Do you have a hot water or steam heating system? What is the size of your house and the size the boiler? What is the model number of your Lux thermostat? The answers to these questions may help in determining what is the issue and a possible solution.

The overshoot and undershoot you are seeing I believe is more of a function you heating system and not the thermostat. I have not seen a thermostat that allows you to change the window of temperature range where the thermostat will call for heat and turn off, but there may thermostats that are better than others. If you make the range too small the boiler would be constantly turning on and off. That would waste energy and create a lot of wear and tear on the boiler.

We have a hot water system, medium size boiler and 2,000 square foot house. Our previous thermostat would click on as soon as the room temp fell below the set temp - so if we set it for 65, it would click on as soon as the thermostat registered that the room temp was 64. With the Lux thermostat, it registers that the temp is 64 but it waits before turning on. I haven't timed it, but it's quite a long time. The problem is, we have radiators, which take a while to warm up, so by the time the furnace clicks on and the radiators heat up, the room temp is 63.

The reverse happens when the room temp meets the set temp. The furnace stays on, until the thermostat registers that the room temp is 1 degree warmer than the set temp. But, our radiators continue to throw heat, so soon the room temp is 2 degrees warmer. We never had this problem with our old thermostat. It was ugly, but boy I wish I kept it.

It is a setting on my Carrier Thermostat - Carrier Hybrid HP & Oil

I agree with what Mike said. But there are two parts to the issue as regards the overshoot and undershoot.

For the undershoot part, as you've suggested, older thermostats used to have what was called a "heat anticipator" setting that was adjustable in fractions of and up to a whole degree or two. It asked the question "how far below the set temperature should we wait until the heat comes back on?" Newer stats tend not to have that but instead, they have a setting for "cycles per hour". In other words, how sensitive should the stat be about turning off at the setting and coming back on when it changes? The way most work, a setting of 1 means one cycle, one on and one off per hour. So that when an on cycle ends, it won't get checked again for 30 minutes. A setting of two assumes cycles of on/off every 15 minutes but when an on ends, it won't check again for 15 minutes.

Forced air heat responds quickly, radiant doesn't. But no matter what the stat is controlling, you don't want the boiler, or furnace (or AC) cycling on and off too frequently, longer runs are more efficient, easier on the equipment, and more comfortable. I think 2 or not more than 3 cycles per hour (with one stage equipment) are the max, with 2 preferred, for a ducted system. I seem to recall when I was setting up my Honeywells, it recommended just one cycle per hour for radiant heat.

Overshoot, a different story. Hitting the set temp turns off the boiler but the water and the radiators stay hot and emitting heat for a good while. You don't want short cycles on the boiler. You can reduce the overshoot a bit by reducing the boiler's temperature setting.

You really have a choice. If you want the house no hotter than a specific temperature, to deal with the overshoot, set the thermostat at one degree lower. If you want to minimize the undershoot, you need to keep the setting higher and accept it's going to get a bit warmer than you want. I do think turning down the water temp setting will help. You can't expect a radiant system to respond immediately to on and off commands (as forced air does) because they don't work that way.

Anticipators on "old-style" thermostats are small heating devices that trick it into shutting off prematurely ... which solves the overshoot problem with radiators. The adjustment scale sometimes refers to the milliamp rating of the 24-volt control system, but trial-and-error comes into play for finding the appropriate setting.

Newfangled electronic stats have the cycles-per-hr adjustment, also sometimes called temperature swing or sensitivity.

gilach, you didn't cite the model number of your Lux, so anyone here trying to help can't look up details on it to determine if there's a setting adjustment that would help.

Thanks for your input, everyone. The Lux model is TX9600TS Universal 7-Day Programmable Touch Screen Thermostat. My guess is that it's currently set to one cycle per hour.

Here is the TX9600TS manual. You thermostat has a temperature swing setting feature. This allows you to set the cut in and cut out temperature delta. The default is 0.25 degrees, but it can be set as high as 2.25 degrees F. It is described on page 24, and how you can change it is described on page 26.

Check to see what you setting you have. There are also additional settings that could cause the behavior you are seeing. Read the manual carefully. I suggest you make one change at a time and give it a day to see if has made an improvement.

This is an interesting thermostat and it sells for $54 on Amazon. I don't recall ever seeing the swing feature on other thermostats, even those that are much more expensive. You may already have the best thermostat for what you want to do. The problem may be that at $54 it is not built as a precision instrument and it has limitations.

Haha.

Mike,

"I don't recall ever seeing the swing feature on other thermostats, even those that are much more expensive"

As I said above, My Carrier Thermostat has this feature... it is 6-7 years old. (You guys helped me select it.)

SaltiDawg,

Refresh my memory, what is the model number?

I have a 14-years-old Carrier 'stat with a 9-position sensitivity/cycle rate adjustment (1 through 9, 3 is factory/default). It's limited to maximum 4 cycles per hour regardless of the setting. Model TSTAT-PHP01-B, I think is it.

Four Emerson Sensi at work with a 3-position adjustment.

Hello everyone,

I checked the swing. It was set to 4 (+/- 1 degree), so I changed to 3 (+/- 0.75 degree) to see if the heat clicked on immediately when the room temp dropped 1 degree below the set temp. It didn't. I woke up this morning and it was 63 degrees when the set temp was 64. The boiler had not been on in a while, definitely not in the previous hour, so I don't think it was the cycles per hour thing. I'm thisclose to giving up. Is there any other setting that I should adjust? Thanks for your input.

The thermostat displays temperature in whole numbers, but it appears the sensitivity can be as low as 0.25 degrees. So if the temperature is 63.5 degrees the thermostat will not call for heat because the swing is set to 0.75 degrees. But we don't know if the thermostat would display 63 or 64 degrees.

It is possible the thermostat is not operating correctly. I would try a 0.5 or 0.25 swing to see if it makes a difference. I think that is too small a swing and if it is you will see it. It worth doing as an experiment.

It is also possible the thermostat is calling for heat, but for some reason the boiler is not responding. Do keep the thermostat at a constant 64 degrees, or do program different temperatures thought the day?

Just to say ... the Sensi 'stat has only the three choices for cycle-rate/sensitivity -- Slow, Medium, and Fast. Recommendation is Slow for hot water / steam heating. Medium for forced-air, and Fast if desired for tighter/less temp swing between cycles.

I'm going to continue this experiment and set the swing to 0.25 and see if it makes a difference. My daytime temp is set at 64, evening 66, overnight 64, and morning 65. So overall, very little difference. I'll let you know how I make out.