programmable thermostat for weekend house - radiant in floor slab heating

[speeder]

I hope this is the right section to post this in.

I have a house that I only spend weekends at. It has a gas fired boiler about 15 years old, with in slab radiant floor heating. It is about 2000 s.f. bungalow.

What I'm wondering is if it would make any sense to have a programmable thermostat that would only heat the house on the weekends. I realize I may have to program it to turn on a day ahead of time to reach the set temp by friday night. It does take a long time for the house to warm up if the thermostats are turned off during the week. I am in the pacific north west. Would the savings during the week more than offset the amount used to heat up the slab again?

I've read some posts about a programmable thermostat not making sense during daily usage, but this is a little bit different situation.

from the gas bill, average daily usage is about 0.66 GJ per day when the non-programmable thermostats are kept at about 20 celcius.

Thanks in advance for any advice.

[Dana]

It absolutely makes sense! If you're shutting it down for days at a time, the fuel savings are tremendous!

If your current thermostats are simple-minded types that would be used for any hydronic heating system and not a radiant-specific type and you're satisfied with their performance in your system you can go ahead and use any standard programmable T-stat.

But you may want to wire it in series with a smarter T-stat that has algorithms designed for managing the thermal mass of the slab to avoid big over/undershoots of the desired setpoint, using the programmable thermostat basically as a timer that interrupts the call for heat from the smarter Tekmar or Wirsbo or whatever. There may be programmable timers that could provide the same or better function as the programmable T-stat too, since you'd only be using it for it's time of day/week function. If there is real freeze risk you need a different approach though.

[jadnashua]

It's been awhile since I shopped thermostats, but some of them have multiple entries to customize them for the type of heating system you have...I'd look into one of them. There are also some that can be controlled via the internet or telephone which might be useful if say you're going to skip a weekend, or come in early.

One thing to keep in mind that big swings in temperature may affect some things in the home, wood furniture and musical instruments like say a piano. Often, those temperature swings involve changes in relative humidity. From an energy standpoint, it makes sense to set back. On some thermostats, they learn the response of the heating system, and instead of setting the time to turn on discretely (which could change based on the indoor temp and outdoor temp), you set the TOD when you want it a certain temperature. It then monitors the response and on say a mild day, might start 15-minutes before, but on a cold day, maybe hours before. I know some of the Honeywell thermostats do that. This can maximize the setback period while maintaining the desired temp at the requested time without you having to anticipate the response.

[Dana]

Jim- can you share the model #s or a series #s for the smart Honeywells with the response-anticipation feature? (via PM, if you don't want to post it publicly.)

[speeder]

But you may want to wire it in series with a smarter T-stat that has algorithms designed for managing the thermal mass of the slab to avoid big over/undershoots of the desired setpoint, using the programmable thermostat basically as a timer that interrupts the call for heat from the smarter Tekmar or Wirsbo or whatever. There may be programmable timers that could provide the same or better function as the programmable T-stat too, since you'd only be using it for it's time of day/week function. If there is real freeze risk you need a different approach though.

How would you wire the simple thermostat in series with something like a tekmar (which model would you say?) Why would I wire it in series with a tekmar instead of just replacing it with the tekmar? Or do you mean that I can keep all my old T-stats (there are 4 zones) and just use one tekmar?


I know the tekmars have "response-anticipation" and overshoot type algorithms, but do they all have these features (even the least expensive ones). Which model honeywell have this feature also.

[jadnashua]

Most of the Honeywell themostats do this...here's just one 7-Day Programmable Thermostat - RTH8500D. The webpage with all of them is at: http://yourhome.honeywell.com/home/p...s/thermostats/ Some are listed as best installed by a pro, but a savy DIY'er should be able to handle it as well.

Basically, with most programmable Honeywell thermostats, you set the desired time you want the house at a particular temperature rather than say 1-hour before you get up. It then adjusts when it needs to start warming or cooling based on the rate of drop or rise and (on some) outside conditions. So, on a mild night, it might not start until a few minutes before your set time. On a really cold night, it might start lots earlier. They have some that are specific to radiant, as well. I've not read the details on them, as with the way my place is setup, it's not practical, at least the way it is setup now.

[Dana]

How would you wire the simple thermostat in series with something like a tekmar (which model would you say?) Why would I wire it in series with a tekmar instead of just replacing it with the tekmar? Or do you mean that I can keep all my old T-stats (there are 4 zones) and just use one tekmar?


I know the tekmars have "response-anticipation" and overshoot type algorithms, but do they all have these features (even the least expensive ones). Which model honeywell have this feature also.

Standard T-stats are set up with a hysteresis (fixed or programmable) and an anticipation (not usually programmable) more suitable for low to moderate mass radiation, whereas slab radiant is very high-mass requiring small hysteresis but "smart" anticipation. Unless the anticipation period is comparable to the delay from heating up the thermal mass of the slab, every time there is a rapid change in heating load (or setback) the temperature in the room will undershoot and overshoot the setpoint and oscillate around the setpoint, sometimes eventually settling in to a tighter range around the desired temp, but sometimes not. Radiant-specific Tekmars use a proportional-integrating (PI) or proportion-integrating-derivative (PID) approach to control to keep the room in a relatively tight band around the setpoint by adjusting the anticipation and duty-cycling the call for heat sent to the boiler based on how far away the temp is from the setpoint and averaging the recent temperature history relative to the setpoint, (and in the PID case, how fast the temp is approaching the setpoint.) IIRC the Tekmars also "learn" the system and adjust the coefficients of the proportional, integral, and derivative equations dynamically based on how the system responds. (If all of this sounds like Greek, so be it- this is classic system control math.) I'm not very familiar with the Honeywell line of T-stats that are radiant-specific.

Most T-stats are 2-wire devices that open and close a switch/relay based on the programmed time of day function and the room temp. To wire it in series you'd have to figure out which wires from the existing T-stat provides that signal, as well as the programmable one, and use the programmable T-stat to maken & break the connection from the existing one.

[ErikJensen]

I actually own a Honeywell RTH8500D. It's a pretty neat device, I have it programmed to bring the heating down when I leave for work in the morning and then about 30 minutes before I arrive back home in the evening it has turned the heating back on. It has indeed saved me some money in the 8 months I have been using it!

[speeder]

dana - thanks for that thorough explanation of the algorithms in the thermostats. I still don't understand why you need to wire a tekmar in series with the old thermostat. why not just replace it?

erikjensen - are you using that honeywell model with a a concrete slab hot water radiant floor heating system?

[Dana]

SFAIK there are no programmable time-of day/week thermostats that have Tekmar-type control algorithms.

[BadgerBoilerMN]

For certain of our commercial radiant floors, days, are required to satisfy the thermostat. Depending on design temperature and control system a properly designed radiant floor heating system is infinitely controllable. Experienced help would be in order.