the rest of the house, which is already a couple degrees too warm. One of my kids reminded me of this today when he complained it was too hot to brush his teeth! I have to admit, it feels like 80 in the bathrooms, and when I go to bed, I always make sure I close the door to the master bathroom, to keep that warm air locked away.

Also the basement is pretty darn cool. Probably 65. It has an under the slab heating duct, I think you call it transit heating?, as well as one wall vent near the ceiling right outside the utility room that actually houses the HVAC, boiler, etc. Today I was nosing around and realized that the air blowing into the basement from that one wall vent is actually cool air, not warm in the slightest. No idea what to make of that. Seems like a whole other can of worms...

Anyway, like I said, I have given up on the DIY approach and called in the professionals. I am just hoping I get a clever and curious technician, and not one who just says "boy I haven't seen a Pulse furnace in years, you should just replace it, they don't even make parts for them anymore!" Because the furnace cranks out the heat, no problem; my gut is telling me it is a thermostat or control board or relay type of problem, not a replace-the-whole-damn-thing problem. But my gut is pretty ignorant, and could be wrong. I'll post an update later. Thanks for reading.

HotMN

And the final update. As I was showing the technician to the utility closet with the furnace in it, I saw a thermostat on the wall in the basement that I'd never noticed before. It was set to about 77. I turned it down. Mystery solved. I should be embarrassed about paying $89 for a service call for nothing (and wasting everyone on this forum's time!), but honestly I'm just relieved to have it working and not spending hundreds or thousands for a repair or new furnace.

Jamie Hall

I KNEW there was another thermostat in there somewhere! But good to know that you've found it. Thank you for the update!

Leonard

Heating Temperature wars ....

Wife likes it colder than I do by ~ 4 degrees, she'ld always turn it down. So I thought about hiding a 2nd thermostat somewhere wired in parelle ,...... set at MY minimum tolerable temp

JUGHNE

An old house I was rewiring had a double pole double throw knife switch attached to the floor joist.
Tracing the wires it was a transfer switch for the 2 thermostats, one upstairs for the wife and one in the basement for the husband. She may not have know about the bsmt tstat or switch.

ratio

I've fixed more than one too hot/too cold problem by installing one of those new wireless stats in the problem area...without the receiver...