Optimum Control for Underfloor Radiant - On all the time?

Antony Upward

Hi,

Had a new underfloor radiant system installed in my basement late last year...very nice.

Question - is it more energy efficient to set the desired temperature for the floor and have the boiler maintain that temperature 7/24 - or is it more efficient(since we're at work M-F 8-7) to have a timer control when the floor should be brought up to temperature (like we do for hot air)?

Our floor is covered with ceramic and cork tile.

If it should be controlled by a timer...what would you recommend?

Thanks
Antony

Paul Pollets

Best method

is constant pump circulation based on outdoor reset. The control should work with either injection mixing or 4 way mixing, depending upon boiler type and preferences. If the radiant floor is a concrete slab, it requires a certain temperature flow at any given time, depending upon how cold it is outside. "Under floor", Joist-bay or suspended applications require hotter temps than a slab, and should be controlled seperately.

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Antony Upward

Thermostat for Underfloor Control

My question is more to do with the control.

Is it more energy efficient to set the floor temperature and have the floor stay at that temperature 7/24 or

Is it better to have the floor temperature change based on time of day - like one would for a hot air system.

I know the thermal mass of concrete is high...so I can well imagine a greater efficiency with a constant temperature...but since we're only in the underfloor heated area of the house from 7pm-midnight (5 hours out of 24) I thought it might be better to let the floor cool over night and during the day and reheat each afternoon.

Comments?

Antony

Christian Egli

Heat for occupancy

Energy savings will only come if you cycle the temperature up and down to follow occupied and unoccupied times. It is not more fuel efficient to keep the system at one temperature setting at all times, but it is much easier to handle.

The greater savings can be achieved if you turn the system completely down or off when you are not there to enjoy the heat, but this may not be entirely practical.

With floor heat, you have to be mindful of the long lag time between a call for heat and the time the floor start warming up. So, you have to anticipate when you'll be calling for heat and have your programmable thermostat go on an hour, or two, or more, before you'll need your floors warm. The same applies at the end of the cycle. The boiler has to shut off way before you want your floors to be cold.

Having an outdoor thermostat kind of predicts all of this in function of what happens with the weather. It is a nice way to control systems that have a long lag time, however, it does not account for occupancy. There are controls on the market that do all that though, and they modulate the water temperature in function of the weather.

If you already have a programmable indoor thermostat, try playing with your schedule and see what happens.

Antony Upward

Maximizing Savings

Just to check...

If it takes E units of energy to keep a floor at a certain temperature T 24/7 it will always take <E units to keep the same floor at temperature T for a period less than 24/7

I'm thinking of a Tekmar Controls 510...which has a slab sensor, air sensor and has the sort of "early-start" capability you mention - to accomodate the high thermal mass of the slab.

Comments?

Weezbo

depends where you \"Live\"

Where do you live comfortably? Say i like 56¡ãF its comfortable to me.well dial the water temp down till its within a few degrees or as close as possible to there and leave it alone/. if the control dont wanna Go There as the young people say these days,then the control is the thing that needs the attention. if i say feel comfortable at 80¡ãF well ok many controls like that number and Go There. now if you want to have 80 on an80 degree day you really have no real need for circulation unless,...you are trying to evenly distribute That temperature throughout the environment. now if you feel comfortable at 70¡ãF and the sun is shining down on the house and its (99) in the shade well,if you have a means or "Shedding" loads, recirc can help.if it is 40 below zero outside Stuffs different. the point of the idea is this, during some times of the year very little Extra heat is needed for some people to feel comfortable,and during other times of the year Lots of extra heat is required. today the Rain is happening ...i plugged the boiler back in, yesterday it was 80 so i unplugged the boiler and turned on my circ pump,to "Moderate " my environment. forgive me if this isnt clear... i have radiant and baseboard both react and distribute heat somewhat differently. set back thermostats may not react on thier own in the control of heat as one would expect. i can turn the thing to 90 ,that dont mean it will be 90 in 5 mins. it is rather a complex formula the control must preform to get it to 90 and stay there. dialing the thing up and down every 15 mins doesnt really get it as it were either. so sometimes the heat is shooting past the thermostat then diving down to someother depths...i find that to be uncomfortable. once i hook up the variable speed injector and dial down the mix ahead of it ,so it cannot overshoot well then things are ok for a while,however before winter i have to allow the water temp ahead of the variable speed pump to attain a slightly higher temperature.then i am fairly certain that the flow will be continious,my place wont freeze and i will be squeezing every btu through the system that is needed and no more....... so, as lengthy as this is theres alot ive said here that can help.... if you turned the thermostat down in the evening and back up in the early a.m. back down when you left for work ,back up two hours before your arrival home thats about all the Range the thermostat would be able to really deal with. just precisely what the savings would be ,is, would require lots of information that i dont have available. my best guess would be to keep the Range of set backs to about 4-6 degrees.

Mark Wolff

Timer Controls and such

The short answer is concrete slab = continuous temperature
underfloor staple up (plywood floor on floor joists) = timer based reset or even better Programmable T-Stats with time and temp based reset.

Now for the long answer. If your basement is a concrete slab with tubing laid in it, the time required to heat that slab the 4-10 degrees you may try to reset will be too long. Also the slab won't cool quickly enough so you really at that point are just turning the heat off and trying to guess how long it takes to heat back up. Keep in mind that a properly insulated concrete slab can hold enough residual heat to keep a home warm for over 24 hours. With a concrete slab I never set back more than 2 degrees Farenheit.

With a staple up radiant heating system there is a much smaller mass to heat and release heat, therefore greater setbacks can be achieved. Give 1-4 hours for recovery time (greatly dependent upon floor covering ie. tile, hardwood, carpet, rugs, etc.)

For control, one option is a simple on/off switch for your whole system, but a much better solution is to INVEST in programmable electronic thermostats with multiple time and temperature settings per day. Honeywell is a consistant maker of quality t-stats, as are several other brands. I would recommend you NOT purchase the cheapest programmable thermostat you find. You very well may have trouble with it.